Local work function control of indium tin oxide by micro-contact printing for electroluminescent devices

We demonstrate here that light emission of an electroluminescent (EL) device was enhanced on chemically modified ITO areas over a patterned ITO anode with a self-assembled monolayer (SAM) of 4-chlorophenylphosphoryl dichloride (ClC6H4OPOCl2: CPPDC) prepared by micro-contact printing (μ-CP). The EL device was fabricated by vapor-depositing a hole transport layer, a light emitting and electron transport layer, and a C6H5COOLi/Al bilayer cathode on the patterned ITO anode. The enhanced light emission under lower drive voltages on the modified areas can be interpreted by the increase in the work function of the ITO covered with the SAM measured with a Kelvin probe force microscope (KPFM) and thus the decrease in the hole injection barrier height. In addition, we could demonstrate the much smaller pattern images than the previously reported ones [F. Nüesch, Y. Li, L.J. Rothberg, Appl. Phys. Lett. 75 (1999) 1799] by the use of ink-pads.