  We describe surface patterning strategies that permit high photon-collection
efficiency together with high carrier-collection efficiency in an ultra-thin
planar heterojunction organic photovoltaic cell. Optimized designs reach up to
50% photon collection efficiency in a P3HT layer of only 30 nm, representing a
3- to 5-fold improvement over an unpatterned cell of the same thickness. We
compare the enhancement of light confinement in the active layer with an ITO
top layer for TE and TM polarized light, and demonstrate that the light
absorption can increase by a factor of 2 due to a gap-plasmon mode in the
active layer.
