In recent years, alternatives to PEDOT:PSS, which is used in organic light-emitting diodes and polymer solar cells (PSCs), have been actively researched due to its disadvantages in terms of device stability. Nevertheless, PEDOT:PSS is still one of most powerful materials due to its good conductivity, low-temperature processability, and suitable work function that is well-matched with the HOMO energy levels of most donor polymers. In this study, an ultra-thin tungsten oxide/PEDOT:PSS hybrid hole extraction layer (H-HEL) is designed to take advantage of the benefits of PEDOT:PSS and address the device stability problems of PEDOT:PSS. Device stability is dramatically improved and power conversion efficiency is slightly improved in P3HT:PC60BM model system according to the ISOS-D-1 protocol. In order to understand this phenomenon, time-dependent photocurrent-voltage (J-V) measurements are conducted for device stability combined with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) studies. (C) 2013 Elsevier B.V. All rights reserved.