Figure 3. Optoelectronic characterization of the graphene–CQD–ITO photodiode.

(a) Schematic of graphene–CQDs photodiode operation. Incident light creates electron–hole pairs in the PbS quantum dots, which are drifted by the built-in electric field at the interface and by the bias applied across the diode. Source V+ and V− probes of the device are kept floating while I_TD current is measured between top and drain probes. (b) Current–voltage characteristic of the graphene–CQD diode for gate-voltages V_BD specified in the legend. Positive V_TD induces reverse-biasing of the graphene–QD junction. Increasing V_BD induces n-doping in the graphene channel and therefore increases the turn-on voltage of the diode. Inset: IV characteristic in logarithmic scale demonstrating diode leakage threshold around V_TD=1 V. (c) IV characteristic of the graphene–CQD photodiode at different illumination intensities (values in the legend). Inset: IV characteristic in logarithmic scale demonstrating open-circuit voltage. (d) Responsivity and EQE of the graphene–CQD photodiode for illumination wavelength of 635 nm. Inset: band diagrams with and without V_TD bias. Black line is a guide to the eye.