Visible light-modulated phototransistors based on ZnO and CdSe/ZnS quantum dots

Phototransistors that can be modulated by visible light have been fabricated using ZnO film and CdSe/ZnS quantum dots (QDs). Typical ZnO thin film transistors (TFTs) were fabricated using a solution process, and then small-band-gap QDs, which can absorb low-energy visible light, were coated on the ZnO surface. The device induced strong photocurrent under illumination by visible light, whereas it was difficult to observe any photocurrent under illumination by near infrared light. The small band gap of CdSe/ZnS QDs (1.94 eV) enables the absorption of low-energy photons such as visible light, and excites carriers from a valence band to a conduction band. Then the excited carriers can flow through the conduction band of the ZnO film. Thus, the device could be modulated by visible light (λ = 635 nm); its photoresponsivity was 14,602 A/W. These results represent a useful way to fabricate solution-processed visible-light phototransistors for flexible and transparent electronics.