Unveiling the Intrinsic Energy Barrier in Single-Photon Upconversion of Perovskite Quantum Dots for Optical Refrigeration

Perovskite quantum dots (PQDs) feature exceptional single-photon upconversion (SPUC) photoluminescence, rendering them promising candidates for use in optical refrigeration. Despite significant advancements in recent fundamental studies, the specific scenario of uphill carrier activation involved in the SPUC process has not been fully elucidated. Herein, the mechanism of SPUC in PQDs, with particular attention paid to the underlying carrier-transition kinetics, is systematically investigated through time- and energy-resolved spectroscopic techniques. Quantitative analysis results indicate a significant deviation of the upconversion behavior from that predicted by the two-level direct-transition model, a simplified theoretical framework commonly applied to PQDs. Conversely, an additional energy barrier Δ E *, superposed on the SPUC energy gap, plays a pivotal role in determining the carrier activation kinetics. We further verify that Δ E * is associated with the intrinsic properties of the perovskite lattice and is barely influenced by the nanocrystal surface chemistry. The presented findings provide a valuable framework for improving the SPUC performance of PQDs from the perspective of lattice regulation, in addition to the widely used ligand engineering approach, paving the way for the rational design of next-generation optical refrigeration materials.