Surface analysis and surface doping of graphene on indium-tin-oxide

Abstract Surface doped indium tin oxide (ITO)/graphene systems as a hole transporting transparent front electrode in electro-optical applications are potential alternatives to conventional ITO with wet chemically prepared polymers. In this paper we investigate, firstly, chemical impurities of chemical vapor deposition prepared graphene transferred onto glass/ITO substrates under ambient conditions using x-ray photoemission spectroscopy (XPS) at the oxygen and carbon peak positions. For each of the two elements the intensity contribution of the graphene layer could be extracted from a mathematical simulation of the respective XPS spectra. Impurities from the supporting poly(methyl methacrylate) layer for the transfer were identified. Secondly, we study effects of graphene surface-doping on the work function of such transparent front electrodes using ultraviolet photoemission spectroscopy data. As doping material we have deposited ultra-thin layers of low-cost electron acceptors, iron chloride, vanadium oxide, molybdenum oxide (MoO x ) or tetracyano-2,3,5,6-tetrafluoroquinodimethane in order to increase its initial low surface work function of Φ = 4.3 eV up to Φ = 6.3 eV. Finally, we fabricated under vacuum conditions a bulk heterojunction organic photovoltaic device based on a Zn-phtalocyanine:C 60 absorber layer thermally deposited on an ITO/graphene front electrode, which has been electronically adjusted to the Zn-phthalocyanine donor (Φ = 5.2 eV) by applying the matching dopant MoO x . The current-voltage characteristics of the device shows improved open circuit voltage and fill factor, hence improved power conversion efficiency compared to that with pristine ITO/graphene.