Effects of Halogenation in B ← N Embedded Polymer Acceptors on Performance of All-Polymer Solar Cells.

Halogenation, for example, fluorination and chlorination, is an effective strategy to regulate the performance of organic photovoltaic materials. Although fluorination has been widely applied to polymer acceptors, systematic studies on the comparison of nonhalogenated, fluorinated, and chlorinated polymer acceptors have been a blank to now. Herein, a B ← N embedded electron-deficient unit (A), namely, BNIDT was copolymerized with three electron-rich units (D), that is, benzodithiophene (BDT), fluorinated BDT, and chlorinated BDT to obtain three D-A polymers of BN-BDT, BN-BDT-F, and BN-BDT-Cl, respectively. The three polymers exhibit similar LUMOs of ca. -3.77 eV, whereas the HOMOs are remarkably decreased from BN-BDT (-5.46 eV) to BN-BDT-F (-5.71 eV) and further slightly lowered to BN-BDT-Cl (-5.74 eV). All-polymer solar cells (all-PSCs) were fabricated using PBDB-T as the donor and the three B ← N-based polymers as the acceptors. The efficiencies of all-PSCs were significantly promoted from nonhalogenated BN-BDT (1.60%) to fluorinated BN-BDT-F (3.71%) and further elevated to chlorinated BN-BDT-Cl (4.23%). Device characterizations revealed that halogenation on the polymer acceptors leads to enhanced hole-transfer driving forces and better donor/acceptor miscibility, for example, smaller domain sizes and root-mean-square roughness (rms) values, which further gives rise to higher and more balanced hole/electron mobilities and efficient physical processes, for example, efficient exciton dissociation and collection and weaker recombination losses in halogenated devices. This work demonstrates that the photovoltaic performance of nonhalogenated polymer acceptors can be remarkably boosted by fluorination and further enhanced by chlorination. This is the first systematic study on the halogenated polymer acceptors by comprehensively comparing nonhalogenated, fluorinated, and chlorinated ones.