Abstract
APbX 3 (A = Cs, methylammonium {MA}; X = I, Br, Cl) lead halide perovskites are of interest for light-emitting applications due to the tunability of their bandgap across the visible and near-infrared spectrum (IR) coupled with efficient photoluminescence quantum yields (PLQYs). It is widely speculated that photoexcited electrons and holes spatially separate into large (Frölich) negative and positive polarons which are stabilized by the A cations. Polarons are expected to be optically active, with recent IR transient absorption experiments showing spectral features consistent with photoionization of the polaron into the continuum band states. For large polarons in the intermediate coupling regime, it would also be expected to observe spectral signatures of transitions within the polaronic potential well producing polaron excited-states. From the polaron excited-state we predict that large polarons should be capable of spontaneous emission (photoluminescence) in the mid-IR to far-IR regime based on the concept of inverse occupations within the polaron potential well. To test this hypothesis, we use density-functional theory (DFT) based calculations using a CsPbBr 3 nanocrystal atomistic model as a host material for either negative (electron) or positive (hole) polarons. We dynamically couple electronic and nuclear degrees of freedom by computing nonadiabatic couplings which allow us to explore nonradiative relaxation of excited polaronic states. Radiative relaxation of excited polaronic states is found from Einstein coefficients for spontaneous emission. Efficiency of polaron emission is determined from rates of nonradiative recombination ( k NR ) and radiative recombination ( k R ) as k R /( k R + k NR ). It is found that both the positive and negative polaron show bright absorption features and photoluminescence from the relaxed-excited state (RES) to the polaron ground states (PGS), but it is an inefficient process (PLQY ∼ 10 –4 –10 –7 ). Methodology considerations for improving the computed PLQY of polaron emission are discussed, such as Marcus rate corrections and coherence. This work provides computational support for observation of IR polaron absorption and a potential direction toward extending the emission capabilities of APbX 3 perovskites into the mid-IR to far-IR regime.