Thermal Disorder-Induced Strain and Carrier Localisation Activate Reverse Halide Segregation.

The reversal of halide ions has been studied under various conditions. However, the underlying mechanism of heat-induced reversal remains unclear. This work finds that dynamic disorder-induced localisation of self-trapped polarons and thermal disorder-induced strain (TDIS) could be co-acting drivers of reverse segregation. Localization of polarons results in an order of magnitude decrease in excess carrier density (polaron population), causing a reduced impact of the light-induced strain (LIS - responsible for segregation) on the perovskite framework. Meanwhile, exposing the lattice to TDIS exceeding the LIS could eliminate the photoexcitation-induced strain gradient, as thermal fluctuations of the lattice could mask the LIS strain. Under continuous 0.1 W/cm 2 illumination (upon segregation), the strain disorder was estimated to be 0.14%, while at 80°C under dark conditions, the strain was 0.23%. However, in-situ heating of the segregated film to 80°C under continuous illumination (upon reversal) increased the total strain disorder to 0.25%, where TDIS is likely to have a dominant contribution. Therefore, the contribution of entropy to the system's free energy is likely to dominate, respectively. Various temperature-dependent in-situ measurements and simulations further support the results. These findings highlight the importance of strain homogenization for designing stable perovskites under real-world operating conditions. This article is protected by copyright. All rights reserved.