Advanced inorganic materials and structures for photovoltaics

Resume : Performance of thin silicon solar cells depends on effective light trapping in absorber layers. Today, light trapping is mainly based on a combination of refractive-index matching layers at the front side of solar cells, light scattering at randomly textured rough interfaces, and deployment of highly reflective metal contacts at the back side. Recently, photonic and plasmonic structures have sparked an enormous research interest as light trapping structures that can be more effective in solar cells than the conventional randomly textured structures. Randomly textured surfaces can potentially enhance light absorption in a thin semiconductor slab in the weak absorption region by 4n2, where n is the refractive index of the semiconductor. The experimental demonstration of reaching the 4n2 absorption enhancement limit in a thin semiconductor slab, i.e. not in a complete solar cell, will be presented. The key step in achieving the theoretical light absorption enhancement in a thin c-Si slab was the decoupling of the front- and rear-side surface texture. It will be demonstrated that the light trapping concepts based on diffraction on periodic photonic nanostructures and scattering using plasmonic structures have potential to outperform the currently used randomly textured structures. However, to achieve the 4n2 absorption enhancement in silicon solar cells with thin absorbers, the parasitic absorption in the supporting layers of has to be significantly supressed. The current status of thin-film silicon solar cells with implemented advanced light trapping concepts will be presented.

Resume : The goal of this work is to demonstrate that such non-vacuum technology as spray pyrolysis for deposition of TCO layers can be effectively used for the processing of Si based solar cells. For this purpose, morphology, structural, electrical and optical properties of transparent conductive oxides (TCO) such as Indium-Tin-Oxide (ITO) and Indium-Oxide doped with Fluorine (IFO) deposited by spray pyrolysis on glass and Si substrates at different temperatures have been investigated and implemented layers for processing of Si based solar cells. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ellipsometry and resistivity measurements were used for the analysis of individual layers as well as TCO/Si interfaces. It is shown that the resistivity of the TCO layers deposited on a glass substrate is up to 3 times higher than the resistivity of similar layers deposited on a Si substrate. Resistivity of the TCO layers has been found to decrease with the increasing of the deposition temperatures. It is shown that IFO film possesses unique property to form highly rectifying contact to p-type silicon, in contrast to the ITO which forms rectifying contact to n-type silicon. This property is of great importance to obtain high-efficiency solar cells (SC) based on TCO/Si heterojunction as well as for TCO/(n+p(or n)p+)Si solar cells with diffused emitter. Si based bifacial SCs, using the investigated TCO layers as front and back side transparent electrodes, were processed and conversion efficiencies for front/rear illumination 18,9% / 12,2% was measured for IFO/(n+p p+)Cz-Si/ITO and 16,9%/9,8% for heterojunction solar cell IFO/pp+-Si/ITO.

Resume : Silicon heterojunction technology (HJT) uses silicon thin-film deposition techniques to fabricate photovoltaic devices from mono-crystalline silicon wafers (c-Si). This enables energy-conversion efficiencies above 21 %, also at industrial-production level. In this presentation we review the present status of this technology and point out recent trends. We first discuss how the properties of thin hydrogenated amorphous silicon (a-Si:H) films can be exploited to fabricate passivating contacts, which is the key to high-efficiency HJT solar cells. Such contacts enable very high operating voltages, approaching the theoretical limits, and yield small temperature coefficients. With this approach, an increasing number of groups are reporting devices with conversion efficiencies well over 20 % on both-sides contacted n-type cells, Panasonic leading the field with 24.7 %. Exciting results have also been obtained on p-type wafers. Despite these high voltages, important efficiency gains can still be made in fill factor and optical design. This requires improved understanding of carrier transport across device interfaces and reduced parasitic absorption in HJT solar cells. For the latter, several strategies can be followed: Short-wavelength losses can be reduced by replacing the front a-Si:H films with wider-bandgap window layers, such as silicon alloys or even metal oxides. Long-wavelength losses are mitigated by introducing new high-mobility TCO’s such as hydrogenated indium oxide, and also by designing new rear reflectors. Optical shadow losses caused by the front metallization grid are significantly reduced by replacing printed silver electrodes with fine-line plated copper contacts, leading also to possible cost advantages. The ultimate approach to minimize optical losses is the implementation of back-contacted architectures, which are completely devoid of grid shadow losses and parasitic absorption in the front layers can be minimized irrespective of electrical transport requirements. The validity of this approach was convincingly demonstrated by Panasonic, Japan in 2014, reporting on an interdigitated back-contacted HJT cell with an efficiency of 25.6%, setting the new single-junction c-Si record. Finally, given the virtually perfect surface passivation and excellent red response of HJT solar cells, we anticipate these devices will also become the preferred bottom cell in ultra-high efficiency c-Si-based tandem devices, exploiting better the solar spectrum. Such tandem cells have the potential to overcome the fundamental single-junction limit of silicon solar cells (29.4%). Combining HJT cells with perovskite solar cells as top cell appears to be particularly appealing.

Resume : Selective carrier (hole or electron) transport layers (HTL, ETL), which have been widely used in organic devices because of their low temperature and solution-based processability, have attracted interest to be used as emitter layer for both crystalline and amorphous silicon based solar cells. This approach would avoid the requirement of high temperature steps or the use of expensive equipment. An interesting approach consists of replacing the a-Si:H emitter layer in Heterojunction with Intrinsic Thin Films (HITs) solar cells by Hole Transport layers based on Transition Metal Oxides (TMOs). Recently, Battaglia et al. have reported the fabrication of HITs solar cells using substoichiometric molybdenum trioxide (MoOx, x<3) as a hole-selective contact. A power conversion efficiency of 18.8 % and a Voc of 711mV were reported for this kind of solar cells. Our group has also recently reported planar HITs solar cells based on MoO3 HTL with 12.5 % and open circuit voltages of 610 mV. In this work a series of heterojunction crystaline silicon solar cells using different TMOs (MoO3, WO3, ReO3 and Va2O5) as emitters are reported. The diodes were characterized by measuring the current-voltage and capacitance-voltage characteristics for different temperatures. Finally the optoelectronic performance of the solar cells was correlated with the electronic properties (barrier height, ideality factor, saturation current,...) obtained for the different TMOs layers.

Resume : The heterojunction (HJ) between amorphous Si (sub)oxides [...] and crystalline Si (c-Si) is investigated, aiming at clarifying the passivation and hole transport properties. Layers ranging from pure a-Si:H to near-stoichiometric a-SiO2 were grown by varying SiH4/CO2 precursor gas mixtures during chemical vapor deposition. In-system photoelectron spectroscopy was employed to measure the valence band offset ∆EV [1], while the defect density Dit at the a-SiOx/c-Si HJ was determined using photoconductance decay. We measure a systematic increase of ∆EV starting from 0.3 eV for the a-Si:H/c-Si HJ to 4.3 eV for the a-SiO2/c-Si HJ. Concomitantly the electronic quality of the HJ deteriorates, as evidenced by an increased Dit. Furthermore, (p)a-Si:H/(i)a-SiOx:H/(n)c-Si HJ solar cells (cf. [2]), with intrinsic a-SiOx:H passivation layers deposited using the same parameter sets, were fabricated. We report a linear decrease of the fill factor for increasing ∆EV in the range of 300 – 750 meV. The reason is an increase of the barrier height for minority charge carriers (holes) at the (i)a-SiOx:H/(n)c-Si HJ and a simultaneous change of the hole transport mechanism from thermionic emission to defect-assisted tunneling processes across the junction. Our results suggest that Si suboxides are unsuitable as hole contact layers in high-efficiency n-type Si HJ solar cells. [1] T. F. Schulze et al., Phys. Rev. B 83, 165314 (2011). [2] J. P. Seif et al., J. Appl. Phys. 115, 024502 (2014).

Resume : We present the progress and challenges of state-of-the-art kesterite solar cells. During the recent years our team has been pushing the frontiers of CZTSSe photovoltaic technology. Efficiency levels were almost doubled since 2009 to the current records of over 12.6%. We have achieved top performance absorbers by both solution processing and vacuum deposition via precise composition and processing control. While device parameters such as the short circuit current have been improved to levels comparable to high-efficiency CIGS, the Voc deficit remains the main challenge to overcome before kesterites can be commercialized. We discuss possible causes and strategies for addressing this issue. We also include details on some recent applications of our kesterite absorbers such as monolithic tandem solar cells with efficiencies higher than previously reported for similar CIGS devices.

Resume : Copper Indium Gallium Selenide (CIGS) based polycrystalline semiconductor is used for thin film solar cells, which is one of the most popular materials in the recent years. However, indium and gallium are scarce elements in the earth and widely used in other microelectronic applications. This has led to the limitation of mass production of CIGS modules in the mid- to long- term. Therefore, earth abundant photovoltaic materials become more and more popular in the last few years. Among those materials, Copper Zinc Tin Sulfide (CZTS) has arisen as the most promising candidate. CZTS is a p-type semiconductor with kesterite structure, similar to the chalcopyrite structure of CIGS. As widely known, sodium plays an important role in CIGS solar cell. Na replaces Cu forming a more stable NaInSe2 compound and having a larger band gap. Na also forms defects on Cu site and In site and the resulting increase of the effective hole densities. Due to the similarity between CIGS and CZTS, it is expected that sodium may make similar effect on the latter one. However, research work on Na in CZTS film is still quite few compared with Na related research in CIGS solar cell. Few people report Na distribution in CZTS film, surface and CZTS/Mo interface. There are also few references about how to control Na amount in CZTS films. Therefore, it is necessary to get to know more details about the distribution of Na in CZTS and its role in the films. In this work, three kinds of CZTS films were prepared: without sodium, with 40 nm NaF layer in the precursor and using soda lime glass substrates without barrier layer. SEM showed a bi-layer structure of CZTS films. The bottom layer with small grains became thicker with sodium in the film. UV-Raman spectroscopy was used to analyze the phases both on the film surfaces and CZTS/Mo interfaces. A ZnS phase could be observed both in the surface and interface, especially in the sample prepared on the soda lime glass. The sodium distributions in CZTS thin films were investigated by XPS. The results indicated that sodium was rich in the film surface and interface. Simultaneously those areas became Cu poor and Zn rich. We assumed that sodium was one of the main reasons of those chemical modifications in the CZTS film surface and interface.

Resume : To make thin film photovoltaics (TFPV) more competitive with classic Si-based devices, research is focusing on the production of absorber layers. Printable inks containing nanocrystal (NC) precursors are explored, since a variety of NCs can be synthesized in apolar media. When printing photovoltaic absorber layers of NCs such as CIGS, the formation of a dense thin film is required, which is however hampered by the presence of the organic ligands. Our research focusses on CuInS2 NCs, for which the original steric stabilizers are exchanged for inorganic moieties containing sulfide or selenide species. This leads to an ink stabilized by charge that, apart from the solvent, doesn’t introduce unwanted components in the film. In CIGS processing, crystal growth is typically promoted by a gas-phase selenization step after film deposition. However, this introduces an additional process step that involves working with toxic gasses in a closed atmosphere. We show that introducing Se-containing moieties to stabilize CuInS2 dispersions enables us to enhance NC sintering and transformation, without the need for a selenization step. Two inks were prepared: both containing (N2H5)2Se capped CIS NCs and one of them containing Se NPs as well. Upon thermal annealing under He atmosphere phase transformation from CIS to CISe was observed with in situ XRD, resulting in a pure CISe phase in both cases. Finally, we address the prospects of using this approach to form CIGS absorber layers for TFPV.

Resume : CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se2 (CIGS) solar cells. As we have recently reported [1], a non-contiguous coverage of the CBD buffers on CIGS grains with {112} surfaces can be detected. In the present contribution, we report on the effect of annealing of CIGS thin films in air prior to the CBD of CdS and Zn(O,S) layers. A series of glass/Mo/CIGS/buffer stacks with different thicknesses of the CdS and Zn(O,S) layers was fabricated for scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses. The CBD process was carried out directly after the CIGS deposition either on CIGS absorbers in the as-grown or the oxidized state. SEM images of as-grown CIGS with thin CBD buffer layers on top reveal sparse buffer coverage on top of CIGS grains with {112} surfaces, whereas both CdS and Zn(O,S) buffer layers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different CBD buffer growth behavior as a result of changes in the surface energies of CIGS grains due to the annealing step. Reference solar cells were processed similarly and completed with i-ZnO/ZnO:Al layers for CdS and (Zn,Mg)O/ZnO:Al for Zn(O,S). The cells with improved buffer coverage for both CdS and Zn(O,S) exhibit higher efficiencies as compared to the as-grown CIGS samples. [1] W. Witte, D. Abou-Ras, and D. Hariskos, Appl. Phys. Lett. 102, 051607 (2013)

Resume : The solid state sensitized heterojuction photovoltaic cells have been developed rapidly by many research groups because they present a promising avenue towards cost-effective high efficiency solar power conversion. Although many materials have been used in nanostructured device, the goal of attaining high-efficiency thin film solar cells in such a way has yet to be achieved. Among the many candidates for absorber, CH3NH3PbI3 perovskite nanocrystals have recently attracted attention as a new class of light harvesters for high-efficiency nanostructured device. They exhibits a direct band gap and a wide range of light absorption covering the visible-to-near infrared spectrum as well as a high absorption coefficient. In this work, we have fabricated the perovskite-sensitized solar cells using methyl ammonium lead iodide (CH3NH3PbI3) as a light absorber and poly-triarylamine (PTAA) as a hole conductor on the surface of TiO2 electrodes. We have studied about the change of efficiency of the perovskite-sensitized solar cells according to the different TiO2 electrode structures. The perovskite-sensitized solar cell with the mesoporous TiO2 electrode has shown a better efficiency than those with the planar and electrospun TiO2 electrodes.

Resume : Strong limitations to solar cells efficiency are due to the thermalisation process induced by photons having energy higher than the bandgap and to the non absorption of photons having energy lower than the bandgap. Adapting the cell to the solar spectrum by photon conversion processes is one of the possible ways to reduce these limitations. The development of efficient down conversion layers allowing the conversion of ultraviolet (UV) photons into near infrared (IR) is required. In this work, SiOx layers codoped with Cerium (Ce) and Ytterbium (Yb) are studied. Among all rare earth ions, Ce is of particular interest since Ce3+ is characterized by an electric dipolar allowed 5d-4f transition leading to a strong absorption in the violet-blue. Ytterbium is also of prime interest for Si-based solar applications because of its emission at around 980 nm, above the Si bandgap. Host matrix and doping were obtained by e-beam assisted evaporation and by effusion cells, respectively. Samples were studied by photoluminescence (PL), Raman and IR absorption spectroscopies. The influence of the rare earth concentrations and of the annealing treatments was investigated. With an excitation in the UV, the characteristic emission bands of Ce and Yb are measured at room temperature. The indirect excitation of Yb is demonstrated and the energy transfer process from Ce to Yb is studied by PL decay time and PL excitation experiments. The potential of these layers for PV applications is discussed.

Resume : Nowadays, the fabrication of the highest efficiency CIGS based solar cells needs the use of a wet-chemically deposited (CBD) cadmium sulfide (CdS) buffer layer. Beside the toxicity of cadmium which can still be pointed out as an issue, the use of the chemical bath deposition technique for the buffer layer is considered as a drawback for the in-line processing. Different alternative combinations, mixing materials and processing, have already been proposed, whereas the most promising one is the reactively sputtered zinc oxy-sulfide ZnSxO1-x [1,2,3]. However, the incorporation of oxygen in zinc sulfide needs a heating of the substrate which can influence the diffusion of Zn in the CIGS absorbing layer [4,5]. This study aims to assess the deposition of this buffer layer using sputtering technique at room temperature. To reach that goal, two targets, ZnO and ZnS, have alternatively been sputtered using a continuous rotation of the substrate holder. Depending on the applied power to each target, it was possible to deposit 100nm-thick ZnSxO1-x films in which the composition, x, can be tuned between 0 and1. [1] B.K. Meyer et al.,Appl. Phys. Lett. 85 (2004) 4629 [2] A. Grimm et al.,Thin Solid Films 520 (2011) 1330-1333 [3] H.I.Pan et al.,Applied Surface Science 256 (2010)4621-4625 [4] M. Benabdeslem et al.,Journ.of Crystal Growth 274 (2005) 144-148 [5] T. Sugiyamaet al., Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 8

Resume : The ordered interdigitated geometry has represented a holy grail of 'third-generation' photovoltaics. In this regard, we demonstrate the preparation of functional ‘extremely thin absorber’ solar cells consisting of massively parallel arrays of nanocylindrical, coaxial n-TiO2 / i-Sb2S3 / p-CuSCN junctions. Anodic alumina is used as an inert template that provides ordered pores of 80 nm diameter and 1-50 µm length. Atomic layer deposition (ALD) then coats pores of up to 20 µm conformally with thin layers of the electron conductor and the intrinsic light absorber. The crystallization of the initially amorphous Sb2S3 upon annealing is strongly promoted by an underlying crystalline TiO2 layer. After the remaining pore volume is filled with the hole conductor by solution evaporation, the resulting coaxial p-i-n junctions display stable diode and photodiode electrical characteristics. A recombination timescale of 40 ms is extracted from impedance spectroscopy in open circuit conditions, whereas transient absorption spectroscopy indicates that holes are extracted form Sb2S3 with a lifetime of 1 ns. The ordered nanorod solar cells scheme is quite practical experimentally and highly general. Members of the photovoltaics community can now apply it to the generation of series of samples in which each geometric parameter (rod diameter and length, individual layer thicknesses) is varied systematically. The three materials can also be replaced each of with alternatives, given that many n-type oxides are accessible by ALD, as well as several intrinsic semiconductor compounds of the heavier chalcogens featuring large extinction coefficients.

Resume : Industrial Cu(In,Ga)Se2 (CIGS) thin films formation process generally require two steps : 1) Cu-In-Ga precursors deposition 2) crystallization under selenium atmosphere (H2Se). Nevertheless, recent works have shown that it is possible to obtain good quality CIGS absorbers by sputtering in an inert atmosphere without additional selenium supply. This work present CIGS thin films deposited by a simple method that avoid the use of additional selenium: magnetron pulsed DC sputtering from a single quaternary target at room temperature, followed by a thermal annealing in nitrogen atmosphere. The obtained absorbers are compact with grains bigger than 1µm. Composition is Se-rich and Cu-poor.

Resume : Silver nanoparticles (Ag-Nps) can be used as light scattering element to enhance solar cell’s conversion efficiency. These plasmonic nanoparticles have a very strong interaction with light near the resonance frequency. Surface Plasmon resonance is the most outstanding optical property of such nanostructures which can amplify, concentrate and manipulate light at the scale. Incident light can be either absorbed or scattered depending on different features of Nps such as surrounding medium, size, shape and density. Nps on top of silicon substrate take the advantage of light scattering thus by enhancing the optical path length of light and the multiple extinctions. Depending on the position of Nps (on top of the substrate or embedded within a dielectric layer) and the type of the dielectric medium, surface plasmon excitations are significantly modified. In this work, we investigate the effect of the surrounding medium of silver Nps on the morphological and optical properties of Ag-Nps/c-Si structures for solar cells application. The plasmonic structures are characterized by atomic force microscopy (AFM) and UV-Vis spectroscopy. A correlation between the size of the Nps and the surrounding dielectric medium and the plasmon resonance wavelength is obtained. Finally, the electrical properties of the plasmonics-based devices are investigated by I-V measurements.

Resume : In order to improve the conversion efficiency and stability of the hydrogenated amorphous silicon (a-Si:H) solar cells, it is important to analyze their output characteristics and to optimize the conditions under which they are manufactured. Since computer models are an excellent tool for studying transport mechanism and also as a method that can lead to a better device design, amorphous silicon alloy device modeling has been receiving a great deal of attention in the last 20 years. In the literature, there are several computer programs for modeling amorphous silicon solar cells. Therefore in this study, a detailed simulation studies of I-V characterization of single junction p–i–n amorphous silicon solar cells and a-Si:H/c-Si heterojunction solar cells have been presented, in order to get more insight into the factors determining the solar cell performance. AMPS-1D (Analysis of Microelectronic and photonic structure) device simulation program has been used to present data concerning the optimization of electronic properties of amorphous silicon p-i-n solar cells and examines the correlation of the external electrical characteristics to the internal solar cell properties such as band diagram, the electric field profile, free and trapped carrier concentrations, space charge density, recombination rate profiles, electron and hole lifetime and spectral response. The simulation program and its application were extensively described. Applying an optimized graded buffer layer at p+/i interface of single cell, a further efficiency improvement has been realized. A standard procedure of mid gap density of state optimization of p+ window layer and p+/i buffer layer has been performed. We have investigated the degradation kinetics of a-Si:H solar cells with improved buffer design. The influence of the mid gap defect density and the thickness variations in the intrinsic layer were also studied to improve the stability. Our findings show that thin cells can be exposed to light for longer periods before degradation effect dominates their operation. Stability can be improved by reducing the thickness of the intrinsic layer. The design and optimization of a-S:H /c-Si heterojunction solar cell was done with AFORS-HET (Automat FOR Simulation of HETerostructures) simulation program. Detailed simulation studies of I-V characterization of heterojunction solar cells have been carried out with TCO/ (n) a-Si:H/ (i) a-Si:H/ (p) c-Si/BSF/Ag structure. The effect of a-Si:H (i) layer, at a-Si:H/c-Si heterointerface, on the cell performance was investigated in details. It is also investigated the influence of the thickness of all layers, different defect configurations and defect concentration in c-Si (p) and the change of mobility gap for (p++) a-Si:H BSF layer. Morover, the interface defects at the front and back side of the c-Si (p) absorber is also considered. The results of these simulation studies have been seen to be good agreement with the reported studies in the literature.

Resume : Semi-transparent thin-film solar can be easily incorporated in a typical window to generate electricity and yet allow daylight for occupant?s visual comfort. Although full-penetration a-Si:H-based cells solar cells have been studied for application to PV windows owing to their low production cost and high reliability, the remaining issue is that their power conversion efficiency, transparency and color cannot be controlled independently. Carbon alloying of a-Si:H allows to wider the optical gap and to vary the refractive index of the material thus giving more freedom to control the optical and photovoltaic device characteristics. This work reports on preparation and characterization of device-grade a-SiC:H absorber material deposited by plasma enhanced chemical vapor deposition (PECVD). Films with an optical bandgap, E04, ranging from 2 to 2.4 eV were deposited in SiH4 CH4 H2 plasma by varying the CH4-to-SiH4 ratio and RF power. Test cells in the n-i-p configuration with intrinsic a SiC:H layers deposited under the same deposition conditions were also fabricated and characterized. Devices showed excellent forward current-voltage characteristics with a diode ideality factor in the range from 1.4 to 1.8, and low (<500 pA/cm2) reverse dark current. The correlation between the optical bandgap and open circuit voltage was also observed. The density of deep defect states in a SiC:H was estimated from the transient current measurements and correlated with the optical bandgap.

Resume : An increasing trend towards efficient and thinner silicon wafer solar cells demands excellent optical confinement and passivation schemes. In this direction, the present study aims to develop stable and superior passivation coatings. Chemical vapour deposited SiN:H is considered to be one of the promising candidates for the passivation of silicon surface. However, interfacial characteristics of Si/SiN structures are further to be improved to achieve better surface passivation reducing the interface defects. This has been achieved using a combination of SiO2 layers grown by nitric acid oxidation method followed by the deposition of SiN:H layers by PECVD. This study evaluates the surface passivation quality and related interface characteristics in terms of minority carrier lifetime and capacitance-voltage measurements. Effective passivation quality of SiN:H layers has been achieved by introducing nitric acid grown SiO2 layers at the Si/SiN:H interface. Surface recombination velocities as low as 12 cm/s was achieved in this study. The influence of a wide range of processing parameters was explored in order to assess the passivation stability of the deposited layers. It is found that the passivation quality of layers is stable up to 700 oC, above which depassivation of the interface was occurred. The hydrogen diffusing from SiN:H layers through SiO2 layers is influencing the passivation characteristics of the silicon surface.

Resume : Al doped Zinc Oxide (ZnO:Al) and Indium Thin Oxide (ITO) are transparent and conductive oxides used as contact layers in organic and inorganic based solar cell. Our work has been focused on deposition of TCO with low cost magnetron sputtering directly from ZnO:Al and ITO targets. The ITO results in a good conductivity (10-4 ohm cm) even at low film thicknesses and at low deposition temperatures, where the lowest resistivity of the ZnO:Al (10-3 ohm cm) needs larger thickness and temperatures that could affect heterojunction solar cells (amorphous, organic and chalcogenides). In addition band alignment matching with the active absorber is important. The work function and the thickness of the TCOs plays a fundamental role in the energy band alignment and antireflection properties of the devices. The ZnO:Al has a more suitable workfunction in comparison to ITO for n-type doped emitter layer in solar cells to enhance the built-in voltage of the overall structure, but its lower conductivity hampers its utilization. As the workfunction can be changed by varying the deposition parameters and the thickness of the TCOs a detailed study of the variation of the ZnO:Al and ITO workfunction and conductivity with the film deposition parameters has been made in order to optimize the TCOs for their specific use in solar cell.

Resume : Silver nanoparticles (AgNPs) are exhibiting exceptional plasmonic properties either as free standing nanostructures or embedded into a dielectric matrix. In this work we are comparing two different synthetic approaches: the pulsed laser annealing (PLA) and the polyol method (PM), for AgNPs in terms of optical and electronic properties through Spectroscopic Ellipsometry (SE) and surface topography through Atomic Force Microscopy (AFM). For the PLA case it was observed that the energy density (mJ/sqcm) and the number of pulses strongly affect the plasmonic properties and higher values of these parameters are resulting ablation and absence of nanoparticles formation. On the other hand PM AgNPs are strongly depend on the final solution's solvent selection and the concentration. Higher concentrations are resulting stronger plasmonic effects. AgNPs from PLA and PM were applied in the fabrication of Organic Photovoltaics and the effiency, the Voc and Isc are correlated with nanoparticles properties.

Resume : Zinc oxide (ZnO), a wide band gap semiconductor (3.3 eV at room temperature), and zinc oxide doped with aluminum (AZO) are intensively studied for photovoltaic applications. First, n-type conductivity makes ZnO a promising n-type partner for p-type silicon or cadmium zinc telluride. Moreover, AZO films are used as a TCO material to replace too expensive ITO. In this work, we focused on the photovoltaic effect in low costs solar cells based on AZO/n ZnO/p Si. ZnO and AZO layers were grown by atomic layer deposition method (ALD). Unfortunately, conduction band offset appearing in n-ZnO/p-Si heterojunction solar cell is recognized as a serious roadblock to obtain high efficiency solar cells. However, if a magnesium doped zinc oxide layers (ZnxMgx-1O) is used instead of ZnO we can influence the energy band diagram. The conduction band of ZnxMgx-1O can be raised above that of Si, so we significantly reduce influence of the recombination centers in junction. We found optimal Mg concentration in ZnO layers for obtaining high efficiency solar cell structures. The PV efficiency for AZO/Znx-1MgxO/ Si/Al structure is increased by at least 1.5%, as compared to structures with ZnO. This work was partially supported by the National Centre for Research and Development grant (PBS1/A5/27/2012), and (E. Zielony, P. Biegański and E. Popko) by the National Laboratory of Quantum Technologies (POIG. 02.02.00-00-003/08-00).

Resume : Designing and development of new highly active photocatalytic systems for various important applications is one of high-priority issues in modern photocatalysis. High prospects are expected in designing of structurally ordered photocatalytic blocks, which contain a semiconductor’s nanoparticle and the dye-sensitizer applied and fixed on the surface with the electron-conducting film. However, only preliminary assumptions were made about a chemical form of the dye applied on the semiconductor while no information about a character of the dark-phase interaction between the photocatalytic block’s components is available. The information related to the concentration dependencies of the block’s activity should also be supplemented as well as checking of possible utilization of various dyes as sensitizing agents. Some new light sensitive heterostructures of TiO2 and polymethine dyes have been synthesized and a character of influence of the dye's structure on their spectral and electrochemical properties was determined. Energy levels HOMO and LUMO of the dyes were calculated to evaluate the principal possibility of their utilization as sensitizers. Photoactivity of the heterostructures was investigated for the reaction of iodide oxidation under various irradiation intensity and concentrations of the dye. Analysis of the results proves that the above mentioned approach to designing of the light sensitive materials can generally be used for different semiconductors and sensitizers.

Resume : In this work, Ag incorporation in CuInSe2 solar cells was investigated in an attempt to improve device performances since Ag substitution with Cu has been reported to improve crystallinity by lowering melting temperatures of chalcopyrite materials. Ag-incorporated CuInSe2 (i.e. (Ag,Cu)InSe2) films were grown by a typical three-stage co-evaporation. In order to incorporate Ag in CuInSe2 films, Ag and Cu were simultaneously evaporated during the second stage of the three-stage co-evaporation. A Ag/(Ag+Cu) compositional ratio was fixed to about 0.3 because bandgaps of (AgCu)InSe2 films were not greatly affected to that extent. A CuInSe2 film without Ag was found to have relatively fine microstructure, while the (Ag,Cu)InSe2 films exhibited significant recrystallization. Devices were also fabricated using the (Ag,Cu)InSe2 films to understand the effects of the Ag incorporation to device performances. A CuInSe2 cell without the Ag-alloying exhibited a performance as an efficiency of about 11.4% with Jsc of 40.2 mA/cm2 and Voc of 430 mV. In contrast, an (AgCu)InSe2 cell with Ag/(Ag+Cu) = 0.3 exhibited an improved device performance as an efficiency of about 12.8% with Jsc of 39.2 mA/cm2 and Voc of 470 mV. This improvement by the Ag incorporation appeared to be associated with reduced structural and electronic disorders. The more details of the Ag-incorporation effects to a CuInSe2 film will be discussed in the conference.

Resume : The Cu2ZnSnS4 (CZTS) is considered as an ideal solar cell absorber due to its abundance, non-toxicity, high absorption coefficient and proper direct band gap. We report here on the use of pulsed KrF-laser deposition (PLD) technique for the fabrication of high quality CZTS thin film deposited on Mo coated glass substrates without resorting to any post sulfurization process. The ~1µm-thick PLD-CZTS films were deposited at room temperature (RT) and then subjected to post annealing at different temperatures ranging from 100 to 500°C in Argon atmosphere. The X-Ray Diffraction (XRD) and Raman spectroscopy reveals that the PLD films exhibit the characteristic kesterite CZTS structure regardless of their annealing temperature (Ta). However, their cristallinity is found to improve significantly Ta  400°C. Rutherford backscattering spectroscopy measurements also confirmed the stoichiometry of the films. The annealed films are found to exhibit a relatively dense morphology, as revealed by SEM observations, and an AFM measured surface roughness that increases with Ta (RMS increase from ~14 nm at RT to 70 nm at Ta = 500°C with a value around 40 nm for Ta = 300-400°C). The optical band gap energy of CZTS films, derived from their associated UV-Vis transmission spectra, was found to decrease slightly from 1.73 eV for non annealed films to ~1.60 eV for Ta ≥ 300°C. These band gap values are very close to the optimum value for a solar cell absorber, making these PLD-CZTS films attractive for thin film solar cell applications.

Resume : Perovskite-based solar cells have recently attracted tremendous attention in the field of renewable energy technologies. The devices are typically realized using planar or mesoporous TiO2 and 2,2’,7,7’-tetrakis(N,N-p-dimethoxyphenylamino)-9,9’-spirobifluorene (spiro-OMETAD) as the electron transport and the hole transport layer, respectively. The most performing devices are realized using a TiO2 layer that is processed at high temperatures (>450°C), which is not compatible with roll-to-roll processing on flexible substrates. For this reason it is very critical to explore the possibility to realize perovskite-based solar cells at low temperatures. In this work, we report the application of low-temperature-annealed solution-processed TiO2 for planar-heterojuntion perovskite solar cells. The TiO2 layer was realized starting from an alcoholic solution that is very easy to synthesize. The film was deposited by spin coating and annealed at temperature < 200°C. After, we realized photovoltaic devices with the following structure: ITO/TiO2/CH3NH3PbI3/spiro-OMETAD/Au. We made a comparative study of the photovoltaic behavior of devices with TiO2 films deposited using different recipes in order to investigate the influence of this layer on the morphology of the subsequently deposited perovskite film. All the devices were characterized by UV-VIS spectroscopy, IV light, IV dark and quantum efficiency measurements.

Resume : The goal of this work is a solution processed absorber layer based on nanoparticles for a copper indium diselenide (CISe) solar cell that does not require vacuum technology. In order to achieve a homogeneous layer composition it is believed that layer formation from binary metal compounds is beneficial compared to deposition of “complete” CuInSe2 nanoparticles. Different kinds of binary selenide micro- and nanoparticles were synthesized and stabilized. A microwave assisted solvothermal reaction for In2Se3 microparticles was performed, finding that smaller particles could be obtained in shorter times and at lower temperatures than with a conventional autoclave based solvothermal synthesis. Moreover In2Se3 and Cu2-xSe nanoparticles have been synthesized via aqueous redox reactions and tuned to the same Zeta potential with ascorbic acid. Both types of nanoparticles were mixed in ethanol in a molar ratio In/Cu of 1.2 to acquire a indium-rich nanoparticulate ink. This ink was then drop cast on aluminium and molybdenum coated polyimide in ambient atmosphere and annealed in N2-atmosphere. With in-situ XRD a formation of CISe was found starting at lower temperatures than with state-of-the-art processing routes. Special emphasis will be put on the discussion of the reduction of the layer porosity.

Resume : Fabrication of superstrate configuration CdTe/CdS solar cells deposited by close spaced sublimation (CSS) includes CdCl2:O2 thermal treatment at 450 oC resulting in deep recrystallization and sintering of CdS and CdTe layers and substantially improves optoelectronic properties of the cells. Resulting high concentration of chloride in the lattices and residue phases of CdO, CdCl2, (2CdO?CdCl2) and solid solution of CdS in CdCl2 as precipitates on grain boundaries cause low concentration of holes in CdTe and hygroscopicity of the cells. Considering volatility of CdCl2 and reaction products of CdO with CdS and CdTe as Cd, SO2 and TeO2, controlled thermal annealing in relevant ambient of the CdTe/CdS:CdCl2:O2 material should be a possible way for enhancement of optoelectronic properties of the cells. We report systematic investigation results on influence of annealing in different ambients at 200-450o C on the properties of CdCl2:O2 heat treated CdTe/CdS thin films and devices and discuss possible mechanisms behind these changes. The CdTe/CdS thin films and devices were characterized by X-ray diffraction, scanning electron microscopy, Raman and Photoluminescence spectroscopy, I-V and C-V measurements. By combining ambient and temperature in the process tube, substantial enhancement of solar cell characteristics has been achieved.

Resume : Nanoholes (NH) represent the inversion of the popular architecture based on wire-like structures. They are mechanically robust, reduce the handling costs in all steps of manufacture and displays improved light trapping abilities resulting from effective optical coupling [MATER SCI ENG B 178 (2013) 686], as well as a large density of waveguide modes. Its peculiar quasi one-dimensional geometry plays a double role: it allows to effectively trap the visible light, so it can be used as a top layer over a planar junction, or it can be used as building block for the formation of radial junctions, where the light path is orthogonal to the current collection path, allowing to optimize the two independently. Advanced lithographic technique, based on self-assembling methods, have been exploited to form NHs on c-Si [ECS J. Sol. St. Sci. Technol. 1 (2012) Q52]. In this work the new architecture is integrated in a solar cell device. Due to the new and peculiar characteristics of the NH, a proper technique, based on chemical solution processing, is proposed to perform the doping for the formation of the junction. It is based on the molecular doping method which is an easy, rapid, inexpensive and scalable manner to realize highly concentrated and sharp doping profiles [Sol. En. Mat. Sol. Cells 132 (2015) 118]. The NH based Si solar cells electrical characteristics will be presented and discussed together with those of the planar reference cells.

Resume : We present calculated electronic and optical properties of the semiconducting chalcopyrites CuGaSe2, CuGaS2, CuInSe2, and CuInS2. These results were performed in the framework of density functional theory (DFT) using full potential linearized augmented plane wave (FP-LAPW) method. The calculated absorption coefficient gives the most important information in terms of energy band gaps of these chalcopyrites, using the LDA and mBJ corrections.

Resume : Microcrystalline silicon oxide (μc-SiOx:H) is an attractive material for application in thin-film silicon solar cells as doped layers or intermediate reflectors, since its optical band-gap, refractive index, crystallinity and conductivity may be modified over a wide range by varying process deposition parameters. In the present work we investigate the electronic and structural properties of n-type μc-SiOx:H films as a function of oxygen content and layer thickness, varied between 30 to 700nm. μc-SiOx:H films were investigated by dark conductivity measurements, photothermal deflection spectroscopy, Raman and XRD spectroscopies to evaluate optical band gap (E04) and crystallinity, respectively. Our results indicate that in μc-SiOx:H layers the crystallinity increases with layer thickness up to a thickness of around 200nm and tends to saturate at a level of around 20% for the layers thickness between 200 to 700nm, while dark conductivity increases from 10-11 S/cm up to 10-2 S/cm over the entire thickness range. In contrast, n-type μc-Si:H layers (without additional oxygen) show an increase in both crystallinity (up to 70%) and conductivity (up to 5S/cm) with thickness up to 700nm. The dependence of the electrical conductivity on the structural properties and oxygen content for doped µc-SiOx:H is discussed and a transport model is proposed. The applicability of theses layers is demonstrated yielding an a Si:H single junction solar cell with a stabilized efficiency of 10.3%.

Resume : Fabrication of superstrate CdTe/CdS solar cells deposited by close spaced sublimation includes CdCl2:O2 thermal treatment at 450 oC. This process resulted in deep recrystallization and sintering of both CdS and CdTe layers and substantially improved the optoelectronic properties of the solar cells. The high concentration of chloride in the lattices and residue phases of CdO, CdCl2, (2CdO•CdCl2) and solid solution of CdS in CdCl2, as precipitates on grain boundaries, limit the concentration of holes in CdTe (<1015 cm-3) and cause hygroscopicity of the cells. Considering volatility of CdCl2 and reaction products of CdO with CdS and CdTe as Cd, SO2 and TeO2, controlled thermal annealing in relevant ambient of the CdTe/CdS:CdCl2:O2 material is shown as a possible way for removal of the residues - so-called gettering, which can improve the optoelectronic properties of the cells. We report systematic investigation results on the influence of annealing in different ambients at 200-450o C on the properties of CdCl2:O2 heat treated CdTe/CdS thin films and devices and discuss possible mechanisms behind these changes. The CdTe/CdS thin films and devices were characterized by X-ray diffraction, scanning electron microscopy, Raman and Photoluminescence spectroscopy, current-voltage and capacitance-voltage measurements. By combining ambient and temperature in the process tube, substantial enhancement of solar cell characteristics has been achieved.

Resume : Remarkable improvements to solar cell performances could be achieved by extending the spectral sensitivity to the near-infrared region that holds roughly half of the energy of the solar spectrum. Indeed the most promising hybrid bulk hetero-junction solar cells employ PbS (and PbSe) quantum dots (QDs) as inorganic component due their strong near infrared absorption. Pbs QDs also have the additional advantage of high bandgap tunability and good charge mobility. However in polymer/QDs blends the charge transfer process between the two components is not yet well understood. In this work we investigate charge formation and transfer in P3HT/PbS QDs focusing on the effect of a post-deposition ligand treatment of the blend that replaces the native insulating long chain oleic acid, present on PbS, with a shorter conductive ligand (1,2-ethane dithiol, EDT). In particular, we use steady state and ultrafast optical and photocurrent spectroscopy to study the polymer-QDs charge transfer dynamics before and after ligand exchange. We observe that the EDT treatment strongly enhances the charge transfer between the polymer and the QDs, while, very importantly, it does not affects the structure of the pure polymer and its exciton dynamics. These results open up new possibilities for the improvement of the efficiency of hybrid bulk hetero-junction solar cells.

Resume : The traditional measurement of the thermoelectric Seebeck coefficient gives a global value for a given material. In view of the combinatorial approach to discovery of new thermoelectric materials, it is highly desirable to have fast measurement techniques if possible with capabilities to access local fluctuations or gradients in material properties. The optical characterization of semiconducting materials by photoluminescence is the most convenient technique to access the quasi Fermi level splitting and the temperature of the carriers by fitting the spectra with the generalized Planck's law of radiation [1]. These two parameters are directly related to the Seebeck coefficient of the material [2]. We have developed a unique contactless optical measurement technique to determine Seebeck coefficients of materials by recording spectrally resolved photoluminescence images using an absolutely calibrated hyperspectral imager setup [3] with a spatial resolution down to a few micrometers. This contactless setup yields spatial gradients of quasi Fermi level splitting and temperature fluctuations, leading to the local Seebeck coefficient. This method was applied on a multi quantum well structure based on InGaAsP, and on a bulk chalcogenide AgInS2 sample. We will also show the perspectives offered for the research of new thermoelectric materials. [1]Würfel, J. Phys. C : Solid State Phys. (1982) [2]Tauc, Czech J Phys (1955) [3]Delamarre, Appl. Phys. Lett. (2012)

Resume : Different encapsulation glasses were tested in different configurations in order to evaluate the losses on solar cells and solar panels performances after encapsulation. The optical properties and surface morphology of different glasses were investigated in correlation with the current-voltage (I-V) characteristics of solar cells with and without encapsulation, in order to identify the most suitable solar cells encapsulation materials and configurations. The investigated encapsulation configurations were: 1) monocrystalline silicon solar cell / oxide film / glass; 2) monocrystalline silicon solar cell / glass / oxide film; 3) monocrystalline silicon solar cell / rare earth doped fluoride glasses and 4) monocrystalline silicon solar cell / EVA / oxide film / glass; 5) monocrystalline silicon solar cell / EVA / glass / oxide film; 6) monocrystalline silicon solar cell / EVA /rare earth doped glasses (TR3+/Yb3+; TR = Pr, Tm and Cr3+/Yb3+). For the configurations 1), 2) and 4), 5), the oxide films were prepared by reactive sputtering, thermal oxidation and sol-gel. The dependences on temperature of the I-V characteristics were also investigated for the different configurations mentioned above.

Resume : In the thin-film photovoltaic industry, to achieve a high light scattering in one or more of the cell interfaces is one of the strategies that allow an enhancement of light absorption inside the cell and, therefore, a better device behavior and efficiency. Although chemical etching is the standard method to produce this texture, the use of laser light give us the possibility to texture different materials, maintaining a good control of the final topography with a unique, clean, and quite precise process. In this work AZO films with various texture parameters are fabricated. The common parameters used to characterize them, as the rms roughness or the scattering factor, are discussed, and, for a deeper understanding of the scattering mechanisms, the light behavior in the films is simulated using a finite element method code (a detailed description of the simulation method is included). This method gives information about the light intensity in each point of the system, allowing the precise characterization of the scattering behavior near the film surface, and can be used as well to calculate a simulated scattering factor that can be compared with haze factor measurements. A final discussion of the validation of the numerical code, based in a comprehensive comparison with experimental data is included.

Resume : Hybrid composites of conjugated polymers and colloidal semiconductor nanocrystals (NCs) such as lead sulphide (PbS) are of interest for photovoltaic application. One of the most important issues is to control non-covalent and electronic interactions at the organic/inorganic interface. In P3HT/PbS, considered as inadequate previously, the tailoring of the NC surface chemistry with shorter surface ligands (arenethiolate ArS) [1] lead to a new morphology in the film hybrid interface and has allowed to reach power conversion efficiencies of up to 3%. Here we present a complete optical study on the blend in order to understand the physical origin of the efficiency improvement [2]. Time resolved spectroscopy, both in the visible (polymer) and the near infrared (PbS) region, allows to unveil phenomena involved in the charge transfer process and to understand the role of the hybrid interface and morphology of the blend. A comparison with other organic materials, suitable as photovoltaic active materials, allows to extend the conclusions to new types of blends including the low band gap polymers PTB7 and PCDTBT, able to absorb a high portion of the incoming sunlight. A broader understanding of the role of the PbS NC surface treatment in such hybrid blends is significant for application and performance enhancement in photovoltaic devices. [1] C. Giansante et al. J. Phys. Chem. C, 2013, 117 (25) [2] C. Giansante et al. Adv. Funct. Mater., 2014 (25)

Resume : Recently attention has been brought to the preparation of thinner absorber layers in CIGSe solar cells, in order to reduce both the deposition time and the consumption of indium and thus their cost. In previous work, we showed that reducing absorber layer thickness below 500 nm leads to a decrease of cell efficiency due to a decrease of both Jsc and Voc. This reduction is caused by an incomplete absorption of light and a sharpened impact of the interfaces recombination. An enhancement of the cell efficiency lies in the control of the Na and K contents in CIGS. In particular, the beneficial effect of a KF post-deposition treatment (KF PDT) has been proved on thick absorbers. In this study the investigation of K incorporation and its impact on the cell performances are extended to ultra-thin CIGS, with a thickness range of 200 nm to 500 nm. The influence of the Na and K concentration was decoupled by inserting a diffusion barrier layer (alumina) at the interface between Mo and the soda-lime glass substrate. The effect of the KF PDT on the formation of interfacial layers and cell properties has been studied. We show that for optimal performances, the temperature and KF quantity have to be adapted to the CIGS thickness. This treatment leads to an improvement of the Voc and Jsc, and thus to efficiencies higher than 10% for absorber thickness lower than 500 nm. The reason for these improvements is discussed by combining electrical characterization with material properties.

Resume : Micromorph tandem cell is a sequence of two p-i-n junctions, a-Si:H top cell and uc-Si:H bottom cell, which allows solar energy conversion in a wide range of sunlight spectrum (from ~ 300nm to 1100nm). As the cells behave like two current generators in series, the overall cell current is determined by the lower current between the two cells. Consequently, a large mismatch between the cells currents is useless. Typically, the best condition is to have a top cell to bottom cell current ratio of Top/Btm ~ 1. The Top/Btm current ratio is a fundamental parameter in regulating the impact of light induced degradation (LID) of a tandem cell. In fact, when the initial current of the Top cell is lower than that of the bottom cell, i.e. the tandem cell is top limited, the tandem cell LID factor is mainly determined by the Staebler-Wronsky (SW) degradation of a-Si:H top cell. This implies a higher degradation of the tandem cell performances caused by significant short circuit current reduction. One of the key point of thin film silicon based tandem cell is to reduce production cost and at same time to increase the efficiency. We studied a wide range of thickness of uc-Si sub-cell and of processes in order to optimize the extraction efficiency of the uc-Si sub-cell to reach the higher efficiency with the lower LID factor. Actually we measured an initial efficiency between 13% and 14% on small area samples. Moreover we measured on full area modules a LID of about 10%.

Resume : This work presents results concerning sintering of Si powder by Spark Plasma Sintering (SPS) method. It is shown that Si ingots as well as single wafers suitable for the low-cost Si based PV can be processed by this method. Light microscopy, scanning electron microscopy, energy dispersive spectroscopy, Raman spectroscopy were used for the analysis of SPS based Si structures. It is found that during the SPS process, inhomogeneity of sintered ingots and wafers occurs. Nevertheless, inhomogeneity can be smoothed by proper optimized sintered conditions for the large scale (up to 8 inch) ingots and wafers. It is found that the SPS process, being applied to Si powder, provides formation of a Si material, which has structural properties similar to those for multi-Si. It is concluded that SPS Si structures can be potentially considered as a base for the low-cost Si based photovoltaics.

Resume : In this work, one-dimensional device simulator AMPS-1D (Analysis of Microelectronic and Photonic Structures) was employed to study the electrical and optical properties of bifacial SLG/TCO/Cu2ZnSnS4 (CZTS)/CdS/TCO thin film solar cells. The impact of TCO/CZTS interface has been investigated. The combination of optical transparency and electrical conductivity for TCO front and back contact layers are capable of yielding high efficiency. Several transparent conducting oxides (TCOs) materials and metals have been tested respectively as a front and buck contact layers for bifacial CIGS solar cells. The presence of barriers in the front and back contact in the structure can significantly affect the cell performance by limit the carriers current flow. The influence of various parameters for the front and the back side illumination was studied and the corresponding design optimization was provided. The depletion region overlapping between the TCO/CZTS or CdS/TCO junction will result in the decrease of the solar cell performance. The best energy conversion efficiencies have been obtained with SnO2:F contacts. An efficiency of 6.8% (with Voc ≈ 0.86 V, Jsc ≈ 18.5 mA/cm2, FF ≈ 0.62 and QE ≈ 72.5%) have been achieved with ZnSn2O3-based as TCO front contact layer and Zn-based back contact layer. All these simulation results give some important indication to lead to higher efficiency of bifacial CIGS solar cells for feasible fabrication.

Resume : Great demand of photovoltaic materials costs reduction was a driving force for thin film photovoltaics emerging. Over a last few years concepts of plasmonic nanoparticles application have been developing, enabling enhanced absorption in weakly absorbing thin film Si or organic cells. Application of plasmonic metal islands film on the rear of the Si cell resulted in significant photocurrent enhancement in long wavelength region as shown by many authors. However, the concept of plasmonic nanoparticles on the cell front was very underdeveloped. Therefore, the simulation studies were performed in order to determine the optimal structure. Size and surface coverage of silver spherical nanoparticles periodic array were adjusted to maximize the absorption in underlying Si substrate. The experimental part were focused on approaching the simulated optimal structure. For this purpose colloidal suspensions were used. Nanoparticles were deposited by layer by layer technique. In order to obtain proper nanoparticles arrangement the deposition condition were studied as well. Optical properties of the samples as well as actual microstructure features were analyzed in conjunction with dedicated simulation investigations. Finally, the nanoparticles were applied on the front of Si wafer based solar cell resulting in 8% of short circuit current improvement. Much higher enhancement can be possibly obtained by careful microstructure improvement using this easy and cheap technique.

Resume : Semiconductor nanostructures (NS), which exploit the appealing features of a confined system have been widely investigated in the past decades. Beyond the enhanced surface-to-volume ratio or some light management phenomena, one of the most interesting features in nanostructures is the quantum confinement effect (QCE), arising in nanostructures smaller than the exciton Bohr radius (5 nm for Si, 24 nm for Ge). QCE is expected to increase the optical bandgap and the oscillator strength from the bulk values, opening the possibility to tailor the light absorption spectrum, with large potential benefits in photovoltaic applications. Still, up to now this effect has not been largely exploited, since other conditions usually hinder the QCE, as interface related states, large spread in NS size, poor quality of the embedding matrix. In this work, Ge quantum dots (2-10 nm in diameter) in SiO2 grown by plasma-enhanced chemical vapor deposition, will be reviewed evidencing whether and to which extent quantum confinement affects the light-matter interaction. Structural (TEM, RBS, Raman) and optical (absorption spectroscopy, Tauc analysis) characterizations, and effective mass approximation models are employed to describe the QCE-induced variation of optical bandgap (from 1.0 up to 2.5 eV). Indeed, an unprecedented high light absorption efficiency, ten times larger than in the bulk, has been discovered for the smaller Ge QDs in multilayered sample, due to the excitonic effect.

Resume : Germanium nanoparticles or quantum dots (QDs) embedded in transparent dielectric matrix have properties radically different from the bulk semiconductor and present a great potential for application in electronic and optoelectronic devices. Due to quantum confinement properties the optical bandgap of QDs based materials can be tuned by varying the nanoparticle size. These properties may be exploited for the fabrication of nanoscale electronic devices or advanced solar cells. In this work we explored structural and transport properties of QDs based superstructures for advanced solar cells. Magnetron cosputtering was used for deposition and upon suitable thermal treatment a superstructure of QDs was formed. Transport properties were explored by I-V measurement in the dark and in light together with C-V characterization. The obtained results were modeled with the known transport mechanisms for QDs containing materials. A special emphasis is given to space charge limited current and hopping conductivity mechanism.

Resume : One of the main challenges in the development of highly efficient semiconductor quantum dot-based solar cells is controlling the energy transfer between adjacent nanocrystals. This is partly due to the lack of precise, yet large-scale, control in nanoscopic dimensions, which is required to achieve a close-packed, uniform and stable superstructure [1]. We study the assembly of quantum dots (both in a liquid environment and deposited on a substrate) by different techniques, such as dynamic light scattering (DLS), microscopic photoluminescence (μ-PL) in confocal and non-confocal modes and scanning electron microscopy (SEM). Here, we present some of our results on the formation of superstructures from nanocrystals and on how the optoelectronic properties of such systems change as spatial condensation takes place. [1] Liu, Yao, et al., "Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids" Nano Lett. 10 (5), 1960-1969 (2010).

Resume : Multiple exciton generation (MEG) in II-VI nanostructures opens up new opportunities to improve solar cell efficiency beyond the thermodynamic limit. Despite the demonstration of up to 700% MEG efficiency in PbSe quantum dots (QDs), power conversion efficiency of PbSe QDs solar cells remains low, relative to inorganic solar cells. One of the limitations to the concept of MEG solar cells is that, after generation, excitons may recombine either radiatively or via Auger processes, reducing photocurrent generation yield and charge extraction in tightly confined nanostructures. So far charge extraction has hindered MEG benefits in solar cells. In this work we investigate the nature of the MEG in PbSe nanostructures with different dimensionality: QDs (0D), nanowires (NWs, 1D) and nanosheets (NSs, 2D) using steady-state and ultrafast optical and photocurrent spectroscopy and first-principle calculations based on many-body perturbation theory. Our results indicate that MEG generation and recombination strongly depend on light polarization in NWs and NSs, where structural anisotropy reflects in different absorption, emission and carrier delocalization along preferential directions. This proves that efficient MEG can also be achieved along the confined directions of 1D and 2D nanostructures by proper control of light polarization. Ordered NW and NS architectures may be employed to design MEG solar cells in which effective carrier extraction occurs along preferential, delocalized axes.

Resume : Despite the successful application of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) for multijunction solar cells, their efficiency is still low compared to the other solar cell technologies based on inorganic absorber materials. The inferior device performance of thin-film silicon is mainly ascribed to two remaining issues: the light-induced degradation in a-Si:H and the weak infrared absorption of μc-Si:H. By tackling these challenges, we have recently attained several record-breaking efficiencies in single-junction and multijunction thin-film silicon solar cells. To improve the light-soaking metastability of a-Si:H, we have reduced the incorporation of higher-order silane radicals during a-Si:H deposition by using a triode-type remote plasma-enhanced chemical vapor deposition (PECVD). Although the deposition rate is relatively low (0.01-0.03 nm/s) compared to the conventional diode PECVD process (~0.2 nm/s), the light-induced efficiency degradation (Δη/ηini) of single-junction solar cells is substantially reduced. As a result, we have attained independently-confirmed stabilized efficiencies of 10.2% (Δη/ηini~10%) for a-Si:H single-junction and 12.7% (Δη/ηini~3%) for a-Si:H/μc-Si:H double-junction solar cells. For the development of high-efficiency μc-Si:H solar cell, a novel light trapping technique using honeycomb-shaped periodic patterns has been developed and applied to μc-Si:H n-i-p solar cells. The surface morphology of the textured substrate was systematically designed to provide effective light trapping for the infrared light while preserving the high-quality μc-Si:H growth without giving rise to texture-induced microcracks. Through optimization not only of the texture design but also of the absorber deposition, front TCO and p-i interface layers, we have achieved an independently-confirmed efficiency of 11.8% for a μc-Si:H single-junction solar cell. Using this light trapping technique, a stabilized efficiency of 13.7% (not confirmed) has been demonstrated for an a-Si:H/μc-Si:H/μc-Si:H triple-junction solar cell.

Resume : To minimize production costs of flexible solar cells and modules, cheap polymers, such as transparent polyethylene terephthalate (PET), are an attractive substrate material. Besides the advantage of very low cost, using PET film as substrate in a solar cell limits the process temperature range to T<140 °C, reducing the electronic quality of silicon films. We investigate the effect of long term post-deposition annealing on the performance of a Si:H solar cells and single layers, that are fabricated at T<140 °C on PET and glass substrates. The annealing was carried out at T=120 °C in air for periods between 30 and 120 min. Electrical properties of individual a-Si:H layers show improvement upon annealing. Moreover, it was found that all JV parameters of the solar cell are significantly improved upon annealing, increasing the efficiency by up to 35 % after 120 min, mainly due to an increase in fill factor by around 20 %. Additionally, p-, i- and n-layers in the solar cell were consecutively substituted by high temperature layers with superior electrical properties and weakly sensitive to annealing at T=120 °C. We show that the origin of improvement upon annealing is mainly related to changes in the low temperature i- and n-type layers, while the effects of the p-layer annealing are minor. The annealing procedure, along with an advanced light management scheme of nanoimprint texture, results in an efficiency of 6.9 % for a flexible a-Si:H solar cell on a transparent PET substrate.

Resume : In this study, Iron-doped tin dioxide thin films were deposited on glass substrate at 350 °C by spray pyrolysis technique. Structural, optical and electrical properties of the films were investigated as a function of dopant concentration, which was varied between 0 and 5 at % of Iron. X-ray diffraction analysis showed polycrystalline structure with clear characteristic peak of SnO2 cassiterite phases in all films. Atomic Force Microscopy (AFM) reveals that film roughness is not affected by iron doping. All films present a high transmittance in the visible range. From the electrical measurements, it was found that the resistivity increased with Fe2+-doping levels. Hall Effect measurements showed an n-type conductivity

Resume : This paper will discuss our research approaches for printable CZTS/CIGS thin film solar cell, which will include CZTS/CIGS nanoparticle syntheses using thermal injection method; forming high quality CZTS/CIGS layer by solution based method and annealing; Using MoNa electrode or Na ion solution process to further improve the crystalline of the CZTS/CIGS layer ; forming printable Mo back contact using Mo ink; and using solution method to form AZO nanorod sandwich layer to improve light absorption. We have observed significant improvement on the efficiency of CZTS/CIGS solar cell by optimizing MoNa back contact, using Na ion solution, by special ambient annealing process and using AZO nanorod sandwich layer as the window layer. Detail will be addressed at the presentation.

Resume : Copper Zinc Tin Sulfur (CZTS) is a p-type semiconductor with kesterite structure. It has a direct band gap of 1.5 eV and high absorption coefficient, making it an ideal absorption material for photovoltaic application. However, the secondary phase formation during film preparation has been one main problem limiting the CZTS device performance. The optimized composition ranges for high performance devices are Cu/(Zn+Sn) between 0.80 and 0.95, and Zn/Sn between 1.10 and 1.25. Most group try to understand the secondary phase formation and their potential removal to improve CZTS technology. However, few people report more details on the evolution of CZTS surface and CZTS/Mo interface during sulfide process under various temperatures. Therefore, to gain more information on material processing, it is necessary to carefully investigate the crystalline structure of CZTS surface and CZTS/Mo interface under various annealing temperatures. In this work, we reported CZTS films annealing in the range of 200 – 600 °C with a S-containing atmosphere. To investigate the CZTS/Mo interface, CZTS films were splitted from Mo coated substrates, which were named ‘CZTS side’ and ‘Mo side’ respectively. Our aim was to investigate the modifications CZTS surfaces and CZTS/Mo interfaces under various annealing temperatures. The morphologies, structures and chemical compositions were characterized by Scanning Electronic Microscopy (SEM), X-Ray Diffraction (XRD), Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The revolutions of film surface and interface under various temperatures and the mechanism of chemical reactions were discussed in this work.

Resume : Over a decade, much attention has been paid toward synthesis and characterization of one dimensional nanostructure mainly due to their unique magnetic, electronic and optical properties and their potential applications in different fields including magnetic, electronic and optical devices [1?2]. CdSe nanorods array were prepared using impregnation in porous anodic aluminum oxide. CdSe is one of the II?IV semiconductors and because of high photosensitivity it has been widely used in photoconductive devices [3]. Photothermal deflection technique (PTD) is used to determine thermal conductivity and diffusivity. It deduced from the photothermal deflection measurements by comparing experimental amplitude of the photothermal signal to the corresponding theoretical one. In this work we study the effect of time impregnation on thermal and structural properties. The nanostructure as well as the atomic composition characterization was carried out using X-ray diffraction (XRD), this investigations demonstrate that CdSe nanorods are a uniform cubic CdSe crystal. The morphology characterization studied by scanning electron microscopy (SEM), has been shown that the grain size of (nc-Si) growth with the time of impregnation.

Resume : We present a new fabrication route for a carbon-free and dense CuInSe2(CIS) thin films using a nanoparticle-based ink for solar cell applications. CuxSy and In2Se3 binary nanoparticles, approximately 10 nm in size, were synthesized by a low temperature colloidal process. The precursor film was deposited using the coating ink formulated with those binary nanoparticles and pyridine as a solvent, and then annealed in the rapid thermal annealing (RTA) chamber at 540oC for 15mins under selenium (Se) atmosphere. Scanning electron micrographs, X-ray diffraction patterns and Raman spectra showed that a phase-pure, carbon-free and dense CIS thin film was prepared in this method. A solar cell device fabricated using this CIS thin film showed the following photovoltaic characteristics: VOC = 350 mV, JSC = 32.21 mAcm-2, FF = 49.50 % and Eff. = 5.68 % under standard AM 1.5 condition.

Resume : Tin mono-sulfide(SnS) thin film has been investigated as a photovoltaic absorber material due to not only a high optical absorption coefficient but also non-toxicity and earth abundance. However, the highest photovoltaic conversion efficiency is as low as 4.4%. One of the reasons for the low efficiency is considered to be the poor quality of the SnS layer, such as the formation of secondary phases and a high grain boundary density. Thus, many research groups have dedicated their own effort to obtaining a high quality of SnS thin film. Nano-particle based approach can be one of the suitable fabrication methods of SnS thin films because the reaction path and process variable control to modify the crystalline structure are relatively easy. Moreover, SnS thin film can be prepared by a simple coating process using a nano-particle based ink, and the subsequent thermal annealing can promote the grain growth in the film and thereby reduce the grain boundary which acts as a recombination center. In this study, we focused on the effect of the process condition on the properties of the SnS nano-particles and thin films. We have found that the crystalline structure of nano-particle is highly dependent on the precursor composition ratio and the reaction temperature. The partial pressure of hydrogen sulfide(H2S) gas during the thermal annealing affects the grain growth in the SnS thin film. Finally, we demonstrate a SnS thin film solar cell and characterize its photovoltaic performance.

Resume : We deposited an microcrystalline Si:H films at 100 Torr by using high working pressure plasma-enhanced chemical vapor deposition (HWP-PECVD) system with a cylindrical rotary electrode; this system is superior to conventional PECVD because it has the following features: a high deposition rate as a result of the high partial pressure of the reactive gas and a high plasma density by the very high frequency of 100 MHz; the ability to control the film uniformity because of the homogeneous distribution of reactants by the rotary electrode system; and low bombardment damage because of the lower kinetic energy. In order to decrease water vapor inside a chamber, which is one of the origin of oxygen impurity, is a major factor to cause deterioration of Si film quality, we developed the gas circulation system. We have utilized the reactive nature of H2O with SiH4. After the gas circulation for 24 hours, the oxygen content at the intrinsic Si:H film was decreased to 10^18 atom/cm3 from 10^19 atom/cm3. Recently, the cell efficiency of μc-Si:H single cells was progressed to be about 7% by using microcrystalline p-type silicon.

Resume : We report analysis of the photo induced minority carrier effective lifetime (Teff) in pn junction formed on silicon substrates with different bias voltages. The p+n junction was formed on the top surfaces of n-type silicon substrates by ion implantation of boron and phosphorus atoms at top and bottom surfaces followed by activation by microwave heating. Then loop shape Al electrodes with an open middle region were formed at top boron-doped p+ and bottom-phosphorus doped n+ surfaces. The electrical current showed typical rectified diode characteristics when bias voltages were applied to the electrode on the p+ surface. The internal potential barrier was well formed. Although Teff was lower than 1x10-5 s in the reverse bias condition, it markedly increased to 1.5x10-4 s as the forward bias voltage increased to 0.7 V and then it leveled off above 0.7 V when continuous wave 635 nm light was illuminated at the middle open region on the p+ surface. The small optical penetration depth of 2.7 micrometers at 635 nm light limits carrier generation inside the depletion layer. Since most of photo-induced minority carriers flow into the p+ surface electrode region according to the potential slope and Teff becomes low in the reverse bias condition. The forward bias voltage decreases the potential barrier height, which allows minority carriers to diffuse over the whole substrate and Teff increases. Teff is finally limited by recombination defect states at the top and rear surfaces.

Resume : GaInP is a technologically important energy material for fabricating high-efficiency multi-junction solar cells. Meanwhile, it is also an important luminescent material for fabricating red color light emitting devices. Therefore, behaviors of carriers in GaInP are of great importance to performance of solar cells and other device applications. Among various behaviors of carriers in GaInP alloy, localization and radiative recombination are highly interested issues. In this work, we report an interesting observation: remarkable transition of radiative recombination mechanism of carriers in GaInP layer from luminescence of deeply localized states to light emission of shallowly localized states in a GaInP/GaAs solar cell by temperature dependent electroluminescence (EL) measurement. It is found that temperature and injection current have significant influence on this transition process. Either higher temperature or stronger injection current clearly boosts the luminescence mechanism transition. To interpret this mechanism transition, the localized states ensemble (LSE) luminescence model was used. And effective thermal activation barrier was developed to describe the effects of temperature and injection current. Our theoretical analysis agrees well with experimental data and provides a precise picture for anomalous temperature dependent luminescence behaviors in similar systems. This study enhances the existing understanding of carrier localization, thermal activation and luminescence mechanism transition in compounds.

Resume : Current CIGS-based solar cells provide power-conversion efficiencies of more than 15% even though dislocation densities of 10^10 to 10^11 cm^−2 are typically found, experimentally, which is suggesting that dislocations are electrically not active or passivated by point defects. In order to understand the role of dislocations in CIGS-absorbers, we study the nature of perfect screw and 60° dislocations in CuInSe2 and CuGaSe2 by means of first-principles calculations within density functional theory. We present results on structural and electronic properties and compare formation energies of different dislocation types. All dislocations under study exhibit localized-shallow defect states, while two deep defect states are found for the glide 60° dislocation in CuInSe2. The calculated core formation energies show that for p-type conditions screw dislocations always prefer a neutral state. In the case of the 60° dislocations, the largest and most relevant portion of the p-type conditions corresponds also to neutral states. This results give an initial step towards understanding, why high densities of dislocations appear not to be detrimental for the efficiency of such devices.

Resume : Recently, copper antimony sulfide (CuSbS2) is highlighted as a new photovoltaic absorber because of its low toxicity, sustainability, and scalability comparing to CdTe and Cu(In,Ga)Se2 (CIGS) chalcopyrite. Also, CuSbS2 has a direct optical band gap in the range 1.38-1.5eV, which is close to the optimum value required for solar energy conversion (1.4eV), and a high optical absorption co-efficient more than 104 cm-1. Until now, we have tried hybrid ink process as a new beneficial concept to form CIGS thin films. In the hybrid ink, a use of chelating agent is the main concept due to the roles as a binder as well as holding ions. In this study, CuSbS2 photovoltaic cells were fabricated with using our own hybrid ink. In the hybrid ink, two different nanoparticles such as Cu-S and Sb-S were tried separately and monoethanolamine (MEA) was used as a chelating agent to bind the Cu or Sb precursors. The characteristics of CuSbS2 absorber and photovoltaic performances of fabricated cells with each hybrid ink will be presented.

Resume : In recent years, thin film silicon solar cells on glass have proven to constitute a promising solar cell type. The Liquid Phase Crystallization (LPC) technique enabled material qualities equalizing multi-crystalline silicon wafers [1]. However, decreasing the absorber thickness further requires light trapping. Here, we successfully integrated different sub-micrometer periodic light trapping structures in LPC silicon thin-film solar cells on up to 5x10 cm2 large glass substrates by nanoimprint lithography. By structuring the glass substrate single- as well as double-sided textured silicon absorbers were fabricated. Our patterns based on silicon alcoxides proofed to be high temperature stable and compatible with the solar cell production process. Therefore, these structured substrates can either directly be integrated into the solar cell device or modularly attached to the sun-facing air-glass-interface, there providing anti-reflective properties. For 10 µm thick double-sided textured silicon absorbers a significantly increased incoupling of light has been achieved simultaneously allowing for a high electronic material quality with open circuit voltages above 600 mV. [1] Haschke et al., SOLMAT 128, 190 (2014)

Resume : Kesterite Cu2ZnSnS4 (CZTS) is a very promising absorber material for low cost and high efficiency thin film photovoltaic cells due to its direct and adequate band gap and to its high absorption coefficient. In this work, CZTS layers were deposited using a single step electrodeposition process onto Mo-coated glass substrates. Sulfurization treatment was performed under Argon atmosphere at 500°C. The effect of the deposition time in the range of 10 min-40 min was investigated. X-ray diffraction (XRD) and Raman spectroscopy have confirmed the Kesterite structure of all deposited films. The surface morphology of the samples was examined using scanning electron microscopy (SEM). The best chemical composition, determined by energy dispersive X-ray spectroscopy (EDS), was obtained for CZTS films electrodeposited during 30 min. Photoluminescence Spectroscopy (PL) analysis was also performed to characterize the CZTS films and all the curves showed a broad PL band centred around 1.52 eV, corresponding to the optical band gap of the CZTS.

Resume : Plasmonics is an emerging field that benefits from nanoscale properties of metals. Generally metals support surface plasmons that are collective oscillation of excited free electrons and are characterized by a resonant frequency. By affecting the geometry of the nanoparticle, the surface plasmon resonance can be tuned. As the resonances of noble metals are mostly in the visible or infrared region of the electromagnetic spectrum their application in solar cells is expected. The surface plasmon resonance is affected by the shape and size of nanoparticles and by the dielectric properties of surrounding medium. Three different mechanisms can be exploited for photovoltaic application, which are: the scattering from the metal particles, the near field enhancement and direct generation of charge carriers in the semiconductor substrates. Most reported photocurrent enhancement results for inorganic devices are explained by the first mechanism of scattering. We deposited Ag thin films on clean Si substrate and on Si substrate covered with SiO2 films of different thickness. It is shown that the formation of Ag nanoparticles critically depends on the substrate temperature during deposition. All deposited films were subsequently annealed in high vacuum. The morphology of resulting samples was explored using atomic force microscopy, small-angle X-ray scattering and scanning electron microscopy as well as their size distribution and uniformity. Reflectance in UV-Vis range was used to study scattering from the nanoparticles before and after different treatments.

Resume : Cu(InxGa1-x)Se2 (CIGSe) is used as absorber material in highly efficient thin film photovoltaics. Micro-concentrator solar cells are promising for efficiency enhancement and have been investigated by a top-down approach before. We started to develop local deposition processes to build CIGSe micro-absorbers under material and cost reduction, especially to save Indium and Gallium. We investigate two different technologies to fabricate CIGSe micro-absorbers. In the first approach we evaporate indium and/or gallium onto a high temperature substrate to form InxGa1-x islands. Afterwards we deposit a flat Copper film. With a selenisation process CuInSe2- absorber islands are built up. In a second approach an innovative laser transfer technology is used to transport locally Indium or copper onto a molybdenum substrate. Investigations of transferring layer systems will follow.

Resume : Improving the efficiency of solar cells requires the introduction of novel device concepts. Recent developments have shown that in Si solar cell technology there is still room for tremendous improvement. Using the heterojunction with intrinsic thin layer (HIT) approach 25.6 % power conversion efficiency was achieved. However, a-Si as a window and passivation layer comes with disadvantages as a-Si shows low conductivity and high parasitic absorption. Therefore, it is likely that using a crystalline material as window layer with high band gab and high mobility can further improve efficiency. We have studied GaP grown by MOCVD on Si with (001) and (112) orientation. We obtained crystalline layers with carrier mobility around 100 cm2/Vs and which passivate Si as confirmed by carrier lifetime measurements. We performed band alignment studies by X-ray photoelectron spectroscopy yielding a valence band offset of 0.3 eV. Comparing this value with the Schottky-model leads to an interface dipole of 0.59 eV. The open circuit voltage increases with increasing doping and is consistent with the theoretical open circuit voltage deduced from work function difference and interface dipole. We obtain an open circuit voltage of 0.38 V for n-doped GaP with doping levels in the order of 10^17 1/cm^3. In our next steps we will increase the doping level further in order to gain higher open circuit voltage. We will discuss the implications of these findings for GaP/Si heterojunction solar cells.

Resume : Recently, down shift which converts high energy photons to low energy photons has been tried to improve solar cell efficiency further. Phosphors or quantum dots like CdSe are usually used as materials for down shift. One of key issues to adapt the down shift to solar cells is to increase conversion efficiency of phosphors or quantum dots because the conversion efficiency of the nano-sized phosphors or quantum dots is low due to high surface to volume ratio. In this study, we used CdSe quantum dots as material for down shift which was embedded in patterned PDMS. The emission wavelength of CdSe quantum dots was 530 nm. CdSe embedded PDMS was patterned by plasma etching of polystyrene nanosphere array. This patterned PDMS film including CdSe quantum dots was put on typical GaAs solar cells to be used as down shift. GaAs solar cells were not anti-reflected coated and showed efficiency of 18% with the short-circuit current (ISC) of 26mA/cm2. As expected, cell efficiency was increased up to the level comparable to that of AR-coated GaAs solar cell. We expect that higher cell efficiency will be obtained by optimization of down shift film and solar cells.

Resume : Crystalline silicon (c-Si) has many advantages such as naturally abundance, stableness, non-toxicity and strong UV absorption. Therefore, c-Si has been used for several optical devices, especially for photovoltaic devices. Although the c-Si photovoltaic devices achieve high solar energy conversion efficiencies, it requires a high purity of silicon as well as complicated fabrication processes. Also, the p-n junction of c-Si is normally formed by ion implantation or dopant diffusion processes, both of which are expensive and require very high temperature processing. Recently, some research groups have reported a graphene or graphene oxide (GO)/c-Si heterojunction solar cell which shows a schottky diode like characteristic. The graphene/c-Si or GO/c-Si solar cells have advantages of simple process, low cost and high efficiency, as graphene or GO can be used as conductive and transparent electrodes for solar cells. In our study, GO sheets with two different GO sizes (i.e., 0.5 and 3 μm in diameter) are formed on c-Si (100) substrate and thermally reduced to be GO/c-Si hybrid junction solar cells. The reduced GO (rGO) sheets are used for a hole-extraction layer as well as a transparent conductive layer. The solar energy conversion efficiencies of the rGO/Si solar cells with 0.5 and 3 μm GO size are 2.1 and 3.5%, respectively, under illumination with AM 1.5G 100 mW/cm2 simulated solar light. The rGO/Si solar cell with the large size of rGO sheet shows the improved JSC and VOC because the large sizes (3 μm) of rGO sheet make an enhanced sheet conductance and enlarged schottky barrier height.

Resume : The single crystals of the A^IIIB^VI layered semiconductors are composed from A-B-B-A packages. The bonds between packages are weak, of van der Waals type. In the space between packages atoms and molecules can be intercalated. The crystalline structure and surface morphology of the compound obtained by Cd intercalation of InSe single crystals were studied by XRD spectroscopy and SEM, and AFM, microscopy, respectively. As a treatment result, a composite formed by CdSe crystallites and InSe lamella is obtained. The dimensions of CdSe crystallites and InSe lamella depends of treatment conditions (temperature, duration) and represents tens of nanometers to units of micrometers. As CdSe crystallites formation centers serve the defects present on the InSe planar packages, which density is ~10¹⁰ cm ^-2. These defects also serve as formation centers of self-oxides obtained as a result of single crystals thermal treatment in ambient conditions. The intrinsic absorption band edge of InSe crystals is localized at ~1.25 eV (T=293K). In the obtained InSe-CdSe composite, at the same time with InSe characteristic threshold, a weak contoured threshold localized in the CdSe absorption band edge region is present. The photoluminescence specter (T=80K) contains both CdSe and InSe characteristic emission bands. The dynamic transformation of optical spectra (processes related to crystallites dimensions) depending on treatment conditions, is analyzed.

Resume : In the past years, group-IV nanostructures received great attention as new material for efficient optoelectronics devices, photodetectors and solar cells. In particular, Ge quantum dots (QDs) gained a renewed scientific interest over Si QDs due to the lower synthesis temperature, higher absorption coefficient and larger exciton Bohr radius [1]. The optical behavior and the band-gap tuning of Ge QDs do not simply depend on the size, as QCE predicts, but also on QD-QD distance and ordering. To observe the features of QCE by controlling of the QD diameter, multilayers of Ge QDs embedded in SiO2, separated by an SiO2 barrier layers (20 nm thick) were synthesized by plasma enhanced chemical vapor deposition and annealed up to 800°C. We present a detailed study on the structural and optical properties of multilayer samples as function of the annealing temperature. Using UV-Vis-NIR spectrometry and Tauc analysis of the absorption spectra, the optical bandgap and the absorption efficiency of Ge QDs were extracted. We observe an important variation of the optical band gap (from 1.8 eV to 2.5 eV) as a result of the annealing processes. Moreover, the Ge QDs in the multilayered structures, both in the as deposited and annealed samples, show a light absorption efficiency much higher than in bulk Ge. [1] S. Cosentino et al. JAP 115, 043103 (2014)

Resume : Gallium and indium selenide are layered semiconductors with band gaps of about 2 and 1.2 eV at room temperature, respectively. They are perspective materials for photoconducting and photovoltaic devices fabrication in visible and near infrared regions. Understanding of excess charge carriers dynamics mainly controlled by traps in forbidden gap are important for development of effective photoconducting and photovoltaic devices on their bases. The intensity modulated photoconductivity spectra (IMPS) of GaSe and InSe single crystals have been studied depending on modulation frequency and photon energy at different temperatures from liquid nitrogen to room temperature. IMPS are presented in form of real (in-phase) and imaginary (quadrature) components of modulated photocurrent. The IMPS of gallium and indium selenide crystals show significant deviation from Debye law and obey universal fractional power dispersion law in frequency domain as well as their low-frequency dielectric spectra. The correlation in behavior of IMPS and low-frequency dielectric responses with temperature change of both GaSe and InSe single crystals has been revealed. According to proposed model dispersive dielectric media screens the trapped minority excess charge carriers, effectiveness of which depends on duration of charge occupation by trap, since trapping time corresponds to frequency region with dielectric dispersion.

Resume : Cu2ZnSnS4 (CZTS) has been attracting attention due to its special characters, such as large absorption coefficient, optimum band gap and abundant elements, which indicate its potential application in low-cost solar cells [1-3]. In recent reports, CZTS with kesterite or wurtzite phase had been successfully synthesized and the formation mechanism was also studied. However, few researches focus on CZTS quantum dot sensitized solar cell (QDSSCs) due to the poor quality of quantum dots [4, 5]. Therefore, it is very important to synthesize high quality CZTS quantum dots with good dispersion and tunable size. In the present work, wurtzite CZTS quantum dots are synthesized by one-step solvothermal method and the sizes of CZTS quantum dots are tuned by controlling the reaction time. Their optical performance is studied by UV-Vis and photoluminescence spectra. The as-synthesized CZTS quantum dots are deposited on TiO2 nanocrystals by self-assembly directly and then treated with (NH4)2S solution to remove the organic ligands. The Pt/FTO is employed as the counter electrode to fabricate the QDSSCs. The photovoltaic conversion efficiency is also investigated under AM 1.5 illumination. In conclusion, wurtzite CZTS quantum dots with high quality were synthesized and the sizes could be tuned by reaction time. Moreover, we successfully fabricated CZTS quantum dot sensitized solar cells for the first time. References: [1] Lu, X. T.; Zhuang, Z. B.; Peng, Q.; et al. Chem. Commun. 2011, 47, 3141. [2] Christopher, A. C.; Cheng, C.; Simon, M. F.; et al. Chem. Commun. 2013, 49, 3745. [3] Wang, J.; Xin, X. K.; Lin, Z. Q. Nanoscale. 2011, 3, 3040. [4] Zou, Y.; Su, X.; Jiang, J. J. Am. Chem. Soc. 2013, 135, 18377. [5] Luo, Q.; Zeng, Y. Q.; Chen, L. W.; et al. Chem-Asian J. 2014, 8, 2309.

Resume : Cu2ZnSn(S1-xSex)4 (CZTSSe) compound solar cells have been actively studied using both vacuum and non-vacuum processes as absorber fabrication methods. Considerably remarkable results based on non-vacuum processes have been reported [1-3], as the world-best record of 12.6% conversion efficiency of CZTSSe cell was produced using the absorber layer prepared via a non-vacuum process based on hydrazine precursor, while CuIn1−yGaySe2 (CIGS) thin film solar cells produced a record efficiency of 21.7% based on co-evaporation process [1,4]. As hydrazine is a toxic and unstable compound that requires extreme caution for handling, some groups developed a relatively safe non-vacuum processes [2,3]. However, they include a complicated step for nanoparticle synthesis or an iterative process of spin-coating and drying steps to get the desirable thickness. Cu2ZnSnS4 (CZTS) thin films were spin-coated on Mo-coated glass substrates using a simple sol-gel method based on methoxyethanol solution with metal salts and thiourea, followed by post-sulfurization. The iterative spin-coating was not required, but only a coating was applied to get > 1 μm-thick film after post-annealing. CZTS films were prepared and characterized with variation in rapid thermal annealing conditions. Performance of the solar cells fabricated with the absorbers is also investigated in this study. [References] 1. W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu and D.B. Mitzi, Adv. Energy. Mater. 4(7) (2014) 1301465. 2. Q. Guo, G.M. Ford, W.-C. Yang, B.C. Walker, E.A. Stach, H.W. Hillhouse and R. Agrawai, J. Am. Chem. Soc. 132 (2010) 17384. 3. J.V. Caspar et al., “Solution Chemical Routes to High-Efficiency Cu2ZnSn(S,Se)4 Thin-Film Solar Cells”, DuPont Central Research and Engineering, E-MRS 2012-Spring Meeting (13-18 May 2012, Strasbourg, France) 4. M. Powalla et al., “CIGS Thin-Film Solar Cells with an Improved Efficiency of 20.8%”, ZSW, EUPVSEC 2014 (22-26 Sep 2014, Amsterdam, Netherlands)

Resume : A new concept of full silicon a-Si:H/c-Si tandem solar cells is explored in this work. The conventional way to stack a-Si:H and c Si subcells face a problem of current match due to low a-Si:H subcell short circuit current. Here we propose to deposit the top a-Si:H p-i-n subcell on the surface of c-Si substrate with periodic columnar (wire) structures. This approach allows one to enhance the effective area of a-Si:H p-i-n subcell and therefore to increase the short circuit current. The theoretical calculations performed by 2D computer modeling demonstrate an increase of top a-Si:H cell short circuit current by factor of 1.5 in case of wired substrate surface. The a-Si:H/c-Si heterojunctions are used to form the bottom subcell. The details of the proposed solar cell design as well as simulations results, which indicates influence of Si:H p-i-n structure and c-Si wires parameters (distance between wires and their length and diameter) to performance will be described in the paper. A technology of periodic wire structure formation on the c-Si substrate surface is a key issue for photovoltaic application. This technology should be able to provide a large scale production therefore no lithography may be used. This technology was developed based on results obtained by wet chemical and dry plasma chemical etching using micron size polystyrene spheres as a mask. The experimental data of the structural, optical and electrical properties of the developed structures will be present.

Resume : A new technological approach to the fabrication of silicon based heterostructures is explored in this paper. A thin n-type doped epitaxial GaP(N) layer is proposed to growth on the surface of p-type Si substrate using low-temperature methods of plasma enhanced deposition. Thus n-GaP(N) wide gap emitter forms anisotype heterojunction with p-Si. GaP(N) layer being lattice matched to Si and grown at low temperature, should provide low defect density and sharp interface with Si substrate. In addition, a layer of GaP (N) may be used as the nucleation layer for subsequent epitaxy of GaP(NAs) layers lattice matched to Si, in order to form high efficiency multijunction Si solar cells. The GaP layers were grown on Si substrates by time modulated PECVD at 350°C using PH3 and TMG as sources of III and V atoms. The properties of GaP layers and first GaP/Si photovoltaic structures fabricated by PECVD will be discussed in the paper.

Resume : Zinc oxide (ZnO) is a direct wide band gap semiconductor that has been extensively investigated due to its important applications in several fields, including optoelectronics, biomedicine or photovoltaics. In fact, it was one of the first materials to be applied in dye-sensitized solar cells (DSSCs). Besides its low cost and non-toxicity, ZnO is supposed to be a potential substitute for titanium dioxide (TiO2) owning to their similar band gaps and electron affinities. Moreover, this semiconductor exhibits some advantages like the high electron mobility and the ability to easily produce a great variety of nanostructured morphologies, allowing the designing of the surface area which is a key factor to maximize the dye absorption. Even though the energy conversion efficiency of ZnO based DSSCs remains lower than the values found for TiO2, increasing research has been devoted to this material in order to overcome this limitation. In the present work we report the use of laser assisted flow deposition technique to grow ZnO nanostructures (nanoparticles and tetrapods) with high optical and crystalline quality. The structural and morphological characterizations of the grown ZnO were performed by XRD, TEM and SEM analysis and the optical quality was accessed by photoluminescence spectroscopy. DSSCs were produced using a combination of nanostructures, which were subsequently sensitized with the Rose Bengal dye. Different parameters in the cell construction were analysed and discussed.

Resume : Thin film silicon solar cells on low cost foreign and flexible substrates could be attractive for low cost production of photovoltaic electricity. This work aims at the synthesis of high-quality continuous polycrystalline silicon (pc-Si) layers on cheap Aluminium substrates using the aluminium induced crystallization (AIC) process of amorphous silicon. To avoid uncontrolled exchange of Al/Si, an Al-doped ZnO diffusion barrier layer against impurities but still conducting has been used. For the AIC process, a 200 nm thick Al-layer acting as a source of Al, was first electron beam evaporated over the substrates. Amorphous silicon films with thicknesses ranging from 200 nm were deposited by ECR-PECVD on the substrates. The annealing temperature and time were the key parameters of the Al/Si exchange process. Here, the direct crystallization using a conventional furnace was carried out at T=480?C and for a duration of 10h. After the removal of the aggregated impurities (Al and a-Si) and defects (grain boundaries) on the surface by selective etching. The resulting crystallized layers were characterized by Raman spectroscopy. The as-grown AIC polysilicon films were found to be continuous and densely packed without amorphous phase. The AIC process was followed by (1) 10 μm thick a-Si deposition by ECR-PECVD and (2) by n-type a-Si deposition using PVD. These layers were crystallized using solid phase epitaxy. Properties of the crystallized layers for different thermal budgets (1h, 2

Resume : The realization of thin films of polycrystalline silicon on foreign substrates is an attractive alternative to the ingot casting which allows a reduction in costs. The purpose of this work is to form polycrystalline silicon films from the crystallization of amorphous silicon deposited on aluminum substrate. This kind of substrate will then be used as conductive support but also as a reflector on the rear face of the photovoltaic cell. However, because of their miscibility, silicon and aluminum can strongly react during heating leading to an Al-Si alloy rather than to separated materials. In this work, 2 to 5 microns thick amorphous silicon films were deposited by ECR-PECVD on various type of aluminium substrate. Such substrates were non, partially or totally anodized meaning that an alumina layer is formed on top surface. Crystallization of the amorphous silicon was carried out using a temperature of 550 ? C and varied times from 10 to 80 min. The as-crystallized Silicon films are then characterized by Raman spectroscopy, scanning electron microscopy and by electron backscatter diffraction. The results show the feasibility of amorphous silicon crystallization using both a low temperature of 550 ? C and a short duration of 80 min. The analysis shows the formation of two separate layers during the process: a silicon / aluminum alloy as the upper layer and a polysilicon film at the bottom and in contact with the aluminum substrate. We also found that the crystallization process is strongly dependent on the degree of anodization of the Al substrate. Silicon layers with few micrometers grains are thus obtained and can potentially be used as nucleation sites for the fabrication of the absorbing silicon films for photovoltaic applications.

Resume : GaAsPN semiconductors are promising material for the elaboration of high efficiencies tandem solar cells on Si substrates, due to its perfect lattice matching and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. We report promising building blocks for the development of GaAsPN/Si-based dual-junction solar cells. Early stage GaP/GaAsPN/GaP PIN solar cells have been elaborated by MBE on a GaP(001) substrate, prior to the elaboration on a GaP/Si(001) pseudo-substrate. The quantum efficiency (IQE around 40%) shows that carriers have been extracted from a 1 ?m-thick GaAsPN alloy absorber. I-V measurements performed on this sample shows a remarkable open-circuit voltage record at 1.18V. Our best cell was obtained using a 300nm-thick absorber with 2.25% efficiency under AM1.5G. This cell exhibits a remarkable FF of 71%, and Jsc of 3.77 mA/cm?. Assuming that a 1 ?m thick GaAsPN layer is necessary to absorb the main part of the solar spectrum and considering the absence of any anti-reflective coating, this last results is promising. Modeling of the tunnel junction (TJ), either GaP/Si ,or Si/Si, has shown high theoretical current densities with doping levels experimentally attained in the GaP alloy, and considering a n-doped Si bottom absorber. The TJ and, hence, the overall tandem cell with a purposely designed bottom Si subcell, is currently under development, and early results will be presented. Support: ANR project MENHIRS 2011-PRGE-007-01.

Resume : In order to lower the cost of chalcogenide based solar cells, a novel and non-vacuum Electrostatic Spray Assisted Vapour Deposition (ESAVD) process has been exploited to deposit CIGS based absorber layers [1]. ESAVD is a non-vacuum chemical vapour deposition based process in which a mixture of chemical precursors is atomized to form aerosol. The aerosol is charged and directed towards a heated substrate where it would undergo decomposition and chemical reaction to deposit a stable solid film onto the substrate[2]. In this work, we explored the optimization of CIGS absorber using a three-stage deposition by ESAVD process. During the three-stage deposition, a Cu-rich layer is sandwiched between two Cu-poor layers to increase the grain size and to avoid the short circuit led by the existence of excess Cu2Se (which is conducting) in the absorber. After selenization, the absorber layer clearly exhibits the characteristic peaks of CIGSSe from XRD and Raman analysis. From XRF results, it can be seen the ratio of Cu/In in Cu poor layer is 0.68, the relatively higher ratio of Cu/In in the Cu rich layer is 0.85 and the final Cu/In ratio in the film after selenization is 0.78, which is desirable for photovoltaic device applications. From the cross-section SEM image of the three layer CIGS absorber, it can be seen that a Cu rich layer with grain size around 500-600nm is sandwiched between two thin layers of Cu poor CIGS with small grain size. The fabricated solar cell shows above 5% efficiency under 100mw/cm2 AM1.5 illumination. References: [1] M. Wang, X. Hou, J. Liu, K. Choy, P. Gibson, E. Salem, D. Koutsogeorgis, W.Cranton, Physica Status Solidi (a). 1-4 (2014). [2] K.L. Choy, Prog. Mater. Sci. 48 , 57-170(2003). Acknowledgements: This work has been funded by the European Union’s Seventh Framework Programme Scalenano, FP7/2007-2013 under grant agreement nº 284486.

Resume : The concepts of third generation solar cells depend critically on the dynamics of ultrafast carrier relaxation and electron-phonon interactions on very short times scales. The hot carrier solar cell as one of the third generation cells especially depend on the reduction in the energy relaxation rate in an absorber material. Here we investigated the ultrafast carrier dynamics in 1 µm bulk In26.5GaN thin film grown by a new thin-film growth technique called energetic neutral atom-beam lithography/epitaxy (ENABLE) which was done by our collaborators in the Los Alamos national laboratory(LANL), US. The spectroscopies including steady state-photoluminescence (PL), time-resolved PL and transient absorption in the time scale of picoseconds have been measured and analysed. It indicates the relaxation time of our sample is about 22ps through the Maxwell-Boltzmann approximation. This indicates mechanisms, and that variation of Indium content can mediate these. Moreover, the indium fluctuation introduced extrinsic energy state in forbidden energy was observed through tr-PL.

Resume : Intermediate band solar cells (IBSCs) pursue the enhancement of solar cell efficiency by making use of below bandgap energy photons. In this work we review the recent international progress made in the field through: a) the identification of intermediate band material candidates; b) the implementation of intermediate band solar cells based on quantum dots; c) the exploitation of the “band anti-crossing” mechanism and d) the insertion of impurities that create deep centers into semiconductors. Once this background is covered, we also provide our best understanding about future research lines in this field.

Resume : We show a significant advancement in 3C-SiC crystal growth and that high quality 3C-SiC with boron implantation has optically active deep boron centers which generate extra electron-hole pairs. For the first time photovoltaic 3C-SiC is demonstrated. Boron doped 3C-SiC introduces an energy level of 0.7 eV above the valence band. This level forms an intermediate band (IB) in the 3C-SiC bandgap (2.3 eV) at a high boron concentration. The IB allows absorption of photons with energy smaller than the bandgap. Electrons are excited first from the valence band to the IB, and then from the IB to the conduction band. This generates additional electron-hole-pairs to the ones given by absorption of photons generating electrons from the valence band to the conduction band. In total, the efficiency of a solar cell can increase substantially. High quality 3C-SiC was realized by a new growth approach that combines initial nucleation and step-flow growth. Initial domains extend laterally to larger domains and a pronounced quality improvement. We present a comprehensive study of optical properties of as-grown and boron implanted 3C-SiC, and SEM, TEM and absorption measurements. High resolution PL evidences the high quality material and clearly indicates formation of optically active deep boron centers yielding the IB behavior. We discuss boron doped 3C-SiC as potential base material for an avenue of energy application in photovoltaics, biomarkers and hydrogen generation by splitting water.

Resume : Radial junction solar cells based on silicon nanowires offer a self-organized, optically thick and geometrically thin structure with low fabrication cost and fast growth rate [1]. During our previous investigations of electronic properties of Si nanowire-based RJ solar cells grown by plasma-assisted vapour-liquid-solid (VLS) process we have reported on an inhomogeneous distribution of conductivities observed for individual neighbouring RJs by atomic force microscopy (AFM) [2]. Correlation between several AFM techniques and scanning electron microscopy of identical nanowires was sought in order to identify the reason for conductivity variations. We characterized samples constituted by 1 µm long P-I-N radial junctions with 500 nm in diameter. They form a disordered arrays of nanorods pointing at different directions with variations in length and shape. Using sets of microscopic indents as reference points for the local coordination system, we managed to correlate the results from scanning electron microscopy, Kelvin probe force microscopy (KPFM) and PeakForce conductive AFM technique on the same place of the sample. KPFM measurement showed an average surface potential shift of about 40 mV when radial junctions were illuminated with an orange (592 nm) light-emitting diode. Measurements also revealed a correlation between the magnitude of the surface potential shift in radial junctions under the illumination and their length. The surface potential signal exhibits inhomogeneity in values for individual radial junctions similar to that observed in the measured conductivity. However, these variations do not directly correspond to those in conductivity and their relation remains a subject of our further research. Indentation can be also used in order to observe the same place in different stages of a sample preparation process. We employed conductive AFM to analyse the VLS growth of silicon nanowire-based radial junctions step by step. This way we found that conductivity variations - comparable to those observed on complete P-I-N radial junctions - originate even during the formation of the nominally undoped silicon nanowires. [1] S. Misra et al., Journal of Physics D: Applied Physics. 47 (2014), 39301. [2] A. Fejfar et al., Sol. Energy Mat. Sol. Cells. (2014) in press

Resume : We propose a novel approach for light trapping in solar cells. It is based on absorption through multiple resonant modes in the critical coupling regime (perfect absorption at the resonance wavelengths). We have developed a theoretical model that provides an analytical formula for the absorption maximum. We demonstrate that this multi-resonant light trapping strategy exceeds the lambertian limit. We present numerical and experimental examples of multi-resonant absorption in ultra-thin (25nm-45nm) semiconductor layers. We also applied this novel light-trapping strategy to the conception of ultrathin GaAs solar cells with nanostructured back mirror. We will present the fabrication process by nano-imprint lithography, and our latest experimental results with record Jsc.

Resume : A conversion efficiency of 18.3% on a 30x30cm2-sized Cu(In,Ga)(Se,S)2 (CIS-based) submodule with a Zn-based buffer layer was achieved. This is a final goal of Japanese R&D program for renewable energy consigned by new energy and industrial technology development organization from 2010 to 2014. The absolute efficiency of our CIS-based submodule has been boosted about 2.5% since 2010. Some of these technologies have already been installed in our GW-scale manufacturing. Now the 4th plant with a nominal production capacity of 150MW is in ramp-up for commercial production. The latest high efficiency technique and low cost mass-production technology was applied to this plant. In near future, it will serve as a blueprint for future manufacturing facilities with the highest cost-competitiveness in the world. In this paper, we will review our recent progresses on our R&D and manufacturing. As a leading company of the CIS-based solar module, we will keep exploring the potential of the CIS-based solar cell both on lab and commercial scales.

Resume : Inorganic thin film solar cells on flexible substrate are an interesting technology not only for large scale ener-gy production but also to enable applications where flexibility and especially lightweight are a prerequisite. The full potential, however, can only be tapped if the efficiencies of solar cells grown on flexible foils or films are comparable to that of devices grown on rigid substrates. In this contribution we present an overview on the recent developments in the field of highly efficient flexible chalcogenide-based Cu(In,Ga)Se2 (CIGS) and CdTe solar cells. Thin film solar cells based on CIGS absorber material have recently shown higher power conversion efficien-cies than polycrystalline silicon on a laboratory scale. Proper control of alkaline elements added to the CIGS absorber layers is a key aspect for the processing of high efficiency thin film solar cells. Sodium addition has long been known for its positive effect on electronic properties of the absorber layer, whereas the addition of other alkali metals was believed to be less beneficial. Recently, a KF post-deposition treatment (KF PDT) of the absorber layer grown at low temperature has been introduced, allowing the processing of flexible devices with 20.4% efficiency. State-of-the-art CdTe solar cells with efficiencies above 20% are developed in the so called superstrate con-figuration implying that a high optical transmittance of the substrate is mandatory. For flexible and lightweight solar cells this limits the choice of substrates to ultra-thin glass and clear polyimide. However, most flexible substrates with suitable mechanical properties and thermal stability are not meeting the optical requirements. Here we will discuss CdTe solar cells in substrate configuration which allow the use of opaque substrates such as metal foils and present the role of Cu doping on achieving flexible CdTe solar cells on metal foil with effi-ciency of 11.5%. In our presentation, we will especially focus on the role of group IA and IB elements on the absorber carrier concentration and the importance of interface engineering. Further potential for performance improvements will be discussed.

Resume : Solar cells based on chalcogenide (CIGS) thin films are emerging as a new option in photovoltaic technology which can combine the advantages of Si thin films, i.e. low production costs and flexible modules, with high conversion efficiencies (> 20 %). The most common approach in CIGS solar cells manufacturing is to use a CdS/intrinsic ZnO (i-ZnO) and Al-doped ZnO (Al:ZnO) window layer stack on top of the CIGS film. Replacement of CdS by a Cd-free layer with wider band gap (> 2.4 eV) would a) decrease the production cost by avoiding the expensive treatment of toxic wastes and b) increase the overall cell efficiency by enhancing the quantum efficiency in the blue range. Moreover, the use of a ?soft? and highly conformal deposition technique is preferred to improve the electrical properties of the buffer layer/CIGS interface. In this paper we present spatial atmospheric atomic layer deposition of a Zn(O,S) buffer layer as CdS replacement for CIGS solar cells. Spatial ALD is emerging as an industrially scalable deposition technique at atmospheric pressure which combines the advantages of temporal ALD, i.e. excellent control of film composition and uniformity on large area substrates, with high growth rates (up to nm/s). Films are grown by sequentially exposing the substrate to oxygen and sulfur precursors (H2O, H2S) and the zinc metal precursor (i.e., diethylzinc, DEZn). By controlling the kinetics of surface reactions between vaporized precursors and reactive sites at the film surface, the composition of Zn(S,O) can be precisely tuned, as measured by EDX analysis (Fig 1). The incorporation of S into ZnO results in a bowing of the band gap in the range from 3.3 eV (ZnO) to 2.7 (S/O S ~ 0.5) and 3.4 eV (ZnS), as measured by spectrophotometry. The morphology of the Zn(Ox-1,Sx) films varies from polycrystalline (for 0

Resume : GaTe single crystals are composed by layered packages of Te-Ga-Ga-Te type, linked by weak, polarization forces. This property allows obtaining of nanolamellar structures by intercalation of free atoms and molecules. The GaTe-ZnTe composite were obtained by thermal treatment of GaTe single crystalline plates in Zn vapors. As a result, GaTe nano-structuration occurs with formation of ZnTe crystallites, GaTe lamella and Ga which coagulates in drops with sub-micrometric dimensions. The composite crystalline structure was studied by X-ray diffraction (XRD) and surface morphology – by AFM and SEM microscopies. XRD diagrams and Raman spectra indicates the presence of ZnTe and GaTe crystallites with dimensions of tens of nanometers to micrometers, depending on obtaining technology. As ZnTe crystallization centers serves the surface defects of GaTe plates, which density is ~10¹⁰ cm^-2. Structural anisotropy defines specific conditions of ZnTe formation and grows on to GaTe lamella surface and interface. The diffuse reflection and photoluminescence spectra were studied from plate’s natural surface and perpendicularly on it, before and after intercalation. As a result of intercalation the GaTe excitonic absorption is attenuated and an absorption threshold is observed in the absorption edge band region of ZnTe crystals. Correlation between technological process and composite optical spectra structure were established.

Resume : Flexible silicon thin-film solar cells using light-weight, unbreakable and inexpensive substrates have many advantages, such as higher throughput using roll-to-roll processing, ease of handling and transportation and robust. These thin-film solar cells also enable the esthetic and innovative design of solar products with various shapes and sizes. In this presentation, we report on the enhanced performance of n-i-p hydrogenated nanocrystalline silicon (nc-Si:H) thin-film solar cells prepared on the flexible 50-nm-thick polyimide substrates by new approaches including nanotextured ZnO:Al/Ag/Al:Si multilayer back reflectors, wide bandgap p-nc-SiC:H window layers and n-nc-Si:H seed layer for intrinsic Si absorber layers. The effective light trapping in the solar cells were achieved by the nanotextured multilayer back reflectors deposited at low substrate temperature of 75°C, resulting in the enhancement in the photo-induced current. Using wide gap p-nc-SiC:H window layers containing Si nanocrystallites, the increase in the open circuit voltage and short-circuit current density was obtained in comparison to the solar cells with narrow bandgap p-layers. Also, the improved nucleation and growth of intrinsic nc-Si:H absorber layers was observed in the solar cells using highly crystalline n-type seed layers. High conversion efficiency of 7.25% was achieved in the flexible nc-Si:H thin-film solar cells by using these new approaches. The details of the experimental results will be given in this presentation.

Resume : Quantum dot sensitized solar cells (QDSSCs) have received much attention due to the narrow band gap, multiple photo-induced electrons generation and quantum size effect [1-3]. However, the current researches on quantum dots (QDs) such as CdS and CuInS2 contain either high-toxic or less-abundant element. Therefore, efforts have been focused on the investigating of earth-abundant alternative materials with non-toxic. As copper antimony sulfide (CAS) QDs is a p-type semiconductor with high absorption coefficient (over 105 cm-1) [4] and near-optimal bulk band gap energy (1-1.8 eV) [5], and it is believed to be a potential candidate [6]. Therefore, it is very important to investigate the CAS QDs sensitized solar cells. In the present work, the novel tetrahedrite (Cu12Sb4S13) QDs with various sizes were synthesized by a new and facil hot-injection method, which was self-assembled on TiO2 nanocrystal photoanodes, and then the inorganic ligand was exchanged to remove the organic ligand on the QDs surface. The influence of QDs sizes on the optical absorption intensity was studied by UV-Vis spectra. FeS/Fe nanowire arrays film was prepared by a general solution method, which were employed as counter electrodes to fabricate the QDSSCs. The photovoltaic properties for incident photon to current efficiency (IPCE) and photovoltaic conversion efficiency of the solar cells are also studied. In conclusion, a novel route is explored to synthesis the CAS QDs, and it is found that the QDs size may have a great effect on the optical absorption performance, which may also influence the the photovoltaic performance of solar cells. References: [1] Pan, Z. X.; Mora-Seró, I.; Shen, Q.; et al. J. Am. Chem. Soc. 2014, 136, 9203. [2] Pan, Z. X.; Zhao, K.; Wang, J.; et al. ACS Nano 2013, 7, 5215. [3] Li, T. L.; Lee, Y. L.; Teng, H. Energy Environ. Sci. 2012, 5, 5315. [4] Van Embden, J.; Latham, K.; Duffy, N. W.; et al. J. Am. Chem. Soc. 2013, 135, 11562. [5] Ramasamy, K.; Sims, H.; Butler, W. H.; et al. Chem. Mater. 2014, 26, 2891. [6] Yang, B.; Wang, L.; Han, J.; et al. Chem. Mater. 2014, 26, 3135.

Resume : Copper-indium-gallium-selenide (CIGS) is one of the anticipated thin film solar cells due to high absorption coefficient of 10^4~10^5 cm^(-1) and high conversion efficiency with tunable band gap of 1~1.7 eV. Thinning absorber layer is an effective approach to achieve cost reduction and flexibility, while the drawback of diminished light absorption, which will result in a trade-off between cost and conversion efficiency, should be taken into consideration. Plasmonic nanoparticles (PNs) embedded in CIGS absorber layer is one of the significant method to enhance the light absorption over the range of visible region. Considering that Au might have a chance to react with Cu, In and Ga into alloys during high-temperature selenization process. Here, we propose core-shelled PNs with 15 nm-in-diameter Au cores inside surrounded by oxide protection layer with controllable thickness from 4 to 10 nm. The protection oxide layer can sustain the high temperature process and avoid the alloy formation based on SEM images and XRD spectra. With the help of 15 nm-in-diameter Au nanoparticles, an enhanced absorption could be accomplished over the range of 500~600 nm, which also contributes to the highest intensity regime of solar spectrum, yielding 10 % improvement of photovoltaic efficiency.

Resume : Typical Cu(InGa)Se2 (CIGS) cell has a structure of SLG/Mo/CIGS/CdS/i-ZnO/ZnO:Al, and its current collection is primarily limited by several optical and collection losses of individual layers, i.e., shading from front grid, reflection from ZnO, absorption in ZnO and CdS layers, and incomplete generation and collection in CIGS absorber. In this contribution, Nd doped ZnO (ZnO:Nd) layer was explored to replace i-ZnO in the conventional CIGS cell structure for the potential use as a down-shifting or down-converting layer, and thus to increase light absorption and carrier collection. The Nd-doped ZnO films with various Nd contents were deposited by magnetron sputtering system starting from a Zn target covered with small pieces of Nd, and then characterized by several techniques including XRD, Raman, SEM, TEM, RBS, UV-VIS, PL etc. For examples, the PL measurements revealed that the main emission peaks of Nd were detected in the near infrared region. The substrate temperatures during sputtering varied from 25 deg.C to 400 deg.C to determine optimum temperature for effective emission of Nd and improvement of cell performance. For cell fabrication, CIGS light absorber was prepared by typical three stage co-evaporation and CdS buffer layer was added by standard chemical bath deposition. Device performance of CIGS cell was characterized by QE and I-V measurements.

Resume : Cu2ZnSn(SxSe1-x)4 (CZTSSe) compound semiconductor has been investigated as an indium-free alternative to the CIGS absorber layer in thin film solar cells, and many notable research results have been reported. The best solar cell device has shown maximum conversion efficiency of 12.6% based on CZTSSe prepared using the hydrazine-solution process by IBM et al. [1] Although co-evaporation technique seems one of the best methods to control the film composition, only limited studies have been done on thin film solar cells based on CZTSSe deposited by co-evaporation process [2-3]. Cu-Zn-Sn-Se thin films co-evaporated on substrates at a low temperature as absorber precursors were annealed via RTA selenization processes. The absorbers deposited by co-evaporation at high temperatures without any post-annealing were also prepared. As-synthesized Cu2ZnSnSe4 (CZTSe) thin films were characterized, and solar cells were also fabricated with both types of absorbers to investigate the photovoltaic performances. The best efficiencies obtained by evaporation-based two-step and pure co-evaporation processes were 7.72% and 6.64%, respectively. [References] 1. W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Adv. Energy. Mater. 4(7) (2014) 1301465. 2. I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W.-C. Hsu, A. Goodrich, R. Noufi, Sol. Energy Mater. Sol. Cells 101 (2012) 154-159. 3. B. Shin, O. Gunawan, Y. Zhu, N.A. Bojarczuk, S.J. Chey, S. Guha, Prog. Photovolt: Res. Appl. 21(1) (2013) 72-76.

Resume : In the typical direct solution coating approach, metal salts based molecular inks are directly deposited onto the substrate to form films. Mostly carbon containing organic additives are added into the ink for better dissolution and coatability. The prominent feature of the final film is the doubled-layered structure, with top CIS and bottom C-rich residual layer originated from the organic additives. Generally, residual C-layer is considered as the main efficiency limiting factor, i.e., a source of high series resistance. However, the exact influence of this layer on device performance is still not clear and contradictory views are present. Furthermore, not only as the residual layer, C must affect the formation mechanism and the properties of top CIS layer, but no such reports are available. Therefore, in this study, systematic investigation on the electrical properties of C-layer along with its role in the transformation mechanism from precursor to CIS film was done. We found that C-layer itself is electrically conductive and forms ohmic contact with both CIS and Mo films. On the other hand, C was found to play a significant role in the diffusion behavior of Cu, In and Na during selenization in which C hindered the diffusion of Cu while pumped Na towards the top CIS layer. Experiments concluded that carbon residue layer does not act as a resistive element but affects the diffusion behavior of other elements and hence affects the composition of the top photoactive CIS films.

Resume : Several structural and optical properties of ceria (band gap, refractive index and lattice parameter) make this material very promising for applications in optoelectronics and photovoltaics. In particular, CeO2 can be efficiently functionalized by doping with trivalent rare earth ions to give rise to photon management properties. In this work, we show that Sm and Nd ions can be successfully inserted in CeO2 thin films by using pulsed laser deposition starting from RE-doped pellets. We show that the films are characterized by interesting photon management properties even at a relatively low deposition temperature (400 °C), which is compatible with photovoltaics. By combining the information obtained from different structural (XRD, XAS, …) and optical (absorbance, PL, PLE, …) characterization techniques, we demonstrate the presence of the trivalent ions in CeO2 and provide insight in the electronic level structure and in the transfer mechanisms. In particular, we give evidence that the energy transfer mechanisms can be fully explained only by considering the presence of Ce3+ ions in CeO2. The strong absorption cross section of f-d transitions on Ce3+ ions and high mobility of oxygen vacancies in ceria, that allows the formation of Ce3+ close to RE3+ ions, strongly increase the potential of these layers as downshifters and downconverters.

Resume : The structural, optical, surface potential and photo-luminous (PL) characteristics of CdSe nanoparticles (NPs) deposited in ZnS thin films matrix have been investigated. AFM and HRTEM studies show that the size of CdSe nanoparticles (NPs) increases in the size range 4-15 nm on increasing the deposition time of CdSe NPs. X-ray diffraction (XRD) study shows a decrease in the intensity of XRD peaks corresponding to ZnS and emergence of peaks corresponding to CdSe hexagonal phase with increase in the size and volume fraction of CdSe nanoparticles. Decrease in XRD peak intensity may be due to partial screening of ZnS by CdSe and overlapping of peaks corresponding to both materials in same 2θ range. High resolution transmission electron microscopy (HRTEM) characterisation indicates the polycrystalline nature of CdSe nanoparticles. Spectroscopic elliposometry results show band gap modulation and large enhancement of absorption features due to CdSe at lower nanoparticle size. Kelvin probe force microscopy (KPFM) investigations clearly reveal electronic interaction due to Fermi level alignment of CdSe NPs and ZnTe thin film materials. PL emissions corresponding to ZnS band gap diminishes and new emission features corresponding to CdSe NP are observed on the incorporation of CdSe nanoparticles. PL properties show that CdSe NP /ZnS thin film structures can be used for intermediate semiconductor applications.

Resume : High quality epitaxial crystalline Cu(In,Ga)Se2 (CIGS) films were grown on n-type (1 0 0)—Germanium substrates using pulsed electron deposition (PED) technique at a substrate temperature of 300 °C, thanks to the high-energy of adatoms arriving to the substrate. The crystalline quality was confirmed by X-ray diffraction techniques and from Transmission Electron Microscopy and the only defects found were twin boundaries along the (112) direction in these CIGS films; surprisingly neither misfit dislocations nor Kinkerdall voids were observed. A 100 meV optical band located below the band edge was observed by Photoluminescence technique. Current–voltage and capacitance–voltage measurements confirm an intrinsic p-type conductivity of CIGS films, with a free carrier concentration of 3.5 10^16 cm3. These characteristics of crystalline CIGS films are crucial for a variety of potential applications, such as more efficient absorber layers in single-junction and as an integral component of multi-junction thin-film solar cells.

Resume : Organometal halide perovskites are promising candidates as high bandgap absorbers in multijunction solar cells for targeting cost reductions by ultrahigh conversion efficiencies. For monolithic integration with existing silicon based bottom cells planar architectures are more convenient. Additionally, the planar configuration potentially provides enhanced flexibility for device optimization and easier upscaling toward the production stage also in the single junction approach. However, the film morphology is a critical issue. The fabrication methods need to be adapted to avoid pinhole formation and incomplete surface coverage that would hamper the device performance. Solution-based techniques have been here applied to deposit homogeneous CH3NH3PbI3 films on fluorine doped tin oxide covered with compact TiO2: spin coating of PbI2, drying and dipping in a solution of CH3NH3I in 2-propanol followed by a novel washing process. In particular the effect of spinning rate, drying procedure and dipping time has been investigated. Substrate surface Coverage and morphology have been investigated by SEM imaging and FIB-milled cross sections. Optical absorption spectra and crystal structure by X-ray diffraction have been evaluated on films fabricated on glass. The layers have been then tested within simplified planar cells by evaporating Au electrodes. Inorganic hole transport layers and alternative front contacts are currently being explored to improve the photovoltaic performance.

Resume : The partial coverage of the solar spectrum strongly limits the solar cells (SC) efficiency. Sub-bandgap photons can be recovered by coupling to the SC an upconverting (UC) material to shift their energy to higher values. Classical nonlinear UC, as well as UC in rare-earth based materials, requires high excitation densities (MW cm-2) resulting unsuitable for solar applications. To the contrary, sensitized upconversion (sTTA-UC) in organic systems shows quantum yields up to 30% at irradiances of few suns (mW cm-2). In sTTA-UC, converted light is generated through triplet-triplet annihilation between emitter molecules sensitized via energy transfer from a light harvester moiety. However, the absorption band of proper sensitizers is typically narrow, reducing the fraction of recoverable photons. Here we demonstrate a method to broaden sTTA-UC absorption by adding a Rhodamine dye to a model bi-component system (PdTPTBP/perylene). This gives rise to a super-sensitized upconversion (ssUC). Rhodamine absorbs light in the transparency window of the sensitizer and transfers to it the harvested energy. ssUC allows a 2-fold increase of the absorbed light. Thanks to that the excitation intensity required to reach the maximum conversion yield is lowered to 0.7 suns with an overall enhancement of UC light generation yield of 20%. These results show that ssUC is a general route to design efficient broadband UC systems suitable for SC technology.

Resume : Solar energy conversion consists on the production of electrical energy in the form of current and voltage from incident photons, in which structures at a nano-scale offer an advantage because of their high surface-area. However, such structures present disadvantages of energy losses during the photoelectrochemical process. An attempt to reduce the recombination rate consists of using a bilayer of a metal-oxide semi-conductors for high-performance nanomaterial-based DSSC. Two materials with wide band gap ( ZnO/TiO2) that have very similar photochemical properties are proposed to reduce energy losses. To this aim, an accurate ab initio (DFT) calculations were preformed to characterize ZnO/TiO2 interface, the mechanism of the heterojunction and how the physical characteristics are affected by the solide-solide reaction. In term of our study, two stable configurations were identified as energetically more favorable, in which they have different interface hybridizations. Using GGA+U functional, the correct alignment of the energy levels at the valence band (VB) and conduction band (CB) edges are recovered where the CB of TiO2 is found more negative than that of ZnO in accordance with experimental results. A lineup of the average of the electrostatic potential between the two materials is also performed for the sandwich system TiO2/ZnO/TiO2. This is of capital importance for applications in photovoltaic cells since new sensitizers can be used for the optimization of dye loading.

Resume : Zinc sulfide (ZnS) is an important semiconductor material with a wide energy band gap for various optoelectronic devices. In special, ZnS became an important material for a buffer layer in thin film solar cells because it does not have any environmental issue unlike CdS. It was reported that some amount of ZnO and Zn(OH)2 as the impurities, which are denoted in ZnS(O,OH), are required to be present in the ZnS thin films for fabricating the efficient photovoltaic devices. In this study, ZnS(O,OH) thin films were synthesized by a solution-based deposition method designed by a combination of a continuous flow reactor (CFR) process and a spray method, and then CIGS solar cells were fabricated with the prepared ZnS(O,OH) thin films in order to investigate the influences of the experimental parameters such as flow rate, annealing temperature, time, and film thickness on the performance of ZnS(O,OH) thin films. The CIGS absorber layer was deposited by a RF sputtering for this study. The films were characterized by XRD, SEM, EDX, UV-vis Spectroscopy, and XRD. The performances of the prepared CIGS solar cells were measured with Solar Simulator at AM 1.5. Based on the XRD and EDX, it was found that the molar ratio of zinc to sulfur, the annealing temperature of the as-deposited films, and reaction time exerted an effect on the characteristics of ZnS(O.OH) thin films. About 8% of efficiency was obtained up to now and more in depth investigations are in progress.

Resume : We employ reactive pulsed laser deposition for the fabrication of amorphous hydrogenated silicon (a-Si:H) thin films for solar cell applications. We irradiate silicon targets by a large number of nanosecond laser pulses in hydrogen atmosphere. Varying the PLD parameters, such as the laser fluence, number of pulses, substrate temperature, and hydrogen pressure, we optimize the morphology, structure, electric conductivity, and optical properties of the a-Si:H layers for maximum efficiency. From IR transmission measurements we estimate the amount of hydrogen in the films. The hydrogen content of the films increases linearly with the PLD hydrogen pressure up to a certain point. Beyond the PLD hydrogen pressure of 15 Pa, excessive hydrogen removes H atoms from the surface of the films and the final hydrogen content decreases. From optical transmission and reflection measurements we determine the optical bandgap of the films. The optical bandgap increases with increasing hydrogen pressure during deposition, until it reaches a maximum value (at 15 Pa) and then starts decreasing, ranging from 1.5 – 2.5 eV. The presence of hydrogen in a-Si:H films is known to passivate Si dangling bonds, which create midgap electronic states. Therefore, by the reduction of these states, the optical bandgap increases. Electrical measurements obtained at room temperature by the Van der Pauw method, reveal that the conductivity of the films increases with the PLD hydrogen pressure up to the value o

Resume : The present development of non-wafer-based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of flexible, inexpensive and environmentally friendly substrates; thereby extending PV solutions to a broad range of consumer-oriented applications. Paper-based materials are envisaged as promising mechanical supports for indoor solar cells that can power a wide variety of low cost disposable electronic systems. However, their fibrous structure and low thermal robustness make it quite challenging to fabricate good-performing inorganic PV devices as thin film silicon cells on such substrates. The advances presented in this work demonstrate that commercial inkjet printing paper, containing a supercalendered top layer of a hydrophilic nanoporous (HN) material, is a suitable PV support allowing the low-temperature (150 ?C) production of reproducible, stable and efficient hydrogenated amorphous silicon cells with an nip structure. Unlike with conventional inorganic substrates (e.g. glass), an essential requirement when depositing on organic materials is the continuous monitoring of the substances released during the cell deposition, performed here with mass spectroscopy analysis, in order to optimally adapt the deposition procedures. In this way a 3.4% cell efficiency (with 41% fill factor, 0.82V open circuit voltage and 10.2mA/cm2 short-circuit current density) was attained on HN paper surface.

Resume : Two new approaches that may overcome the issues of phase separation and high dislocation density in the absorber and thus grow InGaN-based PIN solar cells with improved properties were investigated. The first approach consists of the growth of a thick multi-layered InGaN/GaN. The periodical insertion of the thin GaN interlayers should absorb the In excess and relieve compressive strain. The second approach consists of the growth of InGaN nano-structures for the achievement of high In content thick InGaN layers. This approach allows the elimination of the preexisting dislocations in the underlying template. It also allows strain relaxation of InGaN layers without any dislocations, leading to higher In incorporation and reduced piezo-electric effect. Several sets of InGaN layers with In content varying up to 30% have been grown using both approaches on GaN and Si templates. The two approaches lead to structural, morphological, and luminescence properties that are significantly improved when compared to those of thick InGaN layers. Then the corresponding full PIN structures have been realized to achieve solar cells by growing a p-type GaN layer on the top the half PIN structures. External quantum efficiency, electro-luminescence, and photo-current characterizations have been carried out on the different structures and reveal an enhancement of the performances of the InGaN PIN cells when the thick InGaN layer is replaced by either InGaN/GaN multi-layered or InGaN nanorod layer.

Resume : The conversion of solar radiation into electric current with high efficiency is one of the most important topics of modern scientific research, as it holds great potential as a source of clean and renewable energy. Currently the exploitation of nanocrystals seems to be a promising route to foster the establishment of third generation photovoltaics. Here we adopt a fully ab-initio scheme to estimate the carrier multiplication dynamics of isolated and interacting silicon and germanium nanocrystals. Energy and charge transfer-based carrier multiplication events are studied as a function of nanocrystal’s separation showing benefits induced by the wave function sharing regime. We prove the relevance of these recombinative mechanisms for photovoltaic applications in the case of nanocrystals arranged in dense arrays, quantifying at an atomistic scale which conditions maximize the outcome [1,2]. 1) M. Govoni, I. Marri, S. Ossicini, Nature Photonics 6 (2012) 672-679 2) I. Marri, M. Govoni, S. Ossicini, Journal of American Chemical Society 136 (2014) 13257-1326

Resume : For the first time carrier multiplication (CM) is observed in germanium nanocrystals (Ge NCs): we demonstrate that this effect occurs in self-assembled solid-state dispersion of Ge NCs in a SiO2 matrix [1]. The technical importance of Ge is growing with its applications for optoelectronics and photovoltaics. CM is of interest since it can significantly enhance the efficiency of energy conversion processes in photo-detectors and solar cells. For solar cells in particular, the maximal conversion efficiency of 44% can be reached by using CM. For this process the ideal bandgap energy is between 0.6-1 eV, which is in reach of Ge NCs, with the bulk Ge bandgap of 0.6 eV. We studied the CM effect in Ge NCs by using ultrafast transient induced absorption spectroscopy. By comparing the photo-induced absorption transients at different pump photon energies, typically below and above the threshold energy for CM, we observed the fingerprint of CM in the form of a fast transient induced by Auger recombination of multiple excitons localized in the same Ge NC. A CM efficiency of 190% is measured for photoexcitation at 2.8 times the optical bandgap of the Ge NCs of 1.25 eV, as determined from their photoluminescence spectrum. The observed CM is considerably more efficient in Ge NCs than in bulk. We discuss the possibilities for allocation of Ge NCs in a new generation of highly efficient infrared detectors and future photovoltaics. [1] S. Saeed et al., Light: Science and Applications 2015

Resume : The process of carrier multiplication (CM) – generation of multiple carriers upon absorption of a single high-energy photon – has been reported for nanocrystals (NCs) of different materials. In this process an absorbed photon can induce multiple excitons appearing in the same or in neighboring NCs, with the relevant processes being referred to as multiple exciton generation (MEG) and space-separated quantum cutting (SSQC), respectively. CM captures a lot of attention since it could enable more efficient photovoltaic conversion of the high-energy end of the solar spectrum. Until now, all evidence on CM is based on intensity (derived) arguments – induced absorption and emission, and precise calibration/measurement of intensity is experimentally challenging and prone to misinterpretations. Here we report on experimental investigations of CM using a new approach; rather than compare absorption and/or emission intensity, as commonly done, we follow the spectral changes induced by the CM process. The study is performed on photoluminescence of several Si NCs ensembles with different characteristics – NC size, size distribution, concentration and packing configuration. We observe a clear spectroscopic evidence of CM, with a small but well-resolved red-shift of the PL band marking the onset of the SSQC process, thus conclusively resolving the ongoing dispute.

Resume : Integration of III-V materials on silicon has been and still is a challenging subject due to lattice and thermal mismatch effects, as well as to the polarity issues at the interface which result in a high density of defects. To overcome these difficulties, we present here an original approach where silicon is epitaxially grown on GaAs by low temperature plasma-enhanced CVD. Without the need for ultra-high vacuum and keeping temperature below 200°C, both a GaAs surface cleaning and a subsequent heteroepitaxial growth were achieved and monitored by in-situ spectroscopic ellipsometry. The good electronic quality of the low temperature epi-Si and epi-SiGe layers has been demonstrated by making heterojunction solar cells on highly doped c-Si substrates [1]. The same approach has been applied to epi-Si cells on GaAs. Targeting a Si on III-V tandem solar cell, we have developed GaAs solar cells deposited by MOCVD exhibiting an efficiency of 20%. Aiming at tandem devices, we have also fabricated III-V based tunnel junctions. High peak currents at low peak voltage were obtained (7 A/cm2 at 50 mV) and interestingly these values were improved up to 20 A/cm2 when the tunnel junction was submitted to hydrogen plasma. Finally, optical and electrical modelling was used to optimize tandem GaAs/epi c-Si tandem solar cells. 1. R. Cariou, J. Tang, N. Ramay, R. Ruggeri, and P. Roca i Cabarrocas. Solar Energy Materials and Solar Cells 134 (2015) 15.

Resume : Pseudomorphic GaP/Si(100) quasisubstrates are suitable for both metamorphic III-V/Si photovoltaic tandem cells and lattice-matched GaPN-based photochemical tandem diodes, which enable high-efficient solar watersplitting[1]. Preparation of the GaP/Si(100) interface, however, crucially impacts subsequent heteroepitaxy and is not understood at the atomic scale. Analyzing reflection anisotropy spectroscopy (RAS) signals of single-domain surface dimers (free of antiphase disorder) in combination with density functional theory (DFT) calculations, we found indirect indications for a kinetically limited formation of abrupt Si-P interfaces[2]. Here, we study the GaP nucleation itself time-resolved with in situ RAS. We reveal that an optical anisotropy arises at the heterointerface during the first alternating P and Ga precursor pulses and that it remains during further GaP layer growth[3]. With X-ray photoelectron spectroscopy (XPS), we directly evidence the existence of Si-P bonds[3]. XPS and RAS also reveal that sub-monolayer coverages of (Ga,P) residuals from previous processes can lead to an inverted sublattice orientation of the GaP epilayer[4], which agrees with a change to Si-Ga interfacial bonds predicted by DFT[2] depending on the chemical potential during nucleation. [1] Supplie et al., JAP 115:113509 (2014) [2] Supplie et al., PRB 90:235301 (2014); EMRS Fall Meetin’14 (Symp. J, 2.3) [3] Supplie et al., JPCLett., accepted, doi:10.1021/jz502526e [4] Supplie et al., submittted