Detailed Raman Investigation on the Search for the Secondary Phases in the Chalcogenide Perovskite BaZrS3

Remarkable progress in photovoltaic has been made by the halide perovskites, paving the way for overcoming the Shockley-Queisser-limit of 33.7 % of a single junction solar cell by in tandem photovoltaic applications. However, the biggest challenge for the commercialization is the stability of the halide perovskites which are rather prone to degradation through air, light, heat, and electric field mechanisms. Alternatively, highly stable chalcogenide perovskite materials, containing S, Se instead of halides might maintain their remarkable optoelectronic properties. Among these materials, BaZrS3 with a bandgap of around 1.9 eV and strong absorption coefficient attracts most attention due to the possible integration as a top cell in tandem applications. However, the biggest hurdle seems to be the synthesis of these materials at moderate temperatures on conductive substrates in order to get phase-pure material. Here, we investigate the formation of secondary phases in the low-temperature synthesis of BaZrS3 at 550 °C, compatible with device integration, with various annealing durations. Raman analysis reveals secondary phases such as BaS3 and ZrS3 which cannot be readily identified from X-ray diffraction. Longer annealing times improve the phase purity of BaZrS3.