Erbium‐Induced Boost in Self‐Trapped Exciton Emission of Double Perovskites for Highly Sensitive Multimodal and Multiplexed Optical Thermography

Abstract Self‐trapped excitons (STEs) of lead‐free perovskites have aroused tremendous interest in remote optical thermometry due to strong exciton–phonon coupling and large Stokes shifts. Herein, a bright multimodal and multiplexed optical thermometer is constructed with high sensitivity (S_r) and self‐calibrating ability based on Cs₂NaInCl₆:Er³⁺ double perovskite, allowing for fast and simplified reading via mobile devices. Intriguingly, Er³⁺ doping not only shows the characteristic green emissions but also introduces nanoelectronic domains through a new localized valance band maximum and breaks the symmetry of In³⁺ site, which facilitates the generation of more STEs. The temperature‐sensitive blue emission of STEs and temperature‐insensitive green emission of Er³⁺ endow multimodal optical thermometry including time‐resolved and ratiometric readout schemes with a maximum S_r of 3.8% K⁻¹, where an upconversion primary thermometry serves as a reference to calibrate other modes. Meanwhile, the designed thermometers show robust photostability, repeatability, and structural stability for long‐time working and storage. In addition, the remarkable thermochromic phenomenon from blue to green enables a quick color‐multiplexed thermography on a smartphone, which is utilized to capture real‐time 2D thermal imaging of microelectronic devices. This work demonstrates the great potential of lead‐free double perovskite for low‐cost and portable optical thermometry.