Distance dependence of excitation energy transfer between spacer-separated conjugated polymer films

We report a systematic study of the scaling with distance of electronic energy transfer between thin films of conjugated polymers separated by a silica spacer. The energy-transfer kinetics were obtained directly from time-resolved photoluminescence measurements and show a 1/z(3) distance dependence of the transfer rate between the excited donor and the acceptor film for z >= 8 nm. This is consistent with Forster theory; but at shorter separations the energy transfer is slower than predicted and can be explained by the breakdown of the point-dipole approximation at z similar to 5 nm. The results are relevant for organic photovoltaics and light-emitting devices, where energy transfer can provide a means of increasing performance.