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

• Published in: Thin solid films 2019-07, Vol.682, p.57-62
• Main Authors: Christodoulou, Christos, Wolter, Bettina, Ioakeimidis, Apostolos, Chouliaras, Georgios, Wiesner, Sven, Lauermann, Iver, Centeno, Alba, Zurutuza, Amaia, Fostiropoulos, Konstantinos
• Format: Article
• Language: English
• Subjects: Doping; Graphene; Organic solar cell; Photoemission; Work function
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2019.05.012

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 (MoOx) 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:C60 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 MoOx. 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. •Graphene layer (GL) electronically adjusted by surface doping (functionalization).•Room temperature graphene doping by vacuum deposition on sensitive substrate•GL for band matching at hybrid interface between metal-oxide and organic layer•GL as buffer between transparent electrode and absorber in organic solar cells.