Surface-Interaction-Driven Polarity Switching in II-V Cd₃P₂ Colloidal Quantum Dots for Infrared Photodiodes

Colloidal quantum dots (CQDs) based on II-V semiconductors offer attractive optical absorption and carrier transport properties for infrared optoelectronics, yet their device-relevant electronic behavior remains poorly understood. In particular, Cd₃P₂ CQDs have been constrained by limited control over nanocrystal growth and carrier polarity. Here, a materials-to-device study establishes polarity control in Cd₃P₂ CQD solids for infrared photodiodes. Precise regulation of oleic acid (OA) concentration during synthesis yields monodisperse Cd₃P₂ CQDs with suppressed nanocrystal fusion and photoluminescence quantum yields up to 62 %. Electrical measurements reveal an oxygen-induced transition from n-type to p-type transport in Cd₃P₂ CQD films. Spectroscopic analysis and first-principles calculations indicate that adsorbed oxygen generates surface acceptor states that drive Fermi-level realignment. Building on these functional Cd₃P₂ CQD solids, a Cd₃P₂-based homojunction CQD photodiode is demonstrated, in which Cd₃P₂ functions as both the infrared absorber and a charge-selective layer. The resulting devices exhibit stable ambient operation, achieving a short-circuit current density of 18 mA cm^-2, an external quantum efficiency (EQE) of 24 %, and a fast temporal response of 23 ns under zero bias. These results identify surface-driven polarity control as a viable design strategy for II-V CQD optoelectronics and position Cd₃P₂ CQDs as a promising platform for low-power infrared conversion technologies.