Optoelectronic Signatures of Photo-Induced Polarons in Two-Dimensional Lead Chloride Perovskites*

Lead halide perovskites are of interest for light-emitting and photovoltaic applications due to the tunability of their bandgap across the visible and near-infrared spectrum coupled with efficient photoluminescence quantum yields.  Coupling of the photo-induced charges to the soft perovskite lattice is suspected to form large polarons, which are expected to show reduced interband radiative relaxation and longer non-radiative lifetimes.  Here we use ab Initio atomistic modeling to investigate polaron photophysics. Simultaneous negative and positive polarons are modeled within the perovskite layer analogous to a photoexcitation.  Spinor Kohn-Sham orbitals are used for the electronic basis and include relativistic corrections and the spin-orbit coupling 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.  Einstein coefficients for spontaneous emission is used to compute the radiative relaxation rates of charge carriers.  Here we develop evidence that will improve the understanding of polaron dynamics in lead halide perovskites and their implications for radiative and nonradiative recombination.