Efficient Planar Heterojunction FA1–xCsxPbI3 Perovskite Solar Cells with Suppressed Carrier Recombination and Enhanced Open Circuit Voltage via Anion-Exchange Process

Introduction of Cs into FAPbI3 displayed great potential to stabilize the black perovskite phase by forming FA1- xCs xPbI3, which has been investigated widely based on solution process. During solution processing, the over-rapid intercalating reaction rate between PbI2 and A cations (FA+ and Cs+) can bring some undesirable structural transitions. However, in vapor-assisted solution process (VASP), the over-rapid intercalating reaction rate can be reduced effectively. In addition, the formation process can be regulated significantly by the intermediate perovskite phase. In this study, FACl was employed together with FAI to improve the FA0.9Cs0.1PbI3 films by VASP. In the vapor deposition process, the FACl and FAI vapor coreacted with the PbI2 solid films, preferentially forming the intermediate perovskite phase FA0.9Cs0.1PbI xCl y. The intermediate perovskite phase FA0.9Cs0.1PbI xCl y supplied a plenty of seeds for rapid nucleation of perovskite, which prolonged the crystallization time of FA0.9Cs0.1PbI3, and thus, a smooth FA0.9Cs0.1PbI3 film with suppressed nonradiative recombination, prolonged carrier lifetime and decreased trap state density was acquired. Corresponding planar heterojunction perovskite solar cells achieved a champion power conversion efficiency (PCE) of 16.39% with a Voc of 0.99 V, Jsc of 22.87 mA/cm2, and fill factor of 74.82% under reverse scanning. Meanwhile, a hysteresis index of the FACl-10 device was decreased to 0.024 compared with 0.075 of the control device. Moreover, under the condition of nitrogen atmosphere, the normalized PCE of FACl-10 device diminished only 4.9% which was more stable comparing with 31.88% diminishing of the control device after 30 days.