The energy level alignment at the interface between an organic layer and an electrode plays a critical role in the performance of organic electronic devices. To improve the charge injection efficiency, the energy barrier between the charge transport level of an organic layer and the Fermi level of an electrode should be reduced. Especially, poly(9-vinylcarbazole) (PVK) is a polymeric semiconductor that is widely used as a hole transport and electron blocking layer in various optoelectronic devices. Thus, an understanding of the energy level alignment between PVK and an electrode is of great importance. In this study, the energy level alignment of PVK and indium tin oxide (ITO) was determined with X-ray and ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy measurements. The effect of UV-ozone (UVO) treatment on the formation of a hole injection barrier (Phi(h)) in ITO was also investigated. In both cases of bare ITO and UVO ITO, only small interface dipole and band bending were observed, which indicates charge transfer between PVK and ITO is miniscule. The UVO treatment significantly increases the work function of ITO from 4.05 eV to 4.40 eV, which results in the reduction of Phi(h) from 1.70 eV to 1.50 eV. This reduced Phi(h) value dramatically improves the current density-voltage characteristics of PVK-based hole-only devices. (C) 2018 Elsevier B.V. All rights reserved.