Bidirectional Halide Ion Exchange in Paired Lead Halide Perovskite Films with Thermal Activation

MAPbBr3 and MAPbI3 films cast onto glassslides and physically paired together undergo halide exchange to formmixed halide films. The change in halide composition in these two∼130 nm thick films occurs concurrently with Br– diffusing toward the MAPbI3 film and I– diffusing toward the MAPbBr3 film. The diffusion of thesehalide species, which is tracked through changes in the absorption,offers a direct measurement of thermally activated halide diffusionin perovskite films.

The increase in the rate constant of halide diffusionwith increasing temperature (from 8.3 × 10–6 s–1 at 23 °C to 3.7 × 10–4 s–1 at 140 °C) follows an Arrhenius relationshipwith activation energy of 51 kJ/mol. The thermally activated halideexchange shows the challenges of employing layers of different metalhalide perovskites in stable tandem solar cells.