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.

Atomic-scale imaging of CH3NH3PbI3 structure and its decomposition pathway.

Understanding the atomic structure and structural instability of organic-inorganic hybrid perovskites is the key to appreciate their remarkable photoelectric properties and understand failure mechanism. Here, using low-dose imaging technique by direct-detection electron-counting camera in a transmission electron microscope, we investigate the atomic structure and decomposition pathway of CH3NH3PbI3 (MAPbI3) at the atomic scale. We successfully image the atomic structure of perovskite in real space under ultra-low electron dose condition, and observe a two-step decomposition process, i.e., initial loss of MA+ followed by the collapse of perovskite structure into 6H-PbI2 with their critical threshold doses also determined. Interestingly, an intermediate phase (MA0.5PbI3) with locally ordered vacancies can robustly exist before perovskite collapses, enlightening strategies for prevention and recovery of perovskite structure during the degradation. Associated with the structure evolution, the bandgap gradually increases from ~1.6 eV to ~2.1 eV. In addition, it is found that C-N bonds can be readily destroyed under irradiation, releasing NH3 and HI and leaving hydrocarbons. These findings enhance our understanding of the photoelectric properties and failure mechanism of MAPbI3, providing potential strategies into material optimization.