Water-induced spinodal decomposition of mixed halide perovskite captured by real-time liquid TEM imaging

Abstract

Hybrid halide perovskite materials face significant challenges in reaching long-term stability, particularly due to their vulnerability to humidity. While encapsulation can provide some protection by delaying the water-induced degradation, understanding the precise reaction pathways is crucial for developing more robust compositions and advanced device architectures. Traditional ex situ methods offer only snapshots of degradation states, failing to capture the initiation and full evolution of the degradation process in real time. In this study, we utilize cutting-edge real-time electron imaging and in situ liquid-cell transmission electron microscopy to dynamically observe the degradation processes of the Cs_0.05(MA_0.17FA_0.83)_0.95Pb(Br_0.17I_0.83)₃ (CsMAFA) perovskite in contact with water molecules. Using in situ live TEM imaging, combined to SAED and 4D-STEM ACOM techniques, we capture the subsequent degradation steps at both nanoscopic and mesoscopic levels from inception to full degradation. Our results particularly support the spinodal nature of the early-stage decomposition of the perovskite, where a spontaneous coexistence of CsMAFA, PbI₂ and CsPb₂Br₅ is observed within a single grain upon contact with water molecules. This step either competes with, or, is followed by a dissolution/recrystallization mechanism from CsMAFA to PbI₂ grains, while the solid-state spinodal decomposition of CsMAFA to CsPb₂Br₅ continues. From live image segmentations, two different growth rates of PbI₂ are highlighted, i.e. in t^1/2 and t^1/3, controlling the final particles morphology into fine polygons and needles. Longer exposure times leads to two stages, firstly the complete degradation of the pristine perovskite, resulting in the two end members, namely PbI₂ and CsPb₂Br₅ as a single-phase particles, secondly the dissolution of the CsPb₂Br₅ particles and disentangling of the remaining PbI₂ particles.