Comparison of Three n-Type Copolymers Based on Benzodithiophene and Naphthalene Diimide/Perylene Diimide/Fused Perylene Diimides for All-Polymer Solar Cells Application

All-polymer solar cells have gained large attention in recent years because of their tunable energy levels and absorption spectra for both polymeric donor and acceptor. Comparing with the numerous polymeric donors, the development of polymeric acceptors was relatively slow. Rylene diimide-based polymers are regarded as the most promising n-type polymers, which were widely investigated in the past decade, and some novel rylene diimide structures are constantly designed. In this work, three n-type polymers with a donor/acceptor (D/A) alternative backbone structure, named PNDI-BDT, PPDI-BDT, and PFPDI-BDT, were synthesized. In these polymers, naphthalene diimide (NDI), perylene diimide (PDI), and recently developed fused perylene diimide (FPDI) were utilized as electron-withdrawing segment, respectively, and benzodithiophene (BDT) with thiophenes as conjugated side chains was utilized as an electron-rich unit. The optical properties, electron energy levels, charge transport properties, photovoltaic performance, charge recombination loss, and surface morphology were systematically investigated. After optimizing the device fabrication conditions, PNDI-BDT-, PPDI-BDT-, and PFPDI-BDT-based photovoltaic cells realized the power conversion efficiencies of 0.88, 3.74, and 5.65%, respectively. Our results indicate that FPDI is a better electron-deficient segment in comparison with NDI and PDI, for the design of n-type photovoltaic polymers.