  We report an ultra long range energy transfer in a layered
donor:spacer:acceptor structures, which consist of the typical organic
photovoltaic material system of P3HT as the donor and PCBM as the acceptor. By
varying the thicknesses of spacer and acceptor layers we show that the energy
transfer is to the full volume of the acceptor and not just to its nearest
interface, and we find an effective energy transfer range on the order of about
100nm. Our efforts to elucidate the origin of this process, both theoretically
and experimentally, are discussed as well. Although it was recently implied
that an exceptionally large energy transfer may take place in this material
system, this is the first time, to the best of our knowledge, that the energy
transfer mechanism is characterized in a quantitative way and compared to the
common models. Our analysis offers new prospects for the familiar photovoltaic
bi-layer configurations, which may be very efficient if the ultra long range
energy transfer is utilized through a suitable architecture.
