Effect of Alkyl Side Chain Length on Intra- and Intermolecular Interactions of Terthiophene-Isoindigo Copolymers

Two terthiophene-isoindigo copolymers (P3TI-O and P3TID) were successfully designed and synthesized. Octyl (O) and dodecyl (D) alkyl side chains were appended at the third position of the first and fourth positions of the last thiophene on the terthiophene donor units in P3TI-O and P3TI-D, respectively. The band gaps of these copolymers were less than 1.7 eV. We found intramolecular charge-transfer states in both copolymers, which were generated with time constants of 4.5 and 13 ps for the copolymers with short and long alkyl side chains on their donor units, respectively. These results indicate that an almost 3-fold faster intramolecular charge-transfer process occurs when the alkyl side chains are shortened by four methyl units. Owing to a better interchain charge transfer, P3TI-O exhibits a more efficient exciton diffusion compared to P3TI-D, thus leading to longer exciton lifetimes in the copolymer with shorter alkyl side chains. Consequently, when blended with PC71BM in bulk heterojunctions, P3TI-O showed a better exciton dissociation efficiency compared to P3TI-D. These results correlate well with the higher short-circuit current densities observed in P3TI-O:PC71BM inverted architecture organic solar cells compared to the P3TID:PC71BM ones.