Organometallic Dimers: Application to Work-Function Reduction of Conducting Oxides Anthony J. Giordano, † Federico Pulvirenti, † Talha M. Khan, ‡ Canek Fuentes-Hernandez, ‡ Karttikay Moudgil, † Jared H. Delcamp, † Bernard Kippelen, ‡ Stephen Barlow,* ,† and Seth R. Marder* ,† † School of Chemistry and Biochemistry and ‡ School of Electrical and Computer Engineering, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States * S Supporting Information ABSTRACT: The dimers of pentamethyliridocene and ruthenium pentamethylcyclo- pentadienyl mesitylene, (IrCp*Cp) 2 and (RuCp*mes) 2 , respectively, are shown here to be effective solution-processable reagents for lowering the work functions of electrode materials; this approach is compared to the use of solution-deposited films of ethoxylated poly(ethylenimine) (PEIE). The work functions of indium tin oxide (ITO), zinc oxide, and gold electrodes can be reduced to 3.3-3.4 eV by immersion in a toluene solution of (IrCp*Cp) 2 ; these values are similar to those that can be obtained by spin- coating a thin layer of PEIE onto the electrodes. The work-function reductions achieved using (IrCp*Cp) 2 are primarily attributable to the interface dipoles associated with the formation of submonolayers of IrCp*Cp + cations on negatively charged substrates, which in turn result from redox reactions between the dimer and the electrode. The electrical properties of C 60 diodes with dimer-modified ITO cathodes are similar to those of analogous devices with PEIE-modified ITO cathodes. KEYWORDS: work function, dopant, electron injection, indium tin oxide, poly(ethylenimine), iridocene 1. INTRODUCTION Materials with low work functions (WFs) are required as electron-collecting or electron-injecting electrodes in devices such as organic photovoltaic cells, light-emitting diodes, and transistors. Although low-WF metals, such as those of groups 1 and 2, can be used as electrodes, they are highly unstable in air and, in some cases, may react with organics, for example, through abstraction of halide ions. Alternatively, surface modification can be used to lower the WF of materials that exhibit moderate or high WFs in their unmodified state; deposition of modifiers including molecules that can interact with the surface to form monolayers, 1-4 polyelectrolytes, 5-7 and nonconjugated neutral polymers such as poly(ethylene oxide) 8 and poly(ethylenimine) 9 can result in an appropriately oriented dipole layer, and consequent shift in vacuum level, at the surface of the material. Modification approaches also allow low WF to be combined with other desirable properties, for example, transparency in the case of tin-doped indium oxide (ITO) and other conducting oxide electrode materials. Strongly reducing molecules can also be used to lower electrode WFs; here electron transfer from the reductant molecules to the electrode results in a layer of the corresponding cations on the negatively charged surface and consequently in a surface dipole. For example, evaporation of tetrakis(dimethylamino)ethylene (TDAE), 10 neutral methyl viologen (MV(0)), 11 and acridine orange base (AOB) 12 (Figure 1) onto gold has been found to result in WF reductions of 1.3, 2.2, and 1.9 eV, respectively, which, in each case, have been attributed to electron transfer from the modifier to the gold. TDAE has also been found to reduce the WF of ITO by 0.9 eV. 13 Stronger reductants are expected to lead to a greater extent of electron transfer and, therefore, potentially larger WF modifications. The dimers of certain 19-electron sandwich compounds (effective redox potentials for the monomer cation/dimer couple, E[M + / Received: December 12, 2014 Accepted: February 3, 2015 Figure 1. Chemical structures of reductants used in the literature and in this work to reduce the WF of various electrode materials. Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/am5087648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX