Excited-state dynamics of photo-induced polarons in lead chloride perovskites

Lead halide perovskites are of interest for light-emitting and photovoltaic applications due to the tunability of their bandgap, due to confinement and composition, across the visible and near-infrared spectrum (IR) coupled with efficient photoluminescence quantum yields (PLQY). In the excited-state, coupling of the photo-induced charges to the soft perovskite lattice is suspected to form large polarons. The spatial separation of photo-induced electrons and holes inherent to polaron formation reduces the efficiency of the interband radiative recombination. Additionally, polarons are expected to show longer non-radiative lifetimes due to phonon screening. Here we use ab Initio atomistic modeling to investigate polaron photophysics in a two-dimensional Dion-Jacobson lead chloride perovskite. Simultaneous negative and positive polarons are modeled within the perovskite layer analogous to a photoexcitation. Spinor Kohn-Sham orbitals (SKSO) are used for the electronic basis and include relativistic corrections and the spin-orbit coupling (SOC) interactions. Nonradiative relaxation of the excited polaronic states are computed in terms of Redfield theory by propagating the excited-state reduced density matrix for electronic degrees of freedom weakly coupled to a heat bath. Nonadiabatic couplings between electronic and nuclear degrees of freedom, computed ‘on-the-fly’, are used to parametrize the rates of population transfer. The excited-state potential energy surface is used for the non-adiabatic dynamics to account for changes in the potential energy surface due to polaron formation. Einstein coefficients for spontaneous emission is used to compute the radiative relaxation rates of charge carriers. Here we observe that the polaronic states exhibit an increased nonradiative recombination rate reducing the PLQY.