Efficient polymer solar cells (PSCs) with enhanced open circuit voltage (Voc) are fabricated by introducing solution processed and UV-ozone (UVO) treated nickel acetate (O-NiAc) as anode buffer layer. According to the XPS data, NiAc partially decomposed into NiOOH during the UVO treatment. NiOOH is a dipole specie, which leads to an increase in work function (confirmed by UPS), thus benefits to the formation of ohmic contact between the anode and the photoactive layer, leading to an increased Voc. In addition, the UVO treatment improves the wettability between the substrate and the solvent of the active layer, which facilitates the formation of upper photoactive layer with better morphology. Further, the O-NiAc layer can decrease the series resistance (Rs) and increase the parallel resistance (Rp) of the device, inducing an enhanced Voc in comparison with the as-prepared NiAc-buffered control devices without UVO treatment. For the PSCs based on P3HT:PCBM system, the Voc increases from 0.50 V to 0.60 V after the NiAc buffer layer undergoes UVO treatment. Similarly, in P3HT:ICBA system, the Voc of the device with UVO treated NiAc buffer layer increases from 0.78 V to 0.88 V, showing an enhanced PCE of 6.64%.