Fig. 2
From: Dual-gate organic phototransistor with high-gain and linear photoresponse
Quasi-static optoelectrical characterisation of dual-gate organic phototransistor. a Transfer curves measured in dark at various top gate biases (VTG), with fixed source-drain bias (Vd = 5V). The arrows indicate the scanning direction. The increasing drain current at low VBG indicates the formation of a p channel due to the accumulation of holes at the top dielectric interface when a sufficiently negative VTG is applied, and the threshold voltage shift of the n channel indicates that the two channels are electrostatically interacting. At VBG = 20 V and VTG = −20 V, the drain current is mainly contributed by electrons in the bottom n channel and only partly contributed by holes in the top p channel. b Output curves measured in dark at various top gate biases (VTG), with fixed bottom gate bias (VBG = 20 V). c Transfer curves measured at various light intensities with VTG = 0 V (top) and VTG = -20 V (bottom), at Vd = 5 V. A positive photocurrent (Iphoto) was generated at all gate bias conditions due to the shift in threshold voltage induced by light. Inset shows the output curve in dark (black) and in light (red) at 0.5 mW cm−2 irradiance at VTG = −20 V and VBG = 20 V. d Logarithmic plots showing the dependence of photocurrent (top) and responsivity (bottom) on light intensity (Pin) at both top gate bias conditions (fixed VBG = 20 V). At VTG = 0 V, photocurrent scaled sublinearly with intensity throughout the entire range displayed (\(I_{\mathrm {photo}} \propto P_{\mathrm {in}}^{ - \alpha }\), with slope α = 0.28 ± 0.03). At VTG = −20 V, photocurrent switched from sublinear dependence at high intensities (> 1 mW cm−2, slope α = 0.32 ± 0.04) to linear dependence at low intensities (< 1 mW cm−2, slope α = 0.97 ± 0.05). The dotted lines show the power-law fits. The linear scaling of photocurrent with light is reflected by the constant high-gain responsivity of about 1 AW−1, which corresponds to an external quantum efficiency (EQE) of ~220% at 543 nm. Similar results were obtained in a dual-gate phototransistor made of DPP-DTT:PCBM blend, but with much-improved performance (~40 AW−1 or ~9000% EQE) due to the enhanced carrier mobility in this blend (see Supplementary Fig. 9)
