Cu/graphene hybrid transparent conducting electrodes for organic photovoltaic devices

Cu/graphene hybrid films which function as both p-type and n-type transparent conducting electrodes were utilized in conventional and inverted organic solar cells. The value of sheet resistance decreased to ∼75 Ω/sq for Cu/graphene hybrid films with an optimum thickness of 7 nm of Cu, which constitutes a dramatic improvement compared to graphene alone (∼180 Ω/sq). Meanwhile, the work functions of the Cu/graphene hybrid electrodes can also be controlled through the range of 4.84–4.95 eV by changing the amount of Cu which was thermally deposited on top of the graphene layer. In addition, the Cu/graphene films showed wrinkle features with the root-mean-square (RMS) roughness ranging from 1.9–5.8 nm and RMS currents are nearly doubled as a function of the thickness of Cu deposition (∼30 nA) by using atomic force microscope (AFM) and conductive-AFM (C-AFM) measurements. Conventional and inverted organic solar cells with 3 nm Cu/graphene hybrid electrodes showed 1.4 and 4.5 times larger power conversion efficiencies (PCE), respectively, than those of the devices with pristine graphene electrodes.