We study the effect of oblique incident light on the optical interference effects and photocurrent of organic planar heterojunction solar cells. We find that the thin layers used in organic solar cells induce a complex light distribution within the device, further altered by incident angle. The responsivity can increase as a function of angle to values up to 15% larger than at normal incidence, and optical simulations are shown to match these trends. Furthermore, we show that the outdoor performance at different tilting angles is higher than expected, which improves the attractiveness of organic solar cells for building-integrated applications. (c) 2008 American Institute of Physics.