Effect of Annealing on Exciton Diffusion in a High Performance Small Molecule Organic Photovoltaic Material.

Singlet exciton diffusion was studied in the efficient organic photovoltaic electron donor material DTS(FBTTh). Three complementary time-resolved fluorescence measurements were performed: quenching in planar heterojunctions with an electron acceptor, exciton-exciton annihilation, and fluorescence depolarization. The average exciton diffusivity increases upon annealing from 1.6 × 10 to 3.6 × 10 cm s, resulting in an enhancement of the mean two-dimensional exciton diffusion length (L = (4Dτ)) from 15 to 27 nm. About 30% of the excitons get trapped very quickly in as-cast films. The high exciton diffusion coefficient of the material leads to it being able to harvest excitons efficiently from large donor domains in bulk heterojunctions.