Inverted polymer solar cells using an ultrathin layer (similar to 1 nm) of low work function metal (Ca, Mg, Al, and Ag) on indium tin oxide (ITO) substrate as the cathode, MoO(3)/Au as the anode, and a blend of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as the active layer were investigated. A comparative study between the conventional forward (ITO/MoO(3)/P3HT:PCBM/Ca/Ag) and inverted (ITO/Ca/P3HT:PCBM/MoO(3)/Au) cells shows that the inverted cell has an increase of 29% in power conversion efficiency and an increase of 750% in the cell's shelf lifetime when compared with the corresponding forward cell. The enhancement of efficiency for the inverted cell is due to reduced charge recombination, while the improvement of air stability is due to the better protection of the air sensitive low work function metal in the inverted cell. (C) 2010 Elsevier B.V. All rights reserved.