Understanding Effects of Cesium in CH(NH2)2PbI3 for Stabilizing CH(NH2)2PbI3/CsPbI3 Interface under UV Illumination

In solution-processed perovskite semiconductors, ultraviolet (UV) radiation can induce structural degradation in the light-absorbing layer, for instance, methylammonium lead iodide (MAPbI 3 ). In this paper, we use modulated surface photovoltage (SPV) spectroscopy to track the mechanisms of photo-generated charge generation and separation as well as the formation of defects in formamidinium lead triiodide (FAPbI 3 ) and cesium (Cs)-containing FAPbI 3 (FA x Cs 1– x PbI 3 ) perovskite when exposed to different durations of UV-light treatment. The measured SPV signals (in-phase and out-phase (shifted by 90°)) were found to be strongly dependent on the addition of Cs and on the UV exposure times. Upon the partial incorporation of Cs, the improved stability in the structural and optical properties was observed. The formation of the δ-CsPbI 3 phase in the FA x Cs 1– x PbI 3 perovskite is attributed to the stabilization of the FAPbI 3 /CsPbI 3 interface, which would efficiently inhibit the phase segregation and provide for a stable medium for the modulated charge separation under UV illumination.