Excited-state forces in organic metal halide perovskites from GW/BSE calculations*

Organic metal halide perovskites are promising materials for solar cells and optoelectronic devices, but one of the main barriers to their use is light-induced degradation. Light-induced structural changes also play an important role in Stokes shifts and exciton transport. They have been attributed to mechanisms such as the creation of polaronic trap states from free carriers, halide ion migration, and changes in stress.

 

To understand those structural changes, we calculate forces induced by absorption of light, with the GW approximation and Bethe-Salpeter equation (GW/BSE). Our excited-state forces method, an improved version of Ismail-Beigi and Louie’s [Phys. Rev. Lett. 90, 076401 (2003)], combines quasiparticle energies and exciton wavefunctions from GW/BSE calculations in BerkeleyGW with electron-phonon matrix elements from Density Functional Perturbation Theory (DFPT)

calculations.

We first benchmark excited-state forces on simple test systems, such as the CO molecule, and then we apply this approach to methylammonium lead iodide (CH₃NH₃PbI₃), exploring different excitations and crystal phases. Our calculated excited-state forces are the starting point for structural changes induced by light absorption in hybrid perovskites