Stable Cesium-Rich Formamidinium/Cesium Pure-Iodide Perovskites for Efficient Photovoltaics

The current reported wide band gap perovskites based on iodide–bromine mixed halide composition would suffer from halide segregation. Here, we reported Cs-rich wide band gap pure-iodide Cs x FA 1– x – y DMA y PbI 3 ( x > 0.5; y < 0.01) (denoted as CsFA) perovskites and successfully solved the problem of phase segregation in wide band gap perovskites. The comprehensive experimental and theoretical results indicated that, because of a mildly deformed octahedral framework, our fabricated CsFA pure-iodide wide band gap perovskites exhibit much higher stability than those mixed halide perovskites with similar band gaps under thermal or illumination stress. On the basis of a representative Cs 0.7 FA 0.3– y DMA y PbI 3 perovskite with a 1.65 eV band gap, the corresponding solar cells delivered a stable output of 19.40% with good photostability. Overall, our reported Cs-rich pure-iodide CsFA perovskites present promising candidates for wide band gap optoelectronic applications.