Low-voltage high-performance UV photodetectors: an interplay between grain boundaries and debye length

Accurate detection of UV light by wearable low-power devices has many important applications including environmental monitoring, space to space communication, and defense. Here, we report the structural engineering of ultraporous ZnO nanoparticle networks for fabrication of very low-voltage high-performance UV photodetectors. A record high photo-to dark-current ratio of 3.3 × 10⁵ and detectivity of 3.2 × 10¹² Jones at an ultralow operation bias of 2 mV and low UV-light intensity of 86 μW⋅cm^-2 are achieved by controlling the interplay between grain boundaries and surface depletion depth of ZnO nanoscale semiconductors. An optimal window of structural properties is determined by varying the particle size of ultraporous nanoparticle networks from 10 to 42 nm. We find that small electron-depleted nanoparticles (≤40 nm) are necessary to minimize the dark-current; however, the rise in photocurrent is tampered with decreasing particle size due to the increasing density of grain boundaries. These findings reveal that nanoparticles with a size close to twice their Debye length are required for high photo-to dark-current ratio and detectivity, while further decreasing their size decreases the photodetector performance.