Thermal Activation of Anti-Stokes Photoluminescence in CsPbBr<sub>3</sub> Perovskite Nanocrystals: The Role of Surface Polaron States.
basic_science · Level V
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- Also identified by DOI 10.1021/acsnano.4c03548.
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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 CsPbBr<sub>3</sub> perovskite nanocrystals (NCs) has provided a path forward in the development of semiconductor-based optical refrigeration strategies. However, the mechanism of ASPL in CsPbBr<sub>3</sub> 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 CsPbBr<sub>3</sub> NCs, we investigated the relationship between the average energy gained per photon during up conversion, Δ<i>E</i>, and the thermal activation energy, <i>E</i><sub>a</sub>. We find that <i>E</i><sub>a</sub> is systematically larger than Δ<i>E</i>, and that <i>E</i><sub>a</sub> increases for larger Δ<i>E</i>. 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 CsPbBr<sub>3</sub> NCs for applications in optical refrigeration.