A comparison of n-type copolymers based on cyclopentadithiophene and naphthalene diimide/perylene diimides for all-polymer solar cell applications

All-polymer solar cells (PSCs), using naphthalene diimide (NDI)- or perylene diimide (PDI)-based polymers as the electron acceptor, have been intensively investigated in recent years. Nevertheless, the lack of comprehensive comparison studies of NDI- and PDI-based polymers has limited the further development of novel acceptor polymers. Here, we conduct a comparative study of two solution-processable cyclopenta[2,1-b:3,4-b'] dithiophene (CPDT)-based n-type copolymers, PCPDT-NDI and PCPDT-PDI, focusing on their optical, electrochemical and photovoltaic properties. Although PCPDT-NDI has better near-infrared (850-1100 nm) light absorption and crystalline properties, the photovoltaic performance is disappointing mainly because of the poor miscibility with the PTB7 donor polymer and the inferior film quality. On the contrary, PCPDT-PDI exhibits a much better photovoltaic performance with a power conversion efficiency of 2.13% when using 1-chloronaphthalene (CN) as an additive to obtain a good film morphology and to improve the electron mobility. The comprehensive comparison of PDI and NDI- based polymers will help understand the structure-property-performance relationship and will contribute to further development of novel rylene diimide-containing n-type polymers.