We report efficient bulk-heterojunction [regioregular of poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C-61 butyric acid methyl ester (PCBM)] solar cells with a highly transparent and electrical conductive NiO film as hole transporting layer (HTL) on optimized Al doped zinc oxide (AZO) substrate. The highest power conversion efficiency (PCE) of 3.15% of AZO/NiO/P3HT:PCBM/Al solar cells was achieved through NiO nano-layer sputtering O-2/Ar flow ratio, film thickness optimization and it was found that room temperature sputtered NiO at an O-2/Ar ratio of 3:7 and with a thickness of 10 nm shows the best photovoltaic properties. (C) 2010 Elsevier B.V. All rights reserved.