Perovskite nanocrystals (NCs) such as those made from CsPbBr₃ have numerous uses. Most involve exploiting their light emission and are motivated by their defect tolerance and large, as-made emission quantum yields (QYs). For the latter, near unity QYs are possible. This leads to ready applications as light emitters but also intriguingly to possible material platforms for demonstrating semiconductor-based optical refrigeration.

     Absent, though, is a fundamental understanding of perovskite NC emitting states, central to these applications. Of particular note are observations of near-universal, size-, temperature-, and composition-dependent absorption/emission Stokes shifts where observed energy differences are those that separate absorbing and emitting states. Moreover, efficient (near-unity efficiency), photoluminescence up-conversion, induced by exciting NCs below gap, motivates better understanding the microscopic nature of perovskite band edge states.

     I will explain work we have done recently within the context of attempts to optically refrigerate a semiconductor. These studies now provide new insight into the band edge states of CsPbBr₃ (and possibly CsPbX₃) NCs. Starting with a microscopic mechanism for explaining how it is possible to obtain efficient, near-unity efficiency photoluminescence up-conversion to we extend the conclusions of these measurements to explain the origin of observed, size-dependent absorption/emission Stokes shifts. Here, despite some initial studies we and others have conducted, very little is known about their true origin. This is to be contrasted to more conventional NCs such as CdSe where band edge exciton fine structure quantitatively accounts for both global and resonant Stokes shifts, with emission emerging from a dark exciton. Although similar perovskite NC fine structure might account for their shifts, both theory and experiment predict bright/dark fine structure splittings easily an order of magnitude too small to account for experiment.

     We now propose that perovskite NC emitting states are polarons, which result from the lattice accommodation of photogenerated charges. Polaron binding energies and lifetimes are, in turn, suggested to be the origin of observed l-, T-, and composition-dependent absorption/emission Stokes shifts and excited state lifetimes. This represents a significant departure from more conventional descriptions of NC band edge states, which exclusively involve exciton fine structure and dark exciton emitting states.