文摘
Two donor鈥揳cceptor copolymers, PBDT and PNDT, containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b鈥瞉dithiophene (BDT) and 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b鈥瞉dithiophene (NDT), respectively, as an electron-rich unit and 5,6-difluoro-2,1,3-benzothiadiazole (2FBT) as an electron-deficient unit, were synthesized and compared. The introduction of the NDT core into the conjugated backbone was found to effectively improve both light harvesting and the charge carrier mobility by enhancing chain planarity and backbone linearity; the NDT copolymer has stronger noncovalent interactions and smaller bond angles than those of the BDT-based polymer. Moreover, the introduction of the NDT core brings about a drastic change in the molecular orientation into the face-on motif and results in polymer:PCBM blend films with well-mixed interpenetrating nanofibrillar bulk鈥揾eterojunction networks with small-scale phase separation, which produce solar cells with higher short-circuit current density and fill factor values. A conventional optimized device structure containing PNDT:PC71BM was found to exhibit a maximum solar efficiency of 6.35%, an open-circuit voltage of 0.84 V, a short-circuit current density of 11.92 mA cm鈥?, and a fill factor of 63.5% with thermal annealing, which demonstrates that the NDT and DT2FBT moieties are a promising electron-donor/acceptor combination for high-performance photovoltaics.