Work function of ITO substrates and band-offsets at the TPD/ITO interface determined by photoelectron spectroscopy

Surface compositions and work functions (Phi) of commercially available indium tin oxide (ITO) substates were measured by photoelectron spectroscopy (UPS/XPS). Whereas substrates cleaned by organic solvents are significantly contaminated and have low Phi values (3.9-4.2 +/- 0.1 eV), substrates cleaned by Ar+ sputtering typically have values of Phi = 4.3 +/- 0.1 eV. Even higher Phi values (up to 4.7 +/- 0.1 eV) are obtained by reactive ion etching with oxygen, likely related to oxygen-containing surface impurities. Evaporated TPD is physisorbed on ITO, but causes a drop of the vacuum potential by 0.2-0.4 eV (depending on the ITO pretreatment) directly at the TPD/ITO interface, in contradiction to the common-vacuum level rule. The TPD highest occupied molecular orbital (HOMO) is found 1.1-1.3 eV below the Fermi level of the ITO, which indicates the presence of a significant barrier for hole injection. (C) 2000 Elsevier Science S.A. All rights reserved.