Strain Relaxation on Perovskite Surface via Light-Enhanced Ionic Homogeneity

The efficiency and stability of perovskite solar cells (PSCs) can be either deteriorated or enhanced by strain at interfaces, which is sensitive to various external conditions, particularly light illumination. Here we investigated the vertical strain distribution in perovskite films synthesized under light illumination with various wavelengths. The films were formed by reacting formamidinium iodide (FAI)/methylammonium chloride (MACl) vapor with vapor-deposited PbI₂ (CsBr) films. Strain in the films was evaluated with incident-angle-dependent grazing-incidence wide-angle X-ray scattering, which showed out-of-plane compressive and in-plane tensile strains, particularly on the surface. Short-wavelength light relaxed the strain on the perovskite surface via promotion of ionic diffusion, including FA, MA, Cs, and I, to reach vertical ionic homogeneity. With the charge trap concentration being reduced, both the efficiency and stability were greatly improved. This finding provides deep insight into the effect of light on strain in PSCs.