Regulation of the Miscibility of the Active Layer by Random Terpolymer Acceptors to Realize High-Performance All-Polymer Solar Cells

The main limitations on the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs) are the weak absorption coefficients of widely used naphthalene diimide (NDI)-based polymer acceptors and the difficulty of morphology control. Herein, a 3,3′-difluoro-2,2′-bithiophene (2FT) unit is introduced into the NDI-based polymer N2200 by random copolymerization, creating three terpolymer acceptors PNDI-2FT-0.1, PNDI-2FT-0.2, and PNDI-2FT-0.3. Incorporation of 2FT into the backbone is found to significantly improve the absorption coefficients of terpolymers. More importantly, it can be observed that random copolymerization of 2FT into the backbone not only can reduce the strong aggregation of the polymer but also can produce more flexible main chains to favor closer contact and better miscibility with the crystalline donor PBDB-T relative to the N2200 bipolymer. Under the optimal condition, a PBDB-T:PNDI-2FT-0.1-based device achieves a notable PCE of 9.46% with a short-circuit current (JSC) of 16.62 mA cm–2. Note that both the PCE and JSC are the outstanding values in NDI-based all-PSCs. Moreover, the optimized morphology of a bulk heterojunction induced by random terpolymers enables an active layer that has good thickness tolerance. These results demonstrate that the simultaneous regulation of light absorption and miscibility between a donor and an acceptor by random copolymerization is a promising strategy to realize high-performance all-PSCs.