Organometallic Dimers: Application to Work-Function Reduction of Conducting Oxides

The dimers of pentamethyliridocene and ruthenium pentamethylcyclopentadienyl 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.