Ultralow Dark Currents in Avalanche Amorphous Selenium Photodetectors Using Solution-Processed Quantum Dot Blocking Layer

We propose a true solid-state alternative to the vacuum photomultiplier tube using amorphous selenium (a-Se) as the bulk avalanche i-layer. A-Se is a unique photosensing material in which carrier transport can be shifted entirely from localized to extended states where only holes get hot and undergo impact ionization, resulting in deterministic and non-Markovian avalanche gain. To achieve reliable and repeatable impact ionization gain without irreversible breakdown, a non-insulating metal oxide n-type hole-blocking/electron-transporting layer is needed. For the first time, we have deposited a solution-processed quantum dot (QD) hole blocking layer over an a-Se film at room temperature, without any surface or bulk crystallization. We have measured the lowest dark current density ever reported (30 pA/cm2 at the onset of avalanche) compared to any other solid-state avalanche sensor at room temperature. Our results provide new strategies for the development of advanced solid-state photomultipliers via efficient QD-based interface layers to fully exploit the deterministic avalanche properties of a-Se.