One of the main causes of the degradation of organic solar cells is the interface between the indium tin oxide (ITO) anode and the poly (3,4-ethylendioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL), which is known to be unstable. On the contact surface, corrosion of the ITO is induced by PEDOT:PSS, and this degrades the performance of the device. In this paper, we solve this problem by introducing an inorganic HTL, nickel oxide (NiO), between the ITO and the active layer. Solution process, a low-cost production method for preparing thin films, is employed to fabricate the NiO HTL for organic solar cells based on poly (3-hexylthiophene) (P3HT) and 16,61-phenyl C-61-butyric acid methyl ester (PCBM). The performance of the resulting organic solar cells is optimized by modifying the baking conditions of the NiO thin films, yielding a power conversion efficiency enhancement of 1.97% and fill factor of 52.11%. Moreover, a remarkable improvement in cell lifetime is observed. X-ray photoelectron spectroscopy studies show that cell performance and stability strongly depend on the oxygen composition ratio of the NiO structure. The transition of the O2- ratio is a key factor that determines the conductivity of NiO. These results compare favorably with those reported in the literature for conventional devices with PEDOT:PSS. (C) 2012 Elsevier B.V. All rights reserved.