Electrochemical half-reaction-assisted sub-bandgap photon sensing in a graphene hybrid phsotodetector

The photogating effect has been previously utilized to realize ultra-high photoresponsivity in a semiconductor-graphene hybrid photodetector. However, the spectral response of the graphene hybrid photodetector was limited by the bandgap of the incorporated semiconductor, which partially compromised the broadband absorption of graphene. Here, we show that this limitation can be overcome in principle by harnessing the electron-accepting ability of the electrochemical half-reaction. In our new graphene phototransistor, the electrochemical half-reaction serves as an effective reversible electron reservoir to accept the photoexcited hot electron from graphene, which promotes the sub-bandgap photosensitivity in a silver chloride (AgCl)-graphene photodetector. The photoconductive gain of ~ 3 × 109 electrons per photon in the AgCl-graphene hybrid is favored by the long lifetime of photoexcited carriers in the chemically reversible redox couple of AgCl/Ag0, enabling a significant visible light (400–600 nm) responsivity that is far beyond the band-edge absorption of AgCl. This work not only presents a new strategy to achieve an electrically tunable sub-bandgap photoresponse in semiconductor-graphene heterostructures but also provides opportunities for utilizing the electrochemical half reaction in other two-dimensional systems and optoelectronic devices. Adding silver chloride nanoparticles to graphene films creates an internal amplification effect that is ideal for see-through solar sensors. Graphene is transparent and light sensitive, but it generates only a low photocurrent in its natural state. One way to boost its responsiveness is with semiconductors that absorb photons and inject ‘hot’ carriers into graphene to create an electronic gain. Jinyao Tang from the University of Hong Kong and colleagues have now discovered how to overcome the semiconductor-limited responsiveness of these hybrids. The team's material uses graphene as a broadband light absorber that reverses the usual flow of hot electrons into a reservoir of silver chloride nanoparticles undergoing electrochemical redox reactions. This setup enables light amplification at smaller wavelengths than typical optical band gaps, with color selections controlled through a simple insulating coating. The redox half-reaction of wide bandgap silver chloride was utilised to promote the photo-emitted carriers from graphene and the photoresponsivity of the phototransistor, circumventing the limitation of semiconductor bandgap and facilitating the pronounced photoresponse to long wavelength photons far beyond the optical absorption edge of AgCl.