Computational, thin-film deposition and characterization approaches have been used to examine the ternary halide semiconductor Cs 3 Sb 2 I 9 . Cs 3 Sb 2 I 9 has two known structural modifications, the 0-D dimer form (space group P6 3 /mmc, No. 194) and the 2-D layered form (P3m1, No. 164), which can be prepared via solution and solid state or gas phase reactions, respectively. Our computational investigations suggest that the layered form, which is a one-third Sb-deficient derivative of the ubiquitous perovskite structure, is a potential candidate for highband-gap photovoltaic (PV) applications. In this work, we describe details of a two-step deposition approach that enables the preparation of large grain (>1 µm) and continuous thin films of the lead-free layered perovskite derivative Cs 3 Sb 2 I 9 . Depending on the deposition conditions, films that are c-axis oriented or randomly oriented can be obtained. The fabricated thin films show enhanced stability under ambient air, compared to methylammonium lead (II) iodide perovskite films stored under similar conditions, and an optical band gap value of 2.05 eV. Photoelectron spectroscopy study yields an ionization energy of 5.6 eV, with the valence band maximum approximately 0.85 eV below the Fermi level, indicating near-intrinsic, weakly p-type character. Density Functional Theory (DFT) analysis points to a nearly direct band gap for this material (less than 0.02 eV difference between the direct and indirect band gaps) and a similar high-level of absorption compared to CH 3 NH 3 PbI 3 . The photoluminescence peak intensity of Cs 3 Sb 2 I 9 is substantially suppressed compared to that of CH 3 NH 3 PbI 3 , likely reflecting the presence of deep level defects that result in non-radiative recombination in the film, with computational results pointing to I i , I Sb , and V I as being likely candidates. A key further finding from this study is that, despite a distinctly layered structure, the electronic transport anisotropy is less pronounced due to the high ionicity of the I atoms and the strong antibonding interactions between the Sb s lone pair states and I p states, which leads to a moderately dispersive valence band.

Through density functional theory calculations, we show that the alloy perovskite system BaZr1–x Ti x S3 (x < 0.25) is a promising candidate for producing high power conversion efficiency (PCE) solar cells with ultrathin absorber layers. To maximize the minority carrier lifetime, which is important for achieving high PCE, the defect calculations show that BaZr1–x Ti x S3 films should be synthesized under moderate (i.e., near stoichiometric) growth conditions to minimize the formation of deep-level defects. The perovskite BaZrS3 is also found to exhibit ambipolar self-doping properties, indicating the ability to form homo p–n junctions. However, our theoretical calculations and experimental solid-state reaction efforts indicate that the doped perovskite BaZr1–x Ti x S3 (x > 0) may not be stable under thermal equilibrium growth conditions. Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr1–x Ti x S3 thin films.

Recent theoretical and experimental reports have shown that the perovskite CH3NH3PbI3 exhibits unique ambipolar self-doping properties. Here, we show by density-functional theory calculation that its sister perovskite, CH3NH3PbBr3, exhibits a unipolar self-doping behavior—CH3NH3PbBr3 presents only good p-type conductivity under thermal equilibrium growth conditions. We further show that despite a large bandgap of 2.2 eV, all dominant defects in CH3NH3PbBr3 create shallow levels, which partially explains the ultra-high open-circuit voltages achieved by CH3NH3PbBr3-based thin-film solar cells. Our results suggest that the perovskite CH3NH3PbBr3 can be both an excellent solar cell absorber and a promising low-cost hole-transport material for lead halide perovskite solar cells.