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

The influence of chemical species present at the interface between the elec troluminescent polymer and the ITO electrode in light-emitting devices on t he band edge energies of overlayers of semiconducting conjugated polymers h as been studied using photoelectron spectroscopy. The formation of InCl3 du ring 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 fun ction of ITO was changed in situ, within a given sample, by exposure to X-r ays. For the polymer spin-coated directly on ITO, the vacuum levels are ali gned. With the electrically conducting polymer blend, poly(3,4-ethplenediox ythiophene) doped with poly(4-styrene sulfonate) spin-coated on ITO, the Fe rmi levels are aligned, as expected. Therefore, with a conducting polymer b lend 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 b arrier to hole injection into the polymer is determined by the work functio n of the conducting polymer blend instead of the work function of the ITO. (C) 2000 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.