Dynamic chloride vacancy evolution enables thermally enhanced upconversion luminescence in Yb3+/Er3+ Co-doped Cs2NaBiCl6 double perovskites

Rare-earth (RE)-doped upconversion luminescence (UCL) materials are essential for advanced optoelectronic technologies, yet their performance is severely compromised by thermal quenching (TQ) at elevated temperatures, hindering practical deployment in high-temperature environments. In this study, we introduce a defect-mediated strategy to achieve thermally enhanced UCL in Yb 3+ /Er 3+ co-doped Cs 2 NaBiCl 6 double perovskites, focusing on the dynamic behavior of chloride vacancy-related centers (V Cl -related centers) and their possible role in modulating energy transfer between RE ions. By systematically tuning V Cl -related centers relative abundance through different processing routes and LiCl doping, we establish data-consistent mechanistic interpretation and a clear correlative relationship between defect-related states and the thermal enhancement of UCL emission. The ground sample, characterized by high defect density, exhibits a 2.1-fold increase in UCL intensity at 573 K compared to room temperature. With moderate LiCl doping (5%), the thermal enhancement factor increases to 5.9-fold—nearly three times greater than that of the undoped ground counterpart. Using electron paramagnetic resonance (EPR), Hall effect measurements, and fluorescence lifetime analysis, we show that defect-related states associated with V Cl are consistent with non-radiative trapping centers affecting the Yb 3+ -sensitized upconversion pathway under ambient conditions. Upon heating, the combined results are consistent with a thermally activated reconfiguration of the defect landscape, plausibly involving partial compensation or transformation of V Cl -related centers by mobile chlorine-related species, thereby suppressing non-radiative pathways and enhancing Yb 3+ →Er 3+ energy transfer efficiency. Critically, singly Er 3+ -doped samples show negligible thermal enhancement, confirming that the observed effect specifically targets Yb 3+ sensitizers rather than Er 3+ activators. This work provides a data-supported working model for thermally enhanced UCL in halide double perovskites and establishes a general defect-engineering approach for designing RE-doped UCL materials with superior resistance to thermal quenching. These advances open new avenues for applying such materials in extreme-environment optoelectronic devices, including high-temperature optical sensors and NIR-driven light sources.