Effect of thermal annealing on exciton diffusion in a diketopyrrolopyrrole derivative.

We show that the method we have developed for measuring the singlet exciton diffusion length in blends with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) can be applied not only to polymeric materials, but also to small molecule organic semiconductors. Small organic molecules have a large potential for molecular re-organization upon thermal annealing. Here we show that the exciton diffusion length is decreased upon annealing from 9 to 3 nm in a thin film of a diketopyrrolopyrrole derivative. Such a variation is attributed to exciton delocalization effects in the crystalline domains that are formed during the annealing process as well as to the exciton quenching at grain boundaries.