We investigate the properties of NiO thin films prepared by sol-gel solution and the photovoltaic performance of bulk-heterojunction (BHJ) solar cells based on NiO thin films as anode buffer layers with various film thickness and amorphous-crystalline phase. Optoelectronic properties and surface morphology of NiO films strongly depend on the thickness and phase of the NiO. The performance of solar cells based on nanocrystaline (nc-) NiO exhibits better photovoltaic characteristics in comparison to those employing an amorphous (a-) NiO. The obtained power conversion efficiency (PCE, eta) of a solar cell with a 20-nm-thick nc-NiO is 2.24%, with a short-circuit density (J(SC)) of 7.77 mA/cm(2), open-circuit voltage (V-OC) of 0.57 V, and fill factor (FF) of 0.50. (C) 2012 Elsevier B.V. All rights reserved.