We report the characterization of triphenylamine-based hole-transporting material, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl4,4'-diamine (TPD), used in the electrochromic devices. The electrochromic properties of different TPD thickness were studied by employing the electrochemical and spectroelectrochemical methods. Cyclic voltammograms of TPD films on an indium tin oxide (ITO)-coated glass exhibited one reversible redox couple in the range of 0-1.4 V versus Ag/AgCl in 0.1 N HClO4. To assemble the dual-type electrochromic devices using different thickness (250, 400, and 1000 nm) of TPD films: ITO/TPDHPU-LiClO4//PEDOT/ITO, the optical contrast of the devices increases with increasing the TPD film thickness, whereas the 1000 nm TPD film showed relatively excellent stability with respect to the electrochromic characteristics as compared to 250 and 400 nm films; the color of the film changed from orange-brown, transparent, and dark blue at -2.0, 0.0, and +2.0 V (vs. PEDOT), respectively, where PEDOT is poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate). (C) 2008 Elsevier B.V. All rights reserved.