Intrinsic properties of double perovskite materials affecting optoelectronic devices
Keywords: structural characterization
optoelectronics
optical characterization
novel materials
semiconductors
perovskite
solar cells

Hybrid halide perovskites have attracted huge attention ever since it was shown that they could be used in highly efficient photovoltaic devices produced via low-cost deposition methods. Their exceptional attributes, including high carrier mobility, an adjustable spectral absorption range, long diffusion lengths, and the simplicity and affordability of fabrication make them one of the most exceptional and market competitive optoelectronic materials for applications in photovoltaic, light emitting diodes, photodetectors, lasers and more. Despite their phenomenal properties, several crucial issues still need to be tackled before their industrial-scale use, including toxicity and instability. Whereas an ever-growing number of studies focus on improving optoelectronic properties, there is still an urgent need for a detailed understanding of these materials at a more basic level. The objective of this thesis is to understand how the intrinsic properties of halide perovskites affect the devices performances. Particular attention is given to double perovskites, as they proved to be particularly stable and they can easily be synthesized without any toxic element. Within this family, Cs2AgBiBr6 has received increasing attention as a promising candidate for optoelectronic applications. In this study, we explored the relationship between the structural and optical properties of this material. In particular, we discovered the presence of a low-temperature cubic-to-tetragonal phase transition, which influences the optical properties quite dramatically. In fact, we found that the exciton energy is mainly dependent on the new rotational degree of freedom present in the tetragonal phase. Moreover, the temperature-dependent time-resolved photoluminescence measurements revealed the presence of a carrier decay component that is only present in the tetragonal phase, which has been associated with the formation of tetragonal twin domains. Cs2AgBiBr6 is an indirect band-gap material, with a bandgap energy around ~ 2 eV. The performances of devices using this double perovskite as the absorber layer are still quite low (PCE ~ 2%). To explore the possibility of tuning the optoelectronic properties of this double perovskite, we explored the use of indium (InIII) alloying. The structural, optical and electronic properties of Cs2AgBi1-xInxBr6 have been explored at room and low temperature. Our structural analysis indicates that the presence of indium shrinks the lattice and shifts the transition temperature towards lower values. Moreover, we found that the structural phase transition in Cs2AgBiBr6 is very close to a tricritical point and acquires the character of a second-order phase transition when indium is substituted on the bismuth site. Our spectroscopic studies indicate that the presence of indium enlarges the band gap of the material, and at the same time strongly enhances the photoluminescence intensity. Moreover, the photovoltaic performance improves, demonstrating a higher PCE in the presence of 10% indium. Nevertheless, the PCE is still considerably low, and it is not clear whether these materials will ever be able to perform as well as lead-halide perovskites. Our thermodynamic modelling calculations investigate the fundamental limitations of the system. Significant losses are present in the open-circuit voltage because of a shallow onset of the absorption edge, combined with significant sub-band gap absorption. For this reason, immediate attention is required to understand and mitigate the presence and role of non-radiative sites in the absorber material. Regardless of whether Cs2AgBiBr6 devices can be optimized, Cs2AgBiBr6 is an interesting model system for understanding the physics of the wider family of halide perovskites. All these compounds have closely related crystal structures and understanding structure−property relations is highly relevant for materials that are already known and for guiding the design of future compounds.