Terahertz Nanoimaging of the Defects Evolution in Vapor‐Deposited Perovskites Thin‐Film Solar Cell

Defect passivation is a challenging issue in vapor‐deposited perovskite solar cells. Herein, the molten salts vapor deposition of FAPbI 3 thin‐film solar cell by RbCl and PbI 2 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.