L. Chkoda et al., Work function of ITO substrates and band-offsets at the TPD/ITO interface determined by photoelectron spectroscopy, SYNTH METAL, 111, 2000, pp. 315-319
Surface compositions and work functions (Phi) of commercially available ind
ium 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 clean
ed by Ar+ sputtering typically have values of Phi = 4.3 +/- 0.1 eV. Even hi
gher Phi values (up to 4.7 +/- 0.1 eV) are obtained by reactive ion etching
with oxygen, likely related to oxygen-containing surface impurities. Evapo
rated 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 i
nterface, 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 leve
l of the ITO, which indicates the presence of a significant barrier for hol
e injection. (C) 2000 Elsevier Science S.A. All rights reserved.