We report an efficient tandem polymer solar cell (PSC), consisting of two active layers of poly-(3-hexylthiophene):[6,6]-phenyl C-61 butyric acid methyl ester (P3HT:PC61BM) and poly[(2,5-bis-(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl} -alt-([2,2'-(1,4-phenylene)-bisthiophene]-5,5'-diyl}]:PC61BM. The two active layers are connected by a Ca (1 nm)/Ag(1 nm)/MoO3 (10 nm) intermediate layer, which is superior to a Ca (1 nm)/A1 (1 nm)/MoO3 (10 nm) intermediate layer for better performance. The tandem PSC with complementary spectral absorption from 300 to 850 nm showed a power conversion efficiency of 4.50% under AM1.5G, 100 mW cm(-2) illumination and an efficiency improvement of about 25.0% over the single-stack sub-cells. (C) 2013 Elsevier B.V. All rights reserved.