We investigated the effects of various surface treatments on the work function and chemical composition of an indium tin oxide (ITO) surface. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the work function of ITO. X-ray photoelectron spectroscopy (XPS) was used to study the electron structures of ITO surface. We performed surface treatments on ITO using O(2) plasma and HCl solution. Our UPS/XPS analysis indicates increases in the work functions by O(2) plasma treatments. It is known that the Fermi energy level is controlled by the donor concentration, and thus the Fermi energy level is shifted toward the valence band minimum. (C) 2011 Elsevier B.V. All rights reserved.