The advent of organic solar cells promises access to inexpensive solar electricity. Major advances in this thin film technology bear on biphasic and bicontinuous mesoscopic structures composed of an electron donor and acceptor material. Here, we demonstrate a high efficiency organic solar cell with simple planar bilayer architecture based on a soluble cyanine dye as electron donor and fullerene C₆₀ as electron acceptor. Upon doping the trimethine cyanine layer and optimizing the device cathode, the cell reaches a power conversion efficiency of 2.0% under simulated AM1.5 solar irradiation. The external quantum efficiency approaches 80% at maximum and is among the highest reported in organic photovoltaic devices. Soluble cyanines are compatible with low-cost coating processes, adding to the merits of this class of photographic dyes for organic solar cell applications.