Imide-linked alkyl chain influence on the properties of pyrrole-based imide-functionalized polymers containing pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione and benzodithiophene units for polymer solar cells

A series of pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD)-based polymers incorporating different alkyl side chains on the imide nitrogen were prepared to investigate the imide-linked alkyl side chain effects on the properties of DPPD-based polymers. DPPD derivatives with heptyl, octyl, decyl and dodecyl substituents on the imide nitrogen were prepared and copolymerized with 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (BDT) to offer polymers P(BDT-C 7 -DPPD), P(BDT-C 8 -DPPD), P(BDT-C 10 -DPPD), and P(BDT-C 12 -DPPD), respectively. All four polymers displayed identical optical band gaps ( Eg ∼ 2.11 eV) and highest occupied/lowest unoccupied molecular orbital energy levels (HOMO/LUMO ∼ −5.39 eV/–3.28 eV). The conventional single layer polymer solar cell (PSC) fabricated with a device structure of ITO/PEDOT:PSS/polymer:PC 70 BM (1:1.5 wt%)/Al offered a power conversion efficiency ( PCE ) of 1.65%, 1.49%, 1.24%, and 0.87%, respectively. The PSC device performance was improved to 5.67%, 5.29%, 4.06%, and 2.46%, respectively, with the use of processive additive such as 1,8-diiodoocate (DIO).