Advanced inorganic materials and structures for photovoltaics

Resume : This paper presents an overview of the state-of-the-art in synthesis methods and structure-function investigations for exotic forms of silicon, including Si clathrates, Si-24, BC8 crystalline structures [1]. The outstanding challenges to realizing these materials as the next generation semiconductors will be presented. For example, one key challenge to uncovering the structure-function relationships of these materials is the ability to produce sufficient quantities of crystalline Si-caged compounds. The synthesis route to large, gram-scale quantities of almost phase pure Si clathrates will be presented. Understanding the fundamental electronic and optical properties of group IV allotropes is another key challenge. Discussion will be given to the role that computational calculations can play in guiding the synthesis protocol of Si clathrates and other exotic forms of Si, as well as exploring the potential of other allotropes that indicate promising structure stability and PV properties. For example, a coupled experimental and theoretical approach suggests the group IV cage-like allotropes can exhibit direct or quasi-direct bandgaps. The role of nanostructured materials on the electronic properties of group IV crystalline materials will be also discussed, with specific examples for Si clathrate and BC8. Recent calculations suggest the possibility of producing BC8 quantum dots, which are optically active, low bandgap semiconductors with much higher multiexciton generation rates than d-Si quantum dots. 1. P.C. Taylor, Exotic Forms of Silicon, Physics Today, 69(12), 34-39, 2016. Acknowledgements: This work was supported by Energy Frontier Research in Extreme Environments (EFree) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science under award number DE-SC0001057.

Resume : With more than 90% of the photovoltaic market, crystalline silicon is a material of choice for solar cells. Its excessive consumption still remains an issue: 180µm-thick wafers are used when 50µm would be enough to absorb most incident photons. Several methods exist for the kerf-free transfer of thin silicon seeds, but are usually too expensive or difficult to control. We demonstrated that an innovative way to obtain ultra-thin silicon layers by guiding a stress-induced spalling technique with low-energy hydrogen implantation allows the production of ultra-shallow substrates. Silicon layers with thicknesses varying between 50 and 120 µm have been obtained using our process by applying an appropriate thermal treatment to the implanted sample before bonding on a stress-inducing substrate. These thin silicon layers were used as p-type active layers for the production of crystalline silicon solar cells. After etching of the implanted surface in order to remove the damaged zone as well as the first 700nm layer, the silicon samples were processed using a classical solar cell production method. The junction was made by diffusion using a phosphorus solution. The first observed results showed that efficiencies of about 10% can be achieved. The optimization of the process is still in progress.

Resume : Seeding is an explored way to control the final structure of Silicon PV ingots. In HPMC–Si ingots, nucleation on numerous seed grains creates grain boundaries that can terminate the propagation of dislocation clusters. In the present work, we focus on the impact of the initial growth in G2 ingots that were prepared by directional solidification with a seed layer of poly-Si chunks. The subsequent grain structure formation was characterized by photoluminescence, metallography, and EBSD. In the remaining seed region, different photoluminescence intensities are evidenced which reveals the existence of two morphologies, a genuine non-melted seed from the poly-Si chunks, and a region within the seed layer that is generated from re-solidified of infiltrated molten silicon. Both grain morphologies in the seed layer have a random orientation and we evidenced that all grains at the solid-liquid interface grow by epitaxy at the start of growth. As a consequence, grains in the first growth layer are also randomly oriented. The columnar grains, which grow from wider grains of the seed layer, are larger. During growth competition, these grains reach a higher solidification height compared to grain that grow on narrow grains in the seed layer. Moreover, the dominant grain boundary types are RAGB, followed by Σ3 twin boundaries in both seed and growth regions. These results give prospects to improve seed arrangement or new coating for application to HPMC-Si process.

Resume : Silicon exhibits a large variety of different bulk phases, allotropes, and composite structures, such as, e.g., clathrates or nanostructures, at both higher and lower densities compared with diamond- like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, which are non-toxic and earth-abundant, with properties tailored precisely towards specific applications. We discuss how such novel Si based materials may be used to significantly improve the efficiency of solar energy conversion devices.

Resume : MeV Hydrogen implantation in silicon followed by a thermal annealing is an economical kerfless approach that can be used to produce ultra-thin substrates. This process have been used to produce ultra-thin (111)Si substrates with thicknesses ranging from 20 to 150 µm. The main advantage of such a process is the production of substrates without raw material losses, which can help to decrease the solar cells prices down. However, if good results were reported on (111)Si, its feasibility on (100)Si (the main silicon used for solar cells) was not yet demonstrated. Recent studies showed that unlike (111)Si, ultra-thin substrates produced with (100)Si break in very small pieces during the delamination process. In this paper, our effort focused on the investigation of optimal parameters leading to the delamination of large surfaces of (100)Si substrates. We particularly studied the effects of hydrogen fluence and the layer thickness on the delamination process. Implantations were carried out with fluences from 7x1016 to 3x1017cm-2 at energies up to 2.5MeV. Full delamination of large areas was obtained only for layer with thicknesses higher than 50 µm and with hydrogen fluences higher than 1×1017 H/cm². Yet, we are able to detach (100)Si layers with surfaces up to 12 cm². Solar cells built using delaminated substrates provide efficiencies close to the reference ones, which demonstrate the good crystalline quality of ultra-thin substrates produced.

Resume : Crystalline silicon solar technology has been dominating the photovoltaics market for decades with no sign of change in the near future. Continuous improvements in efficiency were shown in recent years through the use of passivated contacts, with the reaching of unprecedented efficiencies. Such contacts allow a physical separation between the silicon wafer and the metal, which prevents the strong recombination which unavoidably happens at the metal-silicon interface. Different strategies are used, the most successful one being the use of amorphous-silicon as passivating and selective contact layer. We will discuss a few examples of recent advances towards ideal passivated contacts emphasizing the material requirements to fulfill this goal.

Resume : We present a new passivating contact concept utilizing microcrystalline silicon carbide (grown via hot wire chemical vapor deposition) and an ultra-thin silicon tunnel oxide (grown via wet-chemical oxidation) on a crystalline silicon surface (µc-SiC:H(n)/SiO2/c-Si). In addition to excellent passivation quality (up to 6.7 ms effective lifetime) this stack offers very high transparency combined with sufficient electrical conductivity. Hence, this novel functional stack can be used as a highly transparent passivated contact to be implemented at the front side of either a two-side contacted or an interdigitated back contact solar cell. The fabrication of the film stack does not involve any high temperature step as in the case of TOPCon or POLO concepts and is therefore compatible with silicon heterojunction (a-Si/c-Si) solar cell fabrication. However, the mechanisms leading to high passivation quality using the µc-SiC:H/SiO2 stack are still not fully understood. Therefore we performed extensive studies on the fabrication and characterization of tunnel oxides using different wet chemical solutions and characterization techniques in combination with µc-SiC:H(n), respectively. We correlate the implied open circuit voltage to the oxide thickness, stoichiometry, hydrogen content, and lateral homogeneity. From these new insights we tuned the properties of the tunnel oxide to improve the passivation quality as well as the contact resistivity.

Resume : Previously, we reported a conversion efficiency of 15.5% on Se-free Cu(In,Ga)S2 solar cell via KCN-free process. It was the highest conversion efficiency on the Se-free Cu(In,Ga)S2 solar cells, however, it is still lower compared with Cu(In,Ga)Se2, CdTe and perovskite solar cells. There are three big restricting factors of the Eff enhancement on Se-free Cu(In,Ga)S2 solar cell. First, the carrier density is very low. This low carrier density is assumed that the degradation of all electric parameters, especially, open-circuit voltage and fill-factor. Secondly, the life time of absorber is very short, which is assumed to be the restrict of the open-circuit voltage. At last, Se-free Cu(In,Ga)S2 absorber has surface roughness after sulfurization. It is presumed that the fill-factor degradation was caused by the roughness of absorber. To improve them, we have been trying a variety of experiments on Se-free Cu(In,Ga)S2. In this paper, we mainly focus on the improvement of the carrier density, and report our latest progress. As a result, our champion cell demonstrated over 16% efficiency via in-house measurement, contributed by enhancement of carrier density. Additionally, we conducted some other experiments to boost the efficiency on Se-free Cu(In,Ga)S2 solar cells, for example, rapid thermal sulfurization, Ag doping, Cd-free buffer layer and so on. At the conference, we will show each effect of above experiments and reveal which trial was more effective for the Se-free Cu(In,Ga)S2 solar cells.

Resume : Current Cu(In,Ga)Se2-based solar cells are able to reach power-conversion efficiencies of more than 15% while exhibiting experimental dislocation densities up to 10^10 to 10^11 cm^2 [IEEE J. Photovoltaics 2, 364–370 (2012)]. This finding suggests that dislocations are electrically inactive or passivated by point defects. In order to understand their role, we studied 60° mixed dislocations, being one of the dislocation type found in experiments, by means of first-principles calculations within density functional theory. We do so for both parent compounds CuInSe2 and CuGaSe2 and for all the possible core configurations: glide set, shuffle set, cation-rich and anion-rich. Our results show that in Cu(In,Ga)Se2, the electrical activity of stoichiometric cores is determined by the presence and strength of cation-cation or anion-anion “wrong bonds”. We found that only the cation-rich cores are active when stoichiometric. However, since absorber layers of highly efficient Cu(In,Ga)Se2-based solar cells are non-stoichiometric, we also focused on the role of segregated defects in the cores. A recently proposed structural model, [J. Appl. Phys. 115, 103507 (2014)], associates Copper depletion and Sodium accumulation inside and around the cores to a valence-band offset, which could explain the apparent harmless nature of dislocations in Cu(In,Ga)Se2. However, our results show that the driving force for Sodium segregation into the cores is the strain interaction between the point and the line defect. Therefore, Sodium accumulation is not able to create a cylindrically symmetric valence-band offset, which would be needed for the proposed model to work. Our results point to the fact that Sodium indeed has a beneficial effect. However, our data suggest that this effect comes rather from weakening of the “wrong bonds” which induce deep transition levels. In summary, our results present an initial step towards understanding why dislocations appear to be non-detrimental for the efficiency of Cu(In,Ga)Se2-based solar cells.

Resume : Transition metal dichalcogenides (TMDs) receive much significant attention due to its intriguing properties for fundamental research and potential for application in electronics, catalysis, optoelectronics, and so on. In this work, we focus on growth of PtSe2 because of its chemical activity, small band gap and low resistivity. Recently, Yeliang Wang et al.1 demonstrate single layer of PtSe2 by single crystal Pt substrate and selenization with 270 ºC. Despite of this cases, there are other research groups trying to grow high quality by different methods and it should be pointed that the synthesis temperature of PtSe2 is much lower compared with other TMDs such as MoS2 and WSe2 which can reach below 300 oC. Besides, the electrical performance of PtSe2 is different from the other TMDs. Theoretical calculations of the band structure of PtSe2 shows large changes in the bandgap from 1.3 eV at the monolayer to 0.3 eV at the bilayer and it is expected to be a semi-metallic behavior at multiple PtSe2 monolayers. On the other hand, PtSe2 have potential for high mobility device and other application such as hydrogen evolution reaction, photo-catalytic and gas sensor. In our work, we try to grow high-quality PtSe2 by the plasma assisted selenization process at a low temperature. First, we deposit Pt metal on arbitrary substrate by DC sputtering process within 5 nm thickness. After Pt deposition, we apply plasma assisted selenization process to form a few PtSe2 monolayers at a temperature below 200 oC. Material characterization based on TEM, XPS and Raman analysis was applied to investigate the quality of PtSe2 film grown by different condition. Moreover, we experimentally demonstrate the high performance of back-gated device without transfer process. We also explore the photonic electrical properties of PtSe2 by directing fabricating on Si substrate and flexible substrate. The good photo-responsivity, wide band detection, and good stability are demonstrated. References 1. Wang Y., et al. Monolayer PtSe(2), a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt. Nano Lett 15, 4013-4018 (2015).

Resume : Cu2ZnSn(S,Se)4 (CZTSSe), obtained by non-vacuum methods, is recognized as a promising In-free absorber layer towards scalable and low-cost thin film photovoltaic technology. Herein, the photovoltaic performance of CZTSSe prepared from a single precursor solution is assessed. CZTSSe films are deposited by spin-coating a precursor solution containing metal chlorides and thiourea on to a Mo substrate and annealing under Se atmosphere. Quantitative XRD analysis and Raman spectroscopy confirm the formation of CZTSSe in kesterite phase with Cu-poor and Zn-rich composition. Electron micrograph shows growth of compact and homogenous microcrystalline films. Solar cells with architecture: Mo/CZTSSe/CdS/i-ZnO/Al:ZnO are fabricated with total area of 0.25 cm2. The performance of 20 devices is measured under AM1.5G illumination with the best device featuring a short-circuit current of 27.8 mA/cm2, open-circuit voltage of 396 mV, fill factor of 52 % and power conversion efficiency of 5.7 % (external quantum efficiency >70 %). The overall dispersion in the figure-of-merits for all of the devices was less than 20% of the mean value. Power conversion losses, as probed by temperature dependent impedance spectroscopy, indicate two distinct bulk trap states with activation energies of 38 meV and 160 meV. It was also found that partial selenization of Mo substrate during annealing leads to non-Ohmic back contact barriers of up to 311 meV.

Resume : The compound semiconductor Cu2ZnSn(S1-xSex)4 (CZTSSe) is a promising alternative material for absorber layers in thin film solar cells. The structural property of particular interest is the distribution of the cations Cu and Zn2 in the kesterite type crystal structure. The presented study show that the degree of disordering of Cu and Zn on the structural sites 2c and 2d in CZTSSe (so-called Cu/Zn disorder) is influencing the electrical and optical properties. In the most investigations published up to now the low temperature ordering effects were deducted indirectly by Raman spectroscopy, spectrophotometry, electro-reflectance, and from kinetics simulations. The only direct methods to deduce the Cu/Zn disorder reported till now are neutron diffraction [1] and anomalous X-ray diffraction. The latter was used to determine the temperature dependency of Cu/Zn ordering in CZTSe kesterites [2]. A major reason for this is the difficulty of deriving reliable cation site occupation factors from X-ray diffraction data, because Cu and Zn2 are isoelectronic and thus have essentially the same X-ray scattering factor. But neutron diffraction can solve this problem; the coherent neutron scattering lengths of copper and zinc are sufficiently different. A detailed structural analysis of CZTSSe powder samples, grown by solid state reaction of the elements, was performed by neutron diffraction giving insights into the cation distribution within the crystal structure of CZTSSe, and consequently the Cu/Zn disorder. In the presentation the correlation between Cu/Zn disorder in CZTSSe and transport properties, e. g. characteristic temperature, NNH activation energy and acceptor band width, derived from resistivity measurements, will be discussed. [1] S. Schorr et al., Eur. J. Mineral. 19, 1 (2007). [2] D.M. Többens et al., Phys. Status Solidi B, 1–8 (2016).

Resume : CZTSe has gained a strong interest as absorber material for PV applications. However, the narrow tolerance to stoichiometry variations that favors the formation of secondary phases and cluster defects is a challenging drawback that restrains the device performances. In this context Raman spectroscopy has demonstrated to be a powerful tool for the characterization of CZTSe, allowing the assessment of relevant parameters such as secondary phases, crystal quality. In particular, UV-based Raman spectroscopy is a very promising technique, due to its high sensitivity of the surface region which provides information of the buffer/absorber interface. Until now technical limitations in the measurement of 100-300cm-1 spectral region impeded its full exploitation. In this work, an UV-Raman analysis using a special developed setup is presented. The UV-NIR multi-wavelength Raman analysis of CZTSe has allowed to identify a “non-band gap resonant” effect for the 325nm excitation, enhancing the 174cm-1 and 230cm-1 spectral regions which are attributed to photon coupling with the E1B optical active transitions. In particular, the UV-characterization of more than 200 samples with stoichiometries close to the compositions where highest efficiency are obtained allowed associating the 174cm-1 and 230cm-1 spectral regions intensities with the concentration of VCu and ZnSn point defects, respectively. The correlation of these intensities with the device optoelectronic performance will be discussed.

Resume : In2S3 as alternative for CdS absorber layer in CIGS-based thin film solar cells has gained lots of scientific attentions in recent years. β-In2S3 crystallizes in an ordered vacancy spinel-like structure, which can host impurities diffusing from absorber and/or front contact layers. The presence of impurities even at low concentration can highly influence the optical and electrical properties of In2S3 . Barreau [1] has enumerated the proper incorporation of third elements in In2S3, a route towards high performance cells. Insertion of Na and Cu in large extent can cause the formation of NaIn5S8 and CuIn5S8 compounds on the buffer side of the cell. Since lattice constants and band gap values of these compounds are different from β-In2S3 , their formation (in general, formation of NaxCu( 1−x)In5S8 compounds) can improve band alignment at absorber/buffer interface. Therefore, it is important to study their bulk properties as well as the interface they form with the absorber layer. In this contribution we calculated bulk properties of pure β-In2S3, NaIn5S8 and CuIn5S8 compounds and compared their atomic structures and electronic properties. The calculations were done using Vienna Ab-initio simulation package (VASP) using the projected augmented-wave method. For bulk calculations, exchange-correlation potential was treated with 25% contribution of the Hartree-Fock exchange and an adjusted exchange-screening parameter. To study absorber/buffer interface, we follow the orientation reported by Abou-Ras et al. [2] and to study the influence of Na and Cu incorporation on the absorber/buffer interface, we compare lattice matching for all three CuInSe2/In2S3, CuInSe2 /CuIn5S8 and CuInSe2 /NaIn5S8 interfaces. Through these calculations, we are also able to see how inter-atomic distances and bond lengths change in these three interfaces comparing their strained and coherent states. [1] N. Barreau, Indium sulfide and relatives in the world of photovoltaics, Solar Energy 83, 3 (2009), 363-371 [2] D. Abou-Ras et al., Interfacial layer formations between Cu(In,Ga)Se2 and InxSy layers, Journal of Applied Physics 98, (2005), 123512

Resume : Hybrid organic–inorganic perovskites (HOIPs), such as methylammonium lead halides, have garnered a huge interest not only due to their exceptional optoelectronic and light-absorbing properties but also because this has been coupled with a versatile and facile synthesis scheme that can be easily adapted. Presented here is a study of the formation, degradation and then subsequent regeneration of HOIPs, specifically methylammonium lead iodide, all the while being exposed to ambient conditions (hence humidity, oxygen and light). We have evaluated three specific aspects: (a) Have explicitly varied the nature of the underlying electron transport layer and have observed that surprisingly the morphology and surface roughness of the underlying substrate imparts another condition on the degradation rate of perovskites and therefore it adds to the list of parameters that need to be paid attention to in order to minimize the degradation process. (b) Moreover, we have also evaluated the influence, individually, of UV and visible light on the degradation of these HOIP coated substrates. UV irradiation imposes a more dramatic effect on the degradation rate and this is irrespective of the underlying substrate present. (c) Furthermore, making a successful, degradation resistant device is possible, but involves deposition of additional layers (e.g., hydrophobic layers to minimize interaction of perovskite with moisture), adding to the cost of the overall device. The current study, in addition to describing the substrate dependent degradation, introduces a simple and cheap method to reverse the degradation itself caused by moisture and light. This method involves a simple chemical treatment of degraded perovskite that, depending on the degree of degradation, regenerates the tetragonal lattice of methyl ammonium lead triiodide as confirmed by the XRD and UV-visible absorption.

Resume : Hybrid halide perovskites are currently under intense investigation due to their potential applications in optoelectronics and solar cells. Among them, MAPbI3 where MA stands for the methylammonium cation, exhibits ideal properties for solar cells. In attempt to identify new lead-free halide perovskites we have studied using ab-initio electronic structure calculations in conjunction with hybrid functionals a series of MABX3 compounds where B is a divalent cation and X a halogen atom. Our results suggest that the compounds under study exhibit a variety of lattice constants and energy band gaps. Especially, MAGeCl3 and MAGeBr3 are susceptible to replace MAPbI3 in devices since they show comparable energy gaps. Further calculations on the mixed hybrid halide perovskites show that we can tune the values of the energy gap although no simplified pattern exists. Our results pave the way for further investigation on the use of these materials in technology relevant applications.

Resume : Quantum dots (QDs) have received great interest for the application of alternative light harvesters in dye-sensitized solar cells since they have advantages over conventional organic chromophores such as high extinction coefficients, broad absorption ranges and tunability of the bandgap. For the QD-sensitized solar cells (QDSSCs), a wide bandgap nanostructured electrode (TiO2 or ZnO) is desired to be well covered by QDs. It has been demonstrated that highly surface charged QDs of opposite signs can be alternatively deposited onto TiO2 as a dense and robust fashion. A multilayer of QDs is preferred for QDSSCs over a monolayer counterpart to fully utilize the sunlight incident into a relatively thin-film-based photovoltaic device. The controlled assembly of QD multilayers provides a model system for study and optimization of electron/energy transfers between QD layers. To further improvement in charge transfer, hydrocarbon surface ligands of QDs are replaced by short inorganic molecules such as metal chalcogenide complexes or chalcogenide anions. The effects of inorganic ligands on photovoltaic properties of QDSSCs were studied at the interface of TiO2/QDs and QDs/electrolytes by selective ligand exchange. Comparison studies of three kinds of metal chalcogenide complexes decorated QDSSCs implied that appropriate energy level combination between QDs and inorganic ligands is critical for couplings to QDs, electron transfers and the efficiency of QDSSCs. All-inorganic multilayered QDSSCs were fabricated by metal ion bridging between QDs and the photovoltaic properties were investigated for the dependence on the number of QD layers.

Resume : Single crystalline silicon (Si) wafers were produced by electrical discharge (ED) formed on metal multi-wires. Si bricks with dimension of 50 x 50 x 50 mm3 were sliced from an ingot grown along (100) direction. Effects of ED conditions such as applied voltages, pulse durations, and cut-off current on microstructures of the sliced wafers were investigated. Slurry based multi-wire slicing is well-known as a typical wafering process, which generates a large amount of waste. Recently, diamond wire is applied for the slicing due to no slurry waste. This approach, however, needs high energy consumption because this process is based on a mechanical abrasion between wire and Si. ED is an excellent energy source to saw a Si brick because there is no mechanical stress between wire and Si. ED is well known for precise machining of metal. However, Si is semiconductor, which indicates that its low electrical conductivity does not allow a formation of ED on Si. We have specially developed pulsed-direct current (DC) source for a formation of ED on Si. Their microstructures with various defects were investigated to evaluate the wafers for photovoltaic applications.

Resume : After the discovering of the potential of perovskite photovoltaics, this topic has a growing importance in the solar cell research area because of their simply applying by using wet chemicals. Perovskite materials are inexpensive and also have long charge diffusion length and high charge carrier mobility [1]. In this work we investigated the role of the modification of mesoporous-TiO2 layer with different flouro phenyl boronic acid derivatives self-assemble monolayers (SAM) in perovskite solar cell. Perovskite layer (CH3NH3PbIxCl3-x) was obtained by toluene additive when surface spinning. We characterized the modified and unmodified mp-TiO2/CH3NH3PbIxCl3-x coated perovskite solar cells by using XRD and AFM for the examination of structure and morphology. We also characterized the photovoltaic performances of solar cells and observed that some SAM molecules improve the photovoltaic parameters and efficiency of perovskite solar cells. The maximum power conversion efficiency (PCE) was obtained with 4-fluoro phenyl boronic acid SAM modified cell. Based on all of these results, a better PCE can be performed with provided knowledge. References [1] Jong H. Kim, Po-Wei Liang, Spencer T. Williams, Namchul Cho, Chu-Chen Chueh, Micah S. Glaz, David S. Ginger, Alex K.-Y. Jen, Adv. Mater. 2015, 27, 695–701

Resume : We synthesized fluorinated indoloindole derivative as a high-performance crystalline HTM for perovskite solar cells. A planar π-conjugated backbone linked with a flexible alkyl chain enabled a formation of molecular stacked arrangement by strong π-π interaction, which was revealed in single crystal analysis. In this regard, IDIDF in film state showed a higher mobility than that of p,p-Spiro-OMeTAD. PL quenching occurred more effectively at the perovskite/IDIDF interface, when compared to that at the perovskite/p,p-Spiro-OMeTAD. From CV measurement, a proper HOMO and LUMO energy level for IDIDF was found to be suitable for a HTM. As a result, the device fabricated using IDIDF showed a better performance as compared to p,p-Spiro-OMeTAD, exhibiting a best PCE of 19%. It was thus shown that a planar IDID core-based crystalline HTM is a promising candidate for highly efficient perovskite solar cells.

Resume : In the present study we use a rigorous numerical method to extract from a photoluminescence spectra some important optoelectronic parameters of photovoltaic device, ie Temperature, Quasi-Fermi Levels Splitting and Absorptivity. The theoretical basis used are the generalized Planck’s law for the emission of a non-black body device [1] and a sub-bandgap absorption model developed in ref [2]. Our model improves the resolution of the inverse problem through simultaneous spatial and spectral deconvolution. We show how the noise-to-signal ratio of the spectra influences the correctness and accuracy of the parameters found. This allows to distinguish if the correlations between the parameters are inherent to the model or have physical origin. Our numerical method and the previous results are then applied to analyse the absolute hyperspectral photoluminescence image of a GaAs solar cell as a proof of concept and then a CIGS micro-cell [3]. The maps of the thermodynamics properties and absorption are discussed and compared to other methods, including the previously well-known but less accurate fit of the linearized form of the generalized Planck’s law [3]. [1] P. Würfel P, Journal of Physics C: Solid State Physics 1982; 15:3967–3985 [2] J. K. Katahara and H. W. Hillhouse, ” Journal of Applied Physics, vol. 116, no. 17, p. 173504, Nov. 2014. [3] A. Delamarre et al, Progress in Photovoltaics: Research and Applications, vol. 23, no. 10, pp. 1305–1312, Oct. 2015.

Resume : In this work, we report on the fabrication of silicon tunnel junctions for the realization of tandem cells. The tunnel junction was realized on monocrystalline silicon (P-type samples) using two different processes (i) thermal diffusion of boron and phosphorus solutions from a solid source (ii) implantation of boron and arsenic ions followed by a rapid thermal annealing (RTA) to activate the dopants and cure the defects generated by the ion implantation. The dopants profiles after junctions formation were determined by ECV (Electrochemical Capacity-Voltage) measurements and SIMS (Secondary Ion Mass Spectrometry) analysis. The monitoring of the tunnel diode quality was carried out by Current-Voltage measurements under dark conditions using mesa structures. Several experimental conditions (ions energy and doses, dopants sources, diffusion temperatures…) were investigated to obtain the tunneling effect. We have observed the tunneling behavior only for few operational conditions and combinations of processes. For instance, two consecutive implantations of boron (dose: 2×1015 at/cm2, Energy: 25 kev) and arsenic (dose: 2×1015 at/cm2, Energy: 30kev), thermally annealed at 925°/30 min resulted in a current density values of the peak and that of the peak to valley to be around 50 mA/cm2 and 1.7 respectively. A very narrow depletion zone of 3.9 nm is calculated from the measured doping concentrations. Such silicon tunnel junctions are very suitable towards the realization of III-V or perovskite/ silicon tandem cells. Acknowledgments: This work is carried out within the ANR NOVAGAINS framework.

Resume : The elaboration of crystalline silicon films on aluminum substrates would has the challenge to use the latter as a back contact, in addition toits flexibility and high reflectivity. In this work, we investigate the direct formation of crystalline silicon films versus deposition temperature and gas flow and ratio conditions in an ECR-PECVD reactor. We used silane and hydrogen as precursor gases. The substrates were pure aluminium or aluminium-silicon alloys. Quartz and monocrystalline silicon substrate were used as reference substrates.The grown silicon films were characterized by Raman spectroscopy, SIMS (Secondary Ion Mass Spectrometry),SEM (Scanning Electronic Microscopy), spectroscopic ellispsometry and X-ray diffraction techniques. Silicon thicknesses between 0.4 and 3 μm were obtained on different kind of aluminium substrates.The analyses show that the grown silicon films formed at 480 °C under SiH4: 40 sccm and H2= 10 sccm are amorphous silicon at the initial stage and become crystalline as the deposition proceeds. Dopants distribution profiles show that the aluminium substrate, serving as a back contact for the final cell, also make it possible to create the back field thanks to the diffusion of the aluminum during the silicon growth. We have shown that such process allows the formation of a crystalline silicon film that can be used as an active layer for a PIN solar cell configuration made on an aluminum substrate. Acknowledgments: The research leading to these results has received funding from the European Union Seventh Frame work programme (FP7/2007-2013) under grant agreement n°608593

Resume : For more than ten years, III-V materials have been intensely studied for optoelectronic applications in UV and blue. In 2003, the energy gap of the InN is lowered to 0.7 eV paving the way for alloys that can cover almost the entire solar spectrum. In particular, the InGaN alloy was widely studied for photovoltaic applications thanks to its wide spectral coverage, its good electrical characteristics and its resistance to high power. Nevertheless, this alloy presents some disadvantages such as the absence of suitable substrate, difficult growth for indium percentages greater than 30% and P-doping still somewhat effective. In this work we are interested in the feasibility of manufacturing solar cells based on nitride materials on inexpensive substrates such as silicon or glass. In my context was the InGaN solar cell simulation with a proposed model to determine the influence of different parameters.

Resume : Real-time three-dimensional reciprocal space mapping (3D-RSM) measurement during GaAsSb/GaAs(001) molecular beam epitaxial growth has been performed and the results has been compared with the results of InGaAs/GaAs(001) to investigate the differences of In and Sb atoms on anisotropy in relaxation processes caused by alpha and beta misfit dislocations (MDs). Previously, we have investigated the InGaAs/GaAs(001) using the real-time 3D-RSM technique and found that alpha-MDs formed prior to beta-MDs [1]. InGaAs is III-III’-V type ternary alloys and the core of alpha-MDs is group III atom. On the contrary, GaAsSb is III-V-V’ type. These differences should change the relaxation processes and such basic information will allow us controlling relaxation of lattice mismatched hetero-epitaxial systems. GaAsSb films with 13~15% of Sb content are grown on GaAs (001) substrates. During growth, 3D-RSMs around a 022 reciprocal space point were measured. The experiment was performed at the beamline 11XU of the SPring-8 synchrotron radiation facility. Details of the 3D-RSM measurements are described in our previous paper [1]. At the case of high growth rate (0.3ML/s), the relaxation anisotropy in GaAsSb is the same with that of InGaAs. However, at the case of low growth rate (0.1ML/s), beta-MDs formed prior to alpha-MDs. These suggest that relaxation anisotropy is affected by not only atomic types but also growth rate. [1] H. Suzuki, et al., Appl. Phys. Lett. 97, 041906 (2010)

Resume : ZnO, a wide band gap semiconductor with 3.3 eV band gap at room temperature, is intensively studied for photovoltaic (PV) applications. Most of the studies concentrates on use of ZnO as a transparent electrode (transparent conductive oxide (TCO)). Other possible applications include use of ZnO as a buffer layer and/or as, we demonstrated recently [1-3], as a n type emitter in Si- based solar cells. In the present work we discuss use of a modified ZnO/Si structure with a 3D top electrode consisting of n type ZnO:Al (AZO) layer (deposited by atomic layer deposition method (ALD)) and zinc oxide nanorods (grown by a hydrothermal method). For such solar cells, with electrode deposited on p-type Si wafers (2 Ωcm; (100)), we report light conversion efficiency up to 14%. Various versions of PV cells of the same architecture are investigated, with thickness of silicon layer varied between 50 μm and 200 μm. Electrical parameters are similar to the previously employed silicon wafers (see references [1-3]). For thinner silicon layers the Jsc value deceases from 38 mA/cm2 to 30 mA/cm2, whereas the Voc remains similar, of about 500 mV. The solar cells efficiency varies between ~10% for the cells with the thinnest Si layer to ~14% for the thickest ones. This work was partially supported by the National Science Center (Decision Nos. DEC-2012/06/A/ST7/00398 and DEC-2013/11/B/ST7/01385), and (Wroclaw group) by the National Laboratory of Quantum Technologies (POIG.02.02.00-00-003/08-00) and Statutory grant 0401/0073/16. 1. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 147, (2016) 164-170. 2. R. Pietruszka, et al., Solar Energy Materials & Solar Cells 143, (2015) 99–104. 3. R. Pietruszka, et al., Beilstein J. Nanotechnol. 5, (2014) 173–179.

Resume : To overcome the limitation of wafer manufacturing, the ultra thin wafer is being manufactured with various processes. In this study, an internal processing test was conducted using a picosecond laser applicable to the spalling process for ultra thin wafers. The test was performed by selecting the laser of UV / Green / IR frequency and changing the process conditions of defocusing depth, power and feed speed processes. As a result, the high refractive index of silicon, 3.8, and the low transmittance of IR laser restricted the progress of the test. The result showed that inconsistent internal processing occurred under defocusing depths between 0 and 100 ㎛. The power of the laser was increased by 10 ~ 30% (5~15 W, 18.75 ~ 56.25 J/cm2) because of the low transmittance. The internal processing condition was identified from high energy density, but the relatively high optical absorption led to extremely high surface damage. Internal processing was obtained at the low feed speed of 60 mm/s, but, internal processing with 120 mm/s more occurred in irregular positions with varying feed speed. At 160 mm/s, internal processing did not occur. As a result, the most optimal condition for 50.4 ㎛ internal processing was obtained under the power of 20% (37.5 J/cm2), defocusing depth of 60 ㎛ and feed speed of 100 mm/s.

Resume : The electrical efficiencies of solar cells are strongly affected by their operating temperature. The Nominal Operating Cell Temperature (NOCT), which can be found in the specifications of a solar cell, is determined at the open circuit voltage (Voc) and is obtained under specific conditions (AM1.5G, Air temperature = 20°C, Wind velocity = 1m/s). The origin of solar cells heating under illumination has already been explored in previous studies; and a specific numerical model was developed in order to predict the influence of some parameters on their electro-thermal behaviour. In particular, the operating temperature depends on the environmental factors such as the radiative exchanges or the presence of convection. Besides, the simulations revealed the impact of the bias voltage on many physical properties of the solar cell: the operating temperature varies with the bias voltage. The aim of this work is to present an original set-up which was developed, allowing both thermal and electrical characterisations of silicon-based solar cells, with the possibility to control each environmental factors. Therefore, the influence of the applied bias voltage on the thermal behaviour, and the consequence on its electrical efficiency, will be studied. Finally, the comparison of different solar cells architecture under specific conditions will give the possibility to optimise their electrical properties with thermal criteria E. Skoplaki and J.A. Palyvos, "Operating temperature of photovoltaic modules: A survey of pertinent correlations", Renewable Energy, vol. 34, pp. 23-29 2009. R. Couderc, M. Amara, M. Lemiti, “In-Depth Analysis of Heat Generation in Silicon Solar Cells”, IEEE, vol. 6, pp. 1123-1131, 2016. D. Evans, “Simplified method for predicting photovoltaic array output”, Solar Energy, vol 27, pp. 555-560, 1981.

Resume : Metal oxide films are receiving growing interest in silicon heterojunction (SHJ) solar cells to replace amorphous Silicon (a-Si:H) as charge collector. Key parameters for efficient SHJ are excellent surface passivation, which reflects in high VOC, and efficient carriers collection, which is accomplished by doped a-Si:H layers. Transition metal oxides, among which Nickel Oxide and Molybdenum Oxide, are interesting materials to be adopted as window layers in solar cells due to their higher bandgap than doped a-Si:H emitter. Moreover due to high work function, they have been recently successfully used as hole collectors, able to guarantee built-in voltage of the junction as well as tunneling toward Transparent Conductive Oxide (TCO) electrode. Nevertheless, to successfully replace the doped a-Si:H layer in SHJ solar cell, these transition metal oxide must be carefully considered to avoid undesired performance loss during the entire solar cell manufacturing procedure. In this work we have fabricated SHJ solar cells based on transition metal oxides as hole transport layers and n-type c-Si passivated by amorphous hydrogenated Silicon Oxide (a-SiOx:H) that in our laboratory recently demonstrated high passivation quality and transparency. The top TCO has been chosen with an appropriate work function for charge extraction. Preliminary results show good performances of the structure, which could be further improved by adding efficient electron extraction on the solar cell back side.

Resume : Silicon based heterojunction (a Si:H/c-Si) solar cells are of great interest due to high efficiency and abundance of Si. The further increase of efficiency could be reached reducing absorption in emitter layer or using multijunction approach combining Si with III-V materials. The straight way to solve the both problems is to use GaP (2.26 eV) as wide band gap emitter also as a buffer layer for further III-V materials growth. However photovoltaic application requires a large scale high throughput technology with low thermal budget like PECVD. Here we propose to growth a thin n-type doped GaP layer on the surface of p-type Si substrate using low-temperature plasma enhanced atomic layer deposition (PE-ALD) at 380°C using PH3 and TMG as sources of III and V atoms. Thus n-GaP wide gap emitter forms anisotype heterojunction with p-Si. GaP layer being lattice matched to Si and grown at low temperature, should provide low defect density and sharp interface with Si substrate. The properties and perspectives of the n-GaP/p-Si solar cells fabricated by PE-ALD will be reported in the paper.

Resume : Ferroelectrics (FEs) are a class of materials that show a spontaneous switchable polarization in presence of an external voltage. In particular, some FE perovskite oxides provide the so-called "anomalous Photovoltaic Effect": under illumination they can develop an open circuit voltage much higher than the bandgap of the material, which make them attractive for photovoltaic application. FE solar cells are not limited to this effect and solar cell efficiencies above 8% have been obtained recently [1]. One of the main issues related to FE oxides is their high bandgap (usually >3eV) mostly due to the transition metal-oxygen bonds and the large difference in electronegativity between the elements. Recent investigations concern the development of new FE materials with lower bandgap, in order to better match the solar specrum and hence enhance solar cell efficiencies. In this work, we present the structural, optical and electrical properties of Bi2FeCrO6 (BFCO) oxide materials produced by pulsed laser deposition (PLD). X-ray diffraction indicates high quality epitaxial growth and phase-pure films. We have studied the evolution of parameters such as the bandgap, optical and ferroelectric properties depending on the growth conditions. The ferroelectric properties are investigated by piezoresponse force microscopy (PFM). The films are optimised in order to bring the bandgap as close as possible to 1.3 eV, while maintaining photovoltaic properties. Finally, solar cells structures made of BFCO are fabricated and analysed, in order to bring all oxide solar cells efficiency a new level of performance. [1] R. Nechache, C. Harnagea, S. Li, L. Cardenas, W. Huang, J. Chakrabartty, F. Rosei, Nature Photonics,9, 61 (2015)

Resume : A low open circuit voltage with respect to the band gap is a common phenomenon in Cu2ZnSnSe4 photovoltaic devices, plausible reason for this is a reduction in the effective band gap due to inhomogeneities in structure, phase, or composition. To gain a detailed knowledge of the influence of phase inhomogeneities on the performance of solar cells, the understanding of detection limits of conventionally used characterization methods is essential. In this work we studied the limits of the sensitivity of X-ray diffraction and Raman spectroscopy to the presence of two very common secondary phases for CZTS– ZnSe and Cu2SnSe3. Polycrystalline powder of two CZTSe samples and Cu2SnSe3 sample have been grown using the solid state reaction method, which were used to produce a number of mixtures of corresponding CZTSe with 1-5%, 10% and 20% of ZnSe or Cu2SnSe3 respectively. The structural characterization of the starting materials and mixtures was carried out by powder X-ray diffraction and subsequent Rietveld analysis of the diffraction data using the FullProf suite [1]. The amounts of secondary phases determined by Rietveld refinement have been compared with the initial data, determining in this way the detection limits of PXRD for these secondary phases. To study the crystal structure of the synthesized mixtures at the micrometer scale Raman spectroscopy has been employed. By performing Raman line scan measurements we evaluated characteristic Raman mode intensities corresponding to the different phases and thus are able to estimate the mixture composition. [1] Juan Rodriguez-Carvajal and Thierry Roisnel, www.ill.eu/sites/fullprof/

Resume : Cu(In,Ga)Se2(CIGS) solar cells have a very high potential to reduce production costs for photovoltaic modules. The challenge is to combine the large area issues with high throughput and yield with the high quality of the device. In this study, we have demonstrated a light weight Cu(In,Ga)(SeS)2(CIGSS) photovoltaic devices on flexible stainless steel foils by a nanoparticle-ink coating process. The ink with Cu, In, and Ga oxides was fabricated by a ball milling and dispersing procedure. The CIGSS precursor layer printed by simple and fast doctor blade technique is subsequently reduced under a hydrogen gas ambient to form a Cu-In-Ga alloy layer. The alloy layer was annealed under H2Se and H2S ambient to form the CIGSS absorber layer. Finally, the CIGSS solar cells and mini-modules (10x10cm2) with the use of standard foil/Mo/CIGSS/CdS/i-ZnO/AZO structure were fabricated. We achieved 14.6% efficient mini-module with adapted methods of module patterning, as well as 71% of the module distribution has over 13.0% efficiency.

Resume : It is well known that the performance of solar cells decreases as its operating temperature increases. Solar energy that is not converted into electric energy generates heat, so its operating temperature is higher than the ambient temperature. The power reduction with temperature is attributed to the temperature dependence of material parameters such as the energy band gap, minority carrier mobility and lifetime. This negative effect is characterized by the temperature coefficients of solar cells. In order to develop highly efficient solar cells, and reduce the degradation of the cell performance at high temperature it is important to understand the temperature effect in detail. Here we study the temperature effect on Si solar cells by solving the basic semiconductor device equations for electron-hole transport and the heat transport equation together. In conventional solar cell simulation, the temperature distribution of a solar cell is assumed to be uniform over a solar cell device. In contrast to the conventional assumption, the Lambert-Beer law dictates that more light is absorbed near the front side, so the temperature distribution is not uniform. Using the finite difference method, we first solve the heat transport equation with a heat source term given by the excess energy above the band gap. Given ambient temperature and solar irradiance, we obtain the spatial temperature distribution of a Si solar cell in a steady state. We find that the temperature is highest near the front side and decreases with the depth. This non-uniform temperature distribution gives rise to the spatial dependence of device parameters. We solve the semiconductor device equations by considering spatially varying device parameters and obtain the current-voltage curve. The temperature coefficient is then calculated as a function of solar irradiance and ambient temperature. We analyze how significantly the non-uniform temperature distribution affects the power output.

Resume : Copper indium gallium diselenide (CIGS), with the world record conversion efficiency of 22.6%, is a potential candidate for thin film solar cells, while the indium scarcity makes the higher raw-material cost be an issue for industry application. Thinning absorber layer is one of an effective approach of cost reduction, while the surface recombination at CIGS/Mo interfaces become the dominant recombination paths as we decrease the absorber layer. In this regard, we introduced the Al2O3 passivation layer between the CIGS/Mo interfaces via glancing angle evaporation system to create the nanostructures without using any mold. The negative charged surrounding the Al2O3 will repel the electrons and then reduce the recombination at the rear surface. The thicknesses of the passivation layers are confirmed by Atomic Force Microscopy (AFM), and the surface porosities are checked by Scanning Electron Microscopy (SEM). The optimized Al2O3 layer is inserted into our devices and finally causes an over 10% enhancement in the conversion efficiency in the case of the ultra-thin (~400 nm) CIGS absorber layer. We believe this work has the potential to be utilized in the commercial solar cells in the future.

Resume : CuInSe2, a member of the chalcopyrite family, was prepared via a facile one-step electrochemical process as a thin film and used in dye-sensitized solar cells (DSSCs) as counter electrodes. DSSC is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system. In practice it has proven difficult to make cells suitable for use in all weather from a number of expensive materials, notably platinum and ruthenium, and the liquid electrolyte presents serious challenges. On the other hand, thin film polycrystalline CuInSe2 (CIS) is an attractive material for low-cost photovoltaic applications and related compounds have reached small area conversion efficiencies exceeding 16%. The chalcopyrite structure of ternary I-III-VI2 compounds have high absorption coefficients making CIS well-suited for solar cells. The catalytic activity of the CuInSe2 films based on different precursor concentrations was investigated using electrochemical methods. DSSCs based on optimized CuInSe2 thin film as counter electrodes reached a power conversion efficiency of 4.30%, comparable to that of spin coating Pt (6.07 %).

Resume : Recently a-SiOx:H has attracted interest in heterojunction solar cells fabrication, because of its effective crystalline silicon surface passivation and larger optical bandgap than a-Si:H. Both amorphous layers are commonly deposited by PECVD from silane dissociation in hydrogen dilution. to obtain a-SiOx:H film, CO2 as source of oxygen is added to gas mixture, that heavily modifies the film growth, the composition and hydrogen inclusion, which influence the passivation properties. In this work we have analyzed the role of CO2 and H2 in the film growth and composition. In particular we have compared the effect of the two kind of amorphous layers on the effective lifetime of c-Si wafers, monitoring the lifetime stability and the effect of thermal treatment. FTIR analysis has been used to correlate the passivation properties to chemical bonding among silicon, hydrogen and oxygen. We have found that H2 dilution in the gas mixture during the film growth is the key to obtain high effective lifetime on c-Si wafers passivated with both kind of amorphous films. On samples coated with a-Si:H or a-SiOx:H different metastability has been observed on as-deposited, thermally annealed and TCO covered films. In particular the effective lifetime can improve up to 200% after thermal annealing being almost stable. Finally we have compared heterojunction solar cells with a-Si:H and a-SiOx:H buffer to remark different performances in Jsc and Voc due to film transparency and passivation capability.

Resume : The best candidate for low-cost and new high efficiency generation solar cells is CZTS, a quaternary chalcogenide p-type semiconductor with a character of an absorber layer based on naturally abundant and non-toxic elements [1-3]. Moreover, it is characterized by a direct energy band lies between 1.0 and 1.5 eV, a wide absorption spectrum range from ultraviolet to near-infrared light with an absorption coefficient as high as 104 cm−1. Besides, CZTS materials crystallize in various crystallographic phases: kesterite, stannite and wurtzite [4-5]. The Kesterite phase is more stable thermodynamically compared to the two other structures [6]. The purpose of this work is to investigate the effect of alkali metals as sodium and potassium, with different concentrations from 0% up to 10%, on growth and on properties of CZTS thin films. In this study, CZTS are prepared by simple and non-vacuum technique based on sol-gel method using non-toxic solvent. The prepared CZTS films are then sulfurized for one hour in tubular furnace. The obtained films present good crystallinity along (112) plane of pure kesterite phase, which is proved by XRD and Raman spectrums without secondary phases and films present large grain size. Other characterizations are performed to study optical, structural, electrical and chemical properties of the prepared films (absorption, SEM, EDS, hall effect…). Furthermore, based on the obtained results, this active layer presents the required properties and stoichiometry for solar cell application. ACKNOWLEDGMENT The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 608593. REFERENCES: [1] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93, 2009, pp. 583-587. [2] H. Katagiri, K. Jimbo, W. S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Thin Solid Films 517, 2009, pp. 2455-2460. [3] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, S. Guha, Sol. Energy Mater. Sol. Cells 95, 2011, pp. 1421-1436. [4] J. Paier, R. Asahi, A. Nagoya and G. Kresse, Phys. Rev. B 79, 2009, pp. 115126. [5] X. Lu, Z. Zhuang, Q. Peng and Y. Li, Chemical Communications 47, 2011, pp. 3141-3143. [6] S. Schorr, Sol. Energy Mater. Sol. Cells 95, 2011, pp. 1482-1488.

Resume : Planar structures of microcrystalline composite ZnS-Ga2S3 onto Ga2S3 single crystalline plates were obtained by thermal annealing of Ga2S3 plates in Zn vapors at 850-1350 K. The crystallites’ size depends on treatment conditions (duration and temperature), and vary from tens of nanometers up to micrometers. Composite layer surface morphology and its composition are studied by AFM microscopy and by X-ray diffraction, respectively. The obtained structures manifest itself as n/p junctions photosensitive in the energy range 2.8-4.5 eV. Structures’ photosensitivity depends on crystallites’ dimensions and composite layer thickness. Photosensitivity peak is found around Ga2S3 fundamental absorption edge region (2.9-3.0 eV). The non-equilibrium charge carriers are generated in the Ga2S3 layer of the junction’s interface. By photo- and thermally stimulate luminescence measurements, in the 78-300 K temperature range, the energies for both recombination and trapping levels of non-equilibrium charge carriers are determined. Also, both life time and carriers’ free path are calculated.

Resume : In this study, CZTS thin films are prepared by simple Sol-gel method using non-toxic solvent. CZTS sol-gel mixture contains tin chloride, copper acetate, zinc acetate and thiourea as a source of sulfur purchased from sigma-aldrich. The aim is to investigate the thermal annealing conditions, time and temperature, on properties and growth of CZTS active layer. The prepared films are annealed in tubular furnace at different temperatures from 520°C up to 580°C in sulfur atmosphere. XRD and Raman spectra demonstrate pure kesterite phase. Additional characterizations are performed to study optical, structural, electrical and chemical properties of the prepared films (absorption, SEM, EDS, hall effect…). An optimization of thermal annealing process is achieved to prepare good kestrite phase with large grain size and optimized stoichiometry, best films will be used to complete the cell structure: glass/Mo/CZTS/CdS/TCO/ZnO:Al/Al, and then measure the I-V curve in dark and under AM1.5 illumination. ACKNOWLEDGMENT The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 608593. REFERENCES: [1] K. Tanaka, M. Oonuki, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 93, 2009, pp. 583-587. [2] H. Katagiri, K. Jimbo, W. S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Thin Solid Films 517, 2009, pp. 2455-2460. [3] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, S. Guha, Sol. Energy Mater. Sol. Cells 95, 2011, pp. 1421-1436.

Resume : Solar cell based on polycrystalline CuIn1-xGaxSe2 (CIGSe) absorber layers offer tunable absorption energy threshold by changing the Ga content of CIGSe thin film (i.e. x=[Ga]/([In]+[Ga])). So far, the best labscale energy conversion efficiency (η) is obtained for x=0.3/0.4, and it strongly decreases when x>0.4 [1]. Our work focuses on the potentially detrimental phases that participate in limiting the η. In [2] a damaging Cu-Se phase has been observed in CGSe samples (x=1). Here, we investigate whether this phase could also occur during the growth of CIGSe thin films with different x. We therefore study the structural and chemical differences of CIGSe as a function of three parameters : (i) the Ga ratio x, (ii) the Cu concentration and (iii) the nature of the substrate (i.e. either SLG/Mo or sapphire wafer). Our XRD, EDS and RAMAN observations reveal a x-dependent behavior of the Cu-Se compound. This raises questions about (i) the seemingly different Cu-Se predominant phases for x=0 and 1, and (ii) the Cu-Se intermixing in the CIGSe environment. [1] M. Raghuwanshi, E. Cadel, P. Pareige, S. Duguay, F. Couzinie-Devy, L. Arzel, N. Barreau, Applied Physics Letters 105, 013902 (2014) [2] D. F. Marron , A. Meeder, U. Bloeck, P. Schubert-Bischoff N. Pfander , R. Wurz , S.M. Babu, Th. Schedel-Niedrig , M.Ch. Lux-Steiner, Thin Solid Films 431 – 432 (2003) 237–241

Resume : We have theoretically investigated the 1.55 µm p-type doped GaNAsBi-based Double Quantum Wells (DQWs) using the (16x16) BAC model combined with a self-consistent calculation. We have found that the coupling effect becomes more pronounced by reducing barriers width. The optical performance of the structure is enhanced when the DQWs are coupled and doped. Moreover, a p-i-n heterojunction based on GaNAsBi/GaAs (DQWs) designed for infrared photodetection was developed. The computed gain can reach the value 4.2 〖10〗^4 〖cm〗^(-1). The optimization of well parameters such as the Bismuth composition, the well width and the doping densities give rise to p-i-n heterojunction emitting at the wavelength 1.55 µm. The quantum confined stark effect on the optical properties of studied structures is also discussed.

Resume : We have experimentally studied the multicomponent responses of photoreflectance spectrum using selective phase analysis. After several experimental tests, the phase diagram of GaAs substrate in region of fundamental energy shows only one component. On the other hand, the PR spectrum of GaAsBi/GaAs structure reveals at least two contributions relative to fundamental band-band transition and Franz-Keldysh oscillations for GaAs and/or GaAsBi layers. A successful separation of different components is realized by the help of adequate phase angle. We seem that the separation of contributions combined with multilayers model is useful to extract the values of the physical parameters for each region of each structure. We have detailed the methodology and experimental procedure to identify each contribution.

Resume : Electronic band structure of strain-balanced GaAsN/GaAsBi type II double quantum wells (DQWs) was theoretically investigated by using the band anticrossing model combined with the envelope function approximation. The strain-balanced DQWs were coupled by the optimization of the well and barriers widths. We have discussed the coupling effect on the confined states energies and the oscillator strengths of inter-band transitions. The in-plane carrier effective masses and the optical absorption spectra of 1.3 and 1.55 µm GaAsN/GaAsBi type II DQWs are also examined.

Resume : Under this work, ZnSnTe2 (ZST) thin film samples were fabricated by sequential sputtering of the compound SnTe and ZnTe targets onto the chemically and ultrasonically cleaned glass substrates. The composition of the samples by EDS analysis showed that the as-grown films were in Zn-rich characteristics. XRD analysis indicated that all deposited films have a polycrystalline nature with the high intensity reflection along (111) plane corresponding to the preffered ternary ZST structure. In addition, different secondary phase diffraction peaks associated with ZnTe, SnTe and Te were also observed. Based on the as-grown film characteristics, the ZST films were analyzed under the effect of post-deposition wet etching treatments. In this etching process, the surface morphology and crystallinity of the samples were observed by controlling etching concentration. Applying five different solution concentration, the film behaviours were investigated by using XRD and AFM measurements. Following to the chemical etching treatments on the surface of the films, they were found to be homogeneous without any void and crack formations. Furthermore, the structural characteristics were improved depending on the increasing etchning concentration.

Resume : In this work, the effect of nanowire Si substrate on the structural characteristics of (Cu, Ag)GaTe2 (CAGT) chalcopyrite thin films were invstigated. Thin films were deposited on soda lime glass, bare Si wafer and Si nanowires by using sequential stacked layer evaporation technique. In order to get stoichiometric and polycrystalline CAGT thin film, pure elemental evaporation sources were used by controlling each elemental ratio in the deposited film structure. Furthermore, to fabricate vertically oriented Si nanowires, metal-assisted etching (MAE) method is applied to n-type single crystalline 600 μm Si wafers. In this etching process, although the Si nanowire diameter cannot be precisely controlled, Si nanowire alignment and length were controlled by determining the wet chemical etching recipes for AgNO3 and HF solution and also etching period. The resultant length of Si nanowires etched on planer Si substrate was found to be around 1 μm. The thin film samples deposited onto glass substrates were analyzed to determine the structural and morphological properties, and these results were discussed into growth in the planar and nanowire heterostructures. The chemical composition analysis by EDS showed that CAGT samples deposited on glass were nearly stoichiometric in the detection limit of the measurement system. Analysis of XRD spectrum revealed that the deposited film has polycrystalline structure with the strong preferred orientation along the (111) plane direction. In addition to the main crystalline orientation direction, there were additional diffraction peaks originates from this quaternary film structure. However, there is no other impurities and secondary peaks detected in the XRD spectrum. The EDS and XRD results indicated that CAGT thin film was deposited successfully onto planar Si and Si nanowire substrates. From top-view and cross-sectional SEM images, a continuous film fromation at top of the nanowire arrays was observed. A comparative study of the structural results obtained from XRD, Raman spectroscopy and SEM measurements showed the improved crystallinity of the CAGT thin films deposited onto the Si nanowire arrays.

Resume : The search for green efficient energy sources is becoming a mater of increasing urgency. In this field, water-splitting technology, that is photoelectrochemical conversion of water into O2 and high energetic H2, shows a great interest, as it would permit a direct conversion from solar energy to chemical energy. This technology would offer a way to a sustainable production of H2. H2 is considered as a promising clean fuel candidate to replace hydrocarbons fuels in the future. The photosynthesis of this molecule from water is of major attraction to researchers. Since the discovery of the ability of TiO2 to dissociate water into O2 and H2 upon irradiation, an impressive amount of work has been devoted to the development of photocatalyst semiconductors. Although very active, TiO2’s major drawback is that it is active only with the UV part of the solar spectrum, meaning that the visible part, representing 45% of the solar spectrum is not used. To overcome this problem, the scientific community is developing new semiconductors having smaller band gaps than TiO2. In parallel of the experimental design of new semiconductors, the community of theoretical chemists brought its contribution to this work. Density functional theory (DFT) is a tool complementary to experiment to characterize new semiconductors. High accuracy of hybrid DFT to compute semiconductors properties has been assessed in several published works, paving the way to a theoretical design of semiconductors for specific applications including photocatalytic water splitting. The difference of semiconductor potential of photogenerated holes and electrons must have at least 1.23 eV. But because of the need of excess energy for charge transfer toward the electrocatalyst and kinetic consideration of the redox reaction on top of the electrocatalyst surface, the optimum bandgap for the semiconductor is considered between 1.8 eV and 2.2 eV. The properties of layered oxychalcogenides MCuOS (M = Bi, Y, La , Lu ) for water splitting is investigated. They have been studied for various applications, mostly consisting in transparent p-type semiconductors[1], and thermoelectric materials. BiCuOS compound is very promising candidate for waters splitting applications ,but the major photocatalytic drawback of BiCuOS comes from the indirect nature of its band gap and the small band gap of 1.2 eV. RECuOS (RE=La, Gd, Lu, Y) compounds have bandgaps around 3 eV. These materials have respectively too low and too high bandgaps for water splitting application. By accurate modelling and calculations we successfully design in silico solid solutions having the formula Bi1-xRExCuOS (RE=rare earth element) satisfying all the requirements for water splitting including a bandgap between 1.8 eV and 2.2 eV with high transport properties. The effect of spin-orbit coupling on the most stable configuration’s electronic and optical properties is took into consideration. Full analysis including thermodynamic stability of the solid solution, bandgaps, density of states, effective masses, dielectric constants and exciton binding energies of these materials as photocatalysis semiconductors is provided.

Resume : Currently, the solar industry is proceeding with high efficiency and low cost. In this study, three thin films of SiNx, Al2O3, SiO2 were applied for rear passivation and the laser patterning process was varied to fabricate a solar cell with a local back structure. By applying the rear passivation, it can increase the open circuit voltage due to rear recombination and increase carrier collection efficiency. Moreover, the optimization of open area leads to a reduction in contact resistance and rear recombination. In the case of laser patterns, the open voltage increases as the opening area of the passivation increases. As a result, if the opening area is too large, the increase in recombination rates and contact resistance has resulted in efficiency reduction. On the other hand, if the opening area is too small, the efficiency was reduced by reducing to carrier collect. Accordingly, it is important to optimize the rear passivation and laser patterns for high efficiency crystalline silicon solar cell. The efficiency was increased from 17.4% to 18.8% with optimization of open area.

Resume : Promising path for photovoltaics development is intercalation of silicon nanostructures. Silicon nanowires (SiNWs) allow one to reach a variety of advantages from enhanced light trapping to quantum size effects like variation of the band gap and enhanced direct band absorption. One of the perspective ways of SiNWs formation is dry plasma process, which allows one to control their geometry and density. However, the impact of plasma process to minority carrier lifetime is a very important issue for solar cell applications. In this paper the effect of dry etching process conditions on charge carrier lifetime is reported. The different etching modes like reactive ion etching (RIE) and inductively coupled plasma (ICP) as well as different chemistry (fluorine and chlorine based) were compared. Minority charge carrier lifetime was measured by quasi steady state photoconductivity (QSSPC) technique after SiNWs surface passivation by a-Si:H thin layer. A strong variation of minority charge carrier lifetimes on the etching conditions was observed. In particular, our measurements show that RIE etching provides higher lifetimes compared to ICP etching with the same etching depth. Also an advantage of SF6/O2 gas mixture over Cl2/BCl3 chemistry was demonstrated.

Resume : Many studies indicate that Na has a positive effect on CZTS(e) grain size, non-radiative defects and charge carrier densities. In this work, we study the effect of several Alkali metals on the Voc of Kesterite devices, which remains one of the main hurdles for these technologies. At imec, Kesterite compounds are grown through a sequential process wherein e-beam evaporation is used to first deposit metals on a Mo layer. A SiON barrier below the Mo layer is used to block the natural diffusion of Na and other elements from the glass substrate. This is accompanied by a spin-coating step to introduce alkali metals in a controlled manner. The tri-metal stack is heated in a Se atmosphere to form the semiconductor compound. Completed solar cells undergo another low temperature heat treatment in a N2 atmosphere at 200°C, which is seen to consistently improve cell performance. A careful characterization of these steps reveal the effect of Alkali metals on the improvement of Voc and shunt resistances of the cells. The study is corroborated by J-V, EQE, photoluminescence, carrier concentration and bulk defect density measurements. It is also hypothesized that the uncontrolled presence of Na from the glass substrate in particular, may explain the large unexpected variations seen in the performance of Kesterite devices.

Resume : Solar cells with amorphous silicon PIN radial junctions deposited on silicon nanowire arrays have recently reached the record power conversion efficiency of 9.7 % [1], which is only slightly below the world record efficiency of 10.2 % for amorphous silicon solar cells [2]. Nanowires naturally introduce texturing which improves light absorption; however, they are also enlarging the area of PIN junction, thus increasing charge carrier recombination due to defects at interfaces. Further improvement of the efficiency of the solar cells based on silicon nanowires requires improved control and understanding of important loss mechanisms in the solar cells. We have studied the fabrication of PIN radial junction solar cells on PECVD grown silicon nanowire arrays and the influence of nanowire geometry and size on the solar cell performance. Different nanowire length and absorber a-Si:H layer were fabricated by varying deposition parameters in PECVD. The effects of a catalytic metal layer and nanowire growth were studied as well. Physical dimensions of nanowires have been found to have the biggest influence on the performance of the nanowire array solar cells. An optimal nanowire length has been found to be approximately three times larger than the thickness of intrinsic a-Si:H coating. The mechanism of bulk and interface losses for varying nanowire dimensions in the solar cells are discussed. [1] Misra Ph.D. thesis, Single and tandem radial junction silicon thin film solar cells based on PECVD grown crystalline silicon nanowire arrays, Ecole Polytechnique, Universite Paris Saclay , 2015 [2] Green M.A. et.al., Solar cell efficiency tables (version 49), Prog. Photovolt: Res. Appl. (2017) 25 3-13

Resume : We investigated the potential efficiencies of CIGS-based dual-junction solar cells using the SCAPS-1D. The CIGS solar cells with Eg = 1.5 to 1.7 eV (interval = 0.1 eV) were chosen as the top subcell in the tandem solar cell, while the cells with Eg =1.0 to 1.2 eV (interval = 0.1 eV) were selected as the bottom subcell. After thinning the top-subcell absorbers’ thicknesses, current matching points were found in every possible combination. In this work, two types of current matching points were examined: (1) matching JSC and (2) matching JMP. In general, both of the methods yielded similar device performances in each combination; however, when the two subcells had a relatively large difference in the fill factor, matching the JMP could be more advantageous. Nevertheless, in the present work, any combination showed that the device performance of the tandem is similar to the device performance of an optimized single-junction device (i.e., efficiency of an 1.2 eV CIGS solar cell), ascribable to relatively low device performances of top-subcells. We also discussed how further device improvement is required to achieve a substantially higher efficiency from a tandem device compared with a single-junction device.

Resume : Many electrically polar materials manifest photovoltaic effects with above bandgap photovoltages. The ferroelectric subclass of these materials is particularly interesting because it offers an advanced electric functionality related to polarization switching. In this talk we will show that the 404nm coherent light can be used to polarize electrically the multiferroic BiFeO3 single crystal via the photovoltaic effect. Interestingly, a part of the induced electrical polarization can remain even after light is off. The magnitude of the remanent photopolarization can depend on the light fluence, wavelength and the exact ferroelectric state of the sample. The use of this light induced memory effect will be further discussed in connection with the related photo-deformation and with possible electro-magnetic functionalities.

Resume : In order to overcome the issue of intensive use of fossil fuel energies, scientists need to rapidly propose new energy resources to protect environment and pass down a healthy planet to our future generations. In this context, solar energy appears as an efficient green and renewable alternative energy source to fossil fuels. To date, the most widely used material for solar cells is silicon for which the technology is now very well mastered. Nevertheless, current works tend to focus on new technologies using other semi-conductors materials having key attractive features such as a low cost and an ease of fabrication. For instance, this challenge has been fulfilled with the development of dye-sensitized solar cells (DSSCs), initiated by the works of Grätzel two decades ago and based on the use of TiO2 photosensitized with a dye. Since their inception, many improvements have been allowed due to continuous modifications concerning the different key-elements constituting DSSCs. In particular, means to increase life-time of charge carriers in TiO2 layer are major issues in limiting recombination processes. In this work, it has been shown that incorporation of polyoxometalates (POMs) in TiO2 can decrease recombination processes and consequently increase solar cell yields. POMs constitute a unique class of metal-oxygen polyanionic clusters on the nanoscale, having attractive electrochemical properties. The efficiencies measured for these solar cells have been correlated with investigations of physical processes occuring. In particular, impedance measurements and time-resolved microwave conductivity (TRMC) measurements have been performed. TRMC is a very powerfull microwave reflectance method which appears very relevant to directly investigate the mobility of charge carriers in semi-conductors. Generally used in photocatalysis field, we have usefully introduced this original method in the photovoltaic field to investigate charge carrier dynamic occuring in solar cells.

Resume : Double-side passivated silicon solar cells have been recently widely investigated in the photovoltaic (PV) field, to obtain higher open circuit voltage. This structure results in improved electrical and optical performances and higher efficiency. This is attributed to the reduction of the surface recombination loss. New compounds have been studied as passivating layers and emitters. Among them TiO2 film can play a role as emitter layer in heterojunction solar cells due to its large gap and workfunction. In this work we study TiO2 films obtained by e-beam evaporation for photovoltaic application. Once thermally annealed at T > 300 °C the films show an increase in the conductivity due, probably, to the formation of an anatase phase. At higher temperature the rutile phase appears. Electrical and optical characterization of the annealed samples showed that the best results in terms of conductivity and transmittance can be obtained for the films thermally annealed at T > 400 °C, with a resistivity of about 10-2 cm, a mobility of 3 cm2/Vs and carrier concentration of 1019/cm3. The formation of built-in potential of 0.69V at TiO2/c-Si heterojunction thermally annealed at 400°C indicates that TiO2 film can play a role as emitter layer in heterojunction solar cells. The annealing temperatures represent, at the moment, a bottleneck for its application in heterojunction solar cells based on amorphous or organic layers, since for such cells typical process temperatures must stay below 300 °C. From this point of view a further optimization of deposition parameters, or a different solar cell architecture, where TiO2 deposition and thermal annealing is at the initial stage of the fabrication, are needed.

Resume : As an n-type semiconductor and high dielectric constant (κ) oxide, TiO2 has been frequently employed in photovoltaic applications. High quality oxide films can be grown by atomic layer deposition (ALD). The self-limiting nature of ALD provides thickness control at the sub-angstrom level, and induces perfect conformality and step coverage. Along with amorphous nature, crystalline films of TiO2 can also be grown at relatively low deposition temperatures by ALD. Crystalline TiO2 films exhibit higher κ-values whereas amorphous TiO2 films exhibit lower leakage current than polycrystalline films since the grain boundaries act as diffusion pathways. In this work, effect of ALD growth temperature on materials structural, morphological, optical, and electronic properties were investigated by spectroscopic ellipsometry (SE), AFM, XPS, UV-VIS spectroscopy, and GI-XRD. Tetrakis (dimethylamido) titanium (TDMAT) and water were used as the precursors. TDMAT is known to be highly reactive with water and does not produce corrosive by-products. As the growth temperature was increased, the growth rate was found to decrease as confirmed from SE measurements, which is typical for the use of TDMAT. XPS measurements for different growth temperatures revealed the existence of Ti4 . Refractive indices of the films increased as crystalline phases begin to form with increasing temperature. Band gap values showed a decreasing trend as was expected for the transformation from amorphous to anatase and rutile phases. Formation of crystallites was also proven by GI-XRD and AFM studies. The deposition process was optimized under the light of these results for the resultant film to be utilized as an alternative passivation layer for crystalline silicon (cSi) based solar cells as well as a dielectric layer to enhance the plasmon lifetime in plasmonic nanostructures. Our results show that for thin films of TiO2 grown by ALD on top of few angstroms of wet chemical oxide, minority carrier lifetimes up to 2.3 milliseconds were obtained from n-type cSi wafers.

Resume : Zinc selenide (ZnSe)/cadmium telluride (CdTe) heterojunction based photovoltaic structures were fabricated in superstrat configuration, onto optical glass substrates. The fabricated structures were irradiated using Cockcroft-Walton Tandetron accelerator, with protons with energies of 500 keV and 1012 particles/cm3 fluencies; similar values with those hitting artificial satellites orbiting the Earth. ZnSe and CdTe thin films were deposited by both thermal evaporation and rf-magnetron sputtering and their morphological (atomic force microscopy and scanning electron microscopy), optical (UV-Vis and spectroscopic ellipsometry) and structural (X-ray diffraction patterns) properties were investigated. The irradiation effects onto the fabricated photovoltaic structures were determined and discussed in the frame of electrical and photo-electrical behavior. Specific parameters such as short-circuit current (Isc), open-circuit voltage (Voc), external quantum efficiency (EQE), fill factor (FF) and power conversion efficiency (PCE) were determined and compared for both pristine and irradiated photovoltaic structures. Keywords: ZnSe, CdTe, protons, photovoltaic structures Acknowledgements: This study was partially financially supported by PN-II-288/2014 project.

Resume : Hematite photoelectrode is widely studied as candidate for photoelectrochemical (PEC) cells, showing a favorable combination of low price, abundance, non-toxicity, optimal bandgap of 2.1 eV and excellent chemical stability in neutral and basic aqueous solutions.1 Despite these encouraging characteristics, the performance of hematite is lower than what is thermodynamically expected (16.8 % solar-to-hydrogen efficiency).2 Challenges to overcome are the large overpotential and poor electronic conductivity that lead to high recombination and short charge collection length. Understanding the dynamics of charge carrier processes is crucial for improving the hematite performance3; the prepared thin films, stable over 1000 h2, are suitable for these studies requiring stable and reproducible tests. In this work, transient absorption spectroscopy (TAS) is combined with PEC measurements to study the role of hematite electronic and structural characteristics (e.g. doping, surface treatments, morphology, thickness) on charge carrier dynamics, influencing its late turn-on potential and low photovoltage for water oxidation. Ultrafast (fs-ns) TAS profile is used to address insights into recombination and charge transfer, both in isolated hematite films and three-electrode configuration. Thus, TAS analysis provided strategies to enhance hematite photovoltage and photocurrent by reducing recombination, being also a powerful technique for other photoelectrodes/systems for solar fuels generation. 1 - T. Lopes, L. Andrade, F. Le Formal, M. Gratzel, K. Sivula, A. Mendes, Phys.Chem.Chem.Phys., 2014, 16, 16515M. Barroso, S. R. Pendlebury, A. J. Cowan, J. R. Durrant, Chem. Sci., 2013, 4, 2724. 2 - P. Dias, A. Vilanova, T. Lopes, L. Andrade, A. Mendes, Nano Energy 23 (2016) 70–79. 3 - M. Barroso, S. R. Pendlebury, A. J. Cowan, J. R. Durrant, Chem. Sci., 2013, 4, 2724. Acknowledgements: This work was performed under the project “SunStorage - Harvesting and storage of solar energy”, with reference POCI-01-0145-FEDER-016387, funded by European Regional Development Fund (ERDF), through COMPETE 2020 - Operational Programme for Competitiveness and Internationalisation (OPCI), and by national funds, through FCT - Fundação para a Ciência e a Tecnologia I.P.”.

Resume : Na is known to cast a positive influence on vacuum based CIGSe solar cells in which an additional Na supply step is required, either by deposition of Na-containing precursor layers or by a post NaF evaporation, before and after absorber film formation, respectively. In non-vacuum based process, however, Na can be added into the absorber layer though an easier route by either using sodium salts or by dipping or spraying with Na solutions. In this work, we incorporated Na into our CISe films, simply by dissolving Na salt into the precursor solution and studied its effects on the characteristics of the absorber films and devices. The inherent double-layered structure of CISe films remained the same even with Na addition, however, Na was found to improve the crystallinity of the top CISe film along with the enlargement in the grain size. These improvements in the film properties further increased the device efficiency reaching up to the remarkable value of 12.8 %. The main reason of the efficiency gain was found to be from the increment in the Voc and FF, similar to the reports from the vacuum-deposited CI(G)Se devices. However, a newer feature about Na effect, that we found in this study, is that these improvements are not likely from the reported ‘increased-free-hole concentration,’ rather, from the reduction of the interfacial recombination. The possible explanations of this behavior can be i) the formation of Cu-poor type (OVC) interfacial layer ii) the modification of CISe surface towards having more VCu, which consequently will enhance Cd interfusion from CdS into CISe and will result a buried pn-junction. This difference in the role of Na in vacuum and non-vacuum based CISe solar cells can be attributed to the different growth environments. In this presentation, this new proposed Na effect in the non-vacuum based CISe solar cells as well as the experimental evidences confirming this proposition will be discussed.

Resume : Microwave-assisted solvothermal process was used to synthesize anatase TiO2 nanocrystallites for the application of dye-sensitized solar cells (DSSCs). The morphologies and sizes of TiO2 could be simply controlled by using different kinds of alcohols where no additives were needed. By using isopropanol (IPA) as solvent, TiO2 in size of 20-30 nm with dominant {001}/{010} facets was obtained; whereas ultrafine anatase TiO2 of about 5 nm with dominant {101}-facet was obtained using octanol (OCT). To investigate the influences of TiO2 on the photovoltaic performances of DSSCs, three different pastes were fabricated using IPA, OCT and mixed IPA/OCT as photoanodes. The results revealed that the requirements of TiO2 photoanodes used at 1 sun and room light conditions were quite different. OCT showed the highest power conversion efficiency (PCE) up to 9.58% under 1 sun irradiation because of its high specific surface area that provided high dye-loading capacity. However, the great amount of grain boundaries appeared in OCT became disadvantageous at room light condition. On the other hand, IPA/OCT combined the features of IPA and OCT that was optimal for room light harvesting and its PCE reached 12.46% under 200 lux T5 lamp irradiation. The photovoltaic properties of three different photoanodes in correlation with their band structures, electronic transport behaviors and light harvesting efficiency in different lighting conditions will be carefully discussed in this presentation.

Resume : In recent years, the investigation of the ZnO-based solar cells has attracted more attentions due to the low fabrication process and appropriate electrical efficiency provided by this technology. Unfortunately, ZnO as an absorber layer is quite restricted in visible and IR ranges due to its wide band gap value and the requirement of new approaches to improve the absorbance behavior for infrared and visible lights. Thus, in order to deal with the growing high conversion efficiency requirement, it is very important to develop new approaches and designs to achieve better trade-off between the electrical efficiency and manufacturing cost. In this paper, a new approach based on metallic nanoparticles engineering aspect is proposed to achieve superior absorption for ZnO-based solar cell. The overall device performance comparison with three different metallic layers (Au, Ti, and Ag) is performed numerically. We find that the power conversion efficiency is considerably enhanced as compared to the conventional design. Moreover, the proposed design is optimized design using particle swarm optimization (PSO) approach in order to achieve higher optical and electrical performance of the device. The obtained results make the proposed design methodology as a potential alternative for developing low cost and high performance solar cells.

Resume : CuIn1-xGaxSe2 (CIGSe) is one of the most promising absorbers for solar cells whose absorption properties vary with the Ga ratio x=[Ga]/([In]+[Ga]). Unfortunately, the observed x-dependent cell efficiency curve deviates from the theoretical macroscopic-scale predictions that are based on a perfectly random distribution of In and Ga sites within the structure [1]. Contrary to high-temperature experiments, atomic-scale computer simulations may help to question this assumption, and rule on the existence on local inhomogeneities and subsequent neglected damaging phenomena (phase separation, cluster formation). Therefore we derive the bulk phase diagram of the pseudobinary CuIn1-xGaxSe2 system from a thermodynamical description of the Gibbs energy based on special quasirandom structures [2,3]. We emphasize on the non-negligible contribution of (i) the size-mismatch, (ii) the anion displacement, (iii) the long-range ionic interactions, and (iv) entropic contributions. The resulted phase diagram is a first step to detect the possible influence of internal interfaces in limiting the CIGSe efficiency at larger gaps. [1] M. Raghuwanshi, E. Cadel, P. Pareige, S. Duguay, F. Couzinie-Devy, L. Arzel, N. Barreau, APL 105, 013902 (2014)Mohit [2] A. Zunger, S. -H. Wei, L. G. Ferreira, J. E. Bernard, Physical Review Letters 65, 3 (1990) [3] H. T. Xue, W. J. Lu, F. L. Tang, X. K. Li, Y. Zhang, Y. D. Feng, JAP 116, 053512 (2014)

Resume : Efficiency of single-junction mass manufactured silicon solar cells is near to 22 % and further development of this type of solar devices is constant struggle about efficiency increasing with simultaneous solar cell manufacturing costs reduction. It can be achieved not only by the price reduction of the silicon wafers and processing costs but also exploiting of cheaper materials for contact manufacturing. The silver paste which is used on front contact contributes to approximately 18% of the total cell production cost. In our work we present experimental results concerning replacement of Ag by other, less expensive metal which is abundant and cheap cooper. In this investigation two CuAB composites contained 5 and 10 wt.% of AB fraction respectively for CuAB1 and CuAB2, where AB means modificator, were produced by chemical method. Composites were mixed with commercially available silver paste in 1:1 weight ratio and subsequently screen printed on p-type Si wafers with emitter of 50 ohm/sq. The contacts structures and properties were investigated by SEM, TEM, XPS, TL (Transmission line measurement), while the solar cells parameters were measured by I-V and SR methods. The results have shown that elaborated composite can be cheaper alternative as a material for silicon solar cells electrode fabrication.

Resume : Recently, we have shown the possibility to form Zn nanoparticles in ZnO crystal using Nd:YAG laser radiation. The aim of this work is to elaborate technology of nanoparticles formation in metal oxide semiconductors by laser radiation. The experiments were performed on hydrothermally grown n-type ZnO single crystals with size 5.0x5.0x2.0 mm. The ZnO crystals were irradiated by the fourth harmonic of pulsed Nd:YAG laser with the following parameters: wavelength λ = 266 nm, pulse duration τ = 3 ns and laser intensity up to Imax = 315 MW/cm². Nanostructural features of the samples were studied by field emission scanning electron microscope (FESEM) and Raman spectroscopy. Topography measurements and electrical conductivity mapping were performed by atomic force microscope (AFM). It was found that the formation of Zn nanoparticles characterizes by two thresholds intensities: the first threshold intensity Iₜₕ₁ = 3.5 MW/cm² at which conductivity starts to increase monotonously up to 10³ times, till the second threshold intensity Iₜₕ₂= 290.0 MW/cm², due to the increase of the Zn interstitials (Znᵢ) concentration at the surface of the ZnO sample. Second stage of the process is “black ZnO” appearance at the irradiated surface. This phenomenon is caused by the agglomeration of Znᵢ in nanoparticles wihich the diameter increase with number of laser pulses. An evidence of Zn phase formation in ZnO crystal is appearance of 70 cm⁻ᴵ band in Raman spectra after irradiation by the laser. The work was supported by Latvian National Research Programme in Materials Science (IMIS2) (2014-2017).

Resume : The search for novel materials to serve as the platform for optoelectronic devices has been intensified. The goal is to find structures that will allow for more efficient and environmentally friendly devices. In particular, the search for sources of clean energy has stimulated research on materials and architectures for next-generation solar cells. Here using the DFT approximation, we propose a new mixed-valence metal halide perovskite ABO3 as a photovoltaic material with a direct bandgap of ~1.1 eV and an intermediate band (IB) in the middle of the bandgap. The presence of a band in between the valence band and conduction band opens the possibility of co-doping with donors to make the intermediate band half filled. The compounds with half-filled IBs, are expected to be as efficient as multijunction solar cells. The theoretical conversion efficiency of a single IB solar cell can be up to 62%, and even higher efficiency of up to 72% is predicted for materials with two IBs[1,2,3]. An addition to the optical properties, we also studied the electronic properties of this compound and show how the oxidation states of the A atoms depend on the applied pressure. References: [1] A. Luque and A. Martì, Phys. Rev. Lett. 78, 5014 (997). [2] A. Martì, E. Antolìn, C.R.Stanley, C.D.Farmer, N.Lòpez, P. Dìaz, E. Cànovas, P. G. Linares, and A. Luque, Phys. Rev. Lett. 97, 247701 (2006). [3] N. Lòpez, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, Phys. Rev. Lett. 106, 028701 (2011).

Resume : Tunable optoelectronic properties along with high power conversion efficiencies of more than 22% induces an excited interest towards metal halide perovskites. Band gap tuning can be realized by different ratios of Bromide to Iodide in CH3NH3PbI3-XBrX structure [1]. This can provide for using perovskite thin film in tandem with other absorber layers such as crystalline silicon, CIGS or CdTe [2]. Furthermore, Bromine incorporation in metal halide perovskite has shown improved stability [3]. Methods involving solution processing to fabricate mixed halide perovskites usually lack the control over the layer thickness and the convenience for large area depositions. However, vapor deposition offers a better control over layer thickness, larger area substrate usage and potential reproducibility. Furthermore, low temperature in vapor deposition can provide for flexible substrate employment. In this study, we fabricated CH3NH3PbI3-XBrX films by thermal co-evaporation using PbI2, PbBr2, MAI and MABr precursors and obtained mixed halide perovskites with various bandgaps between 1.55 and 2.3 eV. Deposition parameters such as precursor sublimation rate and the deposition pressure were found to define highly crystalline perovskite with smooth surface during the vapor deposition. Perovskite films with different Br/I ratio were characterized optically and structurally using photoluminescence, X-ray diffraction, reflection and transmission, and scanning electron microscopy measurements. The results clearly demonstrate band gap widening and lattice constant increase with Br concentration. CH3NH3PbI2Br sample with a band gap of 1.65 eV having no Stokes shift is suitable for tandem solar cells. [1] J. H. Noh, et al., Nano Lett., 2013, 13, 1764–1769 [1] C. D. Bailie, et al. Energy Environ. Sci., 2015, 8, 956-963 [2] S. Brittman, et al. MRS Communications (2015), 5, 7–26

Resume : Photovoltaic devices based on ZnO heterojunctions have been demonstrated, however, they used n-type ZnO. It is well known that in case of ZnO n-type doping is easily achieved but obtaining p-type doping in this material has proved to be a very difficult task. As a result lots of efforts are directed towards this issue. In this work we present the experimental results of p-ZnO/n-GaN heterojunctions that have been successfully fabricated. The electrical and optical properties of the junctions have been studied by means of photoluminescence and electrical measurement techniques, such as: current-voltage and capacitance-voltage (C-V) characteristics as well as deep level transient spectroscopy (DLTS). The C-V results proved that the depletion region of the diode is located within the p-ZnO. The DLTS measurements reveal the presence of hole trap-related signals. The activation energies and capture cross sections of these traps were determined and their possible origin has been ascribed. To the best of our knowledge, there were no reports on the latter result with respect to the p-ZnO/n-GaN junctions. The results obtained in this work can contribute in the future to fabrication of homojunctions based on ZnO. Acknowledgements. The research was partially supported by the NCN project DEC-2013/09/D/ST3/03750 and by the Polish National Centre for Research and Development (NCBiR) through the project PBS2/A5/34/2013.

Resume : Two types of heterojunctions (HJs) with thin ZnO or Zn0.9Mg0.1O films directly grown on p-Si(100) substrates by atomic layer deposition method were studied. The aim was to compare which type of the HJ exhibits better electrical properties. According to Knutsen et al.1 addition of Mg to ZnO should result in the improvement of the ZnO/Si HJ. The electrical properties of ZnO/p-Si and Zn0.9Mg0.1O/p-Si heterojunctions (HJs) were investigated via current voltage temperature (I V T) and capacitance voltage temperature (C V T) characteristics. Both HJs exhibited good rectifying properties. The room-temperature turn-on voltages of the n-ZnO/p-Si and n-Zn0.9Mg0.1O/p-Si HJs were found to be 0.46V and 0.56V, respectively. At a voltage bias of 1.4V second barrier was observed. Based on the measurements the transport properties of the HJs were explained in terms of Anderson model. It was found, that the mechanisms of current transport for both types of the HJs are similar. At a low forward bias (less than 0.1V) the I-V characteristics exhibit ohmic behavior. At a medium voltage bias (less than 0.5V), multitunneling capture-emission prevails with the electron trap located at 0.19eV (0,21eV) below the bottom of ZnO (Zn0.9Mg0.1O) conduction band. However the electrical properties of the Zn0.9Mg0.1O/Si HJ are worse as compared to the ZnO/Si HJ. 1K.E.Knutsen et al. Phys. Status Solidi A 210 (2013) 585–588.

Resume : Recently, solar cells based on n-ZnO nanorods and p-Si heterojunction (HJ) were proposed [1]. In order to increase their efficiency, the optimization of the junction interface is essential because it is the main source of undesired recombination losses. N-type ZnO nanorods were grown using hydrothermal method on p-type Si and covered with ZnAlO transparent electrode. The current-voltage characteristics in the darkness and under 1-Sun illumination were measured to obtain the basic parameters of the junction and to confirm the photo conversion of the solar cell. The open circuit voltage Voc was measured at different temperatures. The Voc(T) characteristics extrapolated to 0 K yield energy lower than the energy band gaps. It indicates on the presence of interface recombination mechanism in the junction [2]. Capacitance-voltage and deep level transient spectroscopy measurements performed for the HJs confirmed that the surface states are present at the junction interface. A single trap connected with the ZnO layer was found, and the other related to the surface states. The basic parameters of the traps were determined. [1] R. Pietruszka, et al., Solar Energy Materials and Solar Cells 143 (2015) 99-104. [2] M. Turcu, et al, Apllied Physics Letters 80, (2002) 2598-2600.

Resume : Recently, all-inorganic cesium lead halide nanocrystals (CsPbX3 NCs, X=Cl,Br,I) of perovskite attract much attention due to their high photoluminescence quantum yields and narrow emission bands with wide tunability. Swarnkar et al.,1 from NREL, have demonstrated that the all-inorganic perovskite nanocrystals with a low bandgap energy can (i) be stable and (i) form thin layers with superior electrical properties, thus being very suitable for stable photovoltaic devices. For this purpose, custom-designed quantum structures and solids can be realized by purposeful assemblage of individually characterized and selected NCs. Being of a cubic shape with narrow size distribution, they are ideally suited to form close-packed layers and/or multilayer structures to form e.g. a top cell for a perovskite-based tandem cell. Our recent experiments reveal that upon close contact (upon deposition on a substrate), the CsPbBr3 NCs ‘seamlessly’ arrange themselves in larger arrays and nanowires that can reach micrometer geometries.2 This is investigated using electron energy loss spectroscopy in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope providing a spatial resolution below 1.6 Å. We follow their formation and demonstrate also that selective exposure of NCs to the electron beam can terminate the aggregation process. 1 Swarnkar; A. et al. Science. 2016, 354, 6308 2 C. de Weerd et al., under preparation

Resume : Introducing disorder in light trapping surface textures may improve their ability to enhance absorption in photovoltaic devices. The disorder populates the Fourier spectrum of the geometry, which in turn leads to more available light scattering pathways, even across extended spectral domain. However, not every disorder is equal. Not just the availability, the scattering strength to the different pathways is also important. Disorder may also enhance light scattering within the escape cone and weaken resonant optical responses resulting in less absorption enhancement. Therefore, obtaining the most out of disorder for light trapping purposes requires care and remains a major design challenge, especially due to huge computational costs in numerically addresing the problem. Furthermore, reliable large area fabrication of optimum disordered light trapping surface textures is highly demanding. Here, we give an overview of our recent efforts in tackling these challenges. We present an analytical inverse modeling formalism to deduce optimum surface textures, even with incommensurable components. We show how the inverse problem can be formulated as coupled multivariate polynomial equations of the surface texture Fourier amplitudes. We further present a bottom-up large-area-compatible fabrication strategy for disordered surface textures using nanospheres of different sizes as building blocks. How the scattering response can be tailored via the nanosphere size distribution is discussed.

Resume : Thin-film solar cells based on liquid phase crystallized silicon (LPC-Si) with 8-20µm thick absorber layers demand for advanced light management to achieve high photocurrent densities (jsc). We present random, hexagonal and morphologically flat but optically rough superstrate textures, which provide enhanced light in-coupling and light trapping for LPC-Si thin-film solar cells. Open-circuit voltages >600 mV underline the high material quality of LPC-Si thin-films being grown and crystallized on nano-textured glass superstrates. Nano-textured devices outperform respective planar references by up to 3 mA/cm2 in terms of jsc, as measured by EQE. 500 nm-pitched sinusoidal nano-textures are found to outperform larger pitched superstrate textures with respect to reducing reflection losses at the buried glass-silicon interface. In the wavelength range of interest reflection of incident light is minimized to values close to 4%, which is the reflection at the sun-facing air-glass interface. The superior optical properties of 500 nm pitched sinusoidal gratings have been predicted by optical simulations. Here, the successful experimental realization is presented. Further, the effect of combining individually optimized light management schemes at different interfaces on the optical properties of LPC silicon thin-films is analyzed. In summary, these results constitute a crucial step towards fully exploiting the optical potential of liquid phase crystallized silicon thin-film solar cells.

Resume : Recently, perovskites are emerging as an attractive material for low-cost and highly efficient photovoltaic materials. The advantages of using perovskites (high emission efficiencies and low production costs) and quantum dots (quantum-confinement effects) are combined in all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I, and mixed Cl/Br and Br/I) nanocrystals (NCs) with perovskite structure. They are characterized by high photoluminescence quantum yields (PL QYs, 50-90 %), narrow emission bands and long-term stability. For photovoltaic and optoelectronic applications, low non-radiative losses and long carrier lifetimes are necessary.. These properties can be boosted by a photon recycling process where one photoexcited state undergoes multiple radiative emission-absorption events before the carrier is collected. Here, we demonstrate a repeated recycling between photon absorption and radiative recombination in CsPbBr3 NCs. We observe narrowing of the full width at half maximum as well as a red-shift of the emission spectrum as the density of NCs increases, enabling more efficient photon recycling. The corresponding radiative lifetime is lengthened indicating that energy transport can occur over longer distances, and becomes no longer limited by the carrier diffusion length. Our experimental findings are supported by simulations, which suggest that photon recycling is considerably more efficient in NCs compared to the bulk.

Resume : Cadmium telluride (CdTe) is increasing its expectancy to be used in large scale solar energy considering the record efficiencies of 22.1% and 18.6% for solar cells and modules, respectively [1]. Thanks to its high absorption coefficient, more than 95% of the energy in the solar spectrum above CdTe bandgap (1.5 eV) can be captured with only 1 µm-thick CdTe layer. However, more than 2 µm thick CdTe is typically used for high efficiency planar devices. Trials to reduce the thickness of CdTe to sub-microns resulted in a loss of performance due to deteriorated photocurrent [2]. Electrically conductive ZnO nanorod (NR) arrays are attractive as front contacts for fabrication of extremely thin absorber (eta) CdTe solar cells due to their superior light trapping effects. In this work, a ZnO/CdS/CdTe core-shell NR based structure was optically-modeled and results were compared with the experimental counterparts. The simulations of the 3D ZnO NR and ZnO/CdS NR arrays were carried out by taking into account the random distribution of NRs which unavoidably occurs during fabrication. A full-field electromagnetic wave calculation of the structure using a commercially available finite-difference time-domain (FDTD) simulation tool [3] was performed. An excellent agreement between the simulation and experimental values for the direct transmission and haze of different lengths of ZnO and ZnO/CdS NRs was achieved. The consistency in the simulated and measured haze verified the light trapping effects in the structure, and responsible for the enhanced external quantum efficiency at long wavelengths. The absorption in each layer of the structure was calculated. We showed that even a 40 nm-thick CdTe conformal layer is more absorptive compared to a 100 nm-thick planar CdTe film for wavelengths longer than 500 nm. With further optimization of CdS and ITO layer thicknesses, which accounts for 60% of the absorption in the UV region, the absorbance in the CdTe layer can be enhanced without increasing its thickness. In this presentation, we will discuss the light trapping effects of ZnO NR arrays for eta-CdTe solar cells including the effect of their random nature. [1] M.A. Green, K. Emery et al., Prog. Photovolt: Res. Appl. 25, 3 (2017). [2] V. Plotnikov, X. Liu, N. Paudel, D. Kwon, K.A. Wieland, A.D. Compaan, Thin Solid Films, 519, 7134 (2011). [3] Lumerical Solutions, Inc. http://www.lumerical.com/tcad-products/fdtd/

Resume : In the concept of hot carrier solar cells, photo-generated carriers are extracted at energies higher than the band edges, to maximize the power conversion efficiency[1]. Albeit devices have been proposed as hot carrier solar cells candidates, most published works reported on either purely optical or electrical characterization of such devices. We report on simultaneous optical and electrical measurements on a quantum-well type hot carrier solar cell and discuss the influence of the hot carrier effect on electrical performance. The quantum-well/barriers structure is used as absorber/semi-selective contacts. An original setup allows electrical characterization and spectral analysis of luminescence under different laser excitation and electric bias. Carrier thermodynamic properties are investigated by fitting the full luminescence spectra using the generalized Planck’s law[2]. To obtain a good fit accuracy we take into account the absorption of excitons, free carriers in the quantum well and in the barriers[3,4]. Electrically, first-version devices present a maximal efficiency of 11% under laser illumination equivalent to 15 000 Suns. As expected, temperature and electrochemical potential of carriers are tunable with either laser power or electric bias. The open circuit voltage exceeds absorption threshold under high illumination power. Hot carriers contribution to the Voc and extracted power will be discussed. Reference [1] Ross and Nozik, J App Phys 53, 1982 [2] Würfel, J Phys C, 15, 1982 [3] Colocci et al, J App Phys 68, 1990 [4] Gibelli et al, J Phys Cond Mat, 2017.

Resume : The power conversion efficiency of a single junction photovoltaic (PV) device depends on the bandgap energy of the light-absorbing layer and is limited to approximately 28%. To realize the conversion efficiency over 30%, two or more light absorbing layers have to be embedded into the photovoltaic device. Multi-junction and quantum dots PV devices have been proposed for the purpose, and the conversion efficiency over 30% has been achieved by the three- and four-junction PV devices. Copper oxide semiconductors of 2.1-eV-p-Cu2O and 1.3-eV-p-CuO have attracted increasing attention as light-absorbing layers in photovoltaic devices due to their non-toxicity, abundance, and low-costs, but the conversion efficiencies are limited at low levels for both the single junction photovoltaic devices.[1] The internally stacked Cu2O/CuO layer, which is a bi-layered structure, presents itself as a realistic candidate as the PV layer to realize the high-efficiency, due to the different bandgap energies. Here, we report an internally stacked Cu2O/CuO photovoltaic layer prepared by electrodeposition of Cu2O layer followed by thermal oxidation and expansion of the absorption band originated from the Cu2O and CuO layers. The n-ZnO layer was prepared on transparent conductive Ga:ZnO layer/soda-lime glass substrate by electrodeposition in a simple zinc nitrate aqueous solution, and the consequent Cu2O layer prepared by electrodeposition in an aqueous solution containing copper acetate hydrate and lactic acid. The CuO layer was formed by heating at 573-773 K in air. The photovoltaic devices were then fabricated by vacuum evaporation of Au electrode on the surface of the device. The CuO layer with the characteristic monoclinic lattice could be formed by oxidizing the Cu2O layer, and the Cu2O/CuO bi-layer structure was fabricated by heating at 573-773K in air. The thickness increased with the rise in the heating temperature, and the parallel lattice lines could be observed clearly inside the CuO layer, suggesting that there is a possibility that the lattice relationship was kept while thermal growth of the CuO on the Cu2O layers progressed. Both the optical transmission spectra and external quantum efficiency (EQE) curve for Cu2O/ZnO PV devices showed an absorption edge at approximately 620 nm originating from the 2.1-eV-Cu2O layer. The EQE of approximately 90 % was obtained at 400 nm, but the EQE value was 0 % at the wavelength of over 620 nm. The internally stacked Cu2O/CuO PV layer possessed an absorption edge at the wavelength of 1160 nm originating from 1.3-eV-CuO in addition to the absorption edge at 620 nm, and the EQE was observed from 620 nm to 1160 nm due to the photocurrent generation by the CuO layer. The internally stacked Cu2O/CuO photovoltaic layer could be prepared by a facile method by the electrodeposition of the Cu2O layer followed by heating in air. Both the Cu2O and CuO layers act as the light-absorbing layers and generate electricity from light irradiation. References[1] M. Izaki, T. Shinagawa, K. Mizuno, Y. Ida, M. Inaba, A. Tasaka, J. Phys. D, 40, 3326(2007). Acknowledgments: This work was supported by JSPS KAKENHI Grant Number 25281062.

Resume : During the last years, multiferroic materials have gained great attention due to their fundamental physics and possible integration in advanced application. BiFeO3 (BFO) appears actually as one of the most interesting, because it shows multiferroic properties at room temperature. Recently a peculiar photovoltaic effect has also been revealed in BFO with a large open circuit voltage Voc above the band gap. The very large Voc (up to 16V) was first interpreted as arising from the domain structure and electric field at the domain walls. More recently an interpretation based on the symmetry was put forward to explain the anomalous high Voc. In both cases planar geometry of the PV effect was used and the ferroelectric polarization is responsible of the electric field separating the electrons from the holes in the thin films. A smaller Voc (<1V) was however measured in parallel plate capacitor and the origin of this low PV response may be obscured by the possible existence of a Schottky barrier, defects, depolarizing field and the complex rhombohedral ferroelectric domain structure. To better understand the observed PV effect, we have grown by pulsed laser deposition BFO thin films with different thickness on buffered LaAlO3 substrates. A 20nm thick SrRuO3 layer is used as a bottom electrode while Pt and ITO top electrodes were deposited. Reciprocal space mappings and Raman spectroscopy were used to characterize the domain structure and symmetry. Ferroelectric properties were investigated using a Sawyer-Tower home made system and piezo-force microscopy. Very large spontaneous polarization were measured and I(V) curves were collected at different temperatures to understand the transport properties (interface or bulk limited and the existence of a Schottky barrier). PV effects under laser illumination of different wavelength (from 647nm to 457nm) and powers were investigated at different temperatures. Observed switchable Voc and Isc (short circuit current) will be presented showing that the PV effect arises from the ferroelectric field effect. An attempt to fully understand the electric structure of the BFO films has been performed and an impedance spectroscopy investigation of the ferroelectric PV solar cell will be also presented.

Resume : All-oxide photovoltaics based on cuprous oxide (Cu2O) absorber demonstrate an unprecedented dynamic in the recent years, with efficiencies reaching 8% for solar cells based on thermally oxidized Cu2O sheets. However, the thermal oxidation approach demands high temperatures and material consumption. An up-scalable and low-cost approach to form the absorber is by electrochemical deposition at temperatures close to ambient (ca. 50 °C), using a solution containing copper sulfate, lactic acid and sodium hydroxide. Here, we present our investigation on a solar cell architecture, with sputtered Cr/ITO as bottom electrode, upon which the p-Cu2O is electrodeposited, followed by the atomic layer deposition of a ZnMgO n-type layer. A transparent Al:ZnO front contact is finally deposited by sputtering. Depending on the Mg content of the n-layer, open circuit voltage values up to 650 mV are achieved, with power conversion efficiencies approaching 2%. The morphology and structure of the Cu2O absorber as a function of the electrodeposition parameters are analyzed by SEM, XRD and IR spectroscopy, while the properties of the Cu2O/ZnMgO heterojunction are probed by capacitance-voltage and current-voltage measurements. The work gives insight about the potential and limitations of the electrodeposited Cu2O cells in combination with the Mg-doped ZnO.

Resume : Embedding a strain balanced InGaAs/GaAsP multi quantum wells (MQW) into an absorption region of a solar cell is a promising candidate for realizing a high conversion efficiency. When the barrier width of the MQW is very thin less than around a few nm, referred to a super lattice (SL) structure, miniband between the QW forms and the carrier transport is enhanced drastically. Although the SL formation results in an increase of the conversion efficiency, thorough discussion for the density of states of the miniband, especially for an exciton formation, were not carried out yet. Since the exciton formation is a key feature for determining the absorption wavelength region of the solar cells, we investigate the optical absorption spectra for MQW and SL structures by a piezoelectric photothermal spectroscopy, a useful technique for measuring the absorption spectra of quantum nanostructures in terms of a non-radiative transition. The thicknesses of the wells and barriers were 7.4 and 10.8 nm for MQW and 5.0 and 1.9 nm for SL, respectively. The PPT spectra of the MQW samples showed step like features, reflecting a two dimensional density of states, with a strong exciton absorption line. This suggested the electrons are well localized in the QWs. However, a broad band with two maxima corresponding to two miniband edges were observed in the SL samples. No exciton-like behavior was observed. We concluded that this enhances the conversion efficiency of the solar cell structures.

Resume : An insertion of superlattice (SL) structure in an absorbing layer of solar cell has been proposed to enhance the performance of quantum well solar cell. It is expected that a formation of miniband causes the reduction of carrier recombination. However, currently used current–voltage and quantum efficiency measurements only reveal the macroscopic properties of solar cells. Photoreflectance (PR) and piezoelectric photothermal (PPT) techniques can observe the critical energies of band edges and the density of states in the SL structure, respectively. In this study, we observed the formation of miniband by these techniques and discussed the SL structure suitable for the solar cells. Five samples with different barrier thicknesses from 1.9 to 5.3 nm were prepared. The well widths were kept constant at 5 nm. Two critical energies were obtained at 1.27 and 1.31 eV for 1.9 nm barrier thickness sample. These energies were in good agreement with the calculated transition energies from the miniband center (Γ) and edge (π) of first electron level to the first heavy hole level by using a 3D device simulator nextnano®. For thicker barrier thickness samples, one critical energy was only appeared. Similar behavior was also observed in the PPT spectra. Thus, it was found that an absorption spectrum changed below and above at 3 nm as a critical thickness. We concluded that the barrier thickness of SL structure should be less than 3 nm for enhancing the performance of solar cells.

Resume : The electron photo-generation based on nano-objects semiconductor appears not yet to be clearly understood. Hence, additional investigations are required and spin- scavenging approach by Electronic Paramagnetic Resonance (EPR) can help in this context. Preliminary studies of aqueous suspensions of different TiO2 nanoparticles i.e. with distinct intrinsic characteristics yet showed similar yield of electron photo-generation. Pertinence of primary/aggregates size, fractal dimension, specific surface area, optical properties, light intensity, environment properties (pH and pH of zero charge), direct/indirect charge transfer are discussed. The suspension is not the right state to study the electron photo-generation properties. This is mainly due to interdependent and apparent non-pertinent parameters. Hence, original EPR approach was developed based on thin film depositions of nano-TiO2 within glass capillaries and allowing the control of amount, composition and form factor sample. In this configuration, variation of specific parameters can be probed keeping others constant. Therefore, kinetic models and quantification can be performed to characterize the involved processes.

Resume : The electronic structure and the optical properties of the (BaHfO3)/ (BiFeO3) superlattices have been investigated using the Full Potential Linear Augmented Plane Wave ( FP LAPW) method, implemented in the Wien2k code, in connection with the Generalized Gradient Approximation (GGA). The cubic perovskites BHO have a large gap equal to 6eV, with a polarization enough to classify her as ferroelectric material, and the cubic BFO is a good ferroelectric, is also have a large polarization. The combination between an insulating material BHO and a semiconductor BFO, show that the cubic superlattices (BaHfO3)/ (BiFeO3) with GGA approximation have a metallic comportment, but using the mBJ correction and GGA+U, the (BaHfO3)/ (BiFeO3) gives a semiconductor nature whit energy gap equal 2.1 eV ,with this results we can conclude that this combination of two material allow as to exploit the electric and semiconductor behavior in photovoltaic application.

Resume : This work reviewed the preparation and characterization of Nanostructured Nickel Oxide using Spray Pyrolysis deposition method. Nanostructured Nickel Oxide (NiO) thin films are important materials for applications in various photoelectric, solar cells, optoelectronic and other devices. There are several physical or chemical synthetic methods that have been used to prepare transparent NiO films as described by many researchers but spray pyrolysis stood out to be an efficient and cost effective technique. The X-ray diffraction study has confirmed the formation of NiO with nano-size scale. NiO films are p-type semiconductors and possess a direct band gap of 3.5 to 4.0eV. The influence of deposition conditions on the properties of the films were investigated using x-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible spectrophotometer.

Resume : Abstract: The surface Plasmon effect of noble metal nanoparticles on the photovoltaic properties of silicon solar cells was investigated. The metal nanoparticles were deposited on the p-type silicon base of the n+/p junction using a chemical deposition method followed by a thermal treatment at 500°C under nitrogen atmosphere. Chemical composition and surface morphology of the deposited metal were examined by energy dispersive X-ray (EDX) spectroscopy and scanning electronic microscopy (SEM). The effect of the deposited nanoparticles on the electrical properties was evaluated by the internal quantum efficiency (IQE) and current-voltage (I-V) measurements. The results indicate that the formation of the metal nanoparticles is accompanied by an enhanced light absorption and improved photovoltaic parameters.

Resume : An extremely facile approach to enhance electron extraction in ZnO electron transfer interlayer, and result in improving performance of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is demonstrated in this work, by adding two novel additives cetyltrimethylammonium bromide (CTAB) and Rhodamine 6G (R6G) into sol-gel ZnO precursor solution, respectively. Both additives can enhance absorbance of active layer in the range from 300nm to 800nm in wavelength, and give rise to increasing short-circuit current density (Jsc). In addition, morphological property of ZnO is also improved, the surface of ZnO with additive became smoother, as well as numerous defects were reduced. The best power conversion efficiency (PCE) for the CTAB-modified device is 8.92%, with the open-circuit voltage of 0.79 V, the short-circuit current density of 17.16 mA/cm2, and the fill factor of 65.74%, and the R6G counterpart demonstrate PCE 8.53%, with the open-circuit voltage of 0.80 V, the short-circuit current density of 15.60 mA/cm2, and the fill factor of 67.90%. Our research demonstrates that both additives of CTAB and R6G can dramatically influence optical, electrical and morphological properties of ZnO electron transfer layer, and work as effective additives to enhance the performance of bulk- heterojunction polymer solar cells.

Resume : Beside the success of Cu(In,Ga)Se2-based solar cells, tandem cells combining a wide bandgap top cell with a standard silicon-based bottom cell have been proposed to reach higher efficiencies. Here, we present the preparation and characterization of wide band gap chalcogenides derived from the chalcopyrite Cu(In,Ga)S2 (CIGS), as bulk and thin films materials. The preliminary chemical crystallography investigations were done on Ga-rich compounds. Single crystal investigation was done on a In-free Cu-poor compound. The indexed pattern showed that several reflections are forbidden in the I-42d space group (chalcopyrite). Thus, the corresponding structure was solved in the space group I-42m; Cu and Ga sharing the 4d Wyckoff position. In addition, the Cu deficit is balanced by an extra Ga-content and vacancies exclusively located on the 2a Wyckoff position. On the other hand, thin film materials have been prepared through different deposition techniques. First, CuGaS2 thin films have been grown on sapphir and silicon wafers by co-evaporation process in order to get films of high quality and compare their crystalline properties relative to the conclusions drawn from the crystallography analyses. Additionally, CIGS (Ga/(In+Ga)=0.3) thin films were synthesized by a sputtering and annealing route. Cu-poor compositions were used to avoid the detrimental CuSx secondary phase. From those films, solar cells with a conversion efficiency of 5.4 % were fabricated.

Resume : Tin (II) sulfide (SnS) is a promising material to replace current thin film light absorbing materials in photovoltaics. SnS has a moderate band-gap (1.1-1.3 eV) and high absorption coefficient. SnS thin films have been prepared by metal organic chemical vapor deposition (MOCVD) from the reaction of Sn(dmamp)2 and H2S gas as the source materials. The molecular structure of Sn(dmamp)2 is shown in fig. 1. SnS films were deposited on Si and glass substrates at the deposition temperature of 200-400 oC. Post annealing of SnS thin films was carried out at 400 oC for 1 h under the H2S ambient. Hall measurement using van der Pauw method indicate that the film has a p-type conductivity with a hole mobility of 13 cm2/V·s. Raman spectroscopy and x-ray photoelectron spectroscopy results show that SnS thin film has no impurities or other binary phase detected inside the films.

Resume : The elaboration and deposition effect of silver nanoparticles on optical and electrical properties of porous silicon layer was investigated. The silver nanoparticles were deposited on porous silicon layer using a thermal evaporation method followed by a thermal treatment at 180°C under nitrogen atmosphere. Photoluminescence and UV_Visible analysis of the treated samples were examined to understand the role of silver nanoparticles. The effect of the deposited nanoparticles on the electrical properties was evaluated by the AC impedance spectroscopy. The results indicate that the formation of the metal nanoparticles is accompanied by enhanced photoluminescence intensity and improved electrical parameters.

Resume : A complete and detailed description of the physical processes occurring in the up conversion phenomena is very difficult to accomplish with complete accuracy. On the one hand, it would be necessary to know how it is distributed luminescent centers in different energy sublevels; and secondly, how does the propagation of light in the converter, and how to deal with this phenomenon. Considering these factors, the objective of this work is to obtain comparative results (I x V) for different materials used as up converters (UC) when incorporated in bifacial silicon solar cells using the unidimensional program PC1-D, in order to get the best candidates for use as UC. Up conversion phenomena consists generally of a non-absorbent matrix containing luminescent centers at a given concentration. The absorption and emission resulting from the luminescence were simplified considering the luminescent center as a series of three energy levels. With the mathematical approaches developed, was obtained tables with photon density per unit for all ranges of wavelengths, between 300 and 2100 nm. The simulation was performed assuming a bifacial silicon solar cell already experimentally characterized and contrasted with theoretical data. Forms of illuminations have been modified for this solar cell considering the information of UC, where changed the intensities constants (transmitted sunlight) and conversion efficiency. The simulations of I x V curves demonstrated that UC which absorb at wavelengths between 1800 and 2000 nm have a better potential in relation to others, the efficiencies found (~20 %) for solar cells with UCs implemented are shown below the limit values calculated (~36 %). However, approaches the values found in practice (~16%) and modeled.

Resume : The polycrystalline Cu(In,Ga)Se2 (CIGS) absorber is the most promising material for photovoltaics because of its highest conversion efficiency among all thin-film technologies. Moreover, by using stainless steel as substrates, the flexible CIGS thin film solar cells can be achieved by integrating roll-to-roll technique with high throughput and low thermal budget, leading to the low manufacturing cost and new market of flexible solar cells. Here, we fabricated high efficiency flexible CIGS solar cells by co-sputtering of quaternary CIGS and Ga2Se3 targets to tune the bandgap. The uniqueness of this process is that the Ga profile in CIGS films can be precisely controlled during the CIGS deposition. In addition, no post-selenization is needed, substantially different from the existing processes. The various slope of Ga grading can be realized by simply tuning the sputtering condition of the Ga2Se3 target. Compared to ungraded (single bandgap) CIGS device, we find that the efficiency is improved by forming normal Ga grading, which has higher Ga content at the backside of absorber. The highest efficiency of 13.24% (without anti-reflection coating) on flexible substrates can be achieved with an open circuit voltage of 600 mV, short circuit current density of 30.25 mA/cm2 and fill factor of 73%. The enhanced efficiency in the normal grading CIGS films is attributed to a field-assisted carrier collection and passivation of CIGS backside, leading to the improved light absorption at long wavelength, and thus enhanced Voc and FF. Our approach shines light on the development of low-cost flexible CIGS solar cells.

Resume : Copper bismuth sulfide is a potential material for the development of high-efficiency and low-cost thin film solar cell devices due to its high absorption coefficient. The addition of selenium in the lattice could produce optimized bandgap structures and a higher p-type carrier density in order to further increasing the efficiency. In this work, the precursors were prepared by co-evaporating metal bismuth and CuS materials in a vacuum system. To introduce selenium into the lattice, the precursors were treated in the atmosphere containing selenium inside. To investigate the reaction process involved in selenium diffusion into the precursor bulk, various temperatures were selected in the experiments: 350, 400, 450, 500, 550 °C. In addition, the Cu/Bi atomic ratios had been optimized, which indicated the best atomic ratio of Cu/Bi controlling within the range from 1.0 to 1.5 in the film. Morphologies, compositions and structure of the thin films were characterized via Scanning Electron Microscope, Energy Dispersive System and X-Ray Diffraction. The surface has plenty of plate-like structures. XRD analysis indicated that the main phases of the thin films are CuBiSeO with few bi-phase〖Cu〗_x S. Optical absorptions of the thin films were obtained from the UV-VIS optical transparent measurement. The absorption edges were around 1200nm and consequently the bad gap of CBSS thin films was inferred to be 1.03eV. Electrical properties were measured using Hall Effect. The results indicated that S/Se atomic ratio directly affects resistance of thin films: resistance could vary from 〖10〗^(5 ) Ω to 〖10〗^(4 ) Ω with the thickness of 400 nm. The hole concentrations could reach 〖10〗^18 〖cm〗^(-3) based on Hall Effect analyses. In this work, copper bismuth selenide sulfide had been assumed as a kind of potential photovoltaic material. And the related devices are under preparation.

Resume : Intermediate band solar cells (IBSC) have recently attracted renewed interest as a potential approach to achieve high power conversion efficiency (PCE). In an IB material, sub-bandgap energy photons are absorbed through transitions from valence band (VB) to IB and from IB to conduction band (CB), which together produce the same amount of current as in conventional photons absorbed through the VB–CB transition. In this study, we report on a high performance IBSC based on Cr-doped ZnTe (ZnTe:Cr) fabricated by using pulsed laser deposition (PLD). Chromium content in ZnTe:Cr thin film was about 3.2 at. %. ZnTe:Cr thin film showed p-type electrical conductivity by the effect of Cr doping, while undoped ZnTe thin film had semi-insulating property. From ZnTe:Cr thin film, the higher absorption coefficient appeared than that of undoped ZnTe film in a photon range from ultraviolet (3.5 eV) to near-infrared (1.2 eV). It was suggested that the enhancement of absorption coefficients from ZnTe:Cr thin film is attributed to IB formation in bandgap of ZnTe. Finally we fabricated IBSC structure with ZnO:Al/ZnTe:Cr/Si by PLD. Under illumination with AM 1.5G, it was generated a large short circuit current (JSC) of 21.18 mA/cm2, an open circuit voltage (VOC) of 0.48 V and a fill factor (FF) of 0.58 yielding PCE of 5.9 %, which presents the highest PCE in the IBSC based on impurity-doped ZnTe.

Resume : Band alignment is a crucial point to enhance the performance of heterojunction for chalcopyrite thin film solar cells. In this work, we report ab-initio calculations of the electronic structures of hyperdoped chalcopyrite CuGaS2:Cr with various Cr compositions, and those of CuAlSe2 and ZnSe, and the band alignment between their interfaces. CuGaS2:Cr will be the material with an intermediate band being this the main absorber layer. Previously we have also made calculations for the CuGaS2 host semiconductor. We use density functional theory and the more accurate self-consistent GW scheme to obtain improved bulk band-gaps and band offsets. Band alignments of the interfacial region for CuGaS2:Cr/CuAlSe2 and CuGaS2:Cr/ZnSe systems were carried out by aligning them with respect of an average electrostatic potential. Our results are in good agreement with experimental values for the bulk band-gaps. The theoretical band alignments or this hyperdoped new material with the other two semiconductors show a characteristic staggered band offset which allow them to be used for the design of heterojunction devices in photovoltaic applications

Resume : Quaternary sulfide kesterite, Cu2ZnSnS4, possesses many advantageous characteristics for photovoltaic applications, such as suitable band gap, high absorption coefficient and high radiation stability. In contrast to more examined CIG(S,Se) quaternary semiconductor nanocrystals, it provides a promising alternative because of its environmental acceptance (application of S instead of toxic Se), cheapness and availability (common Zn and Sn instead of scarce In and Ga). Kesterite has been prepared by several techniques, such as solution-based, hot injection synthesis, electrochemical deposition and microwave irradiation. However, these techniques are complex, time-consuming, need high temperatures and toxic organic solvents. In this study, we demonstrate the use of elemental precursors (Cu,Zn,Sn,S) to obtain Cu2ZnSnS4 by a solid-state process at ambient temperature without using solvents. The applied mechanochemical approach enabled us to obtain small scale amount (5 grams) of kesterite in a laboratory mill. The promising laboratory synthesis stimulated further activity in an adapted industrial mill to produce Cu2ZnSnS4 in a larger scale (100 grams). Methods of XRD, UV-Vis spectroscopy, nitrogen absorption, SEM, EDX, HRTEM, XPS, thermal and Soxhlet analysis were applied for the product characterization. The results confirm the possibility of up-scaling the mechanochemical one-pot synthesis process of kesterite Cu2ZnSnS4 which is the perspective absorber for future solar cells.

Resume : Constructing tandem junction devices consisting of a bottom cell based on prevailing solar cell techniques such as silicon (Si) or Cu(In,Ga)(Se,S)2 (CIGS) combined with a perovskite top cell is the most effective approach to improve photovoltaic device performance. An intermediate transparent conducting layer connecting the two single devices, at the same time serving as inorganic hole transport material for the perovskite solar cell is one of the key components to realize this tandem structure. In this contribution we describe the deposition of p-type Cu2O films at room temperature by magnetron sputtering using nitrogen doping to form homogenous surface with highly transparency and good conductivity, and the modeling of a monolithic perovskite-CIGSe tandem solar cell. We applied the in-house software RefDex to illustrate the impact of Cu2O on the performance of a tandem device in module size, and calculated the reflection/transmission/absorption (R/T/Abs).

Resume : We have designed light-trapping configurations for crystalline silicon (c-Si) photovoltaic (PV) cells coupled with solar-pumped lasers (SPLs) emitting at 1064 nm just below the absorption edge of c-Si, and proven the concept. The significant light trapping allows us to use thin c-Si cells to eliminate detrimental impacts of the series resistance and Auger recombination under intense illumination. A combination of an anti-reflection coating (ARC) on the front surface and a diffuse reflector on the back surface of a cell enhances the effective optical path by 4 n2 ~ 50 for c-Si, i.e., the Yablonovitch limit. When an SPL illuminates a stationary PV cell from the normal (or a specific) direction, a bandpass filter (BPF) as an angle-selective filter on the front surface transmits the incident light and reflects the light coming from the inside except for that with very small incident angles, resulting in more significant light trapping and enhancement of the optical path by over 200. Based on this concept, we fabricated 50 um-thick c-Si PV cells and demonstrated quantum efficiency as high as 0.8 up to 20 W/cm2 of the illumination intensity. When SPLs illuminate flying drones and moving electric vehicles equipped with the PV cells for remote power supply, the BPF is replaced with a short-pass filter (SPF) to trap the light with ever-changing incident angle within a certain acceptance angle. The SPF achieves more significant light trapping than the ARC for the acceptance angle < 45º.

Resume : In recent years, despite significant progress in the synthesis, characterization, and integration of various nanowire (NW) material systems, crystal orientation controlled NW growth as well as real-time assessment of their growth−structure−property relationships still presents one of the major challenges in deploying NWs for practical large-scale applications. In this work, we propose, design, and develop a multilayer NW printing scheme for the determination of crystal orientation controlled photovoltaic properties of parallel GaAs NW arrays. By tuning the catalyst thickness and nucleation and growth temperatures in the two-step chemical vapor deposition, crystalline GaAs NWs with uniform, pure ⟨110⟩ and ⟨111⟩ orientations and other mixture ratios can be successfully prepared. Employing lift-off resists, three-layer NW parallel arrays can be easily attained for X-ray diffraction in order to evaluate their growth orientation along with the fabrication of NW parallel array based Schottky photovoltaic devices for the subsequent performance assessment. Notably, the open-circuit voltage of purely ⟨111⟩-oriented NW arrayed cells is far higher than that of ⟨110⟩-oriented NW arrayed counterparts. All this indicates the profound effect of NW crystal orientation on physical and chemical properties of GaAs NWs, suggesting the careful NW design considerations for achieving optimal photovoltaic performances. The approach presented here could also serve as a versatile and powerful platform for in situ characterization of other NW materials.

Resume : The power conversion efficiency in heterojunction inorganic cells, using non-critical materials is still a major issue. Several wide band-gap semiconductors were developed to enhance the solar spectrum coverage, but the narrow absorption range is still a drawback, as only a small fraction of the solar flux is harvested by the active absorbent layer. Thus, one strategy is to make use of tandem architectures where two or more semiconductors get an adequate balance between light absorption and charge carrier mobility. This study focuses on the optimizing the photovoltaic response in an environmental friendly photovoltaic structure. The semiconductors were obtained using common raw materials, non-toxic and cost effective technique (SPD), with low amounts of by-products. The tandem structures contain SnO2, TiO2 and CZTS with different thickness in order to reduce the charge carriers trapping at the interfaces and to allow a high mobility trough the hetero-structure. The crystallinity influence, the elemental composition and the surface energy on the n-n or n-p interfaces is presented, outlining the importance of the structural compatibility during the films deposition and the control of the vacancies, as prerequisite for controlled diffusion at the interface. Morphology is also important mainly to avoid physical gaps. Further on, the I-V curves for each structure were obtained in dark and under irradiation and the results were correlated with the band energy diagram.

Resume : In this work, AMPS-1D was employed to study the electrical and optical properties of bifacial ZnO:Al/CdS/Cu2ZnSnS4 (CZTS)/TCO/SLG thin film solar cells. The impact of barrier height in the CZTS/TCO interface has been investigated for front and back side illuminations. The combination of the optical transparency and the electrical properties for the TCO back contact layer are capable of yielding high efficiency. The presence of barriers in the CZTS/TCO back-side interface of the structure can significantly affect the cell performance by limit the carriers current flow. The optimum CZTS absorber layer thickness used in the simulation has been calculated. The best energy conversion efficiency has been calculated in the case of front and back side illuminations. In the case of front side illumination, an efficiency of 15.8% (with Voc ≈ 0.93 V, Jsc ≈ 18.7 mA/cm2, FF ≈ 0.77 and QE ≈ 82.5% at 570 nm) have been obtained with a barrier height close to 0.4 eV. In the case of back side illumination, we notice a low performances. For 0.4 eV barrier height, we obtain 1.8% (with Voc ≈ 0.95 V, Jsc ≈ 2.6 mA/cm2, FF ≈ 0.74 and QE ≈ 20.6% at 820 nm). Without barrier height we obtain a maximum of efficiency (η ≈ 4%) in the case of back side illumination. All these simulation results give some important indication to lead a higher efficiency of bifacial CdS/CZTS/TCO structure for feasible fabrication.

Resume : Recently, we reported on the crystallographic and optical properties of CuInSe2, CuIn3Se5, CuIn5Se8, and Cu-poor Cu-In-Se compounds in the Cu2Se-In2Se3 system [1]. Increasing the In2Se3 content (decreasing the Cu/In ratio) changed the crystal structure from the chalcopyrite-type to the stannite-type. The band-gap energies of Cu-poor samples, i.e., CuIn3Se5 (1.2 eV) and CuIn5Se8 (1.22-1.24 eV), were larger than that of the chalcopyrite-type CuInSe2 (1.0 eV). In this study, we prepared a single-phase Cu(In1-XGaX)3Se5 (0 ≤ x ≤ 1.0) solid solution with the stannite-type structure using a mechanochemical process and post-heating at 600 oC. We determined their band-gap energies from the diffuse reflectance spectra and measured the ionization energies by photoemission yield spectroscopy (PYS). We estimated the energy levels of the valence band maximum (VBM) from their ionization energies and, by adding the value of the optical band gap to the VBM level, we could determine the conduction band minimum (CBM). The band-gap energies of the samples linearly increased from 1.2 eV for CuIn3Se5 to 1.7 eV for CuGa3Se5. The VBM levels of the Cu(In1-XGaX)3Se5 solid solution did not change significantly. On the other hand, the energy levels of the CBM increased rapidly. We also compare the electronic structure of the Cu(In,Ga)3Se5 solid solution with that of the chalcopyrite-type Cu(In,Ga)Se2. [1] T. Maeda, W. Gong, and T. Wada, Jpn. J. Appl. Phys. 55, 04ES15 (2016).

Resume : Molybdenum disulphide (MoS2) has been received much attention for application in photovoltaics owing to strong absorption property in solar spectral region. Moreover, it reveals direct to indirect band gap transition from 1.29 to 1.90 eV depending on the number of layers. To prepare the MoS2 nanosheets or continuous films, various methods such as mechanical exfoliation, chemical vapor deposition (CVD) and liquid exfoliation have been studied. Among them, the liquid exfoliation method has a large potential on various applications because of its various advantages in mass production and low temperature process. In this study, residue-free MoS2 thin films were formed during the liquid exfoliation process and their electrical properties were characterized with interdigitated electrode. Then, MoS2 film thickness could be controlled by centrifuge condition in the range of 20~40 nm, and its carrier concentration and mobility were measured about 7.36×10^16 cm-3 and 4.67 cm2V-1s-1, respectively. Detailed analysis on the films was done by atomic force microscopy, Raman spectroscopy, and high-resolution transmission electron microscopy measurements. For application of photovoltaic device, the structure of Au/MoS2/silicon/In junction was fabricated, and then it showed the power conversion efficiency of 1.01 % under illumination of 100 mW/cm2.

Resume : Multijunction GaInP/GaAs/Ge solar cells are of the great interest due to their supreme efficiency that has already exceeded 40%. The bottom p-n junction is usually formed by phosphorous diffusion into Ge substrate during epitaxial growth of III-V wide-band-gap window-layer (GaInP) in order not to contaminate the growth chamber with group IV atoms. However, diffusion of group V atoms (Ga) takes place at the same time. Due to higher solid solubility limit of Ga in Ge the potential barrier of ~0.1eV is formed at the III-V/IV interface, which results in decrease of fill factor and can be observed as voltage offset in dark I-V curves at low temperatures (100K). In this paper we propose using AlAs layer as a diffusion barrier at the III-V/IV interface. First GaInP/AlAs/Ge and GaAs/AlAs/Ge solar cell structures were fabricated by MOCVD. It was shown that a 10 nm AlAs layer effectively blocks diffusion of both P and Ga atoms. It was demonstrated that in case of undoped AlAs layer the conductance band offset forms potential barrier for electrons of ~0.2 eV. Admittance spectroscopy technique was used to estimate barrier’s height. Increase of the doping level of AlAs layer reduces effective barrier height. At the doping level of 1019 cm-3 no barrier was detected. Thus, solar cells based on AlAs/Ge heterojunction may have a higher efficiency than conventional solar cells.

Resume : In the quest to push solar cells based on Cu2ZnSn(S,Se)4 (kesterite) to higher efficiencies, there remain some important questions to be answered about basic material properties. One example is the extent of the homogeneity range of single phase kesterite – and its dependence on process conditions – which is so far poorly defined. The homogeneity range is of fundamental importance as, via the influence of defects that dominate in different composition ranges, it gives the total scope for varying the material properties. In this work, we used combinatorial reactive sputtering to prepare Cu-Zn-Sn-S films containing a large range of cation compositions. We varied the anion chemical potential during subsequent thermal treatments by changing the conditions. In the resulting samples, we mapped out the single phase regions by combining spatially resolved compositional and phase analysis and Resonant Raman mapping. Particular observations include a strong effect of the Sn activity on the extent of single phase region, which leads us to propose a chemical equilibrium between gas phase species and E/A-type defect complexes in the solid phase. We also observe large variations in Cu/Zn ordering across the single phase region, which could be attributed to the effects of Cu vacancies and interstitials. This work exemplifies a powerful methodology that has great potential for investigation of kesterite and other new material systems.

Resume : CZTS thin films were deposited by ultrasonic spray technique on Mo-coated substrates, followed by a thermal treatment in elemental sulfur vapor ambient under Argon atmosphere. Various contents of sodium and potassium were added to the starting spray solution to prepare Na and K doped CZTS films. X-ray diffraction and Raman spectroscopy were used to confirm the kesterite structure of our samples. Scanning electronic microscopy and energy dispersive spectroscopy analysis were performed to study the effect of Na and k on morphology, grain’s size and composition of the obtained films. Photoluminescence measurements were also studied.

Resume : General Background Traditional energy is experiencing the depleting condition and peoples’ demands are increasing. Renewable energy have attracted many interests due to its sustainable properties and environment friendly properties.Among these energy, solar energy have caught the interest of public as it can be converted into the electricity directly and simply by photovoltaic devices. In recent decades, hybrid solar cells have attracted the interest of scientists. This kind of solar cell uses a combination of the organic and inorganic materials. Generally, hybrid thin films are semiconducting nanomaterials blended with polymers to make the hybrid thin films. This type of thin film are solution processable and are low cost and simple and allow the band gap to be altered.As hybrid thin film contains the conducted polymer and the nan scale inorganic materials, thus, it generally can combine the properties of these two materials. SnS has become a desirable candidate for PV devices as it meets the requirements discussed earlier. In general, Tin and sulphur are abundant elements and both are non-toxic. In addition, in comparison with the CdTe or CIGS mentioned before, the production process is simple and may be solution based or carried out electrochemically. SnS is a binary compound which has elements from IV and VI groups. It always present the stable orthorhombic crystal structure at room temperature, this kind of structure is stable and symmetric.Referring to the optical and electrical properties, SnS films have a high absorption coefficient.[17] For example, SnS thin films prepared using the e-beam evaporation method have achieved more than 10^4 cm^(-1) whereas thin films prepared using the resistive evaporation deposition method have achieved an absorption efficiency of approximately 10^5 cm^(-1). Relating to the aim of the project and the use of SnS thin film in the PV device, There are many different ways that can be used to produce the SnS thin film, such as atomic layer deposition, chemical vapour deposition, thermal evaporation, electrodeposition and solution based methods. Along these methods, electrochemical methods have attracted the interest of the scientist for its convenient operation and scalable production properties as well as the good control of the semiconductor thin film’s properties via different parameters, for examples, the potential, pH value and temperature of the solution bath and the deposition time. As the SnS thin film will come from the solution base, thus, the aqueous base must contains the compound have Sn and S. According to the paper previously,, are two popular combination solution to provide to Sn and S for synthesis SnS. Before the experiment, the pH in the solution is always been set to the acid condition, which will usually be set from 1.5 to 4.0 in order to help extract the S element and prevent the formation of hydroxyl species as well as the from oxidation. Project methodology In my experiment, three electrode electrochemical cell will be used. The configuration can be shown as figure below. The system contains working electrode, counter electrode and reference electrode. Generally, the substrate must be conduct and widely use Indium Tin Oxide (ITO) orfluorine doped tin oxide. Reference electrode and the counter electrode usually will apply calomel electrode and coiled platinum wire, respectively. Before start the film fabrication, a suitable temperature will be set and the deposition potential always varies from -0.8V to -1.2V from the previous papers.[27] More possibility need to be tried in the future experiment. Thus, the SnS thin film will be electrode-posited into the substrate continuously under the applied voltage. After achieved the SnS thin film, the device will be assembled in order to characterize the properties of PV device. Several characterization methods will be used to identify the properties of the thin film and device. • UV-Visible absorption spectroscopy (UV-Vis) is used to characterize absorption of SnS in the device. • Transient absorption spectroscopy (TAS) will determine the charge generation within the sample. • X-Ray diffraction (XRD) to identify the phase of SnS and give the dimensions of unit cell. • Scanning electron microscope (SEM) to determine the morphology of the SnS in nanoscale. • Solar cell device characterization to determine the power converse efficiency • Other electrochemical characterisations to investigate the electrochemical properties. The aim and the objective of my project • Be able to develop an understanding of the way in which film growth of SnS may be controlled electrochemically. Be able to control the morphology of SnS via the electrochemical deposition method. • Be able to control the thickness of synthesized SnS by concentration, potential, deposition time etc. • Characterise the ECD-SnS (optical properties etc.) and compare with other methods. • Understand the underlying photoelectric mechanism of ECD-SnS • Incorporate electrochemically fabricated SnS into the established cell structure where SnS is synthesized by spin coating.

Resume : Cu(In,Ga)Se2-based (CIGS) solar cells have achieved the highest efficiencies (up to 22.6% on SLG substrate) among the thin film solar technologies. The use of polyimide (PI) substrates broaden the scope of applications for which the weight of the solar panel remains an issue. Yet, PI substrates cannot stand high deposition temperature (<450°C). Because of lower temperature process, lower diffusions of Cu, In and Ga are observed. It implies a strong composition gradient. In this study, we analyze the impacts of the Se flux on the diffusion of the elements in the CIGS layer and on the cell performances. Thus, we have realized a series of experiments: we changed the Se to metal ratio (SMR) but also kept the Se flux constant through the whole process. We have proved a strong influence of the Se flux on the Ga diffusion by Glow Discharge Optical Emission Spectroscopy: a higher quantity of Ga is observed at the back and the front contacts compared to a sample without a control of the Se flux. We also noticed an impact on the crystallinity of the CIGS layer by X-Ray Diffraction: for a SMR = 6, the Full Width at Half Maximum reached its lowest value and the ratio (220)/(112) its highest. We also observed changes in the morphology by SEM. This allowed us to precisely adjust the concentration profiles in the CIGS and to demonstrate a 16.3% efficiency solar cell (without an ARC). It represents a +0.8 % efficiency compared to a baseline sample, mainly due to an increased VOC.

Resume : Zinc oxide thin films co-doped with rare earth (Nd and Tb) have been grown on heated glass substrates (350 °C) by chemical spray pyrolysis method. The effect of doping rate on structural, optical and electrical properties was studied. X-ray diffraction confirmed that all the prepared films have the hexagonal wurtzite structure with a preferred orientation toward the c-axis. No peaks belonging to rare earth or their oxides were observed in the limit of XRD technique detection. UV–visible spectra showed a significant optical transmission above 75 %, which decreased with the increase of Tb concentration. Photoluminescence spectra were dominated by an emission band attributed to the radiative recombination of excitons and a large band attributed to the various defects in ZnO matrix. Electrical resistivity of about 3 10−2 Ω cm was achieved. Keywords: Rare earth, ZnO, Spray, Thin films, Characterization.

Resume : Cu(In1-xGax)Se2 (CIGS) compound semiconductors have attracted much attention as an absorber material in thin film solar cells because of a high absorption coefficient (1 x 105cm-1) and controllable band gap energies (1.0 ~ 1.65eV). Among the several techniques to fabricate CIGS thin films, we have developed one-step sputtering process with Se-contained CIGS single target, in which the conventional post selenization process can be omitted. Furthermore, Cu-Se interlayers with liquid phase around 500oC are used to improve the morphological properties of CIGS thin films. On the basis of these experimental, high quality CIGS thin films with large grain size were fabricated by using Cu-Se layer prior to deposition of CIGS thin films. Under the optimized condition, the CIGS thin films with thickness ratio of Cu-Se and CIGS layers (100 nm : 1200 nm) showed dramatic improvement in grain size up to 1um scale compared to the CIGS films without Cu-Se layers. As a result, the CIGS solar cell showed the best cell efficiency of 9.8% when the CIGS films have the composition ratio (Cu/[In+Ga]) of 0.83 and the grain size of above 1.0um. However, the CIGS thin film containing the Cu-Se interlayer has some problems to be solved such as surface roughness and composition ratio. The single Cu-Se thin film had needle-like crystals when deposited on a heated substrate. On the contrary, post heat-treated Cu-Se films after the R.T deposition showed a partially non-deposited region. Both of these two films were expected to have negative effects on structural and chemical properties of CIGS thin films during the film growth. In this study, we report the characteristics of CIGS thin films with improved surface roughness and controlled compositions through a two-step Cu-Se thin film process, which have two sequential deposition and heat treatment processes. The detailed properties of the CIGS thin films including device performances will be discussed in the presentation.

Resume : PbS quantum dots (QDs) have been considered as promising candidate replacing the conventional Ru/organic dye sensitizer because of their prominent properties such as high optical absorption, facile bandgap tunability by size controlling, multiple exciton generation, and easy charge separation by large dipole moment. Despite great potential of PbS QDs in sensitized solar cells, the PbS QDs have problems in efficient charge injection and transportation due to the long chain organic ligands act as insulator. Therefore, the formation of PbS QDs without insulating ligands is still challenging, so we synthesized PbS QDs by spin-assisted SILAR (successive ionic layer adsorption and reaction) method. And furthermore, to deposit more uniform PbS QDs, lead halide precursors are used by precipitation and anion exchange reaction.

Resume : Metal chalcogenides have attracted a great deal of attention as light absorber because of their excellent optical properties, which can be tuned by controlling the semiconductor dimensions, a high extinction coefficient, and a large intrinsic dipole moment. Especially, Sb2S(e)3 is good light absorber because Sb2S(e)3 has strong absorption coefficient, large dipole moment which enable separate charge easily and suitable band gap for adapting solar cells. However, it is difficult to achieve the efficient solar cells because we can not synthesize the highly pure Sb2S(e)3 by conventional methods such as chemical bath deposition (CBD) and single source precursor (SSP). Conventional methods cause the impurity, which degrade the power conversion efficiency of solar cells. Therefore, we suggest the developed method to synthesize the highly pure Sb2S(e)3 by using hydrazine solution. Through the hydrazine method, we synthesized the highly pure Sb2S(e)3 and adapted these materials for fabricating the solar cells. The Sb2S(e)3 sensitized thin film solar cells exhibited 14.67 Jsc (short circuit current), 0.39 Voc (open circuit voltage), 52.61 F.F. (fiil factor), and 3.01 η(power conversion efficiency).

Resume : Cu(In1-x, Gax)Se2 (CIGS) based thin-film solar cells have gained marked attention for photovoltaic applications due to their high absorption coefficient, tunable bandgap and outstanding stability. High conversion efficiencies of 22% in the lab-scale solar cell and 16% in the large-size module have been reported. Recently, semitransparent CIGS based-solar cells have been considered as a promising one for window and curtain wall applications in the buildings compared to other photovoltaics such as crystalline silicon, amorphous silicon, organic and dye-sensitized solar cells. However, there are several challenges in the preparation of semitransparent CIGS-based solar cells with high conversion efficiency and good transparency in the visible light range. Poor interfacial properties between absorber layer and transparent conducting oxide film and the formation of shunt paths between front and back electrodes induced by thin absorber thickness should be improved. A trade-off between conversion efficiency and visible light transparency of the solar cells is also needed. In this study, the semitransparent CIGS-based solar cells with an absorber thickness below 500 nm were prepared on F-doped tin oxide (FTO) and Sn-doped indium oxide (ITO) substrates by co-evaporation. Material properties of the absorber layers fabricated by varying deposition conditions such as chemical composition, alkali ion dopant and substrate temperature were investigated in detail. Their influence on the cell performance is also examined.

Resume : Cu(In,Ga)Se2 (CIGS) chalcopyrites have been widely investigated in the field of solar renewable energy using photovoltaic cells. Especially, non-vacuum processed CIGS has been studied, despite its low conversion efficiency, because of the high production costs and material waste from vacuum-based processes. Furthermore, wide band gap absorber has been in the spotlight because of the possibility to use as a top-cell absorber in tandem structure. We already reported a non-vacuum hybrid ink to form CIGS thin films. The concept of our hybrid ink is that the combination of the advantages of particle-based and solution-based processes, and it is possible to make dense thin films with little organic residue. The hybrid ink consists of synthesized binary nanoparticles (Cu-Se, Cu-S, In-Se, In-S, Ga-Se, or Ga-S) and an indium (In) precursor solution with a chelating agent, monoethanolamine (MEA). However, Ag-based nanoparticles have not been used yet for wide band gap absorbers in non-vacuum process. Therefore, in this study, (Ag,Cu)(In,Ga)Se2 (ACIGS) thin films using Se@Ag2Se core-shell will be studied and several analyses will be presented.

Resume : CuInSe2 thin film which used in solar cell applications was grown by one-step cathodic electrodeposition on fluorine doped tin oxide coated glass using choronoamperometry mode (fixed potential), with a three electrode potentiostatic system in a suitable acidic medium. The structure, chemical composition, morphology, optical properties, and uniformity of the electrodeposited CuInSe2 film were characterized by X-ray diffraction (XRD), UV–vis–NIR spectrophotometry, scanning electron microscopy (SEM), Raman spectroscopy, respectively. The optimum conditions consisted of an annealing temperature of about 400 ◦C for 30 min under nitrogen ambient. At this temperature, a CuInSe2 thin film, in the form of ternary compound, is formed with the required conditions, namely good crystallinity, good stoichiometry, a suitable band-gap, and depth uniformity. In order to increase the conductivity we grew CIS structure on the graphene. And then the solar cell efficiency of CIS thin films have increased.

Resume : Typical Cu(In,Ga)Se2 (CIGS)-based solar cells have a buffer layer between the CIGS absorber and the transparent conducting oxide, which plays an important role in improving the cell performance, allowing appropriate flow path of photoexcited e-h pairs. Among various buffer materials, Zn-based materials have been frequently studied because of their beneficial properties, for example, good transparency with large direct Eg, less toxicity and cost effectiveness. In particular, Zn(O,S) film is considered as an attractive buffer material able to engineer the bandgap by controlling oxygen/sulfur ratio from ZnS to ZnO. We fabricated a CIGS solar cell with several kinds of Zn-based buffer layers: cracker-Zn(O,S), chemical bath deposited (CBD)-Zn(O,S), and sputter-Zn(O,S) based on optimized conditions previously set up by our group. The efficiency of solar cell indicated the highest value with CBD-Zn(O,S) buffer layer. To find out the correlation between carrier lifetime and cell efficiency depending on buffer types, the optical pump THz probe (OPTP) spectroscopy measurement was carried out, which is specialized in detecting behavior of photocarriers such as trapping at defect states and recombination of e-h pairs. The two fs laser pulses of 400 nm and 800 nm as a pump beam were used to excite charge carriers from buffer layer and CIGS absorber, respectively. From the OPTP results, we discovered the prolonged carrier lifetimes in two cases; (i) at pump beam of 800 nm than that of 400 nm and (ii) after deposition of Zn(O,S) buffer layer than pure CIGS layer in all cases. We interpreted these phenomena in the view point of carrier trapping times at defect states as a function of deposition method of Zn(O,S) buffer layer.

Resume : Organic−inorganic hybrid perovskites have created significant opportunities for low-cost and high-performance solar cell. Great attention has been devoted to the study of both the overall structure of the cell and its individual components. One of the main components is the blocking layer (BL), which selectively transports electrons and is a barrier for holes. The investigations have been focused on the titanium dioxide obtained by sol-gel methods. In particular, perovskite solar cells obtained from the fresh and aged titanium sol revealed significant differences of 8% in the short circuit current. Microstructural and morphological AFM analysis combined with electrochemical impedance spectroscopy and optical properties measurements enabled to the determine sources of these differences. Major differences was found in thickness and crystallinity of investigated blocking layers. Namely, the layer from the aged sol were of greater thickness despite the same spin speed and nanocrystallinity. The results suggest a significant impact of the sol history and the type of substrate on the proper design and preparation of BL for perovskite solar cells. Research was supported by the Polish National Science Centre under the project No. 2014/13/N/ST8/00858 and in the frame of statutory work of the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences.

Resume : Cadmium zinc telluride (CdZnTe) is a ternary semiconductor material which can be used in many important applications, such as solar cells, photoconductors, room temperature gamma ray and X-ray detectors, switching devices, light emitting diodes. CdZnTe has a tunable direct band gap properties which makes it a promising candidate for solar cell applications. CdZnTe can also be used as an absorber layer due to its high intrinsic mobility, high absorption coefficient, high quantum efficiency and high atomic number. Several deposition techniques have been developed to fabricate CdZnTe thin films including sputtering, vacuum evaporation, chemical vapor deposition, close-spaced sublimation (CSS), electrodeposition and molecular beam epitaxy. However, the electron beam vacuum evaporation technique has many advantages over other techniques and it is the preferred technique for large scale solar cell applications due to high efficiency, good reproducibility and low operation cost. In this study, the effect of CdCl2 treatment on CdZnTe thin films grown by electron beam vacuum evaporation have been investigated. Solutions with different CdCl2 concentrations and the effect of exposure time have been studied. CdCl2 treatment procedure is considered vital for the fabrication of high efficiency CdZnTe solar cells and this procedure is believed to improve the recrystallization and grain growth. After chloride treatment, a significant increase in grain size has been observed when compared to as-grown and annealed films. Optical properties using UV-VIS spectrophotometer, crystallographic properties and chemical compositions using X-ray Diffraction (XRD) and Energy Dispersive X-ray analysis (EDX), morphological properties using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) have been investigated. In addition, the rectification properties of Si/CdZnTe thin films after chloride treatment have been studied using I-V measurements under dark conditions. Capacitance-voltage (C-V) measurements (dark at room temperature) have been also performed. As a result, an optimization process has been carried out for CdCl2 treatment of CdZnTe solar cells with respect to exposure time and CdCl2 concentration.

Resume : Current research trends are moving towards earth-abundant and low toxicity materials. Copper-indium-gallium-selenide (CIGS) and cadmium telluride (CdTe) are two leading thin film solar materials that have already been commercialized. Despite several disadvantages such as toxic behavior of Cd in CdTe films [1] and the rare-earth In and Ga in (CIGS) [2,3], the constituent elements of Cu2ZnSn(S,Se)4 (CZTSSe) are non-toxic, earth-abundant, and cost-effective which makes CZTS as an attractive candidate for large-scale solar cell production. In this work, CZTSSe thin films were deposited by thermal evaporation technique onto glass and n-type Si wafer substrates. As an evaporation source, the stoichiometric crystal powder of CZTSSe was used to deposition the film samples. In order to determine the material properties of CZTSSe, thin films deposited onto glass substrates were characterized by using structural, optical and electrical measurements. Moreover, to investigate the device behaviors, the films were deposited onto n-Si substrate. Structural characterization was carried out by X-ray diffraction (XRD) and the presence of CZTSSe phases was defined by Raman scattering measurements. The detailed surface morphology analysis of the films were performed by using scanning electron microscope (SEM) and atomic force microscopy (AFM). UV-Vis spectrometry was used to evaluate the optical characteristics, as transmission, absorption and optical band gap of the thin films. By using room temperature Hall Effect and dark conductivity measurements, electrical properties of the deposited films were determined. In order to deduce the device properties, the In/p-CZTSSe/n-Si/Ag sandwich structure was fabricated, and room temperature current-voltage (I-V) measurements were carried out. Device parameters such as; diode ideality factor, series resistance and shunt resistance values were calculated from I-V analysis. The frequency dependent capacitance voltage (C-V) measurements were measured in the frequency range of 70 kHz and 3000 kHz. It was observed that capacitance values decreased with increasing frequency. Built-in potential and barrier height of diode, space charge density and thickness of depletion region were determined by the results of C-V measurement. References: 1. Wu X. High-efficiency polycrystalline CdTe thin-film solar cells. Solar Energy 2004;77 : 803-814. 2. Jimbo K, Kimura R, Kamimura T, Yamada S, Maw WS, Araki H, Oishi K, Katagiri H. Cu2ZnSnS4-type thin film solar cells using abundant materials. Thin Solid Films 2007;515 : 5997-5999. 3. Tanaka K, Oonuki M, Moritake N, Uchiki H. thin film solar cells prepared by non-vacuum processing. Solar Energy Materials and Solar Cells 2009; 93: 583-587.

Resume : Enhancement of light absorption in thin film solar cells is done by several light management techniques. Surface texturing is one of the most common techniques utilized in light management. Transparent conductive oxide (TCO) layer and/or the substrate itself can be textured in order to reduce scattering and thus enhance optical path length of incoming light in the absorber layer. Texture size and periodicity are the important parameters to create an effective light management TCO interface. Among the other TCO materials, aluminum doped zinc oxide (AZO) is preferred due to its high transparency, non-toxicity, excellent thermal stability and low cost [1]. For thin film silicon solar cell applications, an additional advantage is its compatibility to hydrogen-rich plasma media [2-4]. AZO films are usually deposited by sputtering (RF or DC), vacuum arc plasma evaporation and pulsed laser deposition (PLD) [5-8]. Among these techniques, magnetron sputtering has advantages such as low substrate temperature, good adhesion of films on different substrates, high deposition rates, high uniformity, and adoptable to a large scale fabrication [9] . Therefore, in this study radio frequency (RF) magnetron sputtering is used for low temperature deposition on ultra-thin flexible glass (<100 μm) substrates. AZO surface texturing is usually done by wet chemical etching which is compatible with large area thin solar cell fabrication [10]. However wet chemical etching which usually has HF and HCl as chemicals is not suitable for polymeric and glass flexible substrates. On the other hand, laser texturing affects the TCO surface without introducing any undesired damage to the underlying glass or polymeric substrates. Moreover, laser texturing gives the chance to have a finer control on the fabricated feature periodicity. The main scope of this study is to obtain surface texturing of AZO thin films deposited on flexible glass substrates by RF magnetron sputtering at room temperature for thin film photovoltaic applications. HIPPO Spectra Physics laser UV laser operating at 355 nm was used in this study to obtain surface textures. Laser beam operates at TEM00 spatial mode with less than 15 ns pulse width at 50 kHz. The parameters of the laser (i.e., diode current, repetition rate, motor speed) were tuned to change the incident power on the AZO surface. Incident power was arranged such that no damage is formed on glass substrates. From initial studies, it is deduced that incident power should be as low as possible. In order to characterize the surface texture, film were investigated by SEM. Four-point-probe measurement is used to determine sheet resistance of the films. Optical response of the obtained textures such as total transmittance and haze were obtained using integrating sphere system. Optimization of films is achieved by varying process parameters to reach the demands of proper conductive layer for thin silicon solar cell applications. Laser is moved on a straight line to obtain uniform ablation along the line. In this study, laser does not ablate the thin film layer all the way though the substrate, it only ablates AZO surface partially so that film is not deteriorated. Increasing repetition rate (RR); which is actually the frequency of the laser spot (f), decreases the distance (d) between each spot at certain motor speed (v). Results high haze values can be obtained by using higher frequencies. Haze value can also be increased by decreasing the motor stage speed. Moreover, having overlapping laser pulse spots on AZO surface increases the haze values. Therefore, separated periodic and overlapping textures were compared. As a results of optimizing laser parameters, high conductive, high transmittance, and haze value up to 45% AZO films were obtained. Reference [1] W. Beyer, J. Hüpkes, and H. Stiebig, Thin Solid Films 516, 147 (2007). [2] H.C. Weller, R.H. Mauch, G.H. Bauer, Sol. Energ. Mat. Sol. C 27, 217 (1992) [3] M. De la, L. Olvera, A. Maldonado, and R. Asomoza, J. Ma-ter. Sci. Mater. Electron. 11, 1 (2000). [4] H. Nasser, E. Özkol, A. Bek, and R. Turan, Opt. Mater. Ex-press 5, 932 (2015). [5] W. Yanga, Z. Liua, DL. Pengb, F. Zhanga, H. Huanga, Y. Xiea, and Z. Wua, Appl. Surf. Sci.255, 5669 (2009). [6] Z.L. Pei, X.B. Zhang, G.P. Zhang, J. Gong, C. Sun, R.F. Huang, and L.S. Wen, Thin Solid Films 497, 20 (2006). [7] T. Miyata, Y. Minamino, S. Ida, and T. Minami, J. Vac. Sci. Technol. A 22, 1711 (2004). [8] H Kim, J.S Horwitz, S.B Qadri, and D.B Chrisey, Thin Solid Films 420-421, 107 (2002). [9] K. Ellmer, J. Phys. D: Appl. Phys. 33, R17 (2000) [10] H. Nasser, E. Özkol, A. Bek, and R. Turan, Opt. Mater. Ex-press 5, 932 (2015) [11] J. I. Owen, J. Hüpkes, H. Zhu, E. Bunte, and S. E. Pust, “Novel etch process to tune crater size on magnetron sputtered ZnO:Al,” Phys. Status Solidi A 208(1), 109–113 (2011). [12] W. L. Lu, K. C. Huang, C. H. Yeh, C. I. Hung, and M. P. Houng, “Investigation of textured Al-doped ZnO thin films using chemical wet-etching methods,” Mater. Chem. Phys. 127(1-2), 358–363 (2011). [13] E. Bunte, H. Zhu, J. Hüpkes, and J. Owen, “Novel texturing method for sputtered zinc oxide films prepared at high deposition rate from ceramic tube targets,” EPJ Photovo. 2, 20602 (2011). [14] S. Eckhardt et al. Physics Procedia 41 ( 2013 ) 552 – 557 [15] T. Knüttel, S. Bergfeld and S. Haas JLMN-Journal of Laser Micro/Nanoengineering Vol. 8, No. 3, 2013 [16] S. Eckhardt et al. Light Management in AZO Thin Film Electrodes ADVANCED ENGINEERING MATERIALS 2013, 15, No. 10 [17] S. Lauzurica, M. Lluscàb, D. Cantelia, M.I. Sánchez-Aniortea, J. López-Vidrierb, S. Hernándezb, J.Bertomeu b, C. Molpeceresa Proc. of SPIE Vol. 9180 918006-1

Resume : Recent developments in Cu(In,Ga)Se2 technology — like KF post deposition treatment [1] and point junctions [2] — have suggested the role of the absorber/buffer interface as a limiting factor for highly efficient solar cells. In the light of these findings, we have engineered the Cu(In,Ga)Se2/Zn(O,S) interface by introducing nano-sized point contact openings through a passivation layer, in order to reduce recombination losses at the front interface. To realise this, a solution based approach, similar to Vermang et al.[3], in which monodispersed and highly ordered self-assembled cadmium sulphide (CdS) spherical nanoparticles (SNP’s) are employed, followed by atomic layer deposition (ALD) of a thin layer of aluminium oxide (Al2O3) as the passivation layer. The point contacts are then realised by an etching of the SNP’s. The spectral photoluminescence (PL) from the finished cells is enhanced by the field-effect passivation of Al2O3 at the Cu(In,Ga)Se2/Al2O3 interface, pointing towards a reduction in surface recombination velocity, which might be due to an inversion at the interface induced by Al2O3. At the same time, a 10 % increase in the open circuit voltage (Voc) is observed in comparison to the unpassivated reference cells — which supports the above-mentioned hypothesis. References: [1] Chirila A., Renhard P.,Pianezzi F., Bloesch P., Uhl A.R., Fella C., Kranz L., Keller D., Gretener C., Hagendorfer H., Jaeger D., Erni R., Nishiwaki S., Buecheler S., Tiwari A.N. Pottasium-induced surface modification of Cu(In,Ga)Se2 thin films for high efficiency solar cells, Nature Mater., vol.12, 1107-1111 (2013) [2] Fu Y.,Allsop N., Gledhill S.E.,Köhler T., Krüger M., Saez-Araoz R., Blöck U., Lux-Steiner M.Ch., Fischer Ch-H. ZnS nanodot film as a defect passivation layer for CIGS thin –film solar cells deposited by Spray-ILGAR. Advanced Energy Materials 561-564 (2011). [3] Vermang B., Wätjen J. T., Fjällström V., Rostvall F., Edoff M., Kotipalli R., Henry F., and Flandre D. (2014), Employing Si solar cell technology to increase efficiency of ultra-thin Cu(In,Ga)Se2 solar cells, Prog. Photovolt: Res. Appl., 22, 1023–1029 (2014).

Resume : Binary based metal chalcogenides have attain growing interest in various device applications due to their promising optoelectronic properties. Tin monoselenide is one such material with narrow bandgap and its constituent elements are cheap and widely available in earth crust. In the present investigation, we studied the effect of substrate surfaces onto compositional, morphological, structural, optical and electrical properties of thermally evaporated SnSe thin films. Tin selenide thin films were prepared by high vacuum evaporation method with a thickness of 600nm and at a substrate temperature of 250°C onto glass, FTO, FTO/CdS and Mo - coated soda lime glass substrate surfaces. The compositional, microstructural and optical properties of the as-grown SnSe films were analyzed using HR-SEM, AFM, Raman, XRD , UV-Vis-NIR to know the quality of the films. EDX data show nearly stoichiometric composition of films for Sn and Se for all used substrates. XRD and Raman analysis confirm the orthorhombic crystal structure of SnSe for all the films. Photoelectrochemical measurments show that all deposited SnSe films exibit p-type conductivity. The detailed results on substrate effect on physical properties of films will be presented and discussed.

Resume : Harvesting energy by direct conversion of sunlight to electricity using photovoltaic (PV) technology is emerging as a leading contender for next-generation green power production. Thin film solar cells based on chalcopyrite Cu(In,Ga)Se2 (CIGS) are being considered as the new PV material for the future due to many promising features. High performance at laboratory scale (20.8% efficiency) and remarkable stability of CIGS-based solar cells are the key features that compete directly with (poly)crystalline silicon cells. The quaternary CIGS compound is a direct-band semiconductor with superior absorption characteristics and is considered to be the most suitable absorber material for thin film applications. Many deposition methods are being utilized to grow CIGS thin films; however, only few works on the pulsed laser deposition (PLD) technique have been reported. In this poster presentation, the growth of CuIn0.7Ga0.3Se2 on soda-lime glass substrates using PLD will be presented. The characterization of structure, composition and morphology of CIGS thin films will be performed. PLD growth parameters will be systematically investigated and be correlated with the electrical and optical properties of CIGS films. Optimization of the PLD growth parameters for CIGS films based on an overall assessment of the film charcateristics will be presented as conclusions.

Resume : During the last years, multiferroic materials have gained great attention due to their fundamental physics and possible integration in advanced application. BiFeO3 (BFO) appears actually as one of the most interesting, because it shows multiferroic properties at room temperature. Recently a peculiar photovoltaic effect has also been revealed in BFO with a large open circuit voltage Voc above the band gap. The very large Voc (up to 16V) was first interpreted as arising from the domain structure and electric field at the domain walls. More recently an interpretation based on the symmetry was put forward to explain the anomalous high Voc. In both cases planar geometry of the PV effect was used and the ferroelectric polarization is responsible of the electric field separating the electrons from the holes in the thin films. A smaller Voc (<1V) was however measured in parallel plate capacitor and the origin of this low PV response may be obscured by the possible existence of a Schottky barrier, defects, depolarizing field and the complex rhombohedral ferroelectric domain structure. To better understand the observed PV effect, we have grown by pulsed laser deposition BFO thin films with different thickness on buffered LaAlO3 substrates. A 20nm thick SrRuO3 layer is used as a bottom electrode while Pt and ITO top electrodes were deposited. Reciprocal space mappings and Raman spectroscopy were used to characterize the domain structure and symmetry. Ferroelectric properties were investigated using a Sawyer-Tower home made system and piezo-force microscopy. Very large spontaneous polarization were measured and I(V) curves were collected at different temperatures to understand the transport properties (interface or bulk limited and the existence of a Schottky barrier). PV effects under laser illumination of different wavelength (from 647nm to 457nm) and powers were investigated at different temperatures. Observed switchable Voc and Isc (short circuit current) will be presented showing that the PV effect arises from the ferroelectric field effect. An attempt to fully understand the electric structure of the BFO films has been performed and an impedance spectroscopy investigation of the ferroelectric PV solar cell will be also presented.

Resume : Contact degradation is one of solar cells with Si working area and Ag front layer problems. High diffusivity of Ag is also enhanced by the electric current. Through time, electrical resistivity of contact interface grows, resulting in low conversion efficiency. New contacts generation includes Ag grid coating with Me diffusion barrier and with Graphene monolayer (G) surface cover to overcome presumable of contact conductivity lowering. In order to design new generation contacts it is necessary to investigate properties of Ag/Me, and Me/G interfaces. In previous research diffusion phenomena was investigated in order to find an effective diffusion barrier for Ag contact. The aim of this report – to investigate the electrical conductivity of mentioned interfaces. Combination of effective diffusion barrier properties and electrical properties determines preferable material for practical application. Simulations were conducted in ABINIT software using LDA Troullier-Martins pseudopotentials and PAW pseudopotentials. Models for Ag/Me interface investigation includes both strict and vague interfaces. For all Ag/Me interfaces relative change of conductivity compared to single elements was investigated. Models of Me/G surface interfaces are discussed. Set of metals possible as diffusion barriers are considering from the point of view of their crystal lattice influence on electric properties.

Resume : ZnS/CIGS solar cells have been developed rapidly and it also shows efficiency as high as 22.8%. But, the zinc-based buffer layer has drawbacks such as low deposition rate and insufficient coverage, which must be met for large mass production. In this study, tartaric acid was used as a complexing agent for deposition of Zn(S, O) buffer layers in a “high rate” chemical bath deposition process. Scanning electronic microscopy, X-ray diffraction and UV–visible–NIR spectrophotometer were employed to characterize the qualities of the thin films with various concentrations. As a result, the Zn(S, O) thin films showed good uniformity, good coverage and high transmittance. Besides, the efficiencies of Zn(S, O)/CIGS solar cells were also measured under standard condition. The efficiencies of Zn(S, O)/CIGS solar cells is comparable to a reference CdS/CIGS solar cell. The results hinted that the tartaric acid could be a promising complexing agent for getting good film quality and high throughput, which could be suitable to mass production for CIGS thin film solar cells.

Resume : Graphene is a promising material for application in solar energy, because it has a unique set of properties (such as transparency, high conductivity and mechanical properties). Thus, the research of graphene interaction with metals, including – with metal monocrystals is actual. In our previous research, we have studied planar composites of "bcc Fe/Graphene" type. In this report we represent results for composites of "hcp Ti/Graphene" type. Computer simulations were conducted using molecular dynamics method. Structure transformation of Ti-monocrystal, nearsurface layers has been simulated on different crystallographic surface orientations (0001), (1-100), (11-20) before and after Graphene coated for temperatures 300K and 400K. Such system energy changes were analyzed for packing density dependence at interfaces. Total and axial stresses were calculated for every composite and for every component of composites separately. Also changes of interplanar distances along a normal to the surface and the level of structural deformation in interfaced layers were analyzed. Results for "bcc Fe/Graphene" and "hcp Ti/Graphene" composites comparison were conducted. Computer simulation modeling analysis allows to conclude concerning time-temperature composites stability and their perspective application for solar energy production.

Resume : Plant-inspired light harvesting coatings have recently demonstrated a substantial increase in the performance of organic solar cells. By exploiting the closely-packed micro-cone surface texture of a rose petal an increase of 13%rel in power conversion efficiency was achieved. A key question regarding this enhancement pertains the influence of the different structural disorder types involved in those biostructures. To tackle this research question, we present a combined simulation framework based on ray tracing and on the transfer matrix method to investigate the impact of disorder in such arrays, both with an absorbing back side and a thin film solar cell underneath the structures. Although the propagation angle range is found to be noticeably broadened after the introduction of disorder, the latter has no quantitative influence on the overall light collection properties, regardless of the angle of incidence considered. At the solar cell level, we demonstrate that those effects finally translate into very robust absorption properties with respect to all types of structural disorder considered. Most importantly, our numerical study enables to derive pivotal rules for the design of plant-inspired light-harvesting coatings.

Resume : Localization of light in photonic crystals can be exploited to amplify solar energy conversion in thin third generation solar cells. In this study, light energy conversion at titania photonic crystals sensitized with Q-CdTe, the quantum dots’ stability in the photoelectrochemical medium and growth of type II Q-CdTe/CdSe dots were investigated in the presence of alkaline selenide electrolyte. Quenching of emission of green-emitting (2.3 nm) and red-emitting (4 nm) Q-CdTe showed that selenide effectively scavenges the photohole, explaining the dots observed stability against anodic dissolution in the photoelectrochemical cell. The binding of selenide on Q-CdTe was measured from emission quenching by applying a model of equilibrium complex formation and was found to be size-dependent, with 7-times greater binding constant at the smaller size dots compared to the large size dots, leading to fast growth of type II Q-CdTe/CdSe at the 2.3 nm core. Photoelectrochemical studies were conducted at titania inverse opals with stop band at 600 nm sensitized with different size Q-CdTe in selenide compared to sensitized nanocrystalline titania films. Amplification in light energy conversion was measured at the Q-CdTe sensitized inverse opals, attributed to light trapping in the photonic crystals. A significant photon to current conversion efficiency is reported at Q-CdTe sensitized films after treatment in selenide then annealing, due to the growth of type II Q-CdTe/CdSe.

Resume : The Al doped Zinc Oxide (AZO) is a transparent and conductive oxide used as alternative to the Indium Tin Oxide as electrode in a-Si:H/c-Si heterojunction SHJ solar cells. AZO film is obtained by low cost magnetron sputtering; its resistivity depends on film thickness and process parameters as substrate temperature and DC sputtering power. In SHJ cells low temperature and DC power have to be preferred to avoid thermal stress and damages on thin emitter and buffer layers. Simultaneously metal-like resistivity and high transmittance at low temperature is still challenging for AZO. To reduce the unwanted bombardment Pulsed DC sputtering regime is adopted. This technique consists in the application of a negative voltage pulse to the target for a certain time, then the power is switched to a small positive voltage. Pulsing the magnetron discharge in the mid-frequency range is found to prevent arc events and stabilize the reactive sputtering process improving the optical and electrical properties of polycrystalline material as AZO. In this work we investigated the pulsed regime when 80nm thick AZO film is deposited on clean c-Si surface and corning glass at 180°C and 0.67 W/cm2 power conditions. We characterized the films in terms of resistivity, transparency and workfunction and we verified the effect of chosen process condition on the effective lifetime of c-Si wafers passivated both side with a-Si:H films of thickness comparable to those used in heterojunction solar cells.

Resume : Cooper doped tin oxide (SnO2: Cu) thin films were deposited by spray pyrolysis technique adding various concentrations of cooper chloride (0 – 5 at.%) in the spray solution of tin chloride. X-ray diffraction patterns confirm that the films are polycrystalline in nature exhibiting tetragonal structure. The structural parameters such as crystallite size, texture coefficient and lattice constant (‘a’ and ‘c’) are calculated from X-ray diffraction studies. The AFM results of the films indicate that the surface roughness increases with Cu doping. The prepared films, not only have an average transmittance greater than 76 % in the visible region, but also have an optical band gap between 3.65 and 3.92 eV depending on the doping concentration. From the electrical measurements, it is found that the resistivity is affected significantly by Cu concentration. This work discusses the connection among the material properties of the SnO2 films with different Cu concentration and how these properties are influenced by the Cu doping concentration.

Resume : III-V compound semiconductors grown on Si substrates are attractive for high efficiency solar cells [1]. In order to realize low defect densities in the heteroepitaxially grown III-V absorbers, however, it is necessary to precisely control the Si(100) surface preparation. There is only little knowledge about the impact of arsenic (As) on a well-defined preparation of the Si surface, even though As is a prevalent ingredient in MOCVD environments. Recently, we demonstrated that the influence of As on the atomic structure of the heterointerface enables to control the III-V sublattice orientation and growth of single-domain GaP window layers [2]. Here, we study the surface structure of vicinal Si(100) surfaces in situ during preparation with reflection anisotropy spectroscopy (RAS) to understand the impact of substrate misorientation on surface formation. Moreover, aiming at process control at the microscopic scale, we vary process conditions and benchmark the spectra to UHV based surface science techniques, such as XPS, in order to better understand the microscopic origin of the in situ signals. We conclude that the main contributions are terrace-related, which enables in situ quantification of surface domains. We also find that both As coverage and step density modify the spectral lineshape, which opens perspectives to fine-tune the surface structure as desired for further processing. [1] T. N. D. Tibbits et al., Proc. Eur. Photovolt. Sol. Energy Conf. Exhib. 29, 1975 (2014). [2] O. Supplie et al., APL Mater 3, 126110 (2015)

Resume : The monolithic integration of III-V semiconductors on silicon, and particularly of GaAs has aroused great interest since the 1980s. Potential applications for multijunction solar cells are of great interest since realistic modelizations shown us that AM1.5G conversion efficiencies reaching 29 % and 32 % could be achieved for GaAs on silicon based tandem or triple junctions respectively [Prog. Photovolt: Res. Appl. 2014; 22:810–820]. By using a novel integration method, we have shown that it is possible to achieve heteroepitaxial integration of GaAs crystals (typical size 1 m) on silicon without any structural defect such as antiphase domains, dislocations or stress usually reported. However, concerning their electronic properties, conventional free carrier electrical characterization methods are impractical due to the micrometric size of GaAs crystals. In order to evaluate the GaAs material quality for III-V on Si based multijunctions applications, we have then developed a contactless method allowing us to determine the carrier concentration of a single GaAs crystal by studying its photoluminescence peak energy variation versus the temperature and fitting a theoretical equation with the carrier concentration as unknown parameter. We present in this paper the experimental results of this advantageous contactless technique to precisely monitor the doping level of the GaAs crystals grown on silicon towards the optimization of silicon based multijunction solar cells.

Resume : The concept of multi-junction solar cells as a way to high efficiency is becoming more and more actual as all the mature technologies have recently surpassed 21% cell efficiency. Commercially, this concept was most widely applied in the technology of amorphous and microcrystalline silicon due to its relatively easy implementation. For crystalline silicon, this concept is enabled only recently by emergence of organic-inorganic perovskites. Both technologies will be discussed in this contribution. Based on measurements of voltage as a function of selective illumination we show that the monolithic interconnection through tunnel junction cannot be described by the traditional view as a separate cells in series. Instead, more realistic model assumes overall shunts that are independent from the individual shunts due to the absence of laterally conducting interlayer between individual sub-cells. Point defects then have only local effect on individual shunt resistance thanks to limited lateral conductivity of intermediate tunnel junction but may have significant global effect on the overall shunt resistance.

Resume : Advanced methods of optical spectroscopy were adapted to study optical properties of lanthanide-doped up-converting materials. Quantum yield (QY) and power efficiency (PE) of luminescence were determined under excitation at 980 nm using a set-up with an integrating sphere. Emission is coupled to the double-parallel-detection-paths spectroscopic system covering broad spectral range from UV to NIR (360-1650 nm) [1], which enable to observe both up-converted and down-converted emission and determine EQY separately for each emission band [1]. The technique is demonstrated by characterizing various materials: Na(Y,Lu,Gd)F4 codoped by Yb3+ and Er3+, Tm3+, or Ho3+ and Pr3+:CaF2 codoped by Yb3+ or Eu2+ as sentitizers [2]. The best efficiency was found in NaLuF4: Yb3+ 15 %, Er3+ 1.5 %, which gives the integrated up-converted signal ~17 % in the excitation power range of 70-100 W/cm2, while the down-converted emission power efficiency is about 20 %. Adding an acousto-optical modulator to the detection beam will enable to excite up-conversion materials by long square-shaped pulses and observe lanthanide emission (with characteristic slow kinetics) in order to understand energy transfers and calculate absorption cross-section (ACS) [3]. [1] J. Valenta and M. Greben, AIP Adv. 5 (2015) 047131. [2] A.Guille J. Appl. Phys. 114 (2013) 203509. [3] J. Valenta et al., Appl. Phys. Lett. 108 (2016) 023102.

Resume : RE-doped oxides are perspective candidates for application as luminescent transformers for adoption of incident light to working ranges of photovoltaic solar cells. Oxide materials are usually characterized by excellent thermal, mechanical and optical properties. Luminescent materials based on rare earth vanadates are also considered for these purposes because they are characterized by high matrix absorption in a wide spectral range, effective energy transfer from host to the RE ions, and intensive emission of the RE activators. At the same time, some of luminescent characteristics of vanadates have to be improved in order to achieve their effective work as luminescent transformers for solar cells. With this aim we have synthesized concentration series of the La1-xEuxVO4, Eu1-xCaxVO4 and Eu1-x-yEryCaxVO4 nanoparticles by aqueous nitrate-citrate sol-gel method and investigated their luminescent characteristics. Emission spectra of the investigated compositions are caused by electron transitions in the Eu3+ ions. The Eu3+ ions form two types of emission centers in the investigated samples. Total emission intensity of the investigated Eu1-xCaxVO4 compositions have shown essential increasing when Ca2+ concentration increases up to x = 0.15. The Ca2+ doping enhances efficiency of the regular Eu3+ ions emission by formation of additional Ca2+ - induced channel of excitation energy transfer to the Eu3+ emission centers. Besides, the Ca2+ cations induce formation of the second type of Eu3+ luminescence centers those also contribute in the total spectra. The Sylgard 184 dielectric gel coating is considered for test applications of nanoparticles with the best characteristics on silicone solar cells.

Resume : CIGS solar cells have some losses in the UV range due to the buffer layers. Mainly, the CdS bandgap is around 2.4eV, causing the absorption of photons for wavelength below 500nm. This is the reason why the Luminescent Downshifting Layer (LDSL), a polymeric matrix and fluorescent materials which are directly deposited on the top surface of the solar cell, could be a solution to mitigate this problem. The LDSL addition would be a way to harvest UV photons, those representing approximately 160W/m². Our study focused on the performances of two types of fluorophores: organic dyes (perylene or naphtalimide structure) and core/shell quantum dots of CdSe/ZnS. Experimental results have been compared to the theoretical results with the aim to understand the LDSL behavior. The encapsulating process with a PDMS layer allows to increase, experimentally, the short-circuit courant of 1.177mA/cm² or 3.9% in absolute, due to antireflection effect. The dye effect have also been studied and no Jsc gain have been measured. A model allowed us to understand the optical coupling effect and dye effect on the Jsc variation. This analytical model is built on the figures of merit (FoM). From the optical properties of our homemade layer, FoM have been calculated (Self-absorption, Parasitic absorption, absorption and emission matching). The Jsc gain have been determined with the calculated FoM. Finally, there is an excellent correlation between theoretical and experimental results. Commercial CdSe/ZnS quantum dots unfortunately presents a low PLQY (around 20%) causing losses on the Jsc. Some mixes of organic dyes have been tested to increase the absorbed fraction. Blue and yellow dyes seems to be the best mix allowing to shift UV-blue photons close to the maximum of CIGS EQE. Other fluorescent materials are being tested. These materials seem to be good candidates mainly for their photoluminescence quantum yield, which are close to unity. FoM method will be tested on these new materials.

Resume : Gold nanoparticles have been shown to significantly enhance the efficiency of dye-sensitised solar cells (DSSCs) through plasmon-related absorption and reduction in recombination. However, they generally require complex chemical synthesis and deposition on the TiO2 scaffold of the DSSC. Here we report a simple method for in-situ deposition of Au nanoparticles on a mesoporous TiO2 scaffold via thermal decomposition of a gold precursor at only 200C. This produces an obvious colour change in the TiO2 film from white to purple, which is attributed to the plasmon absorption of Au, as shown by UV-Vis absorption. The size and coverage of the gold nanoparticles are increased by repeating the coating procedure up to 6 times, leading to an increasing reduction in defect-related recombination in the TiO2 demonstrated by photoluminescence analysis. DSSCs are produced by sensitising the structures with N719 dye, and completing with iodide electrolyte and Pt counter electrodes. The maximum efficiency is produced with 3 repeats of the Au nanoparticle coating, giving 9.08% efficiency, compared to 6.12% for uncoated TiO2. These results are supported by external quantum efficiency measurements and electrochemical impedance analysis. This process therefore demonstrates a simple procedure for deposition of Au nanoparticles for the enhancement of DSSC efficiency, which could also be applied to a number of other PV systems to enhance light absorption and charge transfer, and reduce recombination.

Resume : The presentation focuses on the use of quantum engineered dilute nitride solar cells to alleviate minority carrier lifetime/ poor carrier collection issues encountered in these devices and aims at identifying novel pathways toward the development of high efficiency tandems. First and within the framework of the development of 1 to 1.2 eV III-V dilute nitride solar cells, we will discuss the strength of carefully designed quantum- engineered dilute nitride absorbers for achieving a near unity collection of photo-generated carriers. We will then demonstrate experimentally the possibility of realizing devices with open circuit voltages that approach the radiative limit (Woc= Eg-Voc~0.4 eV), thus setting a path for extending the multi-junction solar cell efficiencies toward 50%. Finally and within the framework of the development of tandems devices operating in conjunction with silicon, we investigate the potential of such quantum engineering strategies for enabling ultra-thin and yet highly efficient 1.6-1.9 eV GaInPNAs devices that are lattice matched to Si and will evaluate their prospects toward boosting the performance of Si-based photovoltaics.

Resume : A surface-activated bonding (SAB) at room temperature, in which surfaces of substrates are activated by Ar atom irradiation prior to bonding, is applied to form low-resistance Si/GaAs interfaces for InGaP/GaAs/Si hybrid solar cells with a conversion efficiency >26%.[1] Here we examined separately the Si surface and the GaAs one in the SAB interfaces by plane-view transmission electron microscopy with the reflection of [220] for GaAs and that for Si, respectively, using Si/GaAs interfaces fabricated with substrates of Si(001) and GaAs(001)5°-off towards [110]. At an as-bonded SAB interface, there was an amorphized Si (a-Si) layer (~3nm thick) on the Si substrate, introduced in the surface activation process, and a thin Si oxide layer (<0.5nm thick) existing at the GaAs/a-Si interface, introduced in the post-activation processes. While the Si/a-Si interface was smooth, the GaAs/a-Si interface was strained due to defects on the GaAs substrate introduced by Ar atoms and Si oxides on the Si substrate. Those strains were reduced by annealing according to the recrystallization of the a-Si layer, and this would result in the reduction of the interface resistance. It is therefore hypothesized that the resistance would be originated from the defects at the GaAs/a-Si interface, and the resistance can be reduced by smoothing a surface on the GaAs substrate and removing the Si oxides on the Si substrate, via optimization of the SAB condition. [1] N. Shigekawa, et al., JJAP 54 (2015) 08KE03.

Resume : Thin-film silicon solar cells (TFSSCs) based on plasma-enhanced chemical vapor deposition (PECVD) have been used in a wide variety of applications ranging from large-area photovoltaic systems to consumer products, including light-weight flexible solar cells on plastic substrates, owing to their relatively low growth temperature (~200 °C). However, the efficiency of hydrogenated amorphous silicon (a-Si:H) solar cells degrades by 10?20% after a prolonged exposure to light, which is known as the Staebler?Wronski effect. This detrimental effect has been impeding the improvement of efficiency until now. Compared to pure amorphous materials, hydrogenated microcrystalline silicon (nc-Si:H), which is also grown by PECVD at low temperatures, has higher tolerance to light soaking. Therefore, the multi-junction approach using a stack of a-Si:H and nc-Si:H cells is an effective way to mitigate light-induced degradation and to improve stabilized efficiency . Recently, there are a series of efficiency improvements in TFSSCs, which are triggered by several novel technologies: (1) Single junction a-Si:H solar cell: from 10.1% (2009) to 10.2% (2014) [1] (2) Single junction nc-Si:H solar cell: from 10.1% (1997) to 10.7% (2012), 10.8% (2013), 11.0% (2014), 11.4% (2014), and 11.8% (2014) [2] (3) a-Si/nc-Si dual junction cell: from 12.3% (2011) to 12.6% (2014) and 12.7% (2014) [1] (4) a-Si/nc-Si/nc-Si triple junction cell: from 12.4% (2011) to 13.4% (2012), 13.6% (2015) [3], and 14.0% (2016) [4] Note that the efficiencies in (1)(3)(4) are stabilized efficiencies after prolonged light soaking, while those in (2) are initial efficiencies without light soaking. From these results, it is obvious that the efficiency of nc-Si:H solar cells has been improved significantly in recent 5 years with a relative gain of 16%. The main breakthrough behind this is an improved textured substrate, which is called as ?honeycomb texture?. A carefully-designed honeycomb texture enables us to grow high-quality ?c-Si:H films without detrimental effects related to the texture, resulting in an increased photocurrent density and a relatively high open circuit voltage. This technology was successfully transferred to an a-Si/nc-Si/nc-Si triple junction cell and set a new record stabilized efficiency of 13.6% [3]. Compared with ?c-Si:H cells, the recent efficiency improvement in a-Si:H cells is rather moderate. However, there was a progress in suppressing the light-induced degradation. It was reported that so-called triode PECVD technique is useful to grow a-Si:H material which shows less light-induced degradation than those grown by standard PECVD. This material has contributed to set the current record efficiency in a-Si/nc-Si dual junction cell with a stabilized efficiency of 12.7% [1]. Just recently, we have tried to combine the above-mentioned technologies developed in a-Si:H and nc-Si:H solar cells into triple junction cells. As a result, a new record stabilized efficiency of 14.04% was realized last year [4]. In this talk, we show the details behind the recent record-efficiency TFSSCs which were reported by our group especially in ?c-Si:H and a-Si/nc-Si/nc-Si triple junction cells. We also discuss several technologies which were developed in the framework of TFSSC research but could be applied to the other types of solar cells. [1] T. Matsui et al., Jpn. J. Appl. Phys. 54, 08KB10 (2015). [2] H. Sai et al., Jpn. J. Appl. Phys. 54, 08KB05 (2015). [3] H. Sai et al., Appl. Phys. Lett. 106, 203902 (2015). [4] H. Sai et al., Appl. Phys. Lett. 109, 183506 (2016).

Resume : Fourier Transform Photocurrent Spectroscopy (FTPS) has been introduced to investigate the absorption coefficient vs photon energy (α(hv)) of thin film semiconductors. We have developed at Geeps a FTPS experiment to study the α(hv) spectrum of hydrogenated amorphous silicon (a-Si:H) thin films, to subsequently deduce information on the density of states in the sub bandgap region. In this work we studied if the same α(hv) spectroscopy could be found on both: a coplanar a-Si:H film deposited on glass, and in NIP diodes incorporating the same material as an intrinsic layer. The back and front contacts of the diodes where transparent, made either of ITO or ZnO. The results show that the Urbach tail of both coplanar and NIP diodes corresponds to each other but that α at low photon energy was slightly lower for both diodes. In addition, we found that the diode with ZnO contacts presented a rapid increase of α below 0.85 eV. Transmission-reflection measurements performed on the diodes showed that though the reflection with both types of contacts remains the same, the diode with ZnO contacts exhibited a weak transmission in the wavelength range where we observed an increase of α, signature of a strong absorption of the ZnO layer. This results show that FTPS measurements performed on diodes must be taken very cautiously and that one has to take into account the optical properties (e.g. absorption) of the contacts to properly deduce reliable α(hv) spectroscopy.

Resume : Surface passivation of crystalline silicon (c-Si) by amorphous silicon (a-Si) is common in photovoltaics. The morphology of the a-Si strongly affects the nature and the ease of hydrogen bonding in the layer and can be influenced during deposition process. Physical vapor deposition (PVD) yields a columnar growth of the a-Si while chemical vapor deposition (CVD) supports a non-columnar one. In consequence, the hydrogen embedding and bonding manly effects characteristics as e.g. the long time stability of surface passivation. Moreover, literature reports the growth of an a-Si layer within the very moment of plasma ignition in a CVD process. Up to now, only indirect characterization methods like growth rate analysis, nuclear resonant reaction analysis and/or Fourier transformed infrared spectroscopy allows to determine the specific morphology of an a-Si layer. A proof by a direct method, like electron microscopy observations, was not possible, as the morphological effects are on an atomic scale. This changed now as the comparing of different a-Si morphologies in a multi-layer stack enables a direct observation of an in situ grown layer in the early beginning of a CVD a-Si deposition by tunnel electron microscopy. The authors would like to present their latest results and insights emerged by using a sub atomic resolution transmission electron microscope located in the noise-free laboratories of the Binnig and Rohrer Nanotechnology Center at IBM Research-Zurich.

Resume : Solar cells made from multicrystalline silicon material, which still represent the majority of solar cells produced today, are by nature inhomogeneous devices. Their bulk excess carrier lifetime, which decisively influences the short circuit current density, the saturation current density, and the effective bulk diffusion length, may vary from position to position by an order of magnitude or more. Moreover, such cells may contain ohmic shunts and, in particular in the edge regions, positions of significant depletion region recombination current. A reliable quantitative local characterization of inhomogeneous cells can be performed by combining dark lock-in thermography (DLIT) with electro- and photoluminescence (EL and PL) imaging. In this contribution well-established and most recent findings on the application of these methods are reviewed and typical application examples are presented. It is found that luminescence imaging is most appropriate for local series resistance imaging and for qualitatively imaging all kinds of recombination-active defects. DLIT, on the other hand, shows a lower spatial resolution but is most appropriate for imaging local two-diode parameters like depletion region- and bulk saturation current densities, the latter allowing to draw conclusions also to the local short circuit current density. A comprehensive local characterization of silicon solar cells is possible only by applying both DLIT and luminescence imaging, which complement each other.

Resume : Current crystalline heterojunction solar cells (SHJ) are based on amorphous silicon (a-Si:H)/crystalline Silicon (c-Si) and achieve more than 25% efficiency. III-V materials, such as Gallium Phosphide (GaP), on c-Si SHJ have been shown to be potential candidates to improve the efficiency of SHJ cells. The high bandgap of GaP (2.26 eV) provides higher ultraviolet transparency and better field effect passivation, which makes it a good candidate for alternative emitter and window material. The close lattice constant of GaP with silicon enables its direct epitaxy on nominal or miscut substrates. However, bulk lifetime degradation has been reported, which occurs during the silicon pre-annealing step before epitaxy. This pre-annealing is necessary for surface reconstruction to optimize the epitaxy. The degradation reduces the effective minority carrier lifetime in the bulk silicon from over 1 millisecond down to a few tens millisecond, which limits the cells efficiencies below 10%. We investigate here the origin and dynamics of the bulk minority carrier lifetime degradation, and report on a solution to restore the bulk lifetime to the SHJ standard with the integration of gettering steps into the process flow while trying to preserve the GaP layer in cells precursors. High bulk lifetime is successfully achieved in subsequent solar cells bulks. To extend lifetime improvements, post-epitaxy treatments are explored to decrease interface recombinations between GaP and silicon.

Resume : The high work function metal oxides MoO3 [1] and WO3 [2] are used as hole contacts in silicon heterojunction [3] and perovskite [4] solar cells. So far these layers were prepared by thermal evaporation, which does not enable control of the oxygen vacancies and the conductivity of WO3 [5]. This contribution investigates reactively sputtered WO3 enabling variation of the oxygen vacancy density and conductivity of WO3. To this end WO3 was deposited using different oxygen gas flows during sputtering and measurements of the band bending in the WO3/Si-junction, implied fill factors of WO3/Si structures and solar cell j(V)-parameters were conducted. It was found that the optimal density of oxygen vacancies leads to a resistivity of 1 mOhmcm, while still maintains sufficient band bending, full WO3 stoichiometry and implied fill factors of 84 %. For solar cells it was found that the optimal WO3 thickness is about 20 nm, since thicker layers lead to resistive losses and lower thicknesses lead to losses in implied fill factors by increased recombination under low injection conditions due to decreased band bending. [1] J. Bullock, Nature Energy 2 (2016) 15031 [2] M. Bivour, SolMat 142 (2015) 34 [3] J. Geissbühler, Appl. Phys. Lett. 107 (2015) 81601 [4] J. Werner, ACS Appl. Mat. Int. 8 (2016) 17260 [5] M. Mews, SolMat 158 (2016) 77

Resume : Cu(In,Ga)Se2 (CIGS) solar cells have reached stabilized record efficiencies beyond 22% and show potential to contribute a significant share to the photovoltaic market. Despite being direct band-gap semiconductors with a high absorption coefficient, there is still need for light management to optimize light intake and in particular to address material shortage. The supply risk of indium may restrain production in the upper GW scale and needs to be tackled by concepts of material saving combined with efficiency enhancement. An obvious approach for material saving is the reduction of absorber thickness, which brings the quest for optical concepts leading to light localization in the ultra-thin CIGS. Plasmonic nanoparticles giving rise to resonant modes are promising, yet not without challenges when it comes to stable integration into CIGS solar cells. As a highly inert and lossless alternative photonic dielectric nanoparticles have turned out most beneficial for absorption enhancement in ultra-thin CIGS solar cells. Another approach offering even higher material reduction is the concept of microconcentrator solar cells. The active absorber area is restricted to about one hundredth of the total device area and light focused onto the micro solar cells by concentrator lenses. Compared to large-scale photovoltaics, this micro-sized concept offers highly compact modules with improved heat management. Overall, optical concepts will foster the GW production of CIGS solar cells.

Resume : Photonic structures are introduced in solar cells in order to boost the short-circuit current. Besides commonly used random pyramids with features in micrometer size, different nanotextures (e.g. black silicon) are investigated and optimized for wafer-based silicon solar cells. Optical modelling of such structures including micro, nano and combined textures requires special approaches in optical modelling. In the first part of the contribution, Rigorous Coupled Wave Analysis (RCWA) is applied and tested for accurate optical simulation of a heterojunction Si solar cell, including encapsulation and front glass. Its applicability to different realistic textures (micro pyramids, inverted nano pyramids and others) is verified and the analysis of the required number of sublayers and modes used in simulation is carried out. Limitations of the method to larger textures are revealed. New modelling approach where RCWA is coupled with ray tracing and transfer matrix method is presented as a solution for reliable 3D optical simulations of solar cells including micro, nano or combined multiscale textures. The modelling approach is applicable to various types of solar cells; however, in this contribution result of different texture application is shown for the case of heterojunction Si based device. Quantitative improvements in quantum efficiency curves and short-circuit currents is determined for different textures and their combinations.

Resume : Photonic structures in the regime of wave optics are promising to reduce the thickness of PV devices while improving their efficiency. High-index dielectric structures have been computationally optimized and experimentally implemented on thin film silicon solar cells with distinct thicknesses, composed of either spheroidal-based elements or voids. The results anticipate up to ~50% efficiency enhancements due to the following benefits relative to conventional cells [1]: 1) Optically, the wavelength-sized structures allow geometrical index matching for light travelling towards the PV absorber layer, almost eliminating reflection at short wavelengths (UV-Vis); while pronouncedly scattering longer wavelengths (NIR) and boosting their absorption via optical path length enhancement. Studies at oblique incidence also reveal considerable acceptance angle improvements. 2) Electrically, a key advantage is the incorporation of the photonic structures in the transparent conductive oxide (TCO) of the front contact of n-i-p cells, enabling higher TCO volume which decreases its resistance. Such location also prevents the structures from increasing the cell roughness or surface area, therefore they do not contribute to carrier recombination which is a crucial drawback in many state-of-art light trapping strategies [1]. Thin film silicon cells were the test bed for such novel concepts, but they can be readily implemented in any other type of PV devices. [1] M. J. Mendes et al. Nano Energy (2016)

Resume : Light trapping in planar ultrathin-film solar cells is limited due to the small number of optical modes available in a thin film. A problem that becomes more dramatic if the spectral region of interest spans from visible to NIR frequencies. A nanostructured thin-film design could surpass this limit by providing a broadband increase in the local density of states in a subwavelength volume while maintaining an efficient light coupling. In this work, we report a photonic architecture design which leads to an efficient and broadband absorption enhancement in the VIS and NIR regions by direct coupling of incoming light to a series of subwavelength scale resonant modes. The nanostructured architecture is capable of sustaining (i) phase accumulation resonances, enabling resonant modes for film thicknesses below a quarter-wave thickness and (ii) the resonances excited by the geometry of the periodical nanostructure. We provide an insight in the resonant modes sustained by this architecture and the optimum configurations for optoelectronic applications. Finally, we present the experimental absorption measurements from 3x3 mm prototypes fabricated in a high dielectric constant semiconductor ultrathin layer (below 100 nm thick) by means of soft-nanoimprint lithography. The nanostructured films exhibit an integrated light absorption enhancement of up to 70% over a flat film of semiconductor, in a broad bandwidth, from 400 to 1800 nm, useful for photovoltaics and photodetection.

Resume : Transparent electrodes (TE) are critical materials for many devices in strategic technological areas such as photovoltaics (PV) and flexible electronics. In the case of thin film PV, the thickness of the TE must be reduced with respect to standard bulk solar cells, this being a big issue in terms of electrical and optical properties. In the field of flexible electronics, the robustness and reliability under bending conditions is another hot topic to be considered. The need of simultaneously good electrical, optical and structural properties for TE pushes to study new materials with respect to the standard TCOs (transparent conductive oxides) largely employed today, often containing expensive and toxic elements such as In. Among these new materials, AZO/Ag/AZO multilayer structure (AZO = Al-doped ZnO) has shown very promising capacity of facing all the requirements for being one of the next generation TE. We grew AZO/Ag/AZO films on Si, quartz and plastic substrates in order to measure, electrical optical and bending properties. Samples contain 10nm of Ag intralayer in between 2 AZO films, each of different thickness in the range 25-85 nm. The ultrathin metal layer accounts for a very low electrical resistance (lower than a single TCO layer 10 times thicker ) but high optical transmittance in the visible range. Also the flexibility benefits of the ductility of the metallic intralayer and of the very low thickness of the whole structure (about 100 nm) with respect to standard TCO. Finally, by properly choosing the thickness of the AZO top and bottom layers, we show as the optical reflectance of the film can strongly be lowered to match the requirements for a specific application (e.g. thin film photovoltaics). In summary, we show how very thin AZO/Ag/AZO multilayers, with selected thickness of the two AZO layers and only 10 nm of Ag, produces one of the most electrical reliable, structural flexible and optically tunable material to be employed as TE.

Resume : Er3+-doped upconverters that absorb two photons at around 1.55 um and emit a single 0.98 um photon are applied to solar-pumped lasers and crystalline silicon (c-Si) solar cells for more efficient solar energy harvesting. However, only a fraction of the solar spectrum can be utilized because of the narrow absorption band of Er3+. To overcome this shortcoming, we have designed sensitization mechanism to the Er3+, and demonstrated broadband-sensitivy ranging from 1.1 um to 1.6 um. Six-coordinated Ni2+ ions are candidates for the sensitizers that harvest 1.11.45 um photons not absorbed by either c-Si or Er3+ and transfer the energies to the Er3+ emitters. The first priority to select suitable host materials is high solubility of Er3+ and six-coordinated Ni2+. ABO3 type perovskites, in which the Er3+ and Ni2+ dopants substitute A and B ions, respectively, can fulfill this requisite. Another consideration is to control relative locations of the Er3+ and Ni2+ absorption/emission bands to promote resonance energy transfer from the Ni2+ sensitizers to the Er3+ emitters while suppressing the transfer in the reverse direction that deteriorates the upconversion performance. We have demonstrated Ni2+-sensitized Er3+-upconversion emission using La(Ga,Sc)O3 and Ca(Ti,Zr)O3 hosts with co-dopants of Nb5+ and Li+ for charge compensation. Time-resolved PL measurements revealed that efficiency of the energy transfer from the Ni2+ to the Er3+ is close to unity, and the reverse transfer is minor.

Resume : Efficient management of sunlight photons is a fundamental issue in photovoltaics. The single junction PV cells have limited conversion performances because of their spectral mismatch with the solar spectrum. One solution to this matter is to use the photon conversion approach such as downshifting (DS), downconversion (DC) and upconversion (UC). Nowadays, the DC and UC materials have not enough quantum yield (QY) for use as photon converters, but DS materials have already displayed encouraging improvements in solar cell performance.[1] Today, the organic dyes with almost unity QY and Eu(III) complexes with QYs of 80-90 % are in strong competition. Valuably, the latter has large pseudo-Stokes’ shifts without self-absorption, and show 5D0→7F0-6 narrow emission bands, and milliseconds photoluminescence lifetime. The aim of this work consists in the design and study of new Eu(III) β-diketonate based complexes in order to enhance the conversion efficiency of CIGS solar cells in the UV region. Herein, we will show an improvement of the EQE at 360 nm from 14 % of CIGS alone up to 58 % with the best compound DS layer. The calculated Jsc in the 300-400 nm region increased up to 0.77 mA/cm2 compared to 0.23 mA/cm2 of CIGS alone. This represents around 56 % of AM1.5G sunlight Jsc or 71 % of a perfect DS Jsc. In the best case a boost of the CIGS solar cell conversion efficiency by 0.8 % absolute is observed thanks to the DS layer. [1] A. Gavriluta et all, Adv. Optical Mater. 2016, 4, 1846.

Resume : SnO2 as a standard TCO material have recently attracted special attention, thanks to the particular dual valency of tin and to the reversible transformation of the material that can occurs from p-type SnO to n-type SnO2 with excellent transport properties. Besides, SnO2 can be functionalized with Rare Earth elements (RE), which gives rise to new properties. In the field of solar cells, this class of materials can be used as conversion layers to adapt the incident solar spectrum to the absorption of the cell so as to reduce the carriers’ thermalization losses. In this work we present properties of SnO2 films co-doped with Nd (0.62 at.%) and Yb (1.3 at.%), elaborated by reactive magnetron sputtering. This co-doping process in SnO2 is reported for the first time. The films were elaborated at 100°C for the sake of compatibility with low temperature used for devices fabrication. The XRD analysis showed the phase transformation from amorphous SnO to crystalline SnO2 as a function of the oxygen gas flow during elaboration. We gain better insight into the oxides proportions as well as the RE profile distribution through chemical analysis by XPS spectroscopy, which also showed that both REs (Nd and Yb) are well inserted in the structure and possess the 3+ valence state, predicting that are optically active. Indeed, analysis by PL spectroscopy shows that under UV excitation of 325 nm, the SnO2:Nd:Yb films exhibit a wide and intense emission lines in the infrared region characteristic of Nd and Yb. Thanks to PLE measurements, an efficient energy transfer from the SnOx host matrix to each rare earth has been identified. In addition to that, second energy transfer was observed; Nd3+ ions are found to efficiently sensitize the Yb3+ ions. The films exhibit transparency laying between 75-90% with excellent transport properties, resistivities as low as 0,01 Ohm.cm and mobilities as high as 29,6 cm2/V.s were measured. Such optical and electrical results are of potential interest to solar cells using Nd and Yb co-doped SnO2 films as TCO and photon down shifter.

Resume : We integrate semiconductor quantum-dots within dielectric nanocylinders, arranged in a 2D array, and demonstrate that the resulting metamaterial tackles the requirements of a light down-converter for solar cells. Specifically we utilize silicon nanocrystals (Si-NCs), supporting a form of multiple-exciton generation called space-separated quantum cutting, embedded in SiO2 nanocylinders and demonstrate: i) total absorption of photons with energy E > 2 Egap and ii) total transmission of photons with E < 2 Egap, where Egap is the cell bandgap (Egap ~ 1.11 eV for Si). We experimentally demonstrate that the metamaterial supports absorption peaks, whose spectral positions are tailored by geometrical design parameters. Our calculation predicts that the absorption can reach up to 50%, for a free-standing metamaterial, and it can reach total absorption by using an impedance-matched substrate. We experimentally prove that that the increased absorption directly couples to the Si-NCs, resulting in an enhancement of the photoluminescence intensity. Although in this work we propose and use a specific material platform, the principle demonstrated is general and can be applied to other semiconductor quantum dots and emitting species, such as rare-earth ions and organic molecules. We envision the application of this principle to other applications in solar energy conversion, such as spectral shaping for photovoltaics, but also photocatalysis and artificial photosynthesis.

Resume : Solar cells, devices capable to harvest the renewable solar energy by photovoltaic effects, have been attracting much fundamental and practical interest. Solution-processable third-generation solar cells based on organic-inorganic hybrid lead halide perovskites can now yield power-conversion efficiencies over 20%, holding great promise to their commercial application. However, while hybrid lead halide perovskite solar cells can efficiently harvest the solar radiation from the visible spectrum, they cannot absorb sunlight of wavelength beyond 800 nm. As a result, nearly 40% of the solar energy that lies in the near-infrared (NIR) spectrum is outside their absorption window. Ways to harvest this part of NIR sun light are currently under intense research. In this work, we propose a series of lanthanide-doped fluoride upconversion nanoparticles that can be easily integrated into lead halide perovskite solar cells. Due to the upconversion effect, these particles can allow some additional light harvest in the NIR range for the perovskite solar cell without hampering the solar cell functioning. We will first discuss aspects including material characteristics, upconversion-perovskite solar cell device structure, device characteristics. Combining material and device characteristics and ultra-fast spectroscopic means such as time-resolved photoluminescence, we will discuss the physical roles played by these upconversion nanoparticles in a functional perovskite solar cell.

Resume : Colloidal quantum dot solar cells with solution-producible absorber layers consisting of inorganic semiconductor nanocrystals are an innovative concept in photovoltaics. With a depleted heterojunction between lead chalcogenide nanocrystals and titanium dioxide, up to ~10% power conversion efficiency has been reached at laboratory scale. An approach to overcome the drawback of the toxicity of the lead-based materials used is research on more environmentally friendly materials like copper indium disulfide (CuInS2, CIS). In this work, depleted heterojunction solar cells made of colloidal CIS and ZnO nanocrystals were fabricated [1], their power conversion efficiency still lacking behind that of devices with lead compounds. Using quantum efficiency measurements and optical simulations, we found that only a narrow collection zone in the absorber layer contributes to the photocurrent. A possible reason for this behavior might be the low charge carrier mobility in the CIS nanocrystal layer, as revealed by studies of single carrier diodes. Charge carrier mobility is expected to be related to organic ligand molecules capping the surface of the nanocrystals. Strategies for its improvement as well as recent results on the role of different surface modifications will be presented. [1] D. Scheunemann, S. Wilken, J. Parisi, and H. Borchert; Investigation of the Spatially Dependent Charge Collection Probability in CuInS2/ZnO Colloidal Nanocrystal Solar Cells, ACS Photonics 2, 864-875 (2015).

Resume : Crystalline silicon (c-Si) photovoltaics is the dominant technology today for solar power generation. However, the power conversion efficiency of c-Si solar cells is approaching the theoretical limit. Recently, multi-junction solar cells employing perovskite and c-Si solar cells have shown to bear the exciting potential to surpass the efficiency limit of market-leading single-junction c-Si solar cells besides being cost-efficient compared to other multi-junction solar cell technologies. However, scaling up this technology and maintaining high efficiency over large areas is challenging, as evidenced by the small-area (< 1.5 cm²) perovskite/Si multi-junction solar cells reported so far. For the economic viability of the perovskite-based multi-junction technology, an efficient transition from lab-scale cells to industrial-scale modules is crucial. Additionally, perovskite-based modules can also be combined with other PV technologies in multi-junctions surpassing the single junction performances. In this work, we present the steps towards making four-terminal perovskite-based multi-junction solar modules fully scalable to commercial solar module dimensions, both for stacks on top of c-Si as on CIGS. We show that these multi-junction solar module concepts are compatible with industrially scalable fabrication techniques thus, enabling the production of high-performance large-area perovskite-based multi-junction solar modules.

Resume : Organometal-halide perovskite and Si are the top material candidates for cost effective and >30% efficient tandem solar cells (TSCs). We have analyzed the influence of spectral and temperature changes on the limiting efficiency of perovskite/Si TSCs with different perovskite bandgaps at two locations with distinctly different climate conditions: Utrecht, The Netherlands and Denver, Colorado, US. We compare voltage- or current-matched two-terminal and unconstrained four-terminal tandem assembly strategies using the detailed-balance model. Spectral and temperature variations measured over the course of an entire year lead to efficiency losses between 3.7% and 8.2% depending on the considered tandem configuration and location, suggesting that different locations may benefit from different tandem solar cell configuration. The monolithically integrated configuration is most affected by spectral variation since it is subject to current matching whereas the voltage-matched configuration is the least stable against temperature changes. Yet spectral variations are far more important than temperature changes. Furthermore we show that the annual energy yield of realistic TSCs strongly depends on the fill factor of the sub-cells.

Resume : Depositing perovskite solar cells on top of crystalline Si or thin film Cu(In,Ga)Se2 is an attractive approach to realize highly efficient and cost effective photovoltaic devices. Currently the state-of-the-art perovskite solar cells are processed on relatively small substrate size, and the solvent-dripping technique1,2 poses enormous challenges to scale-up the process from lab-scale to industrial-scale. Here we report partial ion exchange (PIE) to synthesize a series of mixed-perovskite absorbers from non-hazardous solvents by using organic halide post-deposition treatment. We illustrate this method on the CH3NH3PbI3-xBrx system to reveal the key features of PIE reaction. We obtain photo-stable, semi-transparent planar perovskite solar cells with steady-state power conversion efficiency of 16.8% in substrate configuration and demonstrate 22.7% efficiency together with Cu(In,Ga)Se2 solar cell in 4-terminal tandem configuration. We apply the here described PIE method to 5cm × 5cm substrate, and obtain 15.7% efficient semi-transparent device (cell area > 0.5cm2) with good photo- and thermal-stability under continuous illumination. Our work represents a step forward towards fast and scalable production of perovskite solar cells with tailored composition using non-hazardous solvents, and provides insights into partial ion exchange reaction in thin films, which greatly broadens the scope of the materials that can be successfully prepared by this technique. 1 Bi, D. et al. Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%. Nature Energy 1, 16142 (2016). 2 Saliba, M. et al. Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science 354, 206-209 (2016).

Resume : Methyl ammonium lead iodide (MAPbI3) is one of the most popular materials for thin film solar cells. It is relatively easier to prepare a high purity MAPbI3 material that can be applied in solar cells to achieve a photovoltaic conversion efficiency of more than 10%. High efficiency tandem cell structure requires current matching between the component cells. However, it is known that high efficiency silicon based tandem solar cells are difficult to achieve, and the efficiency of such a tandem cell remains lower than expected. Therefore, we investigated a tandem solar cell with a combination of MAPbI3 and heterojunction with intrinsic thin Si (HIT) sub-cells. As current matching between the top and bottom sub-cells is necessary to obtain higher device efficiency, we obtained the optimized structure by optical analysis in combination with simulated electronic characteristics of the solar cells. The single junction MAPbI3 device structure was ITO∕TiO2∕CH3NH3PbI3 ∕Spiro-OMeTAD∕Ag. The tandem cell structure was Spiro-OMeTAD∕ CH3NH3PbI3 ∕TiO2 ∕p-a-Si∶H∕i-a-Si∶H∕n-cSi∕n-a-Si∶H. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and efficiency () of the investigated single junction MAPbI3 cells were around 21 mA/cm2, 0.75 to 1.2 Volts, 0.74 to 0.81 and 12 to 21% respectively, whereas for the tandem cell these parameters were 18 mA/cm2, 1.9 Volt, 0.88 and 30% respectively.

Resume : In this work we present the most recent progress on introducing functionalized transparent conductive oxides as electrical back contact for Cu2ZnSn(SxSe1-x)4 (CZTSSe) Kesterite-based solar cells. Given the possibility of band gap engineering between 1.0 and 1.5 eV in CZTSSe absorbers by changing the composition, varying radiant energy can pass through the Kesterite solar cell and be absorbed by a high efficiency Si-based solar cell for 4-terminal tandem devices. In the first part of the work, the functionalization of the Kesterite back contact was achieved by the insertion of thin layers (5-20 nm) of Mo, Mo:Na (by PVD) or various transition metal oxides like MoO3, NiO, TiO2 and V2O5 (by thermal evaporation) onto clean SnO2:F (FTO) and In2O3:SnO2 (ITO) substrates. Followed by a deposition step of the abundant and inexpensive Kesterite absorber material synthetized by a two-step process (sputtering + reactive annealing); demonstrating a completely new route towards better carrier collection at long wavelengths thanks to the fictionalized substrates. In the second part of the work, several types of low-cost Si based solar cells were fabricated and tested as bottom parts of 4-terminal CZTSSe-Si tandem dual junction device structures with no concentration, demonstrating that this configuration better matches the sun’s spectrum than the individual gap systems.

Resume : The three terminal heterojunction bipolar transistor solar cell (3T-HBTSC) is a novel solar cell structure that resembles that of a (npn or pnp) bipolar transistor. In this npn (pnp) structure, the “top” cell consists of the top np (pn) layers and, which are made of a high bandgap semiconductor. The bottom n(p) layer is made of a low bandgap semiconductor and, together with the middle p (n) layer, forms the “bottom” heterojunction solar cell. In this respect, while a pn junction could be considered the building block of single gap solar cells and conventional multi-junction solar cells, the 3T-HBTSC could be considered the double-junction building block of a new generation of multi-junction solar cells. In addition, the transistor structure allows some simplifications in the layer structure with respect to that of conventional multi-junction solar cells since, for example, tunnel junctions are not necessary. On the down side, the price to pay is that a third terminal is added to the solar cell structure although this is considered advantageous in the long term since it allows for a better matching to solar spectrum changes along the day. In spite of the name, in the 3T-HBTSC the transistor effect has to be avoided since this would result in the voltage of the top cell being limited by the voltage of the bottom cell. From the perspective of the operation of the solar cell, avoiding the transistor effect also favors achieving a low series resistance in the middle layer of the cell (since this layer can be highly doped). The later point is relevant because the total photocurrent produced in the cell has to be extracted through that middle layer. This work presents the fundamental theory of the 3T-HBTSC.

Resume : Closing session.