Chemical species at polymer/ITO interfaces: consequences for the band alignment in light-emitting devices

Abstract

The influence of chemical species present at the interface between the electroluminescent polymer and the ITO electrode in light-emitting devices on the band edge energies of overlayers of semiconducting conjugated polymers has been studied using photoelectron spectroscopy. The formation of InCl 3 during the conversion of precursor-PPV on ITO was directly monitored with XPS. Ultrathin films of poly( bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) were studied directly on ITO, as well as with an intermediate layer of an electrically conducting polymer using UPS. The initial work function of the ITO was varied chemically from 4.4 eV to 4.8 eV. In addition, the work function of ITO was changed in situ, within a given sample, by exposure to X-rays. For the polymer spin-coated directly on ITO, the vacuum levels are aligned. With the electrically conducting polymer blend, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate) spin-coated on ITO, the Fermi levels are aligned, as expected. Therefore, with a conducting polymer blend intermediate layer between the polymer and the ITO, the polymer bands align to the vacuum level of the conducting polymer blend on ITO, and the barrier to hole injection into the polymer is determined by the work function of the conducting polymer blend instead of the work function of the ITO.