Ab initio excited state forces towards light-induced changes in methylammonium lead iodide (MAPI) perovskites

Perovskites are promising materials for use in solar cells, although their degradation limits their use on a large scale. Several works have studied its optical and vibrational properties but the understanding of the exciton-phonon effects is still limited. We study methylammonium lead iodide perovskite (MAPI) in its three phases (cubic, orthorhombic and tetragonal) using excited state (ES) forces from the GW/Bethe-Salpeter equation approach. These forces are the gradient of the ES potential energy surface and indicate the direction atoms tend to move due to light absorption and are useful for the understanding of the microscopic mechanism of exciton self-trapping. Our implementation combines results from the BerkeleyGW code and electron-phonon coefficients from Quantum ESPRESSO. The main advantage of our approach is that our calculations just demand one GW/BSE + DFPT calculation in contrast to finite differences that would demand 3N calculations. For the three phases, we observe strong exciton-phonon coupling with low-frequency modes, which may be related to the degradation of those materials. Our results agree with time-resolved optical experimental data of the tetragonal and orthorhombic phases of exciton coupling to coherent phonons (in particular, the distortion of the I-Pb-I angle). We also study the relaxation of the system on the ES potential energy surfaces and light-induced stress.