Aluminum doped ZnO (AZO) 2 wt.% and un-doped ZnO thin films of different thickness were deposited by Pulsed DC Magnetron Sputtering onto Poly-Ethylene Terephthalate (PET) substrates, whereas PEDOT: PSS was spin-coated onto the AZO/PEr. The optical properties of the samples were measured by in-situ Spectroscopic Ellipsometry in the Vis-fUV energy range (1.5-6.5 eV), where it was found that the ZnO energy gap decreases after a critical thickness while it remains constant for AZO thin films. Electrical characterization of ZnO films shows that the films exhibit metallic behavior independently of their thickness. Also, High Resolution Transmission Electron Microscopy revealed the growth of polycrystalline ZnO onto PET substrates. Concerning the PEDOT: PSS/AZO/PET materials, the thickness of the AZO films has been found to affects neither the fundamental energy gap nor the other absorption peaks of PEDOT: PSS. (C) 2009 Elsevier B.V. All rights reserved.