Cd2+ causes deep traps on the surface of CdTe quantum dots (QDs) often leading to a long response time for a photodetector. Poly(3-hexylthiophene) (P3HT) can be used to selectively passivate the Cd2+-related deep traps by forming a Cd-S bond, while maintaining the shallow traps. By tailoring the trap depth of the CdTe QDs, a high gain, fast response, and low noise P3HT: CdTe nano-composite photodetector is achieved.