Stable, Bromine-Free, Tetragonal Perovskites with 1.7 eV Bandgaps via A‑Site Cation Substitution

Metal halide perovskite absorbers with wide bandgaps (1.6–1.7 eV) that are suitable for tandem devices typically require high Br concentrations; this renders the material prone to halide segregation and degradation. Inorganic, bromine-free CsPbI3 has a wide bandgap of 1.7 eV and does not suffer from halide segregation; however, these active layers are not stable at room temperature. Here, we report a method to create stable tetragonal perovskites with a bandgap near 1.7 eV: we add small amounts of large A-site cations having ionic radii between 272 and 278 pmdimethylammonium (DMA) and guanidinium (Gua)into the crystal lattice. When we deploy perovskites using mixed Cs and these large organic cations, we achieve stable, wide bandgap perovskites with power conversion efficiencies of 15.2% and V_OC of 1.19 V. This study extends materials selection for wide bandgap Cs-based perovskites.