Novel 1D van der Waals SbSeI micro-columnar solar cells by a self-catalyzed high pressure process

Emerging 1D van der Waals chalco-halide semiconductors are attracting a lot of interest as photovoltaic absorbers, due to their unique structural, electrical and optical properties. In particular, the mixed chalco-halide compound SbSeI, that tends to easily form highly crystalline nanowires, have demonstrated efficiencies exceeding 4%, with a bandgap of 1.80 eV, and a synthesis temperature below 300°C. These properties are very interesting for its possible future application in tandem solar cells concepts among others. In this work, SbSeI micro-columnar solar cells are obtained by an innovative process based on the selective iodination of Sb2Se3 layers at high pressures, by using SbI3 as iodine source. Annealing parameters such as temperature, time and pressure are investigated, and a complete morphological, structural and compositional characterization of the absorbers is performed. We observe a self-catalyzed solid-liquid-vapor transformation process of Sb2Se3 thin films into micro-columnar SbSeI at annealing temperatures above 250°C. Highly crystalline micro-columnar structures are obtained as it is demonstrated by XRD and TEM analysis, which sizes strongly depends on the annealing temperature, time and pressure. The extracted activation energy of the processes suggests that the SbSeI micro-columns are formed very fast thanks to the release of liquid Se during the decomposition of Sb2Se3 under iodine atmosphere. First solar cell prototypes were fabricated using standard thin film solar cell substrate configuration, demonstrating devices with an open-circuit-voltage higher than 550 mV and conversion efficiencies close to 1%, which are currently limited by the low short-circuit current and fill factor, due to the deficient coverage of the front contact. Finally, a complete analysis of the SbSeI materials and solar cell devices will be presented, together with strategies to control de size and orientation of the micro-columns, and the improvement of the front contact interfaces.