Photon-Induced Defects Facilitate Upconversion in Two-Dimensional Hybrid Perovskites with Anti-Stokes Shifts of Up to 431 meV

The low efficiency of the single-photon upconversion (SPUC) process poses a significant barrier to the application of two-dimensional perovskite materials in solid-state laser cooling technology. In this study, we investigated the magnitude of the anti-Stokes shift in SPUC, which elucidates the role of phonons in the SPUC mechanism. Additionally, we conducted a comparative analysis of the SPUC photoluminescence (PL) before and after defect generation in OA2MA2Pb3Br10 (where OA = octylamine and MA = methylamine), clarifying the influence of defect states on the SPUC process. Our findings indicate that the defect energy level does not serve as an intermediate state; instead, it functions as a luminescent energy level from which electrons transition downward. The thermal motion of organic long-chain molecules induces distortions in the inorganic lattice, resulting in changes to the bandgap energy. These fluctuations in bandgap energy provide the necessary energy for SPUC. Our results further elucidate the mechanism of SPUC PL, confirm that a significant anti-Stokes shift of 431 meV can be achieved in two-dimensional perovskites, and propose a strategy to enhance the SPUC PL efficiency of perovskites through the careful regulation of defect concentrations. This study deepens our understanding of the physical mechanisms underlying SPUC in two-dimensional perovskite materials.