Energy level alignments of poly[N-9 ''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benz othiadiazole)] (PCDTBT) cast on top of Au and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT: PSS) were investigated using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) as a function of the concentration of PCDTBT solutions. Although UPS data show similar work functions for Au and PEDOT: PSS, the hole transport barrier (phi(h)) at the PCDTBT/PEDOT:PSS interface is clearly smaller than that at the PCDTBT/Au interface. The XPS spectra show that energy level relaxation occurs at the interface between PCDTBT and electrodes. (C) 2010 Elsevier B. V. All rights reserved.