Effect Of Extending Fluorinated Thiophene Pi-Bridges Of Bdt- And Tt-Based Polymers For Nonfullerene Organic Solar Cells

A series of novel benzo[1,2-b:4,5-b]dithiophene (BDT) and thieno[3,4-b]thiophene (TT)-based medium-bandgap polymers (PBDTTT-2F and PBDTTT-3F) were synthesized using a Stille cross-coupling reaction; these polymers contain fluorinated thiophene pi-bridges. The number-average molecular weights (M-n) of PBDTTT-2F and PBDTTT-3F are 37 kDa and 21 kDa, with dispersities of 2.5 and 2.7, respectively. The bandgap energies of PBDTTT-2F and PBDTTT-3F are 1.75 eV and 1.76 eV, respectively. The absorption regions of PBDTTT-2F and PBDTTT-3F cover 300-700 nm, which is complementary to the absorption range of the nonfullerene acceptor (IT-4F). The polymers were combined with IT-4F to fabricate bulk heterojunction solar cells. The PBDTTT-2F:IT-4F and PBDTTT-3F:IT-4F organic solar cells exhibited power conversion efficiencies (PCEs) of 9.52% and 9.08%, with short-circuit current densities of 17.13 and 16.58 mA cm(-2), open-circuit voltages of 0.78 and 0.81 V, and fill factors of 72% and 68%, respectively. The superior PCE of the PBDTTT-2F:IT-4F device compared with that of the PBDTTT-3F:IT-4F device is attributed to the enhanced exciton dissociation, reduced charge recombination, and high carrier mobility in the PBDTTT-2F:IT-4F device.