Photoinduced Strain in Organometal Halide Perovskites

There is a lack of fundamental understanding of mechano-electro-optical multifield coupling for organometallic halide perovskites (OHPs). In this study, the effect of light irradiation on OHPs' mechanical properties was investigated by atomic force microscopy. In the dark, an MAPbI₃ film was dominated by grains with a Young's modulus of approximately 5.94 GPa, which decreased to 2.97 GPa under light illumination. The photoinduced strain distribution within the polycrystalline MAPbI₃ film was not uniform, and the maximum strain generated inside individual grains was 5.8%. Furthermore, the illumination-induced strain promoted the formation of ferroelastic domains. The Young's modulus of one domain increased from 8.99 to 25.27 GPa, whereas the Young's modulus of an adjacent domain decreased from 14.9 to 1.30 GPa. According to the density-functional-theory calculations, the observed photoinduced strain-promoted variations in mechanical properties were caused by the reversible migration of MA⁺ cations. These findings can help establish the relationship among the mechanical-chemical-optoelectronic characteristics of OHPs.