Figure 1: Dark conductance and photoconductance in PbS NC OFETs.

(a) A schematic of PbS–NC OFETs. S, D and G denote source, drain and gate electrodes, respectively. Illumination of PbS–NC films results in photoinduced current between the source and the drain electrodes, which is monitored as a function of light intensity and energy of incident photon, while simultaneously varying the gate voltage. (b) Absorption spectrum of PbS NCs in solution. The band-edge 1S peak is at 1.3 eV. The inset shows a representative transmission electron micrograph of the NCs (scale bar represents 10 nm). The mean NC diameter is 3.3±0.6 nm. (c) The I_sd versus V_g characteristic in dark (black curve) and under 2.25 eV illumination as a function of light intensity (relative units; 1 corresponds to 30 μW cm⁻²) in a three-dimensional representation. Light illumination results in increased carrier concentration and a resultant orders-of-magnitude increase in photocurrent at the flat-band gate voltage (V_g=V_gmin; corresponds to the photocurrent minimum) due to the photoconductive effect. Under illumination, an extra positive voltage, which compensates for the photovoltage due to charge accumulation under the source/drain electrode, is required to achieve the flat-band condition (traced by the black line in the contour plot at the bottom). (d) Capacitance measurements for the NC film in dark indicate accumulation of holes in the channel under negative gate bias. The magnitude of the accumulated charge can be inferred from the integral .