Surface‐Interaction‐Driven Polarity Switching in II–V Cd ₃ P ₂ Colloidal Quantum Dots for Infrared Photodiodes

ABSTRACT 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, Cd ₃ P ₂ CQDs have been constrained by limited control over nanocrystal growth and carrier polarity. Here, a materials‐to‐device study establishes polarity control in Cd ₃ P ₂ CQD solids for infrared photodiodes. Precise regulation of oleic acid (OA) concentration during synthesis yields monodisperse Cd ₃ P ₂ 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 Cd ₃ P ₂ 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 Cd ₃ P ₂ CQD solids, a Cd ₃ P ₂ ‐based homojunction CQD photodiode is demonstrated, in which Cd ₃ P ₂ 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 Cd ₃ P ₂ CQDs as a promising platform for low‐power infrared conversion technologies.