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(FBTTh₂)₂. 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^-3 to 3.6 × 10^-3 cm² s^-1, resulting in an enhancement of the mean two-dimensional exciton diffusion length (L_D = (4Dτ)^1/2) 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.