We present a merocyanine:C 60 /zinc phthalocyanine:C 60 tandem solar cell, comprising two complementary absorbing bulk heterojunction subcells connected in series. High-efficiency devices were realized in a rather simple tandem setup, consisting of only three organic layers that were successively deposited in an ultrahigh-vacuum chamber. The optimized tandem solar cell features an efficiency of 4.5%, demonstrating a performance improvement by ca. 50% compared to the individual optimized single-junction solar cells. The experimental data are in excellent agreement with optical simulations, assuming an internal quantum efficiency near unity in the optimized tandem device.