Corrado Comparotto: Synthesis of BaZrS3 perovskite thin films for photovoltaic applications

Abstract

BaZrS3 has emerged as a promising lead-free perovskite with optoelectronic properties well-suited for photovoltaic applications. This thesis aims to develop a processing method for fabricating BaZrS3 thin films for integration into solar cells, as well as to investigate the optoelectronic properties of this novel material. A two-step approach was adopted, beginning with the sputter deposition of precursors followed by thermal treatment to induce crystallization. Various synthetic routes were explored. Initially, amorphous Ba-Zr-S precursors were grown via reactive co-sputter deposition and subsequently annealed in an inert atmosphere. This process resulted in one of the first reports of BaZrS3 thin films. The synthesized material exhibited strong photoluminescence, with both the peak position and intensity dependent on the annealing temperature. The strongest signal was observed at 900 °C, with an emission peak centered at 1.84 eV. X-ray diffraction confirmed that the highest degree of crystallinity was also achieved at approximately 900 °C. To decrease the synthesis temperature and facilitate the integration of BaZrS3 into solar cells, alternative approaches were explored. Ultimately, sulfurization of co-sputter deposited Ba-Zr precursors—capped with SnS for protection against excessive oxidation—resulted in the formation of the perovskite at temperatures below 600 °C, enabling the fabrication of the first BaZrS3 film grown on conductive substrates. Furthermore, the chemical reactions leading to perovskite formation at moderate temperatures were examined in detail. It was demonstrated that the intermediate solid phase BaS3 accelerates BaZrS3 crystallization compared to BaS2, which in turn is more beneficial than BaS. During the process, the first pressure-temperature phase diagram for the Ba-S system was constructed, providing a valuable framework for synthesizing not only BaZrS3 but also a variety of Ba-containing sulfides. The synthesis methods developed in this thesis enabled the fabrication of high-quality BaZrS3 films suitable for optoelectronic characterization. Notably, the carrier concentration could be tuned from intrinsic to metallic levels—a highly desirable attribute for device fabrication, including photovoltaics. Looking forward, future work should focus on identifying a compatible semiconductor partner to from an effective p-n junction, as well as on increasing grain size to enhance carrier collection in BaZrS3-based solar cells.