Polymer acceptor based on naphthalene diimide and diethynylbenzo[1,2-b:4,5-b'] dithiophene units for efficient all-polymer solar cells

Abstract

Five polymer acceptors based on naphthalene diimide (NDI) and diethynylbenzo[1,2-b:4,5-b']dithiophene (DEBDT) alternating units were designed and synthesized. The resultant polymers have identical polymer backbones but different side chains on both NDI and DEBDT units. Their photoelectronic properties can be fine-tuned via systematic side chain engineering. In these polymers, the size of the alkyl side chains on NDI units have minimal impacts on light-harvesting properties and molecular energy levels, while introducing side chain groups into DEBDT units can significantly broad the absorption profile and tune the lowest unoccupied molecular orbital energy levels of polymers. All-polymer solar cells (all-PSCs) based on these polymer acceptors and PBDB-T as the electron-donating polymer were fabricated, where the power conversion efficiencies (PCEs) varied from 0.42% to 5.84%. All-PSCs based on PBDB-T:PNEOD-C8 exhibits the best device performance, which can be ascribed to the lowered energy loss, good light-harvesting capability, balanced charge carrier mobility and proper blend film morphology. These observations indicate that modifying the side chains on both donor and acceptor unit of polymer acceptors is an effective strategy for optimizing their optical and electrical properties, thus lead to high-performance all-PSCs.