Defect Engineering Enables High‐Precision Optical Thermometry in Zn _0.8 Cd _0.2 S:Ga

ABSTRACT Optical thermometry is highly attractive for harsh‐environment sensing, yet its accuracy and reliability are constrained by conventional single‐parameter readouts, motivating the development toward multi‐parametric optical thermometry. Here, we report an ultra‐high temperature sensitive Ga 3+ ‐doped Zn 0.8 Cd 0.2 S phosphor based on defect engineering. The photoluminescence quantum yield (PLQY) is boosted from 0.2% to 63.9% by Ga 3+ doping, accompanied by strong and continuous thermal response over an ultrabroad temperature range (–193°C to 200°C). The defect‐tailored emission enables high‐performance, multi‐parametric optical thermometry based on emission intensity, spectral bandwidth, and peak position. This approach delivers a maximum relative sensitivity ( S r ) of 3.4%°C −1 based on intensity, alongside high absolute sensitivities ( S a ) of 0.4°C −1 from bandwidth and 0.7°C −1 from peak position, with a minimum experimentally measured temperature uncertainty ( δT ) of 0.03°C at 15°C. Mechanistic analysis reveals that defect‐mediated recombination pathways coupled with strong electron–phonon interactions underpin the exceptional thermal responsiveness. The as‐explored Zn 0.8 Cd 0.2 S:Ga phosphors have been successfully assembled into a portable fiber‐optic sensor with 98% repeatability, establishing defect engineering as an effective paradigm for highly sensitive, self‐validated multi‐parametric optical thermometry under extreme conditions.