Probing Buried Interfaces and Phase Gradients in MAPbI3 Films via Combined Back- and Surface-Incidence GIXRD

Liujiang Zhang; Guanhaojie Zheng; Zhenhuang Su; Zhen Wang; Chenyue Wang; Li Chen; Jianhua He; Xingyu Gao

Abstract

Interfaces between the perovskite absorber and charge-transport layers play a critical role in determining the performance of planar perovskite solar cells (PSCs). However, the lack of non-destructive techniques capable of directly probing the microstructure of buried interfaces beneath perovskite films has significantly hindered a comprehensive understanding of their interfacial properties. Here, we employ flexible substrates to enable synchrotron-based grazing-incidence X-ray diffraction (GIXRD) measurements from the backside of the substrate, allowing direct investigation of buried interfaces. By combining back- and surface-incidence GIXRD, we construct a depth-resolved phase diagram spanning from the film surface to the buried interface in one-step deposited MAPbI₃ films. The film surface is dominated by a thin tetragonal perovskite layer, whereas a cubic phase gradually emerges with increasing depth, leading to phase coexistence within the film interior. Closer to the buried interface, the cubic phase progressively dominates, with the tetragonal phase completely disappearing at the interface. Meanwhile, high-resolution synchrotron X-ray diffraction measurements of MAPbI₃ powders at variable temperatures reveal that cubic and tetragonal phases can coexist, with their relative fractions strongly dependent on thermal history, despite the thermodynamic preference for the tetragonal phase at room temperature. The observed depth-dependent phase distribution in MAPbI₃ films is therefore attributed to an incomplete cubic-to-tetragonal phase transition during cooling after annealing, which becomes increasingly suppressed with depth due to substrate confinement. These findings provide new insights into the structure – performance relationship of one-step deposited MAPbI₃-based PSCs and demonstrate that back-incidence GIXRD is a powerful and broadly applicable tool for probing buried interfaces in thin-film systems.

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