NbF5: A Novel α‐Phase Stabilizer for FA‐Based Perovskite Solar Cells with High Efficiency

The HC(NH2)2+(FA+) is a well‐known substitute to CH3NH3+(MA+) for its capability to extend light utilization for improved power conversion efficiency for perovskite solar cells; unfortunately, the dark cubic phase (α‐phase) can easily transition to the yellow orthorhombic phase (δ‐phase) at room temperature, an issue that prevents its commercial application. In this report, an inorganic material (NbF5) is developed to stabilize the desired α‐phase perovskite material by incorporating NbF5 additive into the perovskite films. It is found that the NbF5 additive effectively suppresses the formation of the yellow δ‐phase in the perovskite synthesis and aging process, thus enhancing the humidity and light‐soaking stability of the perovskite film. As a result, the perovskite solar cells with the NbF5 additive exhibit improved air stability by tenfold, retaining nearly 80% of their initial efficiency after aging in air for 50 d. In addition, under full‐sun AM 1.5 G illumination of a xenon lamp without any UV‐reduction, the perovskite solar cells with the NbF5 additive also show fivefold improved illumination stability than the control devices without NbF5. The application of formamidinium (FA)‐based perovskite solar cells has largely been hindered by phase transition from the dark cubic phase to yellow orthorhombic phase. Here, a highly efficient and phase stable FA‐based perovskite solar cell is fabricated by using NbF5 as a novel additive. NbF5 can improve the quality of perovskite films and effectively suppress the formation of the yellow δ‐phase.