Graphene as Infrared and Electron Transparent Electrode Applied to the Design of Narrow Bandgap Nanocrystal‐Based Photodiode

Abstract Colloidal nanocrystals (NCs) are a promising platform for infrared optoelectronics. Current efforts focus on designing NCs that absorb in the short‐ and mid‐wave infrared and integrating them into diode stacks. A major challenge is to coupling these sensors to read‐out integrated circuits (ROICs) for infrared imaging, which requires infrared‐transparent top electrodes. Conventional materials like tin‐doped indium oxide lose transparency at longer wavelengths, limiting their effectiveness. Metallic grids have emerged as an alternative but struggle to maintain a uniform potential, as shown by nanobeam X‐ray photoemission microscopy. To address this, graphene is explored as a transparent electrode. A novel diode stack is proposed to maintain a backside mirror, accommodate HgTe NCs’ chemical constraints, and incorporate electrodes that efficiently extract both electrons and holes. Unlike conventional designs limited to near‐zero bias, this stack operates optimally under CMOS read‐out‐integrated‐circuit (ROIC) conditions. Additionally, its transparent electrode allows photoelectron emission from within the diode, enabling in situ electric field analysis. This capability enables to rationalize the optimization process of photodiode design.