M. Morales-Masis: Transparant electrodes for high-efficiency photovoltaics

In this presentation we discuss the status and recent developments in the field of transparent electrodes for high-efficiency solar cells, with special focus on transparent conductive oxides. Thin-film PV technologies (hybrid halide perovskites, CdTe, CIGS) but also high-efficiency crystalline silicon heterojunction (SHJ), perovskite-on-silicon tandem cells, as well as new PV designs that are bifacial and semitransparent, rely on the use of transparent conductive oxides (TCO). The implementation of TCOs that combine exceptionally low optical absorption from the UV to the IR with high lateral conductivity is crucial to avoid parasitic absorption or electrical losses in the solar cell. The interfaces between the transparent electrode and the adjacent device layers should ensure minimum contact resistance and light in-coupling need to be optimized, by for example tuning of the refractive index of the TCO electrode. The main challenge for developing high-performance TCOs, is to fulfill simultaneously many, if not all of these requirements, with their final properties being driven by the inherent trade-off between the electrical and optical properties. Hydrogen-doped and zirconium-doped In2O3 TCOs are leading the race of the ideal transparent electrodes for solar cells, mainly due to their exceptionally high electron mobility (> 100 cm2/Vs) that can be achieved at low deposition temperatures (< 200 °C), their wide band gap (between 3.5 and 3.9 eV) and their low sub-bandgap absorption. These properties are strongly linked to a low density of defects, which results from a high crystalline quality and passivated grain boundaries. Here we compare these high mobility TCOs, from their fundamental properties to their integration as front electrode in SHJ cells and in semitransparent hybrid halide perovskites for tandem cells. Finally, we present recent efforts to replace indium in TCOs by applying material design and combinatorial synthesis with Sn-based TCOs as main example.