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Defect passivation is a challenging issue in vapor‐deposited perovskite solar cells. Herein, the molten salts vapor deposition of FAPbI3 thin‐film solar cell by RbCl and PbI2 is reported. It is found that the introduction of RbCl suppresses the generation of uncoordinated Pb atom, and improves the power conversion efficiency (PCE) to 20.16% with improved stability. Terahertz scattering‐type scanning near‐filed optical microscopy (THz s‐SNOM) technique reveals that randomly distributed defects on the control sample is passivated in the Rb‐passivated sample, with only a few restricted in the grain boundaries (GBs). In situ imaging of the accelerated degradation reveals a reverse defect generation behavior on the grains before the phase transition. In the results, it is helped to understand the passivation mechanism on the vapor‐deposited solar cells, and THz s‐SNOM is revealed as a powerful technique for nanoscale investigation in perovskite devices. [ABSTRACT FROM AUTHOR]

To fabricate stable neat FAPbI3 perovskite with a pure α‐phase (pure α‐FAPbI3) is important in the field of photovoltaic commercialization because of its better bandgap than its alloying counterpart with cesium (Cs) or methylammonium (MA) cations. In this study, the first vapor deposited pure α‐FAPbI3 thin film solar cell with a power conversion efficiency (PCE) over 20% is achieved by regulating the phase transition process. It is found that under high humidity conditions, a fast phase transition between high‐purity α‐ and δ‐phase FAPbI3 can be realized. Moreover, theoretical calculations interestingly reveal a phase transition shortcut induced by the abnormal volume contraction that is ascribed to the formation of double hydrogen bonds at a certain H2O concentration. Therefore, a high‐humidity post‐treatment strategy is proposed to fabricate α‐FAPbI3 solar cells with a champion PCE of 20.19% (0.1 cm2) and 18.91% (1 cm2), which is currently the highest recorded value in vapor deposited pure α‐FAPbI3 perovskite solar cells. This study helps to redefine the effect of a water molecule on FAPbI3 solar cells. In addition, the demonstrated scaling‐up possibility provides another perspective for fabricating uniform high‐performance pure α‐FAPbI3 perovskite solar cells. [ABSTRACT FROM AUTHOR]