Tandem organic solar cells with peak conversion efficiencies (η) of 5.15% are demonstrated. This is achieved by stacking two different planar heterojunction devices, each with a high open-circuit voltage (Voc). The phthalocyanine based donor materials in the employed subcells possess complementary absorption, a quality of critical importance to optimize photocurrent in a series connected tandem cell. The tandem structure produces Voc values of nearly 2 V, while fill factor remains above 60%. The measured η corresponds to a 40% increase compared to η of the optimal single cells.