Advanced materials and characterization techniques for solar cells II

Resume : Light trapping is of fundamental importance in many types of solar cells to allow maximum efficiencies, and hence lowest costs, to be reached. We show that that light trapping can lead to substantial efficiency increases using rear surface scattering, near-field enhancement and for tandem solar cells. A doubling of the photocurrent due to light trapping is demonstrated by the combination of silver nanoparticles with a highly reflective back scatterer on the rear of a silicon thin film solar cell. We also propose a planar ultra-thin absorber concept exploiting plasmonic resonance absorption enhancement. We calculate a maximum absorption of 90% for TM polarized normally incident light in a 5 nm thin-film absorber with a single-pass absorption of only 1.7%. Broadband and wide-angle absorption is demonstrated. Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. We evaluate inorganic thin-film top cells in a tandem stack with a high-efficiency c-Si bottom cell. We show when light trapping is incorporated, even relatively low quality earth-abundant semiconductor materials with luminescence efficiencies of 10-5 and diffusion lengths below 100nm are compatible with tandem cell efficiencies above 30%

Resume : Plasmonic light trapping is nowadays considered a promising solution for retaining high efficiency in thin film silicon solar cells while reducing their volume of semiconductor material. However, to produce significant efficiency enhancement, it is essential to achieve broadband light scattering from subwavelength metallic nanoparticles (NPs) together with suppression of the parasitic absorption in the particles. Here we study the performance of a-Si:H solar cells, with a substrate n-i-p configuration, using self-assembled silver NPs incorporated in the cells’ rear contact, forming a plasmonic back reflector (PBR). The optical properties of the PBRs are optimized according to the morphology of the nanostructures, which can be tuned by the parameters of the solid state dewetting process [1]. A broadband photocurrent enhancement of 22.3% is achieved and attributed to both the plasmon-assisted light scattering from the NPs and the front surface texture originated from the conformal growth of the cell material over the particles. Based on the analysis of more than 40 cells built on distinct PBRs we demonstrate a linear relation between the PBRs’ diffuse reflection and the photocurrent enhancement in the a-Si:H light trapping window (600–800 nm), which reaches values as high as 60%. Additionally, remarkably high values of Jsc and Voc are achieved, relative to those reported in the literature for the same type of devices. [1] S. Morawiec et. al., Nanotechnology, 24 (2013) 26560

Resume : Many potential semiconductors for photocatalysis of water splitting have band gaps that are too large for absorption of visible light and hence give low efficiencies under sunlight. Two such examples are ZnO and ZnS. Solid solution formation is one approach to reducing band gaps and enhancing visible-light absorption. For example, it has been shown that GaN-ZnO solid solutions are promising visible-light photocatalysts [1]. In this work, we show based on density functional theory calculations that both ZnO-AlN and ZnS-GaP solid solutions are promising photocatalysts. The solid solutions have small energies of mixing from the two constituents (lower than ZnO-GaN in the case of ZnO-AlN). The solid solutions have tunable band gaps that depend on both composition and atomic ordering. Addition of only a small amount of AlN to ZnO and GaP to ZnS reduces the band gap into the correct energy range for absorption and emission of visible light and close to the optimum for photocatalysis of water splitting under sunlight [2]. Significantly, the band gaps of the solid solutions are smaller than those of either constituent on their own. Furthermore, addition of a small amount of ZnS to GaP produces a direct band gap semiconductor, in contrast to pure GaP which has an indirect band gap, and this should therefore increase the efficiency of light absorption. [1] K. Maeda, et al., Nature, 440 (2006) 295. [2] J. N. Hart, N. L. Allan, Advanced Materials, 25 (2013) 2989.

Resume : Kesterites (Cu2ZnSn(S,Se)4) are compound semiconductors which do not make use of any rare metals, have band gaps suitable for the use in solar cells and a crystal structure similar to the successful Cu(In,Ga)Se2 compounds. Therefore theses materials are worldwide intensely studied as solar cell absorbers. Efficiencies up to 12% have been reached by IBM, and around 10% by a number of labs worldwide. However, to be competitive the efficiencies have to improved to a level comparable to CIGS and CdTe. The presentation will summarise the current state of the art of kesterite solar cells and the challenges on the path to higher efficiencies.

Resume : Organometal trihalide perovskites, such as CH3NH3 Pb X3 (X = I-, Br-, Cl-), are attracting growing interest to prepare low cost solar cells that are capable of converting sun light to electricity at the highest efficiencies. Despite negligible effort on enhancing materials purity or passivation of surfaces, high efficiencies have already been achieved. However, we show that trap states at the perovskite surface generate charge accumulation and consequent recombination losses in working solar cells. We identify that under-coordinated iodine ions within the perovskite structure are responsible and establish a supramolecular strategy to successfully passivate these sites. Following this strategy we demonstrate solar cells with stabilized power conversion efficiency over 15% under simulated full sun light.

Resume : Management of the solar spectrum is one of the key issues to improve the solar cell efficiency. Among the existing solutions, one consists in a layer absorbing energetic UV photons and converting them into two IR photons that match the absorption band of classical Si solar cells. This mechanism called frequency- or down- conversion process may increase the cell efficiency by almost doubling the number of IR photons absorbed as well as reducing the thermalisation losses. In this paper, we propose an innovative approach using a co-doped SiN matrix that presents the advantages of being compatible with Si solar cells fabrication process. Layers were deposited on Si substrates by reactive magnetron co-sputtering of Si, Tb, and Yb targets under a nitrogen-rich plasma. The microstructure of the grown layers were investigated using TEM, EDX and Glow Discharge Mass Spectroscopy while their optical properties and excitation mechanisms were analysed by means of ellipsometry, photoluminescence, and photoluminescence excitation spectroscopies. The quantum efficiency of the system has been studied through time-resolved photoluminescence experiments. Finally, optimized layers have been deposited on a solar cell to measure the carrier lifetime as well as the external quantum efficiency under AMG 1.5 illumination.

Resume : ZnO is a promising material for photovoltaic and photocatalytic applications since its band gap is in the near UV. It is already applied in large quantities for UV protection. We report about a study of nanocrystalline commercial ZnO which has been subjected to an annealing treatment in reducing atmosphere. Intensive blue-green luminescence is found after that treatment, which is attributed to defects. The material has been characterized in detail using photoluminescence spectroscopy, and subsequently been combined with a commercial Si solar cell. A quantitative model has been developed to compare the experimental results with theoretical predictions. For this purpose, we have determined the photoluminescence quantum efficiency of the powders. In addition, we have determined the transmission spectrum of glass slides coated with PDMS films containing varying amounts of the luminescent powder. By combining the measured data with spectrally resolved photocurrent measurements of plain and ZnO-covered Si cells, we are able to quantitatively model the behavior of the system. I t is found that under the present conditions the strong back-scattering from the ZnO layer leads to a reduction of the overall efficiency of the system. However, in the UV region we find an enhancement of the photocurrent. The model allows us to predict the conditions for improving cell efficiency by this method and amount of overall increase of the cell efficiency which can be reached.

Resume : Metallic nanoparticles (MNPs) are gaining importance for application in silicon solar cells as an efficient method to enhance the light absorption in the active layer due to their pronounced scattering properties at the surface plasmon resonance. It has been recently demonstrated that the preferential location for the MNPs in solar cells is in the rear contact, embedded in the transparent conductive oxide (TCO) layer which separates the back mirror from the absorber, forming what is known as a plasmonic back reflector (PBR) [1]. Here we study how light trapping in silicon thin films is influenced by the distinct layers composing the PBR structures. The PBRs were fabricated by sequential depositions of silver (Ag) and aluminium doped zinc oxide (AZO) and the MNPs were formed in a self-assembly manner by solid-state dewetting of thin Ag films. A 1 m thick hydrogenated nanocrystalline silicon (nc-Si:H) layer was deposited on top of the PBR structures at different stages of completion. The obtained surface topographies were investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The light trapping induced by the MNPs is evidenced by the enhancement of the Raman signal (acquired with weakly absorbed excitation laser excitation at 785 nm) in the nc-Si:H film and spectrally analyzed by diffused and total reflectance in the UV-Vis-NIR wavelength range. [1] S. Morawiec et. al., Nanotechnology, 24 (2013) 265601

Resume : Changing the size and morphology of silver nanoparticles leads to kinds of distinctive scattering properties and localized surface plasmon resonant effect, which has demonstrated a huge potential in photovoltaic applications. We have studied lumpy silver particles as rear located strengthen materials for silicon thin-film solar cell. Though theoretical simulations, we find that the large-size lumpy silver particle has a more advantageous property of scattering incident light back than the spherical particle in a broad wavelength range. This kind of large-size silver particles can be used as rear-position strengthening materials for silicon thin-film solar cells. We demonstrate that when the Ag particles' coverage density is 10%, the light absorption enhancement is optimal.

Resume : In last few years group I metal (Ag, Au, Cu) nanoparticle based plasmonic layers received considerable interest due to the increased light trapping in the underlying semiconducting absorber layer. Environmental stability of these nanoparticles can be increased by coating them with ultrathin protective dielectric (or semiconductor) layer. In addition such layer can be used for additional control of the position of the surface plasmon resonance by changing refractive index of the media surrounding nanoparticle. Along with the coated core shell nanoparticles, metal and dielectric nanocomposites can be used for such a purpose. Diamond like carbon is prospective matrix material of such a nanocomposite due to the high optical transmittance, high hardness, corrosion resistance as well as possibility control refractive index and electrical properties in wide range. In present study diamond like carbon films containing Ag nanoclusters (DLC:Ag) were deposited by reactive unbalanced magnetron sputtering. Optical properties of synthesized nanocomposite films were investigated in 180-1100 nm range. Structure of the films was studied by Raman spectroscopy, x-ray diffractometry and atomic force microscopy. Effects of the annealing on optical and electrical properties as well as structure of DLC:Ag films were studied. Significant changes of the position, shape and width of the plasmon resonance peak were observed.

Resume : There are currently many investigations in the field of photovoltaics making current technology solar cells more efficient and cheaper. Multijunction solar cells based on GaPNAs/Si lattice matched heterostructures are very promising. However GaPNAs grown to-date appears to have very short diffusion lengths. Thus it is very hard to provide current matching of GaPNAs- and Si-subcells. This paper presents a 2D-simulation of new design two junction nanowire (NW) solar cells based on GaPNAs/Si lattice matched heterostructures. The effects of NW's height and distance on the electrical characteristics were investigated. The efficiency of NW solar cells increases with increasing of NW height due to collecting more carriers and reaches a plateau at 4 microns. Decreasing the distance between NWs leads to increase of solar cells efficiency due to increase of current spreading. Also it was shown that NW solar cells with lower thickness of GaPNAs layer are able to reach higher efficiency compared to its flat design.

Resume : A comprehensive study on the impact of GaAs and InGaAs capping layers on the carrier transport in quantum dots (QDs) for infrared detectors and solar cell application is carried out. The effect of variation in dot monolayer coverage has been studied. For GaAs capped QDs, the activation energies obtained from temperature-dependent I-V measurements is found to increase with monolayer(ML) coverage upto 3.0 ML. This can be attributed to increase in width of the potential well and corresponding increase in bound-to-continuum energy gap. The spectral peaks were observed to be decreasing from 8.37μm to 7.38μm which affirms the above trend. For InGaAs capped QDs, the corresponding activation energies with increase in ML coverage were found to be much higher than those for the GaAs capped structures. Due to higher lattice mismatch in GaAs capped QDs, strain enhancement at apex of dots leads to significant In adatom migration reducing the height of QDs. InGaAs capping having smaller lattice mismatch with InAs QDs, the In atom migration is hindered and the QD size remains significantly unchanged which justifies the higher activation energies in these structures. The spectral peaks obtained here at lower wavelengths complements the above results. A decreasing trend in activation energy is observed for InGaAs capped structures. This can be ascribed to the increase in defect states arising out of higher strain induced due to higher ML coverage. Riber, France and DST, India is acknowledged.

Resume : We have studied the effect of post-growth rapid thermal annealing (RTA) on the stability of InGaAs capped quantum dots (QDs) for infrared photodetectors and solar cell applications. For comparison, GaAs capped quantum dots heterostructures have also been investigated. QDs with different monolayer (ML) coverage were grown using molecular beam epitaxy (MBE). It is observed that with increasing monolayer coverage the shift in the photoluminescence (PL) peak with RTA is suppressed. This is attributed to higher stability attained due to larger size of dots. Higher ML coverage also prevents the interdiffusion of In and Ga, which results in a reduced blueshift even at temperatures upto 7500C. A temperature of 8000C is, however, sufficient to elevate the interdiffusion enhancing the blueshift. The activation energies calculated from temperature-dependent PL measurements show minor variation with annealing upto 7500C for higher monolayer dots which is in excellent agreement with the above discussion. With the InGaAs capping, the PL peak shift is further reduced in contrast to the GaAs capped dots. The capping layer prevents In interdiffusion to a higher extent consequently reducing shift in the PL peaks. The restrained shift in luminescence even at higher annealing temperatures indicates good thermal stability which is preferable for QD device applications. Riber, France and DST, India are acknowledged.

Resume : By their potential to overcome the Shockley?Queisser limit of a single junction solar cell, the third generation solar cells would enable a lower material usage and/or installation cost share. To this end, a solution consists in the MBE-epitaxial growth of III-V heterostructures towards the elaboration of innovative concepts of solar cells. First, this paper describes our approach to obtain high efficiencies tandem solar cells on Si substrates. GaAsPN diluted nitride alloy is studied as the top junction material due to its perfect lattice matching with the Si substrate and due to its ideal bandgap energy for current matching with the Si bottom cell. Through structural analyses (XRD and TEM), we have developed a growth strategy which dramatically reduces the III-V/Si defects interface. An optimisation of the GaAsPN absorber using a post-growth annealing treatment and first results on p-i-n junctions grown onto GaP substrates are also reported. A second approach consists in the study of InGaAsP/InP MQWs, towards the elaboration of Hot-Carrier Solar Cells, which aim to reduce both the non-absorption of the incoming light (small bandgap) and the thermalization process (electron-phonons interactions). Using different PL techniques, the quasi Fermi level splitting Δμ is estimated as a function of the excitation power. High value of Δμ is found as well as high carrier temperature. These results are compared to electrical measurements. Support: ANR project MENHIRS 2011-PRGE-007-01.

Resume : The goal of our work is to increase light-matter interactions in organic photovoltaics. Our first design involves self-assembled gap antennas with interparticle distances of a few nanometers controlled by a molecular cross-linker (dithiothreitol). The spacing molecules ensure a minimum distance that plays a fundamental role in the formation of intensity hot spots in the nanogap as well as large and red-shifted scattering peaks. This device exhibited an efficiency 14% higher than the reference one showing a relevant enhancement in the red part of the EQE measurements. As second design we build up a photonic crystal and a metal cavity around a transparent organic solar cell. We enclosed the active material in between two metal electrodes forming an optical cavity designed to optimize photon trapping inside the cell. To increase near IR light trapping, while maintaining transparency in the visible, an anti-reflection coating was deposited on top of the front metal contact while a non-periodic multi-layer was inserted in between the back metal contact and the substrate. The cavity configuration was designed specifically for the cell architecture used and we achieved semi-transparent cells with 5.3% PCE, corresponding to 90% the PCE of the opaque cell.

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Resume : Capacitance techniques have been widely applied to crystalline semiconductors homojunctions in order to study defect levels or doping profiles. The theory was also extended to crystalline semiconductors heterojunctions, where for instance it was proposed how to use the bias dependence to determine band offsets. In amorphous semiconductors, the theory was modified in order to take into account the predominant role of the continuum of defects in the band gap. In this presentation, we will briefly review the basic concepts of capacitance techniques and the peculiarities related to amorphous semiconductors, paying tribute to J. D. Cohen and to his pioneering work. Then, we will extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells that gather the ingredients of both amorphous semiconductors and heterojunctions. By presenting both modeling and experimental results, we demonstrate that the conventional theory of heterojunction capacitance based on the depletion approximation, cannot reproduce the measured temperature and bias dependence, and leads to strong errors if applied to the determination of band offsets. We show that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer. SiHET solar cells thus offer a unique opportunity to reconsider the theory of junction capacitance and to explain how, in this case, capacitance measurements can bring insight into the device physics.

Resume : Micro-crystalline silicon (uc-Si) has already been discovered in 1968 by Veprek and Maracek, but it is still an interesting and promising material for solar cells, because of the low processing temperatures and the abundance of the feedstock material. uc-Si, however, also has some drawbacks, and one of them is that the material consists of small grains, requiring excellent passivation of the grain boundaries. The latter is accomplished by tuning the conditions of the PECVD process such that the material contains a certain fraction of hydrogenated amorphous silicon (a-Si:H). a-Si:H is capable of passivating the grain boundaries of uc-Si very well, but in practice an over-proportional fraction of around 30 % a-Si:H in the layers is required to obtain reasonable open-circuit voltages (Voc). This high fraction of a-Si:H not only leads to a non-negligible light induced degradation of the solar cells, but also to a certain loss in short-circuit current (Jsc): charge carriers generated in the a-Si:H tissue have little chance to contribute to the current of the solar cell. In order to reduce the a-Si:H fraction in the uc-Si layers, but maintaining the grain boundary passivation, we have conducted post-deposition hydrogenation experiments. We made uc-Si layers with a relatively high crystalline fraction and subsequently hydrogenated these layers by two different methods: microwave (MW)-PECVD and hot wire(HW)-CVD. Both methods resulted in a significant improvement of the ratio of the photo to dark conductivity. Before hydrogenation this ratio was about 50 and after hydrogenation this ratio improved to a value of 400 for HW-CVD and to 775 for MW-PECVD. Cell fabrication is ongoing and the results of these experiments will be reported at the conference.

Resume : Thin film silicon solar cells on low cost foreign substrates could be attractive for highly efficient and low cost production of photovoltaic electricity. This work aims at the synthesis of high-quality continuous polycrystalline silicon (pc-Si) layers on flexible aluminium based substrates using direct crystallization (DC) or through the aluminium induced crystallization (AIC) procès of amorphous silicon. Pure aluminium (Al) or anodic alumina (Al2O3/Al structures) were used as substrates. Amorphous silicon films with thicknesses ranging from 200 to 1000 nm were deposited by ECR-PECVD and PVD on the substrates The direct crystallization using a conventional furnace was carried out at temperatures ranging from 450 to 550°C and durations from 1h to 5h. Rapid thermal annealing using halogen lamps at températures of 700-800°C and durations lower than 15min were attempted. For the AIC process, a 200nm aluminium layer was first evaporated on the substrates prior to depositing amorphous silicon. Similar thermal treatments than above were performed. The resulting crystallized layers were characterized by Raman, UV reflectance spectroscopy as well as by XRD analysis. It was found that the complete crystallisation of the a-Si layers is strongly dependent on the initial layer thickness and the thermal budget. On the other hand, the as-grown AIC pc-Si films were found to be continuous and densely packed without amorphous phase. The migration of impurities from the substrate to the pc- Si films were studied systematically in terms of chemical and stress level analysis, which are the important aspects to be considered when metallic foils are used as substrates. Complementary experiments and analysis are currently carried out to improve the quality of DC and AIC pc-Si films.

Resume : Radial junctions based on silicon nanowires (SiNWs) are an example of nanostructured design of solar cells with excellent light trapping and efficient photogenerated charge collection. A single pump-down process used to prepare a randomly grown matrix of SiNWs and conformal p-i-n radial junctions led to cells with efficiencies over 8% [1]. Considerable influence of irregularities in SiNWs lengths, orientations, shapes and mutual interactions on the photovoltaic action can be expected. Direct measurement of these effects requires microscopic measurements of photoresponse. This is possible using atomic force microscopy (AFM) with conductive cantilever which serves as a contact to individual radial junctions [2]. At the same time the cantilever can measure the local nanomechanical properties, including local stiffness of the wires, which can only sustain contact forces up to ~1 nN. Resulting conductivity maps show substantial variation of the local electronic properties. The AFM tip cannot reach deeper into the SiNWs matrix and correlation with scanning electron microscopy of the identical nanowires was sought in order to identify the reason for conductivity variations. The results are discussed in terms of random photodiode arrays connected in parallel with overall performance limited by weak diodes. [1] S. Misra et al., Sol. Energy Mat. Sol Cells. 118 (2013) 90–95. [2] A. Fejfar et al., Sol. Energy Mat. Sol. Cells. (2013) 228–234.

Resume : Quantum confinement effect (QCE) can tune many properties relevant to light harvesting, such as optical bandgap, efficiency of luminescence and oscillator strength of the optical transition. Since Ge has a quite large Bohr exciton radius (24 nm), the quantum confinement regime in Ge nanostructures (NS) is relatively easy to get. In addition, Ge shows an absorption coefficient more than 10 times higher with respect to Si in the solar energy range. Thus, Ge NS are really attractive as active absorber for efficient light harvester, solar cells and novel optoelectronic devices. This work aims to show how and to what extent the QCE modifies the optical behavior of a variety of Ge NS confined in SiO2. Materials and devices, prepared by sputtering deposition of Ge-rich SiO2 thin films in single or multiple layer configuration, comprise: Ge quantum dots (QD, 2-10 nm in size) with QD-QD distance accurately fixed between 3 and 20 nm, and single Ge quantum well (2-30 nm in thickness). Light absorption spectroscopy with effective mass simulation evidenced the relationships among synthesis parameters and response to light absorption in prototypal devices, getting light detection with photoconductive gain up to 1500%. Our data fix the NS size as a critical parameters for QCE, but also demonstrate that proximity and interaction among NS play key roles in the efficiency of photon absorption process. Ref. S. Cosentino et al. Nanoscale Res. Lett. 8, 128 (2013) S. Mirabella et al. Appl. Phys. Lett. 102, 193105 (2013) S. Cosentino et al. Appl. Phys. Lett. 98, 221107 (2011)

Resume : The use of silicon nanocrystals (SiNC) in optoelectronic devices has risen from a decade thanks to the discovery of photoluminescence in porous silicon in 1991 [1]. However the SiNCs exhibit a low quantum yield, which prevent from currently using them in optoelectronic devices. This problem can be bypassed by using localized surface plasmons (LSP). Indeed LSPs can modify emitters’ photoluminescence by changing the optical local density of states and/or increasing the local excitation field. Numerous studies have been performed with different emitters, but the analyze of LSP coupled to SiNC is scarce. Seminal studies by Biteen and coworkers [2,3] have paved the way for increasing SiNCs’ performances with LSPs. In these preliminary works only the SiNCs’ emission wavelength was coupled to LSPs. In this work, we first present a study where LSPs resonances of gold nanodisks (GNDs) are coupled to SiNCs [4]. Then, we show results concerning the coupling of both the absorption and emission wavelengths of SiNCs to gold nanorods. In both cases, the use of an original fabrication method gives us the control all the geometrical parameters that modify the SiNCs – LSPs coupling. In the first study, we use GNDs with different diameters and located at different distances to the SiNCs. We then evaluate the maximum quantum yield enhancement for the sample with the optimized geometry. The use of nanorods with different shapes and metallic materials allows us to obtain in the same structure two surface plasmon modes exhibiting different polarizations. To understand them, we studied the coupling of LSP to the SiNC’s photoluminescence intensity, polarization and spatial redirection. [1] A. G. Cullis, L. T. Canham, Nature 1991, 353, 335–338. [2] J. S. Biteen, D. Pacifici, N. S. Lewis, H. A. Atwater, Nano Lett. 2005, 5, 1768–1773. [3] J. S. Biteen, N. S. Lewis, H. A. Atwater, H. Mertens, A. Polman, Appl. Phys. Lett. 2006, 88, 131109. [4] J. Goffard, D. Gérard, P. Miska, A.-L. Baudrion, R. Deturche, and J. Plain, Scientific Reports 3, 2672 (2013).

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Resume : Bulk silicon is notoriously known as an inefficient light emitter due to its indirect bandgap, which is usually perceived as an obstacle towards the utilization of this material in optoelectronics and bioimaging, despite the evident advantages of this approach. Some alleviation of the poor light emission performance of bulk silicon can be achieved by switching to silicon nanocrystals, whose light-emission efficiency can be reasonably high, but whose photon emission rates are still usually low due to the remaining character of the indirect badstructure. In this contribution, we show that the bandstructure of silicon nanocrystals can be strain-engineered to achieve fundamental direct bandgap, leading to a 10 000x increase in the photon emission rate. We will first briefly discuss the validity of the bandstructure concept on the nanoscale [1] and then present both theoretical and experimental evidence that the joint action of quantum confinement and tensile strain induced by surface capping lead to indirect-to-direct bandgap cross-over [2]. [1] P. Hapala et al.: `Theoretical analysis of electronic bandstructure of 2-to-3-nm Si nanocrystals,' Phys. Rev. B 2013, 87(19) 195420. [2] K. Kusova et al.: `Direct Bandgap Silicon: Tensile-Strained Silicon Nanocrystals,' Adv. Mater. Int. 2014, in press, DOI: 10.1002/admi.201300042.

Resume : Nowadays, solar cell technology attracts much attention in reliable and high efficiency photovoltaic applications. Thin-film SiGe solar cells have important advantages such as high photocurrent and their compatibility with the process developed for pure Si cells. In order to improve the electrical efficiency performances of the conventional SiGe solar cell, we have introduced a new multi-trench technique. In the proposed method, the multi-trench is created in the silicon layer and filled with n-type doped SiGe. The p-type trenches under the SiGe layer improve the electrical performance of the proposed design. By 2-D numerical simulation, we have investigated the electrical performances of the proposed design and compared with it a conventional thin-film SiGe solar cell. The proposed accurate numerical models have been used as objective function to optimize the electrical performance of the SiGe-based solar cell. The obtained results show that the proposed design can be considered as a potential candidate for high performance photovoltaic applications.

Resume : The light harvest in a solar cell can be greatly improved if the sunlight is made obliquely incident into the absorption layer. Not only enhancing light absorption by increasing the optical path length, this configuration is also favorable for the extraction of charges as they are generated closer to the collecting electrode. Despite many advantages, the deflection of incident light has not been employed for dye-sensitized solar cell (DSSC) because of the inherent difficulty of incorporating a relevant structure into DSSC. Here we report that a refractive-index grating can be embedded into the TiO2 electrode and this architecture highly increases light absorption in DSSC by diffracting the incident light. A thin TiO2 film spin-coated on conducting glass was molded by imprinting and treated with a higher TiCl4 concentration than the later-coated thicker TiO2 absorption layer. Due to the refractive index difference between two layers, the incident light was widely diffracted, giving diffraction efficiencies over 70% in the visible range. As a result, the IPCE and current density of the cells were much improved. This facile approach can be effectively utilized to increase the light harvest of DSSC and its energy conversion efficiency.

Resume : The Al doped Zinc Oxide (ZnO:Al) is a transparent and conductive oxide used in alternative to the Indium Tin Oxide (ITO) as contact and antireflection layer in amorphous silicon heterojunction and chalcogenide based solar cell. Generally good film quality can be obtained by low cost magnetron sputtering, even though it is difficult to obtain a resistivity below 10-3 ohm cm when grown at the temperature below 200°C commonly used in solar cells manufacturing to avoid thermal stress. Following the recent theoretical and experimental confirmations of the Hydrogen doping property in Al:ZnO films, in this work the Hydrogen effect on the Al:ZnO film as optical, electrical and structural properties is investigated. Two different growth techniques have been considered: DC and Pulsed DC magnetron sputtering. A clear improvement in the electrical properties of the layer has been noted in particular when films have been grown by DC pulsed magnetron sputtering. Resistivity of 6.7x10-4 ohm cm, carrier concentration of 5.4x1020 cm-3 and electrical mobility of 17 cm2/Vs have been obtained even on film thinner than 100nm. While the low mobility value suggests that the film grain size still needs to be increased, XRD measurements show better quality material. Finally the stability of the hydrogen doped ZnO:Al film properties is evaluated when the film is subjected to thermal stress as in solar cell manufacturing applications.

Resume : The combination of low electrical resistivity with high optical transparency make Al-doped ZnO (AZO) films widely used as transparent electrodes in solar cells. In order to improve conductivity and transmittance of these films, annealing treatments are normally employed during or after the growth process. We report on the increase of the conductivity of ultra-thin AZO films upon irradiation with high energy O2+ ions. This improvement is linked to the modifications of both optical and structural properties of the material. The AZO thin films (60 nm) have been grown by RF-magnetron sputtering at room temperature, then irradiated with 350 MeV passing O2+ ions at fluencies of 3x1015, 1x1016 and 3x1016 ions/cm2. By X-ray diffraction analysis, all films have shown hexagonal wurtzite structure, improving the crystalline quality as a function of the ions fluencies. AFM analysis for the surface morphology was also performed. In particular, we observed an enhancement of the electrical conductivity of more than two orders of magnitude, along with grain size increasing and in-plane strain change from compressive to tensile values. The optical bandgap increases with the ion doses, partly due to the Burstein-Moss effect connected with an improved crystalline quality. Annealing effects, up to 400°C, are also reported and compared with the irradiation ones. Our results indicate that the structural quality and conductivity of the AZO thin films can be improved by specific ion implantation processes.

Resume : Solar cells on lightweight and flexible substrates have advantages over the glass- or wafer-based photovoltaic devices in both terrestrial and space applications. Here, we report on amorphous silicon solar cells on plastic foils in the substrate configuration having a front metal grid. A two-dimensional distributed circuit model of the photovoltaic cell has been developed for performance analysis and device design optimization. The circuit simulator SPICE is used to calculate current and potential distributions in a network of sub-cell circuits. The equivalent circuit of each sub-cell includes a diode, a current source representing current generation in the n-i-p structure, and resistors representing the p-layer, transparent-conducting oxide (TCO) electrode, top metallization and shunting resistances. The input parameters also include performance characteristics of the n-i-p structure, which can be determined experimentally. This approach enables a realistic device model that predicts output current-voltage characteristics and maps Joule losses in the TCO electrode and the metal grid. As an example of usage the optimization of contact grid geometry at various TCO sheet resistances has been performed. These results are used to determine the optimal thicknesses for the ZnO:Al electrode and top metallization in the developed photovoltaic module thus boosting its power conversion efficiency.

Resume : Metal oxide semiconductors are widely used as transparent contact materials for solar cells. However the vast majority are n-type. This is because of the typical band structure of metal oxides and the high electronegativity of oxygen. The development of transparent conducting p-type oxides could be of use in a range of PV applications such as bi-facial and multijunction cells, as well as possible high work-function back contact materials for organic or CdTe cells. Cupric oxide (CuO) is a p-type material with an indirect band gap of 1.2 eV. This study examines the effects of doping on the resistivity of sputtered CuO, and aims to increase the material band gap by co-sputtering with SnO2. It was found that a target doped with 2% Na produced films with resistivities of four orders of magnitude lower than equivalent undoped films. Addition of oxygen to the films was found to reduce the resistivity further. The best films were found to have resistivities of 4.3x10-2Ω.cm, which is an order of magnitude better than has been reported for other p-type oxides. Addition of SnO2 was found to increase the band gap significantly, although it also caused an increase in the resistivity. Despite not being as conductive as n-type oxides such as ITO or AZO, this study demonstrates that cupric oxide-based materials have potential as effective p-type transparent conductors.

Resume : We present a composite design methodology for the simulation and optimization of the solar cell performances. Our method is based on the synergic use of different simulation techniques and it is especially designed for the thin-film cell technology. In particular, we aim to efficiently simulate light trapping and plasmonic effects to enhance the light harvesting of the cell. Our method is based on the sequential application of a hierarchy of approaches: a) full Maxwell simulations are applied to study of the light scattering in systems presenting textured interfaces; b) calibrated Photonic Monte Carlo is used in conjunction with the scattering matrixes method to evaluate coherent and scattered phonon absorption in the full cell architectures; c) the results of these advanced optical simulations are used as the pair generation terms in model implemented in a conventional TCAD tool for the derivation of the cell performances; d) genetic-type algorithms are applied to the automatic optimization procedure and the cell design. The material calibration includes optical and electrical properties of common substances used in thin film solar cells, while both full Maxwell simulations and experimental measurements are used to calibrate the angular scattering probability used in the Photonic Monte Carlo method. We discuss the application of our methodology to the study and the optimization of different typologies of solar cell.

Resume : Si based heterojunction solar cell structures with ITO and a-Si:H layers have been analysed by spectroscopic variable angle ellipsometry in order to trace dependence of free carriers’ distribution along the film depth as a function of film thickness as well as its change upon annealing. ITO layers were deposited at RT or 200°C preheated silicon substrates. Properties of HJ based solar cell structures have been compared with the ITO film optical constants’ inhomogeneity, i.e. material properties along the film thickness. The obtained results show that optical as well as electrical properties of thin ITO films prepared by pulsed DC sputtering are depth dependent. In particular, the deposition conditions used ensure a well-determined reproducible non-uniform distribution of free carriers within the film thickness. It has been found that distribution of free carriers in annealed around 300 °C samples is qualitatively different from that of as-deposited layers. It is shown that such annealing leads to an improvement of the HJ solar cell efficiencies. It is concluded that in-depth variations of the optical function of ITO layers responsible for the improvement of Si-based heterojunction ITO/a-Si:H/Si structures upon anneals.

Resume : Transparent contact electrodes are fascinating oxides with interesting physical properties. AZO, ITO and SnO2 were tested for the development of photovoltaic devices, such as solar cells. They have good electrical properties and are usually used as transparent contact electrodes in such devices. In the present paper we discuss the application of RF magnetron deposited AZO, ITO and SnO2 thin films as transparent contact electrodes in high quality chalchogenide solar cells. Therefore, a reliable assessment of their crystallographic and optical properties is essential. The influence of growth parameters (i.e. oxygen pressure, flow rates, substrate type) on the morpho-structural characteristics of the thin films were also investigated. AZO, ITO and SnO2 films with a thickness of ~ 500nm were deposited by RF magnetron sputtering onto glass substrate, using a constant deposition rate of 6 Å/s. Morphological features were investigated by Atomic Force Microscopy and Scanning Electron Microscopy, which showed profiles that are densely packed and well adherent to the surface of the substrate. The X-ray diffraction analyzes revealed that all films are polycrystalline and show a strong orientation after the main planes perpendicular to the substrate. It was highlighted that ITO and SnO2 have good conductivity and are transparent (90-94%) materials, whereas AZO depends on the Al (3%) doping to exhibit improved properties.

Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance – Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.

Resume : Nitric acid oxidation of silicon (NAOSi) is an attractive method to oxidize silicon to form ultra-thin SiO2 layers with good interface quality at lower temperatures. In the present investigation, (100) n-type Si wafers were subjected to two-step nitric acid oxidation, involving a treatment in 40 % HNO3 for 10 min followed by 68 % HNO3 treatment at 121 oC for several time periods. The grown samples were annealed in different ambients. SiO2 thickness practically stopped after attaining a critical thickness. Quasi-steady state photoconductivity measurements on the oxidized samples indicated improvement in minority carrier lifetime after annealing at 400 oC for 30 min in forming gas. The passivation observed in n-type silicon surface was attributed to chemical passivation. Capacitance ? Voltage characteristics revealed that SiO2 layers formed using NAOSi route has positive fixed charges. A short high-temperature annealing affected the local bonding configuration and enhanced the positive charge density of SiO2 layers after hydrogen annealing. The results indicate that NAOSi grown oxide layers may find application in silicon solar cells as passivating layers alone or as buffer layer in conjunction with other surface passivation layers such as Al2O3 or Si3N4.

Resume : In this work we present a model which describes the current-voltage characteristics of an organic solar cell under illumination with 0.25 – 2 sun equivalents. The IV characteristics of an ideal solar cell, under illumination, are described by the sum of the dark current (J_dark) and photo-generated current (J_photo). In a practical solar cell it is also necessary to consider the recombination rate (R) of the photo-generated charge carriers, which in fact reduces the power output of the solar cell (i.e. decreased fill factor). Practically it is a challenging task to determine the recombination rate. Here we present a novel approach based on charge extraction (CE) and transient photo-voltage measurements (TPV) to calculate R. For the calculation of R, the voltage dependent carrier density n(V) must be known as well as the recombination order, often described by the recombination coefficient α. Latter is derived from the charge carrier lifetime dependence on the charge density. Both parameters are accessed by TPV and CE measurements. Based on this, the reconstructed IV characteristic is in excellent agreement with the measured data. It is noted, that the herein introduced model does not need any free fit parameters and allows a consistent recreation of the IV characteristics of an organic solar cell.

Resume : We report the mechanical stacking technology used for fabricating large size multi-junction solar cells. 1) We developed transparent conductive adhesive and stacking technology. 20 micrometer diameter ITO conductive particles were dispersed in epoxy-type transparent adhesive jell named Cemedine. We also developed a gas press method for stacking semiconductor substrates up to 8 inch size. Two low-resistivity silicon substrates were pasted together by the Cemedine adhesive including 6 wt% ITO particles as an intermediate layer. Then, the sample was placed in a pressure proof chamber. Air gas was introduced in the camber at 0.7 MPa for 2 h to hardening the intermediate Cemedine adhesive at room temperature. Current voltage measurement of the sample with the structure Si/adhesive/Si revealed a good ohmic characteristic with a connecting resistivity of about 1 ohm cm2, which was enough low for fabricating multi-junction solar cells. 2) We experimentally demonstrate multi-junction solar cells using our mechanical stacking technique as described above. Commercial GaInP/GaAs/Ge three junction solar cells with a size of 8x4 cm2 were prepared. The initial solar cell characteristics were first measured under AM1.5 light at 100 mW/cm2. The open circuit voltage (Voc), fill factor (FF), and efficiency were 2.51V, 0.85, 31.6%, respectively. Then, the rear metal electrodes were removed and Ge rear surfaces were pasted to a low resistivity silicon substrates using the Cemedine adhesive including 6 wt% ITO particles. Measurement of solar cell characteristics under AM1.5 light at 100 mW/cm2 resulted in Voc, FF, and efficiency as 2.53V, 0.84, 31.4%. The solar cell performances were almost the same as the initial ones. Those results show that our transparent conductive adhesive has a capability of fabrication of large size multi-junction solar cells. In the conference, we will also report solar cells fabricated by stacking GaInP/GaAs epitaxial thin solar cells on Ge cells and stacking amorphous silicon thin film solar cells on crystalline silicon solar cells using our transparent conductive adhesive.

Resume : Nano-sized TiO2 films have been grown by DC-Reactive Sputtering at ~150 °C on [0001] ZnO:Al substrates to be applied in Dye Sensitized Solar Cells. The effects of post-deposition thermal treatment at 500°C on the optical and structural (density, grain size) properties of the TiO2/ZnO:Al bi-layers have been investigated in details. The effects of such modifications on the surface properties, have been evaluated by dye-sensitization with 5,10,15,20 Tetrakis(4-carboxyphenyl) porphyrin (TCPP). Different interactions of the probe-molecules with the as grown and annealed TiO2 surfaces have been observed. In particular, Quantum Yield (QY) analysis of TCPP-sensitized films showed that the emitted/absorbed photons ratio in the as grown layers is a factor of ~3 lower than in the annealed layers. Based on simplified test solar cells, the electron injection is thought to be the most appropriate mechanism to explain the QY results. Our approach gives particular emphasis to the role of the surfaces and the interfaces involved in the TCPP/ TiO2/AZO multilayer structures with the intent of proposing the as sputtered TiO2 layer as blocking layer in complete architecture for low thermal budget solutions (<200°C).