A dramatic increase in the power conversion efficiency (PCE) of inverted organic solar cells (IOSCs) is realized by a gallium (Ga)-doped zinc oxide (GZO) buffer layer acting as an electron-transport layer. The GZO nanostructured thin-film buffer layer was synthesized via an aqueous solution method at 90 °C. Both an increase of electrical conductivity and a smooth surface morphology were realized by Ga doping. The PCE of a GZO-based IOSC was improved by about 110% at simulated air mass 1.5 global full-sun illumination compared with that of undoped zinc oxide-based IOSCs. The increase of the short-circuit current in GZO-based IOSCs is due to the higher electron conductivity and favorable surface morphology of the buffer layer through Ga-doping, resulting in the dramatic enhancement of the PCE.