Enhancing charge-emitting shallow traps in metal halide perovskites by >100 times by surface strain

The low density of deep trapping defects in metal halide perovskites (MHPs) is essential for high-performance optoelectronic devices. Shallow traps in MHPs are speculated to enhance charge recombination lifetime. However, chemical nature and distribution of these shallow traps as well as their impact on solar cell operation remain unknown. Herein, we report that shallow traps are much richer in MHPs than traditional semiconductors, and their density can be enhanced by >100 times through local surface strain, indicating that shallow traps are mainly located at the surface. The surface strain is introduced by anchoring two-amine-terminated molecules onto formamidinium cations, and the shallow traps are formed by the band edge downshifting toward defect levels. The high-density shallow traps temporarily hold one type of charge and increased the concentration of the other type of free carrier in working solar cells by keeping photogenerated charges from bimolecular recombination, resulting in a reduced open-circuit voltage loss to 317 mV.