Materials and devices that upconvert photons from low to high frequencies are of tremendous interest for broad applications in bio- and chemical sensing, infrared imaging, display, and photovoltaics. Here we develop self-powered, micrometer-scale optoelectronic upconversion devices for infrared-to-visible photon upconversion. The realized microscale optoelectronic device exhibits a practical efficiency of 1.5% under incoherent low-power illumination, with emission intensity linearly dependent on excitation. Other remarkable advances over conventional approaches include broadband absorption, wide-emission spectral tunability (red, yellow, and blue), and fast dynamics. Encapsulated ultraminiaturized devices can be implanted into subdermal tissues and provide stable operation in behaving animals. Demonstrations of optogenetic stimulation in living animals present their potential for biomedical uses.