Enhanced exciton diffusion in an organic photovoltaic cell by energy transfer using a phosphorescent sensitizer

We demonstrate enhanced exciton diffusion in an organic photovoltaic cell through the incorporation of a phosphorescent sensitizer. The increase in exciton diffusion length (L-D) is realized using a composite electron donor layer consisting of a N,N-'-bis(naphthalen-1-yl)-N,N-'-bis(phenyl)-benzidine (NPD) host doped with the phosphorescent guest fac-tris(2-phenylpyridine) iridium [Ir(ppy)(3)]. The presence of the phosphor at low concentration allows for the population of the long-lived NPD triplet state and an increase in L-D. An increase in the NPD L-D from 6.5 +/- 0.3 to 11.8 +/- 0.6 nm is extracted from measurements of the external quantum efficiency for donor layers containing 5 wt % Ir(ppy)(3). This enhancement leads to a similar to 80% improvement in the power conversion efficiency relative to devices containing an undoped donor layer.