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

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.