Nickel oxide (NiO) was thermally evaporated between the active layer and the top anode in inverted polymer solar cells. We demonstrate that NiO behaves as an anode buffer layer. The NiO thin film can module the Schottky barrier and form an ohmic contact at the organic/metal interface, which make it a great holes transport layer. The device performance with NiO of different thicknesses are investigated and compared. The results show that the power conversion efficiency (PCE) increases by fivefold with 2 nm NiO compared to the control cell without NiO. The optimum device performance with 2 nm NiO shows the J(sc) of 7.92 mA/cm(2), V-oc of 0.46 V, FF of 39.8%, and thus the PCE of 1.45% under AM1.5G illumination of 100 mW/cm(2). (C) 2011 Elsevier B.V. All rights reserved.