Atomic-Scale\nInsights into Surface Instability in\nHalide Perovskites

The\npathway of surface structure evolution plays a vital\nrole in\ndetermining the stability of the halide perovskites. Understanding\nthe mechanism of surface instability and how external stimuli interact\nwith the surface is essential for developing strategies to mitigate\nthe degradation of halide perovskites. Here, we directly observed\nstructural evolutions on the surface of CsPbBr<sub>3</sub> via an\nintegrated differential phase contrast scanning transmission electron\nmicroscope. We find that surface degradation, different from bulk\ndecomposition, follows a layer-by-layer pathway in the CsPbBr<sub>3</sub> perovskite. This layer-by-layer decomposition method further\nfacilitated a coating strategy to mitigate the degradation via preserving\nthe integrity of the surface layer, as verified through in situ methods.\nThus, we have achieved the improved stability of perovskites and as-fabricated\nlight-emitting diodes. These findings advance the fundamental understanding\n on the mechanism of surface instability and effectiveness of coating,\nthus providing guidance for future study of surface protection toward\nstable optoelectronic devices.