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 VOC of 1.19 V. This study extends materials selection for wide bandgap Cs-based perovskites.