Intrachain and Interchain Excited-State Dynamics of Temperature-Dependent Aggregation Copolymer in Solution

Poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis (2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)] (PBDB-T) is a donor-acceptor copolymer widely used as a donor material in high-efficiency organic solar cells. In this work, we studied the temperature-dependent aggregation properties of PBDB-T in solution. Through the characterization of UV-vis absorption and the photoluminescence spectrum, we found that PBDB-T formed strong interchain interactions that facilitate aggregation at room temperature. In contrast, warmer temperatures cause PBDB-T to coil and increase intrachain interactions, thus reducing aggregation. We further use transient absorption spectroscopy to explore the effect of temperature-dependent aggregation behavior on excited-state dynamics. The results show that the intrachain interaction is beneficial to increase the production of polaron pairs, and the interchain interaction is beneficial to accelerate the production of free polarons. Finally, we investigated the corresponding films and demonstrated that regulating the solution aggregation is an effective way to control the crystallinity, and morphology of the corresponding films.