Role of impurities in determining the exciton diffusion length in organic semiconductors

The design and performance of organic photovoltaic cells is dictated, in part, by the magnitude of the exciton diffusion length (L_D). Despite the importance of this parameter, there have been few investigations connecting L_D and materials purity. Here, we investigate L_D for the organic small molecule N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine as native impurities are systematically removed from the material. Thin films deposited from the as-synthesized material yield a value for L_D, as measured by photoluminescence quenching, of (3.9 ± 0.5) nm with a corresponding photoluminescence efficiency (η_PL) of (25 ± 1)% and thin film purity of (97.1 ± 1.2)%, measured by high performance liquid chromatography. After purification by thermal gradient sublimation, the value of L_D is increased to (4.7 ± 0.5) nm with a corresponding η_PL of (33 ± 1)% and purity of (98.3 ± 0.8)%. Interestingly, a similar behavior is also observed as a function of the deposition boat temperature. Films deposited from the purified material at a high temperature give L_D = (5.3 ± 0.8) nm with η_PL = (37 ± 1)% for films with a purity of (99.0 ± 0.3)% purity. Using a model of diffusion by Förster energy transfer, the variation of L_D with purity is predicted as a function of η_PL and is in good agreement with measurements. The removal of impurities acts to decrease the non-radiative exciton decay rate and increase the radiative decay rate, leading to increases in both the diffusivity and exciton lifetime. The results of this work highlight the role of impurities in determining L_D, while also providing insight into the degree of materials purification necessary to achieve optimized exciton transport.