Indium-based transparent conductive oxides developed for perovskite and perovskite-silicon tandem solar cell applications

Silicon solar cells are a mature PV technology; however, they are approaching their fundamental efficiency limit. Further efficiency improvements require a technological change towards silicon-based tandem solar cells, where a second absorber material with a higher band gap is stacked on top of silicon to reduce thermalization losses. For this purpose, perovskite-based solar cells have gained growing interest due to their rapid improvements in power conversion efficiencies, and promises of lower levelized costs of electricity. However, for perovskite-silicon tandem solar cells to surpass the current industry standard of silicon solar cells, efficiencies higher than 30 %, and long-term device stability are required.
This paper focuses on the goal of improving the overall device efficiency of perovskite-silicon tandem solar cells by optimization of the top transparent conductive oxide (TCO) electrode. Sputtering processes for the TCOs indium tin oxide (ITO) (standard DC sputtering with ceramic target) and indium zinc oxide (IZO) (reactive serial co-sputtering process - magnetron) were developed and investigated. TCO films were optimized with regard to their optical properties, quantified here as the absorption weighted with solar spectral data, as well as to their electrical characteristics assessed with 4-pt resistivity and Hall-effect measurements to quantify resistivity, carrier concentration, and mobility. The best performing TCOs were then integrated as front electrode in perovskite and perovskite-silicon tandem solar cells to evaluate their impact on the device level. The optimized IZO film resulted in up to 1 mA/cm2 improvement of short circuit current density of the tandem solar cells.