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The optical constants of many metals commonly used in solar cells, e.g. as contacts, rear side planar reflectors, or more complex nanopatterned light-trapping structures, can vary depending on deposition method, thickness and other factors, and as such are not documented consistently in the literature. In the case of nanometallic light-trapping structures specifically designed to improve absorption in a solar cell, the choice of optical constants used in simulations significantly affects the predicted enhancement, as well as the structure's optimal dimensions. The trade-off between coupling into guided modes in the photovoltaic material and the number of photons absorbed parasitically in the metal leads to small differences in the optical constants giving significantly different results for the quantum efficiency and photogenerated current. This work documents several optical constant sources for silver, aluminium, gold and titanium, and the effect this has on plasmon quality factors. The effect of choosing different optical constant sources on modelling outcomes is quantified by considering the optimization of a test structure comprising a grid of metal nanodisks on the front surface of a thinned-down GaAs cell. Finally, we define a new spectrally-integrated figure of merit for comparing the expected performance of metals in light-trapping structures based on their optical constants, which we name the spectral absorption enhancement factor (SAEF).

The optical constants of many metals commonly used in solar cells, e.g. as contacts or for light trapping structures, are not documented consistently in the literature, with different sources giving different values. In the case of metallic structures designed to improve absorption in a solar cell junction, the use of data from different sources can give strongly varying results for the effectiveness of nanophotonic light-trapping structures. The trade-off between diffraction into more oblique orders in the junction, enhancing absorption in the photovoltaic material, and the number of photons absorbed parasitically in the metal means small differences in the optical constants can lead to different very conclusions about the EQE and $J_{SC}$. This work documents the different optical constants for silver, aluminium, gold and titanium from several sources, the effect this has on plasmon quality factors, and quantifies the effect on modelling outcomes by considering the optimization of a test structure using a grid of metal nanodisks on the front surface of a thinned-down GaAs cell. Finally, we consider the effect for a structure previously predicted to give a very high $J_{SC}$ for a solar cell with an ultra-thin GaAs layer.

We report thin-film InAs/GaAs quantum dot (QD) solar cells with n − i − p + deep junction structure and planar back reflector fabricated by epitaxial lift-off (ELO) of full 3-in wafers. External quantum efficiency measurements demonstrate twofold enhancement of the QD photocurrent in the ELO QD cell compared to the wafer-based QD cell. In the GaAs wavelength range, the ELO QD cell perfectly preserves the current collection efficiency of the baseline single-junction ELO cell. We demonstrate by full-wave optical simulations that integrating a micro-patterned diffraction grating in the ELO cell rearside provides more than tenfold enhancement of the near-infrared light harvesting by QDs. Experimental results are thoroughly discussed with the help of physics-based simulations to single out the impact of QD dynamics and defects on the cell photovoltaic behavior. It is demonstrated that non radiative recombination in the QD stack is the bottleneck for the open circuit voltage ( V oc ) of the reported devices. More important, our theoretical calculations demonstrate that the V oc offset of 0.3 V from the QD ground state identified by Tanabe et al., 2012, from a collection of experimental data of high quality III-V QD solar cells is a reliable – albeit conservative – metric to gauge the attainable V oc and to quantify the scope for improvement by reducing non radiative recombination. Provided that material quality issues are solved, we demonstrate – by transport and rigorous electromagnetic simulations – that light-trapping enhanced thin-film cells with twenty InAs/GaAs QD layers reach efficiency higher than 28% under unconcentrated light, ambient temperature. If photon recycling can be fully exploited, 30% efficiency is deemed to be feasible.

We report on an investigation of a periodic diffractive Si grating structure used to enhance the absorption in thin film Si cells with an active layer 2 μm by modelling. The media of the grating can be air (n=1.0) with detached back reflector or coated with a layer of dielectric material, SiO2 (n=1.4) as a buffering layer. Gratings with periods of 500, 800 and 1200 nm were located on the rear side of the cell, with depths from 150 nm to 300 nm. The reason to use a shallow grating depth is to ensure smallest material loss. The influence of the grating period, depth and surrounding media on the short circuit was investigated and the optimal grating parameters were obtained. A structure which has a 500 nm period, 150 nm depth, in air media gives the best light trapping, with 76.5% enhancement of short current density (Jsc).

Thick wafer-silicon is the dominant solar cell technology. It is of great interest to develop ultra-thin solar cells that can reduce materials usage, but still achieve acceptable performance and high solar absorption. Accordingly, we developed a highly absorbing ultra-thin crystalline Si based solar cell architecture using periodically patterned front and rear dielectric nanocone arrays which provide enhanced light trapping. The rear nanocones are embedded in a silver back reflector. In contrast to previous approaches, we utilize dielectric photonic crystals with a completely flat silicon absorber layer, providing expected high electronic quality and low carrier recombination. This architecture creates a dense mesh of wave-guided modes at near-infrared wavelengths in the absorber layer, generating enhanced absorption. For thin silicon (100 μm) cells. There is potential for 20 μm thick cells to provide 30 mA/cm2 photo-current and >20% efficiency. This architecture has great promise for ultra-thin silicon solar panels with reduced material utilization and enhanced light-trapping.

Effective light trapping is critical for polycrystalline silicon thin-film solar cells to generate sufficiently high photocurrent. Glass substrate texturing is a standard and very effective light-trapping approach for poly-Si solar cells fabricated by plasma enhanced chemical vapour deposition but it cannot be applied to poly-Si cells deposited by electron beam evaporation, which is a preferred deposition process. In this study light-trapping is implemented by texturing of the rear surface of e-beam poly-Si films deposited on planar glass. Water-based solutions of KOH, NH4F and NH4F/H2O2 are found to be able to texture poly-Si films and, thus, to significantly improve light-trapping. The related texturing processes and resulting textures are characterised by Si etching rates, the surface roughness versus removed Si thickness, texture angle distributions, optical absorption and spectral response enhancement. The RMS roughness increases with the removed thickness and can be as large as 276 nm. Also, the texture angle distribution can reach its maximum at about 20° and has a long tail of larger angles. The absorption at 800 nm can increase up to 75% compared to 30-40% in planar films. The short-circuit current of 26.6 mA/cm2 was demonstrated for a cell made of 3.6 μm thick poly-Si film textured by the KOH solution, which is ∼21% enhancement compared to a reference planar cell with a rear reflector. A larger roughness and steeper texture angles produced by NH4F-based etching solutions compared to KOH-based textures indicate that even higher currents are achievable for e-beam poly-Si thin-film cells on planar glass.

Developments in small R&D KAITM systems have resulted in NREL-confirmed stabilised cell efficiencies of 10.09% for amorphous p-i-n and 11.91% for Micromorph tandem devices. Up-scaling of the processes to 1.4 m2 R&D equipment has so far lead to modules having initial powers of 139.1 W for amorphous silicon and 163 W for Micromorph tandem respectively. At present Oerlikon customers produced in total more than 4.5 million modules (a-Si:H p-i-n or Micromorph tandem) which all together correspond to a cumulative total power of over 450 MWp. Recently Oerlikon Solar introduced its new improved production concept, the so-called ThinFabTM, which brings module production costs down to remarkable 0.5 €/Wp at a capacity of 120 MWp.

Adult triatomines occasionally fly into artificially lit premises in Amazonia. This can result in Trypanosoma cruzi transmission to humans either by direct contact or via foodstuff contamination, but the frequency of such behaviour has not been quantified. To address this issue, a light-trap was set 45 m above ground in primary rainforest near Manaus, state of Amazonas, Brazil and operated monthly for three consecutive nights over the course of one year (432 trap-hours). The most commonly caught reduviids were triatomines, including 38 Panstrongylus geniculatus, nine Panstrongylus lignarius, three Panstrongylus rufotuberculatus, five Rhodnius robustus, two Rhodnius pictipes, one Rhodnius amazonicus and 17 Eratyrus mucronatus. Males were collected more frequently than females. The only month without any catches was May. Attraction of most of the known local T. cruzi vectors to artificial light sources is common and year-round in the Amazon rainforest, implying that they may often invade premises built near forest edges and thus become involved in disease transmission. Consequently, effective Chagas disease prevention in Amazonia will require integrating entomological surveillance with the currently used epidemiological surveillance.

We report on the unconventional optical properties exhibited by a two-dimensional array of thin Si nanowires arranged in a random fractal geometry and fabricated using an inexpensive, fast and maskless process compatible with Si technology. The structure allows for a high light-trapping efficiency across the entire visible range, attaining total reflectance values as low as 0.1% when the wavelength in the medium matches the length scale of maximum heterogeneity in the system. We show that the random fractal structure of our nanowire array is responsible for a strong in-plane multiple scattering, which is related to the material refractive index fluctuations and leads to a greatly enhanced Raman scattering and a bright photoluminescence. These strong emissions are correlated on all length scales according to the refractive index fluctuations. The relevance and the perspectives of the reported results are discussed as promising for Si-based photovoltaic and photonic applications.

Over the last few years, micro- and nanophotonics have roused a strong interest in the scientific community for their promising impact on the development of novel kinds of solar cells. Certain thin- and ultrathin-film solar cells are made of innovative, often cheap, materials which suffer from a low energy conversion efficiency. Light-trapping mechanisms based on nanophotonics principles are particularly suited to enhance the absorption of electromagnetic waves in these thin media without changing the material composition. In this review, the latest results achieved in this field are reported, with particular attention to the realization of prototypes, spanning from deterministic to disordered photonic architectures, and from dielectric to metallic nanostructures.

One of the foremost challenges in designing thin-film silicon solar cells (TFSC) is devising efficient light-trapping schemes due to the short optical path length imposed by the thin absorber thickness. The strategy relies on a combination of a high-performance back reflector and an optimized texture surface, which are commonly used to reflect and scatter light effectively within the absorption layer, respectively. In this paper, highly promising light-trapping structures based on a photonic crystal (PC) for TFSCs were investigated via simulation and experiment. Firstly, a highly-reflective one-dimensional photonic crystal (1D-PC) was designed and fabricated. Then, two types of 1D-PC-based back reflectors (BRs) were proposed: Flat 1D-PC with random-textured aluminum-doped zinc oxide (AZO) or random-textured 1D-PC with AZO. These two newly-designed BRs demonstrated not only high reflectivity and sufficient conductivity, but also a strong light scattering property, which made them efficient candidates as the electrical contact and back reflector since the intrinsic losses due to the surface plasmon modes of the rough metal BRs can be avoided. Secondly, conical two-dimensional photonic crystal (2D-PC)-based BRs were investigated and optimized for amorphous a-SiGe:H solar cells. The maximal absorption value can be obtained with an aspect ratio of 1/2 and a period of 0.75 µm. To improve the full-spectral optical properties of solar cells, a periodically-modulated PC back reflector was proposed and experimentally demonstrated in the a-SiGe:H solar cell. This periodically-modulated PC back reflector, also called the quasi-crystal structure (QCS), consists of a large periodic conical PC and a randomly-textured Ag layer with a feature size of 500–1000 nm. The large periodic conical PC enables conformal growth of the layer, while the small feature size of Ag can further enhance the light scattering. In summary, a comprehensive study of the design, simulation and fabrication of 1D-PC- and 2D-PC-based back reflectors for TFSCs was carried out. Total absorption and device performance enhancement were achieved with the novel PC light-trapping systems because of their high reflectivity or high scattering property. Further research is necessary to illuminate the optimal structure design of PC-based back reflectors and high solar cell efficiency.

Effective light trapping is critical for polycrystalline silicon (poly-Si) thin-film solar cells to generate sufficiently high photocurrent. Glass substrate texturing is a standard and very effective light-trapping approach for poly-Si solar cells fabricated by plasma enhanced chemical vapour deposition but it cannot be applied to poly-Si cells deposited by electron beam (e-beam) evaporation, which is a preferred deposition process. Attempts to develop effective light trapping for e-beam poly-Si cells, by texturing of either glass substrate or poly-Si film itself has been studied in this thesis. A spray etching method is designed for glass texturing, but it is proved not to be suitable for producing light trapping textures. Light trapping is then implemented by texturing of the rear surface of e-beam poly-Si films deposited on planar glass. Water-based solutions of KOH, NH4F and NH4F/H2O2 are found to be able to texture poly-Si films and, thus, to significantly improve light-trapping. The related texturing processes and resulting textures are characterised by Si etching rates, the surface roughness versus removed Si thickness, texture angle distributions, optical absorption and spectral response enhancement. Both 2.5% KOH and NH4F-only texturing are found to provide roughness over 200 nm RMS, steep peak feature angles up to 20˚ and higher absorption enhancement of 30~40% compared to planar films. With a removed Si thickness of less than 2 µm, NH4F-only texturing provides more advantages than 2.5% KOH etching. In order to apply this etch-back texturing to functional e-beam cells, a new spin-on diffused back-surface-field (BSF) subsequent to the texturing process has been successfully developed and therefore the short-circuit current (Jsc) enhancement with various etch-back texturing can be compared. A 3.6 µm thick e-beam cell with KOH etch-back texturing was demonstrated ~21% Jsc enhancement compared to a reference planar cell with a rear reflector and a 3 µm thick e-beam cell with NH4F-based texturing is achieved even higher Jsc enhancement of 28%. Thus, it is demonstrated that a wet-chemical etch-back texturing is a very promising approach to light-trapping in e-beam cells on planar substrates.

The photovoltaics (PV) research community is currently pursuing many approaches to reduce the cost of PV and increase the energy conversion efficiency. Single-crystalline silicon (sc-Si) solar cells are able to achieve high efficiency but have a higher cost relative to other technologies. It may be possible to drastically reduce the cost of sc-Si PV by fabricating solar cells which are an order of magnitude thinner than conventional solar cells, i.e. thinner than 30 microns. Aside from new fabrication paradigms, ultra-thin sc-Si solar cells require advanced light-trapping techniques to enhance the absorption of long-wave radiation which is otherwise transmitted through the cell. In this thesis, a novel process flow for the fabrication of ultra-thin sc-Si solar cells in the laboratory was designed and implemented with the aim of testing light-trapping structures in the context of actual ultra-thin sc-Si devices. The process flow uses 10 micron thick sc-Si membranes, 0.95 cm in diameter, fabricated on silicon-on-insulator wafers using double-sided processing. The best fabricated device incorporated a back surface field, a white paint diffuse rear reflector and a silicon nitride antireflection coating. It achieved a fill factor, efficiency, short circuit current and open circuit voltage of 0.67, 9.9%, 27.9 mA cm-2 and 0.53 V respectively. Simulations suggest the device efficiency can approach 15.4% without light-trapping and 16.5% with a diffuse rear reflector as a light trapping structure. This process flow is intended to be used as a platform on which to test further light-trapping structures with the continuation of this project. Master of Applied Science (MASc)

The solar energy industry strives to produce more and more efficient and yet cost effective photovoltaic (PV) panels. Integration of specific micro/nano optical structures on the top surface of the PV panels is one of the efficient ways to increase their PV performance through enhancing light trapping and in-coupling. In this study, periodic triangular gratings (PTGs) in polymethyl methacrylate (PMMA) were numerically simulated and optimized. The goal of this study is to enhance the ability of solar panels to convert maximum obtainable amount of solar energy by improving the optical in-coupling of light to PV material. Initial optical simulation results shown that a flat PV panel (without any enhancing micro-optical structures) exhibits an average incident light power of 0.327 W over a range of the incident light angles between 15° and 90°. Introduction of the PTG allows capturing the incoming sunlight and reflecting it back onto the PV material for a second or more chances for absorption and conversion into electricity. The light trapping and redirection is achieved through the total internal reflection (TIR) phenomenon. Geometry of the PTG was initially optimized with respect to an incident sunlight orientation of 15°, 30°, 45°, 60°, 75°, and 90°. Optical performance of the particular optimized PTGs was analyzed over daylight conditions and several optical parameters, such as average incident power and intensity, were calculated when sunlight orientation angle was changing from 15° to 90°. By adding the PTG optimized for 15° incidents light, an average incident power of 0.342 W was achieved (4.6% improvement of optical performance). Functional PTG prototypes were fabricated with optical surface quality (below 10 nm R a). The simulation results allow understanding how the overall daytime photovoltaic performance of solar panels can be improved. © 2012 SPIE., Physics, Simulation, and Photonic Engineering of Photovoltaic Devices, January 23-26, 2012, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 8256

Light-incoupling and light-trapping are major prerequisites for high efficiencies in solar cells and modules. This is especially the case for silicon-based thin film solar cells, because the absorber thickness must be smaller than the absorption length of sunlight, for both physical and economic reasons. This thesis investigates the directional and spectral dependence of light-incoupling and light-trapping in solar cells. The directional dependence of light-incoupling for glass-covered solar cells is dominated by the reduced transmittance under angles exceeding the Brewster angle. The reduced transmittance results in a loss in the annual yield of solar cells of about 2 %. The light-trapping does not notably change under increased angles of incidence. To enhance the incoupling at the front of the solar cell, the effects of a textured surface structure on the cover glass of the solar cell are investigated. The texture reduces the reflectance at the air-glass interface and, additionally, reduces the reflection losses originating at the interface between the glass and the transparent conductive oxide (TCO) as well as the TCO and the silicon (Si) absorber due to the randomization of light. On samples without a textured TCO/Si interface, the textured foil induces additional light-trapping in the photovoltaically active absorber material. This effect is not observed for samples with a textured TCO/Si interface. In this case, using tandem solar cells, a redistribution of light absorption in the top and bottom subcells is detected. The anti-reflective texture increases the short circuit current density in thin film silicon tandem solar cells by up to 1 mA/cm², and the conversion efficiency by up to 0.7 % absolute. The increase in the annual yield of solar cells is estimated to be up to 10 %. Further, the spectral dependence of the efficiency and annual yield of a tandem solar cell was investigated. The daily variation of the incident spectrum causes a change in the current matching of the serial connected subcells. Simulations determine the optimum subcell layer thicknesses of tandem solar cells. The thicknesses optimized in respect to the annual yield overlap in a wide range for both investigated locations with those for the AM1.5g standard spectrum. Though, a slight top limitation is favorable. The daily variation is more crucial than the geographic location. Technologically it is not straight forward to design a tandem solar cell with optimum subcell thicknesses for maximum efficiency. Matching the short circuit currents of the subcells maximizes the overall current, but minimizes the fill factor. This thesis introduces a new definition for the matching condition of tandem solar cells. This definition optimizes the power output of the solar cell, instead of the short circuit current density. A new method is proposed to quantify the power mismatch of a given solar cell by varying the composition of the incident spectrum. Experimentally, it is shown exemplary on a silicon thin film tandem solar cell that the efficiency can be increased by 0.5 % absolute by adopting the layer thicknesses to the power matching instead of the current matching. Moreover, the efficiency under standard conditions can be interpolated from the measurements without the need for time-consuming calibrations. It is further shown in this thesis that an angle and energy selectivity of the surface of a solar cell can enhance the light-trapping, and lead to efficiencies in very thin cells above the theoretical limit for non-concentrating solar cells. Simulations determine a theoretically achievable gain of up to 33 % in the annual yield if the cell is tracked to the solar path. For non-tracked solar cells, simulations show that there is a wide angle and energy range of acceptable restrictions that lead to only minor losses but also not to gains. In experiments on hydrogenated amorphous silicon thin film solar cells an enhancement of the overall path length in the device up to a factor of 3.5 was demonstrated using a Bragg-like angle selective filter. This result emphasizes the potential for directional selectivity to improve light-trapping. The total benefit of this potential is, however, limited by parasitic absorption in the adjacent doped layers, in the transparent conductive oxide and at the back reflector. The effective path length enhancement was a maximum factor of about 1.5, in the wavelength regime close to the band gap. Nonetheless, this thesis demonstrates an improvement in the short circuit current density of hydrogenated amorphous silicon solar cells by 0.2 mA/cm². This result is the first experimental proof that the concept of directional selectivity is able to improve solar cell performance.

The object of this study lasting several years was to evaluate the suitability of light-trapping methods for studying the ecology of moths in general as well as questions concerning nature conservation in particular. It addresses the question as to whether and to what extent the habitats of adult moths and their respective pre-imaginal stages coincide. To answer this question the small-scale spatial distributions of pre-imaginal and imaginal stages of moths were compared on the basis of field data obtained in a 16,9 ha study area situated to the west of the city of Aachen, Germany. Whilst pre-imaginal stages were searched by means of many species-specific methods throughout the whole study area, adult moths were mainly light-trapped at a few selected sites. They were sampled with the help of six weekly and simultaneously operated Minnesota-type light-traps equipped with 8-W blacklight lamps. In addition a so-called “gauze tower” with high-powered light sources was used fortnightly at one site in the centre of the study area. In the course of some additional mark-recapture experiments 13.281 adult moths trapped in the study area as well as 450 laboratory-grown moths were individually marked and released. A total of 30.086 individuals and 326 species was recorded throughout the whole study period: 19.586 adult moths and 10.500 pre-imaginal individuals of 301 and 215 species respectively. 198 species were found in a preimaginal stage at eight different study sites (16 to 103 per study site). Their spatial distribution not only depended on the definite stage of development (egg, larval instars, pupa), but sometimes it even varied subject to generation and specific sampling date (before or after hibernation). By means of detailed autecological data analysis it could be shown that the habitat preferences of many species are narrower than generally assumed. A total of 277 species of adult moths was caught in the core study area by means of light-trapping: 246 species (102 to 167 per site) in the traps and 218 at the “gauze tower”. The six light-traps proved to be almost as effective as the “gauze tower” and have the additional advantage of providing data on the spatial distribution and thus habitat preferences of the moths. The results of the mark-recapture experiments showed that the probability of recapture was highest at the release site when released at the trapping site whilst artificially transferred moths often showed a tendency to return to their original trapping-sites. Including other results this leads to the conclusion that the habitat preferences of moths are not only due to mere site fidelity but also to real close ties with their habitats, which, however, seem to be much weaker than those of their pre-imaginal stages. The comparison between the adult and larval fauna of moths revealed extensive overlapping. For example, one can assume that more than 90% of the light-trapped moth species can at least occasionally reproduce and develop within the study area. The species overlapping between the adult fauna of the light-trap sites and the pre-imaginal fauna of the related surrounding areas increased with expanding radius of the area under consideration mainly in consequence of the higher number of observed pre-imaginal individuals and species in the larger areas, but the species overlapping often met a maximum at a radius of only 100 m. The majority of moth species even showed a positive correlation between the spatial distribution of pre-imaginal and adult stages: the more specimens of pre-imaginal stages were found in the surroundings of a specific light-trap site, the more adult individuals of the same species were collected in the respective light-trap. All in all the light-trapped moth fauna can be regarded as a good copy of the pre-imaginal fauna inhabiting the vicinity of the respective light-trapping site. There is much evidence that the majority of the light-trapped adult moths had already entered the biotopes adjacent to the trapping site without having been influenced by the light. The light-trap catches of so-called “guest species” (= species which do not reproduce in the vicinity of the light-trapping site) are therefore rather based on the natural vagility of the adult moths than on the attracting effect of the light. However, the mobility of adult moths seems to be limited, and even though the adult moths seem to have much weaker ties with their habitats than their pre-imaginal stages, they often stay in the vicinity of their larval habitats. This site fidelity of adult moths seems to be the main reason for the observed spatial overlap of the imaginal and pre-imaginal habitat spectra whereas the “habitat fidelity” of the moths makes only a small contribution to this phenomenon. However, for a better understanding of the specific habitat requirements of both larval and adult moths light-trapping can neither substitute the labour-intensive search for the larval stages nor the direct observation of the behaviour of adult moths. Nevertheless, light-trapping can be a useful tool in ecological research and therefore it should not be regarded as a purely faunistic collecting method.

A light-trapping scheme for normal incidence Ge-on-Si photodiodes, utilizing needle-like black silicon nanostructures is presented. Simulations reveal that light absorption in thin rear Ge films can be enhanced several times due to both the antireflection and the scattering effect of the nanostructure. It is shown that especially films with thicknesses ? 100 nm benefit from the black silicon nanostructure, e.g., resulting in a 5 to 10 times higher absorptance at 1500 nm for a 100 nm thick film. Theoretical predictions are experimentally proved by reflectance-transmittance measurements of crystalline Ge films sputtered on black silicon substrates.

We propose and demonstrate colloidal quantum dot hybridized, radial p-n junction based, nanopillar solar cells with photovoltaic performance enhanced by intimately integrating nanocrystals to serve as light harvesting agents around the light trapping pillars. By furnishing Si based nanopillar photovoltaic diodes with CdSe quantum dots, we experimentally showed up to sixfold enhancement in UV responsivity and similar to 13% enhancement in overall solar conversion efficiency. The maximum responsivity enhancement achieved by incorporation of nanocrystals in the nanopillar architecture is found to be spectrally more than four times larger than the responsivity enhancement obtained using planar architecture of the same device. (C) 2010 American Institute of Physics. [doi:10.1063/1.3485294]

The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm2 cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

High-efficiency thin-film silicon triple-junction solar cells in p-i-n configuration have been fabricated using amorphous silicon top cell absorber layers, as well as microcrystalline silicon middle and bottom cell absorbers. The triple-junction cells were fabricated on boron doped zinc oxide (ZnO) films with different surface morphologies. To this end, the naturally grown rough ZnO surfaces were flattened using an Ar plasma for three different treatment times. For the shortest time, we achieved a summed current density over 30 mA/cm(2) and initial and stabilized conversion efficiencies of 13.5% and 12.5%, respectively. For the medium treatment time, we obtained the highest efficiencies (13.7% initial and 12.8% stable), whereas the longest treatment time led to the highest open-circuit voltage (V-OC) of 1.91 V but lower current densities, leading to efficiencies of 12.9% initial and 12.2% stable, respectively. These results were obtained by combining various recently developed features and approaches: first of all, we implemented high-quality mu c-Si: H cells with novel buffer layers, leading to very high efficiencies. Second, we applied randomly textured pyramids on the front glass to improve light in-coupling, and finally, we used very thin (similar to 140 nm) top cells that led to a low light-induced degradation (5%-7% relative loss in efficiency).