Efficiency Enhancement of Organic/GaAs Hybrid Photovoltaic Cells Using Transparent Graphene as Front Electrode

Abstract

Large-area graphene of high quality and uniformity was successfully grown by chemical vapor deposition (CVD) using surface oxidation treatment of copper foil prior to the graphene growth step. The graphene was transferred to the polyethylene terephthalate (PET) substrate (G/PET) to act as a transparent front electrode of hybrid heterojunction photovoltaic (PV) cells; these cells were based on a structural motif of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, GaAs (1 0 0) as the inorganic n-type semiconductor, and a silver comb electrode atop of PEDOT:PSS. By using G/PET as a transparent front electrode, the power conversion efficiency (PCE), under simulated AM1.5G illumination conditions, was greatly enhanced by up to 26% (from 6.85% to 8.60%). All PV characteristics, including open-circuit voltage ( $V_{{\rm oc}}$ ), short-circuit current ( $J_{{\rm sc}}$ ), and fill factor (FF), contributed to this PCE enhancement. The reflectance, external quantum efficiency, and dark current were investigated to explain this observed PCE enhancement. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance in multilayer cells resulted in lower short-circuit current density, leading to lower PCE, in comparison with those with only one layer of graphene.