Organic thin film thickness-dependent photocurrents polarity in graphene heterojunction phototransistor

Abstract To overcome the weak absorption deficiency of 2D materials, combining bilayer/bulk heterojunction and 2D materials in a photoconductive framework facilitates the separation and transfer of photo-generated carriers for prominent performance photo-detector. Nevertheless, apart from improving the inferior built-in electric field, the multilayer device may suffer a low efficiency due to depletion loss considering the effective exciton length limitation. Here, we demonstrate the thickness-dependent photocurrent polarity based on a bilayer enhanced graphene/C60/ZnPc phototransistor. A fast response time down to 7.29 μs and a responsivity of 6537 A/W are achieved in the device with the help of type-II band alignment and valid interfaces between organic layers. Moreover, three controlled thicknesses of intermediate charge transfer layer are investigated for the purpose of probing the transfer direction in multilayer heterojunction, which suggests that the photocurrent polarity is highly related to the thickness of C60 and even input optical power density. Owing to the phenomena of photocurrents polarity, a distinct bending curve of responsivity-power density relation is observed near the positive-negative altering point compared to general logarithm linear relation in most photoconductive devices. Our results may enable further exploration of charge transfer mechanism in multilayer system and development of high speed photoconductive transistor.