Synthesis and characterization of fluorine atom substituted new BDT-based polymers for use in organic solar cells

Abstract

A series of benzo[1,2-b:4,5-b′]dithiophene (BDT)-based donor–acceptor conjugated polymers were synthesized using a Stille cross-coupling reaction. The new fluorinated polymer (PBDTFT-ttTPD) contained 3,4-difluoroalkylthienyl benzo[1,2-b:4,5-b′]dithiophene (BDTFT) donor units and thienothienylthieno[3,4-c]pyrrole-4,6-dione (ttTPD) as acceptor units. A nonfluorinated alkylthienyl-substituted BDT-based polymer (PBDTT-ttTPD) was synthesized and its optoelectronic and photovoltaic properties were compared with those of PBDTFT-ttTPD. The number-averaged molecular weights (Mn) of PBDTT-ttTPD and PBDTFT-ttTPD were found to be 22,500 and 24,100g/mol, with dispersities of 2.3 and 2.0, respectively. The band gap energies of PBDTT-ttTPD and PBDTFT-ttTPD were 1.87eV and 1.89eV, respectively. The HOMO levels of PBDTT-ttTPD and PBDTFT-ttTPD were −5.68eV and −5.78eV, respectively. These results indicated that the inclusion of 3,4-difluorothienyl-BDT units in the polymers provided a very effective approach to lowering the HOMO and LUMO energy levels. The field-effect mobilities of PBDTT-ttTPD and PBDTFT-ttTPD were determined to be 9.2×10−4cm2V−1s−1 and 2.7×10−4cm2V−1s−1, respectively. A polymer solar cell device prepared using PBDTFT-ttTPD as the active layer was found to exhibit a power conversion efficiency (PCE) of 3.27% with an open circuit voltage of 0.79V under AM 1.5G (100mW/cm2) conditions.