Probing Phase Distribution at the Surface of (BA) ₂ (MA) _n‐1 Pb _n I _3n+1 Perovskite Film via Interlayer Exciton Emission

Abstract Quasi‐ 2D halide perovskite films have emerged as an excellent platform for optoelectronic devices, owing to their better environmental stability and unique physical properties. Nevertheless, mixed n‐value phases are typically generated during preparing quasi‐2D perovskite films, and the spatial distribution of these phases at the surface of films is usually random, which has a significant impact on the electrical contact. Currently, no efficient experimental technique probes the phase distribution at the surface of quasi‐2D perovskite films. Here, by utilizing interlayer excitons (IXs) formed between the top perovskite layer and monolayer WSe ₂ , direct mapping of surface phase distribution is achieved. By analyzing the emission peak energy of interlayer exciton emission, the n‐value of the perovskite surface can be precisely characterized. Significantly, by covering the spin‐coated quasi‐2D perovskite films with monolayer WSe ₂ , non‐destructive, layer‐specific detection of the n‐value phases distribution at the top surface layer is successfully accomplished. This study not only corroborates that interlayer excitons can form between 2D perovskite with different n value and monolayer TMDs, but also offers an efficient avenue to detect the phase distribution at the surface of the quasi‐2D perovskite films, which can help optimize the growth conditions to synthesize quasi‐2D perovskite films with designed surface phases.