Highly Air-Stable Carbon-Based α-CsPbI₃ Perovskite Solar Cells with a Broadened Optical Spectrum

Inorganic cesium lead halide perovskites with superb thermal stability show promise to fabricate long-term operational photovoltaic devices. However, the cubic phase (α) of CsPbI3 with an appropriate band gap is unstable in air. We discover that highly stable α-CsPbI3 can be obtained in dry air (temperature: 20–30 °C; humidity: 10–20%) by replacing PbI2 with HPbI3 in a one-step deposition solution. Furthermore, the band gap of HPbI3-processed α-CsPbI3 is advantageously reduced from 1.72 to 1.68 eV due to the existence of tensile lattice strain. By employing such an α-CsPbI3 film in carbon-based perovskite solar cells (C-PSCs), a power conversion efficiency (PCE) of 9.5% is achieved, which is a record value for the α-CsPbI3 PSCs without hole transport material. Most importantly, over 90% of the initial PCE is retained for nonencapsulated devices after 3000 h of storage in dry air. Therefore, HPbI3-based one-step deposition presents a promising strategy to prepare high-performance and air-stable α-CsPbI3 PSCs.