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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 π bridge can significantly influence the photoelectric properties of D-A type polymers, π 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 π 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 π 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 π bridge of thiophene. These results demonstrate that the precise modulation of asymmetric π bridges is a promising strategy to construct high-efficiency D-A type polymer donors. |
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