Enhanced photovoltaic performance of donor-acceptor type polymer donors by employing asymmetric π bridges

摘要

In comparison with the common-used electron donor (D)-electron acceptor (A) type symmetric polymers with regular structures, their asymmetric counterparts have received much less attention and seldom been applied in polymer solar cells. Since the precise modulation of pi bridge can significantly influence the photoelectric properties of D-A type polymers, pi bridges of tailorable thiophenes are introduced into benzodithiophene (BDT)-benzodithiophene-4,8-dione (BDD) copolymers in both symmetric and asymmetric patterns. Together with the halogen substituent of fluorine or chlorine on BDT, four polymer donors are designed and synthesized. As expected, PTB2T-F and PTB2T-Cl with the asymmetric pi bridges of thiophene and hexyl-bithiophene exhibit better planarity, stronger intermolecular interaction and higher hole mobility than P2TB2T-F and P2TB2T-Cl with the symmetric pi bridges of hexyl-bithiophene. The more suitable phase separation and optimized nanomorphology is determined by the reasonable miscibility between asymmetric polymer and acceptor, which is evaluated by Flory-Huggins interaction, contributing to the extremely higher performance than that of symmetric counterpart. Moreover, paired with IT-4F as the acceptor, PTB2T-F affords better device performances than the reference BDT-BDD polymer PM6 with the symmetry pi bridge of thiophene. These results demonstrate that the precise modulation of asymmetric pi bridges is a promising strategy to construct high-efficiency D-A type polymer donors.