This paper presents an optoelectronic thermometer utilizing infrared-to-visible upconversion, achieved through integrated light receiving and emission devices. Microscale thin-film devices exhibit temperature-dependent light emission with a 1.5% °C⁻¹ intensity change and a 0.18 nm °C⁻¹ spectral shift. The sensing mechanism is attributed to temperature-dependent optoelectronic properties of the semiconductor band structure and circuit operation. Spatially resolved temperature mapping is demonstrated using patterned device arrays. *In vitro* and *in vivo* experiments using integrated fiber-optic sensors showcase real-time thermal detection in dynamic human activity and the deep brain of animals.

He Ding, Guoqing Lv, Xue Cai, Junyu Chen, Ziyi Cheng, Yanxiu Peng, Guo Tang, Zhao Shi, Yang Xie, Xin Fu, Lan Yin, Jian Yang, Yongtian Wang, Xing Sheng