Importance of separating contacts from the photosensitive layer in heterojunction phototransistors

We simulate the detection of light in solution-processed poly(3-hexylthiophene) (P3HT)-capped indium oxide (In2O3) phototransistors with and without direct electrical contact between the P3HT and the source and drain contacts. Under illumination, the photogenerated carriers enhance the drain current of the transistor, enabling photodetection. We demonstrate improved phototransistor performance by eliminating direct electrical contact between the P3HT and the source and drain contacts due to the inhibition of hole current flow between the contacts in the off-state. The photoresponsivity and detectivity are improved by up to 2.5 and 7 orders of magnitude, respectively, and reach a maximum of 4.4 A/W and 8.7 × 1017 Jones. Photodetection down to incident light powers as low as 5 pW/cm2 is possible in the absence of direct contacts while detection is limited to 50 μW/cm2 otherwise. We determine the photoresponse rise and fall time as 0.4 μs and 0.6 μs, respectively. Our study highlights the importance of using source/drain contacts that are separated from the photosensitive layer to realize optimal phototransistor performance. •Heterostructure phototransistor performance improved.•Off-state current reduced by 9 orders of magnitude.•Photodetectivity improved by 7 orders of magnitude.•Photodetection to powers as low as 5 pW/cm2.