Dependence of Exciton Diffusion Length on Crystalline Order in Conjugated Polymers

Exciton diffusion in organic semiconductors is crucial to the performance of organic solar cells. Here, we measured the exciton diffusion length in poly(3-hexylthiophene) (P3HT) as a function of the crystalline order using spectrally resolved photoluminescence quenching (SR-PLQ) techniques. The crystalline order in the P3HT films, characterized according to the mean crystal size and normalized crystallinity, was varied by changes in thermal treatment temperatures. The exciton diffusion length increased from 3 to 7 nm as the mean crystal size increased more than twice and the crystallinity increased by a factor of 6. A higher crystalline order improved the spectral overlap and reduced the distance between chromophores, enhancing Forster-mediated exciton diffusion. The higher crystalline order also lengthened the conjugated segments and reduced the energetic disorder, producing favorable condition for exciton hopping.