Emerging Earth-Abundant Solar Absorbers

Developing solar absorbers that are efficient, low cost, stable, and comprised of non-toxic, Earth-abundant elements has long been the holy grail of next-generation photovoltaics (PV) research. 1This effort has been disrupted by the advent and rapid rise in performance of solution-processable lead-halide perovskites. 2One of the key enabling properties is the ability of these halide perovskites to tolerate point defects, enabling efficient PV performance despite high defect densities. 3,4This discovery has reinvigorated efforts within the Earthabundant PV community to design efficient solar absorbers, drawing inspiration from the halide perovskites, with particular focus on defect tolerance and achieving materials with long diffusion lengths.At the same time, the broad families of Earth-abundant solar absorbers provide valuable opportunities to overcome the toxicity and stability limitations of the lead-halide perovskites, without remaining confined solely within the perovskite family of compounds.The merging of these two communities has produced exciting new frontiers, which were explored in the recent symposium on "Earth-abundant next generation materials for solar energy" (Symposium F) at the 2021 Fall European Materials Research Society Meeting (held virtually).Here, we feature some of the key emerging areas discussed at the Symposium: chalcogenide perovskites, II-IV-N 2 compounds, antimony chalcogenides, and the computational search for novel defect-tolerant solar absorbers. Chalcogenide PerovskitesWhilst much of the effort to discover lead-free perovskite-inspired materials has focussed on halides, 4 there is growing interest in chalcogenide perovskites, 5 including BaZrS 3 , SrZrS 3 , BaHfS 3 and SrHfS 3 , among many other Ti-, Zr-and Hf-based compounds. 6,7These materials 1 have captured attention owing to their: 1) low levels of disorder (Urbach energies between 18-34 meV, see Fig. 1a), 6 2) high thermal and environmental stability under ambient conditions, 5 and 3) high absorption coefficients on the order of 10 5 cm -1 . 8These materials also hold promise for exhibiting defect tolerance. 5,9With typical bandgaps close to 2 eV, chalcogenide perovskites are promising for applications as top-cells in tandem with silicon, 10 for indoor photovoltaics, 11 or photoelectrochemical cells. 12However, directly growing highquality thin films of these materials face several important challenges: 1) there is a large mismatch in the vapor pressures of the metal cation and chalcogen precursors, 2) chalcogen precursors (e.g., H 2 S) are corrosive and damaging to the processing equipment, 3) there is a strong propensity for chalcogenide perovskites (as well as their precursor metal elements) to form oxides instead. 13,14cently, Ravichandran et al. 13 and Jaramillo et al. 14 achieved epitaxial thin films of phase-pure BaZrS 3 through pulsed laser deposition (PLD) and molecular beam epitaxy (MBE), respectively, on single-crystalline LaAlO 3 , SrLaAlO 4 or SrTiO 3 substrates.The MBE films grew by buffered epitaxy (mode M1; Fig. 1b) or direct epitaxy (mode M2; Fig. 1c), with M1 found to be faster. 14The PLD films (cross-sectional micrograph in Fig. 1d) were shown to have room temperature photoluminescence (PL; Fig. 1e), along with PL lifetimes up to 8 ns (Fig. 1f), making them promising for PV applications. 13However, processing temperatures in the range of 700-900 C were required for the growth of these MBE and PLD films.The high processing temperatures required limits the ability to incorporate these materials in superstrate device architectures, owing to the common transparent conducting oxide substrates used (indium tin oxide and fluorine-doped tin oxide) being compatible with processing temperatures up to only 500 C.Recent efforts have therefore focussed on developing lower-temperature processing routes for achieving high-quality chalcogenide perovskite thin films.One such route is to presynthesize crystalline nanocrystals, which, if colloidally-dispersible, can be deposited to form a thin film.Nag et al. reported that the growth of BaZrS 3 nanocrystals colloidally in