Perovskite/Silicon Tandem Solar Cells: Toward Affordable Ultra-High Efficiency Photovoltaics ? (2018)

Nowadays the photovoltaics market is dominated by crystalline silicon solar cells, which are approaching their theoretical efficiency limit. Novel solutions have to be found for photovoltaics to further enhance its competitiveness with other energy sources. One of the most promising approaches lies in combining market-proven silicon solar cell technology, having a low optical band gap, with an efficient near-infrared transparent wide-band gap top cell to form a tandem cell. Organic-inorganic halide perovskite solar cells are promising candidates for top cells, showing high efficiencies with simple and cost-effective device fabrication. We first develop perovskite solar cells specifically for tandem applications. We develop and optimize a hybrid sequential deposition method combining thermal evaporation and solution processing to fabricate perovskite absorber materials with specifically tailored optoelectronic properties. These materials are systematically investigated for their optical, structural and electronic properties, and then applied in perovskite solar cells both in n-i-p and p-i-n configurations, for which we show several charge transport materials combinations. As a replacement for the standard metal opaque rear electrode of perovskite cells, we develop a TCO-based transparent electrode, for which sputtered IZO is presented as a good candidate thanks to its high carrier mobility and broadband transparency. Sputtering-induced damages are reduced by the introduction of a buffer layer: MoOx for n-i-p cells and SnO2 for p-i-n cells. The parasitic absorption losses in the MoOx are described in details. We then discuss the parasitic absorption losses in charge transport layers and TCOs, with experimental comparison of various materials. The optimisation of the perovskite absorber deposition method and the improvement of the charge transport layers and transparent electrode allows the fabrication 1cm2 area semitransparent perovskite solar cells with &gt;16% efficiency. We then integrate the semitransparent perovskite cells in mechanically stacked 4-terminal tandem solar cells. The challenges in reducing the strong parasitic absorption and reflection losses are first discussed. We then demonstrate 4-terminal tandem measurements with &gt;25% total efficiency with a small area top cell. With larger 1cm2 area top cells, we fabricate integrated 4-terminal tandem devices with both subcells having similar size and total efficiency &gt;23%. The integration of perovskite solar cells in 2-terminal monolithically connected tandem solar cells with silicon heterojunction bottom cells is finally presented. First, we show the development of a TCO-based recombination layer and the important reflection losses and interference effects observed in all-flat devices. The origins of parasitic absorption losses in monolithic tandems are then explained and new architectures and materials are investigated supported by optical simulations. We then replace the polished wafers by fully textured silicon bottom cells for better light management. The development of a tandem device with the top cell conformally coated onto the textured bottom cell is explained, leading to &gt;25% certified power conversion efficiency. The end of the thesis presents preliminary results on the up-scalability and light soaking stability of the developed textured tandems, as well as a proof-of-concept of a first perovskite/perovskite/silicon triple junction solar cell on textured wafers.