The intriguing electronic properties of graphene and the strong light-matter interaction in layered transition-metal dichalcogenide (TMDC) make them a natural partner for hybrid devices, not only for optoelectronic device applications, but also to understand the conversion of light to electricity in this atomic scale prototype of a donor-acceptor complex. Here, we describe graphene-on-${\mathrm{WS}}_2$ binary heterostructure FET device, displaying gate-tunable persistent photoconductivity. Our time-dependent photovoltage relaxation experiments suggest that the charge-transfer time scale in this heterostructure is dependent on the input optical power, contrary to what is observed for the bare TMDC and is orders of magnitude slower than that observed for various other vdW hybrids. The optoelectronic responsivity of this device at low optical power is found to be as high as ${10}^{10}\mathrm{V/W}$, and thus shows the potential to be one of the most sensitive visible range photodetectors, while the gate tunability of the persistent photoconductivity can be utilized in the memory device applications. We identify that the photoresponse is the outcome of a photogating mechanism, due to the exciton dissociation under optical excitation, followed by the trapping of holes in ${\mathrm{WS}}_2$ and subsequent electron transfer to graphene.

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• Carrier dynamics
• Optoelectronics
• Photoconductivity
• Photovoltaic effect
• Devices
• Graphene
• Transition metal dichalcogenides
• van der Waals systems