Synergistic Multi‐Strategy Engineering of Temperature‐Responsive Self‐Trapped Excitons in Tin‐Based Double Perovskite Nanocrystals Enables High‐Sensitivity Fluorescence Thermometry

Fluorescence thermometry based on self‐trapped exciton (STE) emission in halide perovskites holds strong promise for precision temperature sensing, yet simultaneously achieving both high sensitivity and measurement accuracy remains challenging. Here, we implement a synergistic multi‐strategy design to construct Bi 3+ ‐doped Cs 2 SnCl 6 vacancy‐ordered double perovskite nanocrystals (NCs) with tailored coupling among excitonic states, defect states, and electron–phonon interactions, achieving a record thermal sensitivity of 45.8% K −1 at 410 K. This performance arises from the synergistic effects of the newly designed Sn(II)‐based precursor synthesis, combined with an acetone‐assisted NC surface reconstruction, which collectively suppress nonradiative defects and induce pronounced electron–phonon coupling compared with Sn(IV)‐based precursor synthesis route, thereby significantly increasing the temperature responsiveness of the STE emission dynamics. Moreover, we uncover, for the first time, that temperature‐induced reconstruction of the relaxed excited state stabilizes two STE states at elevated temperatures, thereby enabling reversible thermochromism. Building on this novel mechanistic insight, we develop a dual‐parameter strategy integrating fluorescence lifetime with Commission Internationale de I'Eclairage (CIE) y chromaticity, which reduces measurement uncertainty at elevated temperatures by more than 60% compared with the conventional lifetime‐only approach. These results provide a mechanism‐driven framework for designing STE‐based materials with enhanced thermometric performance and precision.