Synthesis of Calcohalides by Sequential Co-Evaporation and High-Pressure Annealing Process for Photovoltaic Applications

The interest in low-dimensional materials has been regained due to the re-discovery of the high potential of these materials as absorbers for high-efficiency photovoltaic conversion. In particular, recent progresses in research on the quasi-1D material Sb₂(S,Se)₃, whose efficiency has been steadily increasing over the last decade, has attracted the interest of the scientific community to this class of materials. Among other interesting properties, these materials have the advantage of being constituted by abundant elements of low or no toxicity. Theoretical modelling has shown the very interesting properties of a new class of semiconductors based on semi-metal chalcohalides ([Bi,Sb][S,Se]X, with X = Br, I). Among them, they present unique electrical properties due to their anisotropic crystalline structure, potential defect tolerance and ferroelectricity similarly to the perovskite family. The potential applications of these materials range their implementation in tandem, semitransparent PV technologies and photocatalysis due to their bandgaps ranging from 1.6 to 2.3 eV.<br/>In this work the properties of SbSeBr synthesized by co-evaporation of Sb₂Se₃, followed by a reactive annealing under halide atmosphere at different pressures above 1 atm that allows a better control on the incorporation of the halogen element into the crystal structure. A wide range of synthesis conditions are explored by modifying the reactive annealing pressure, temperature and duration. The first findings reveal that both the pressure and the temperature have a great effect on the size of the small irregular grains. Compared to analogous material such as SbSeI, this material requires relatively long process times to achieve a complete transformation of Sb₂Se₃ to SbSeBr. It is interesting to note that, so far, no reports on the synthesis of this particular material by means of the versatile, effective and easily scalable chemical routes are published.<br/>The composition and crystal structure are characterized by combining X-Ray Fluoresence, X-Ray diffraction, Raman spectroscopy, and Energy-dispersive X-ray spectroscopy. The first results show that by the method developed in our lab it is possible to obtain a single-phase material with a very high degree of both purity and crystallinity. An identification of the main Raman modes will also be presented.<br/>The absorber layers have been used to fabricate solar cells, reporting the first working devices for this material with efficiencies above 0.5% and with a very encouraging Voc (&gt;600 mV). The main current limiting factor for the efficiency of the devices is the complex morphology of the SbSeBr layers, suggesting that a proper optimization of the novel synthesis process and an appropriate choice of Hole Transport Layer and Electron Transport Layer can boost the efficiency of these promising materials.<br/>Finally, once the processing conditions are fully optimized, solar cell devices will be completed in both substrate and superstrate configurations, opening promising pathways for the development of highly efficient solar cells based on quasi-1D materials by a proper selection of device configuration, transport layers and material synthesis conditions.