A two-step thermal annealing and HNO3 doping treatment for graphene electrode and its application in small-molecule organic solar cells

• A combined thermal annealing and doping treatment for graphene has been employed.
• The thermal annealing treatment improves graphene surface smoothness.
• HNO3 doping treatment enhances graphene conductivity and surface wetting.
• The two-step treated graphene is applied to OSCs as anodes.
• The OSCs with the graphene anode exhibit a comparable performance to that of ITO.

The transferred chemical vapor deposition (CVD)-grown graphene from copper foils to other desired substrates by using poly(methyl methacrylate) (PMMA) as supporter, suffers from problems of rough surface, low conductivity and poor wettability, which hinders its applications in optoelectronic devices. In this work, a combined thermal annealing and HNO3 doping treatment for graphene electrode has been employed to simultaneously improve its smoothness, conductivity and wettability. The thermal annealing process results in the improved surface smoothness by removing the PMMA residue. HNO3 doping treatment for the graphene enhances its conductivity and surface wetting to PEDOT:PSS. Furthermore, the work function (WF) of two-step treated graphene increased from ∼4.4 eV to ∼5.1 eV. Small-molecule organic solar cells (OSCs) based on the two-step treated graphene anodes exhibit a power conversion efficiency (PCE) of 2.43%, which is comparable to that of ITO electrode (2.48%). This work demonstrates the bright future of graphene transparent electrodes as a replacement for ITO.

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