Photoinduced Lattice Symmetry Enhancement in Mixed Hybrid Perovskites and Its Beneficial Effect on the Recombination Behavior

The strategy of mixing cations and halides in hybrid organic lead halide perovskites has recently been shown to be very successful in achieving high efficiency with long-term perovskite solar cell stability. In this study, the effect of photoexcitation on the crystal structure of a perovskite film based on triple cations and double halides is investigated and correlated to the recombination behavior in the material. Under continuous photon excitation, a gradual increase in photoluminescence is observed from the perovskite film, coupled with a minute red-shift. The X-ray diffraction pattern also shows a gradual shift toward lower angles under light soaking, suggesting a photoinduced-structural change. Noticeably, a lattice expansion occurs in a preferred orientation, which accordingly influences the crystal symmetry. The trap density of a complete perovskite solar cell is measured under continuous light. The traps are observed in the intermediate frequency, corresponding to the dielectric polarization of the bulk perovskite. A gradual decrease of the trap density is observed under light soaking accompanied by a substantial increase in photocurrent, which is in accordance with the increased recombination resistance monitored during the light soaking. The photoinduced-structural response of the perovskite material and its resulting beneficial recombination behavior is addressed.