Review: Progress in solar cells from hydrogenated amorphous silicon

Hydrogenated amorphous silicon (alpha-Si:H) has been used for decades doped and as intrinsic absorber layers in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that alpha-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

Contact Design for Silicon Heterojunction Solar Cells

Luca Massimiliano Antognini

Today more than ever the world needs clean energy sources and thus a fast deployment and scaling up of the photovoltaic industry. In this context improving solar cell efficiency plays a major role. In order to achieve the maximum single junction efficiency for this material, technologies featuring carrier selective passivating contacts are foreseen to be the next successors of actual industrial cells. Among them, the silicon heterojunction (SHJ) solar cell is a promising technology which demonstrated efficiencies up to 26.7%. Its main specificity consists in the use of intrinsic and doped amorphous silicon (a-Si:H) to provide surface passivation and carrier selectivity to the c-Si, which allows to reach very high operating voltages in comparison to other c-Si based technology. The major drawbacks of SHJ solar cell is the large parasitic absorption occurring in those layers. To address this challenge, a strong focus on alternative non-silicon based thin films with wider band gap has built up in the literature. A first part of this thesis focuses on reviewing the fundamental process behind carrier selectivity and discuss the ability of different models from the literature to draw informations directly from the peculiar shapes of IV curves commonly observed in the solar cells featuring newly developed materials. We first discuss the simple four ideal diodes model of Roe et al to explain the occurrence of S-shapes depending on the contact quality. We discuss from a theoretical point of view how to improve the model by modifying the ideal diodes by other circuit elements as well as adding bulk recombinations. We test the theory experimentally and identified modifications that should be incorporated in the model.In a second part of the thesis, we explore the development of doped nano-crystalline silicon (nc-Si) layers to replace classical amorphous silicon layers. We investigate the influence of TMB and BF3 as dopant sources on the transparency and contact properties of nc-Si:H(p) layers as well as their integration in solar cells. We expose the roles of both gas to modify the crystallinity of the layer and find different optimum for both of them to lead high efficiencies, illustrated by a certified 23.9%-efficient solar cell. We report also on further optimization steps of this front junction architecture resulting in certified efficiency of 24.44%.We explore next the n-type nc-Si layers for application as window layer. In particular, we perform thickness and doping series to unravel the layer properties along its growth direction. In order to improve the low doping of the nucleation zone of nc-Si:H(n) at the i/n interface, a thin a-Si:H(n) buffer layer is introduced and shown to improve the passivation, selectivity and contact resistivity. Finally, we also report on the beneficial effect of an additional SiOx capping layer which improves both the reflection properties as well as all contact properties, broadening the optimal parameter window. The absorption loss in the front silicon layers remains however the highest share of losses, and therefore we explore theoretically the opportunity of using thinner front side layers or using thin TCO / silicon nitride bilayers to reduce the absorption losses. We finish the discussion by presenting an optimization roadmap, showing a possibility for double-side contacted SHJ cells to reach efficiencies above 26% without requiring any patterning or localization step.

EPFL2022

Development of high band gap protocrystalline material for p-i-n thin film silicon solar cells

Fabio Maurizio

High band gap thin film silicon solar cells are of high interest for the use as top cell in triple junctions solar cells. Protocrystalline silicon, in the transition phase between amorphous and microcrystalline silicon, is such a high band gap material. During this project, protocrystalline intrinsic layers have been developed and optimized in p-i-n single junction solar cells by plasma-enhanced chemical vapour deposition (PECVD) in a new multi-chamber deposition system. In the first series, cells and layers were deposited with different dilutions going from silane to hydrogen flux ratios of 1:1 to 1:64. At the dilutions 1:16 and 1:32, protocrystalline ma-terial was obtained which showed amorphous characteristics only up to a certain thickness, where crystallites started to evolve. With dilution 1:16, a second cell series has been deposited with variation of the intrinsic layer thick-ness. The best cells in terms of open-circuit voltage and fill factor product were obtained with an i-layer thickness of 100 nm. These cells have open-circuit voltages and fill factors up to 0.975 V and 75.8 % (0.932 V and 69.9 %) for the initial (degraded) state. These are promising values for the use of this material in top cells. However, the low current density means that the optimum i-layer thickness is probably higher for triple cell application. Further cells have been deposited at different temperatures. These results imply that there is still room for further optimization, when current limitations of the heating system will be overcome.

2012

Development of Highly Efficient Perovskite-on-Silicon Tandem Solar Cells

Florent Sahli

Crystalline Silicon (c-Si) solar cells are dominating the photovoltaic (PV) market. Owing to their large manufacturing capacity, reliability and efficiency, c-Si solar cells are now cost-competitive with other non-renewable electricity sources in many places. The price of c-Si modules has been decreasing drastically for the past decades. They now account for less than half of the cost of a PV system. Other costs come from the balance-of-system and these are rather inflexible. And so increasing efficiency of solar modules is the most efficient approach to lower the cost of PV electricity. One issue is that c-Si cells are approaching their efficiency limit. One strategy to increase efficiency beyond this limit relies on adding another solar cell on c-Si to form a tandem solar cell as thermalisation losses are reduced. Metal lead halide perovskite solar cells are promising top cell candidates for c-Si based on their high optoelectronic properties, band gap tunability, high efficiencies, ease of manufacturing and low material costs. By combining both technologies, efficiencies >30% are realistic, well above best-in-class c-Si cells. This thesis aims to produce high efficiency perovskite solar cells for 2-terminal monolithic tandems on c-Si. First, we develop a perovskite fabrication method, which employs thermal evaporation to produce a lead halide template and then organohalide spin-coating. By varying composition at each step, the perovskite band gap can be tuned in the range 1.6-1.8 eV, ideal values for 2-terminal tandems. The use of the template makes the deposition compatible with various substrate textures. Then, we develop a recombination junction that features nanocrystalline hydrogenated silicon layers (nc-Si:H). When used with a font-side polished c-Si bottom cell, it shows a superior optical performance compared to standard transparent conductive oxides thanks to a better matching of refractive indices. Owing to its low conductivity, the top cell leakage current is reduced, enabling to scale-up the cell area. Then, perovskite/c-Si tandems featuring a double-side textured c-Si wafer are demonstrated, achieving a certified efficiency of 25.2%. This efficiency, the highest at the time of publication, is enabled by low reflection losses and efficient light trapping thanks to the c-Si pyramids present on both sides. More importantly, this double-side texture yields an optically close-to-optimum system that is simpler and more efficient compared to alternatives. Furthermore, this top cell process flow does not require any modification to existing c-Si manufacturing production lines as these use front-side textured c-Si. In the second part, we replace the spin-coating of organohalides by a vapour transport deposition (VTD) using a home-made vapour transport deposition setup that offers large processing flexibility. Using a thermally evaporated lead iodide template and methylammonium iodide vapours, perovskite solar cells with an efficiency > 12% are made. Thanks to the presence of a showerhead, a homogeneous perovskite growth is obtained on 6 inch textured c-Si substrates, the industry standard. The VTD of formaminidium iodide (FAI) is more challenging. A trimerisation of FAI forms sym-triazine, which does not react with the template to form the perovskite. Still, in the presence of ammonium cations, sym-triazine can be cleaved to form formamidine, thus offering an alternative pathway to deposit perovskite layers.