Microregion Characterization of Grain Boundary Defects and Electron Capture of CsPbI 2 Br Perovskite

All‐inorganic CsPbI 2 Br perovskite solar cell shows superior thermal stability, but the power conversion efficiency falls behind organic devices, due to the existence of abundant defects. Halogen and metal ions at the grain boundaries induce the formation of defect states, which increases the probability of carrier recombination. Grain boundaries are also the channels of ion migration. Herein, as relatively simple interface engineering strategy is used at the molecular level, using a multifunctional 2,5‐thiophene dicarboxylic acid to passivate the interface between the perovskite surface and the carbon electrode. The 2,5‐thiophene dicarboxylic acid can effectively coordinate the undercoordinated Pb 2+ , reduce the defect concentration of perovskite, and suppress the nonradiative recombination. The device with carbon electrode exhibits a power conversion efficiency of 13.4%. The microscopic passivation mechanism is investigated by Kelvin probe force microscopy. It is observed that the surface treatment effectively passivates the defects at the grain boundary and reduces the capture rate of photogenerated carriers of the grain boundary.