Effect of annealing on exciton diffusion in a high performance small molecule organic photovoltaic material

Abstract

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 cm2 s-1 to 3.6 × 10-3 cm2 s-1, resulting in an enhancement of the mean two–dimensional exciton diffusion length (LD=√4Dτ) from 15 nm 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.