Cover story: Liu JJ, Yang XX, Xu QL et al. Unraveling the efficiency losses and improving methods in quantum dot-based infrared up-conversion photodetectors. Opto-Electron Sci 3, 230029 (2024)

Infrared photodetectors (PDs) promise lots of applications in the field of bio-sensing and imaging. Infrared signal needs to be converted into visible light to realize the imaging of infrared target. Traditional infrared imagers are usually constructed by bonding an infrared PD with each pixel in a thin film transistor (TFT)-based active-matrix backplane. Then the electrical signal generated in PDs at each pixel is collected by the transistors in the backplane, which is then converted to driving voltage of each pixel in the display panel. Due to the lattice mismatch between the infrared detecting materials and silicon-based TFT matrix, expensive flip-chip bonding is used to bond the pixelated PDs and the TFTs on backplane. A feasible way to avoid the costly pixilation and the complicated integration between PDs and display panels is to use infrared up-conversion photodetector, in which an infrared PD and a light-emitting diode (LED) with large effective areas are stacked to form a single tandem device. With excitations from infrared signals, the holes from photoexcitation in the PD unit and externally injected electrons are radiatively recombined in the LED unit, so that the infrared up-conversion photodetectors emit visible light. Without infrared signals, the up-conversion photodetectors keep dark. Therefore, infrared imaging based on up-conversion photodetectors neither needs discrete pixels, nor the expensive bonding process.