Lanthanides‐Induced Enhancement in Self‐Trapped Excitons Emission and Anti‐Thermal Quenching of Cs ₂ KInCl ₆ for Highly Sensitive Optical Thermometry

Abstract Doping lanthanide ions in double perovskites (DPs) offers a promising approach to tailor optical and optoelectronic properties for versatile applications. However, achieving efficient and thermal stable DPs remains a significant challenge. In this paper, a series of Cs 2 KInCl 6 :Yb 3+ ,Er 3+ DPs with a negative thermal quenching behavior and high sensitivity are fabricated. The intrinsic luminescence properties of Cs 2 KInCl 6 are studied with a broad intrinsic self‐trapped excitons (STEs) emission peak at 500 nm, which can be enhanced by doping Er 3+ ions accompanied with greater distortion of the [InCl 6 ] 3− octahedrons as well as multiple characteristic emission modes of Er 3+ under ultraviolet and near‐infrared excitation. Notably, the addition of sensitizer Yb 3+ ions leads to the enhanced up‐conversion emission of Er 3+ (up‐conversion quantum yield up to 1.66%), anti‐thermal quenching performance brought by the phonon‐assisted energy transfer up‐conversion process, as well as high sensitivity and reproducibility based on fluorescence intensity ratio technology. This work provides an effective strategy to enhance the STEs emission of DPs and achieve efficient optical temperature sensors through lanthanide ions doping.