Emerging Chalcogenides: A New Family of Perovskite-Inspired Sustainable Energy Materials

In the last ten years, lead-halide perovskites have achieved outstanding performance in photovoltaics, X-ray detectors and other optoelectronic applications. Significantly, they can be synthesized via cost-effective low-temperature methodologies, which represent an improvement in terms of sustainability compared to physical vapor processes, normally requiring high temperatures and vacuum. However, the presence of Pb and stability issues have raised the question of whether the exceptional properties present in lead-halide perovskites could be replicated in other materials. Among the different known approaches to develop perovskite-inspired materials, here we have fabricated emerging chalcogenide semiconductors with structure analogous to perovskite, and others which have different structure but similar electronic features.<br/><br/>Belonging to the first group, silver and copper chalcohalides - (Ag,Cu)₃SX (X=I,Br) - stand out for their crystalline structure analogous to perovskites, albeit switching cation sites by anions and vice-versa. They are constituted by earth-abundant and non-toxic components, and their bandgap in the 0.9 - 1.1 eV range indicates that they could be suitable for single-junction solar cells or tandem configurations. So far, powders and pellets have been mostly fabricated using costly and time-consuming approaches which involve high temperature and vacuum, and lack adaptability in terms of composition and doping. Alternatively, in this work we present a novel procedure to prepare (Ag,Cu)₃SX (X=I,Br) polycrystalline thin films by molecular precursor ink deposition, using thiol-amine solvent mixtures. This methodology offers advantages in terms of cost-effectiveness, versatility, scalability and sustainability, opening the door to explore new compositions, properties and applications of the ecologically benign chalcogenide anti-perovskite system. Samples have been extensively studied by X-ray diffraction, microscopy and optoelectronic characterizations.<br/><br/>Another approach to develop perovskite-inspired materials consists in developing materials which are electronically analogous, i.e., they have similar electronic characteristics at the band structure. In this regard, it is particularly interesting to seek materials which exhibit the same features which make lead-halide perovskites tolerant to defects. Indeed, despite having high defect densities, hybrid perovskites exhibit low non-radiative recombination rates. This phenomenon has been suggested to emerge from having bonding orbitals at the conduction band, and antibonding contributions at the valence band.¹ Therefore, it is expected that materials with similar electronic structure, such as ternary chalcohalides SbSX and BiSX (X=I,Br), can replicate the beneficial defect tolerance of perovskites. In addition, these materials are constituted by relatively earth-abundant and low toxic components, they are stable, have bandgaps in the 1.5 - 2.0 eV range, and can be synthesized at low temperatures, which makes them ideal as a viable alternative to perovskites. In this work, experimental and theoretical analyses will be presented, including the characteristics of the band structure (obtained by first-principle calculations), structural and optical properties (X-ray diffraction, spectroscopy analyses), and prototype PV devices, demonstrating their potential as a sustainable alternative for energy harvesting applications.<br/><br/>To sum up, this work presents two different families of emerging chalcogenide materials, constituted by relatively earth-abundant elements and manufactured by low-temperature environmentally-friendly techniques, which are inspired in different ways by the successful lead-halide perovskites (structure analogous and electronic analogous respectively), suggesting new, more sustainable ways to develop semiconductor materials for energy applications.<br/><br/>1 <i>Chem. Mater.</i> <b>29</b>, 11, 4667-4674 (2017)