Inverted polymer solar cells with low-band-gap polymers were characterized. Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) were deposited between the active layer and the silver top electrode in inverted polymer solar cells (PSCs) with titanium oxide (TiOx) and indium tin oxide (ITO) as a buffer layer and a cathode, respectively. The performances of different anode buffer layers and three different polymers were compared. The best performance was achieved for the device with poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(5′,8′-di-2-thienyl-2,3-bis(4-octyloxyl)phenyl)quinoxaline] (P2) as a donor polymer and a 20 nm MoO3 layer between the active layer and Ag, that device exhibited a open circuit voltage (Voc) of 0.84 V, a short circuit current (Jsc) of 8.15 mA/cm2, and a fill factor (FF) of 0.41. The overall power conversion efficiency (PCE) for this cell was 2.79%.