Thermal Activation of Anti-Stokes Photoluminescence in CsPbBr3 Perovskite Nanocrystals: The Role of Surface Polaron States.

Abstract

Optically driven cooling of a material, or optical refrigeration, is possible when optical up-conversion via anti-Stokes photoluminescence (ASPL) is achieved with near-unity quantum yield. The recent demonstration of optical cooling of CsPbBr₃ perovskite nanocrystals (NCs) has provided a path forward in the development of semiconductor-based optical refrigeration strategies. However, the mechanism of ASPL in CsPbBr₃ NCs is not yet settled, and the prospects for cooling technologies strongly depend on details of the mechanism. By analyzing the Arrhenius behavior of ASPL in CsPbBr₃ NCs, we investigated the relationship between the average energy gained per photon during up conversion, ΔE, and the thermal activation energy, E_a. We find that E_a is systematically larger than ΔE, and that E_a increases for larger ΔE. We suggest that the additional energetic cost is due to a rearrangement of the crystal lattice as charge carriers pass from surface localized, structurally distinct sub-gap polaron states to the free exciton state during up-conversion. Our interpretation is further corroborated by quantifying the impact of ligand coverage on the NC surface. These findings help inform the development of CsPbBr₃ NCs for applications in optical refrigeration.