Photovoltaic Property And Morphology Of D-A Copolymers With Different Bridge Rings Based On Fluorided Benzothiadiazole

Polymer solar cells have attracted considerable attention due to the ligeht-weight, low-cost, solution processability and large-area manufacture on the flexible substractes. But the low photoelectric conversion efficiency compared with inorganic solar cells limits its commercial application. To obtain high photoelectric conversion efficiency, it is necessary to design and synthsis new ideal polymers with narrow band gap, broad absorption, high mobility, suitable energy level and good morphology. To broaden the response wavelength range of conjugated polymer and adjust the energy level, one attractive approach is to construct donor-acceptor(D-A) conjugated polymers by alternate a conjugated electronrich donor(D) unit and a conjugated electron-deficient acceptor(A) unit in the same polymer backbone. Furthermore, introducting of π-extended fuse rings to backbone of D-A conjugated polymers should be one of the most effective strategy to extend the length of π-congugate and enhance strong intermolecular interactions between the neighboring polymer main chains which is beneficial to lower the energy band gap.In this article, three new D-A conjugated polymers: PTBDT-ffBT-FT12, PTBDT-ffBT-TT12 and PTBDT-ffBT-T12 T have been synthesized taking the alkyl thiophene substituted benzene [1, 2-b: 4, 5-b’]-dithiophene(TBDT) as donor unit, ffBT with strong electron-withdrawing ability as acceptor and four repeated thiophene or furan rings as bridged groups to the backbone. Photoelectric properties and morphology of these polymers have been studied, and the influence of the main chain and substituent position to molecule level,band-gap and stacking have also been investigated. All of studied copolymers showed broad absorption bands in the range of 300-800 nm, and the energy level and band gap can be tuned by the introduction of fluorine atoms and thiophene or furan rings. Among the three copolymers based devices, the best power conversion efficiency(PCE) of 2.1% is obtained from the device based on PTBDT-ffBT-T12T/PC61BM(1:2, w:w). The bad surface morphology of copolymer/PC61 BM blends may be the main reason for low PCEs. Therefore, when tune the band gap and energy level of copolymers using the method of increase more fused rings to form large π-conjugation structure, it may be counterproductive, Other factors should be involved, like the molecular molecular geometry and the steric hindrance of alkyl groups.