Abstract
Two narrow band gap conjugated ternary copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The polymers with the repeating unit in a D1-A-D2-A manner were constructed by copolymerizing a bisstannyled-D1 (D1 = n-alkyl-substituted cyclopentadithiophene) and a dibromo-monomer (Br-A-D2-A-Br, D2 = branched-alkyl-substituted cyclopentadithiophene, A =[1,2,5]selenadiazolo[3,4-c]pyridine or 5-fluorobenzo[c][1,2,5]selenadiazole) through a palladium-catalyzed Stille polymerization. This approach that enables variations in the donor fragment substituents can not only control the polymer regiochemistry but also the solubility. Two ternary copolymers exhibited absorbance up to near-infrared region along with relatively narrow band gap in the range of 1.02–1.26 eV. The polymeric photovoltaic cells based on CDTPSE/PC61BM show the short circuit density of 1.45 mA cm−2, open current voltage of 0.53 V, and photocurrent spectra response from 300 to 1,150 nm under AM 1.5 simulator (100 mW cm−2). It is indicated that it can be potentially applied to near infrared photodetectors.
Highlights
Conjugated polymers have attracted both industrial and academic interests due to the potential for large-area, flexible, and low-cost applications in recent years
Great progress has been made in related research areas based on conjugated polymers including polymer light-emitting diode (PLED), polymeric field-effect transistor (PFET), polymer solar cell (PSC) (Zhao et al, 2016)
The open circuit voltage (Voc) of OPVs including photodetectors is directly related to the bang gap between the highest occupied molecular orbital (HOMO) of donor materials and the lowest unoccupied molecular orbital (LUMO) energies of the acceptor
Summary
Conjugated polymers have attracted both industrial and academic interests due to the potential for large-area, flexible, and low-cost applications in recent years. The common methods to reduce the band gap of conjugated polymers such as extension of conjugation length or introduction of strong donor and acceptor groups can often lead to a rise in the highest occupied molecular orbital (HOMO) energy level (Dong et al, 2013). The open circuit voltage (Voc) of OPVs including photodetectors is directly related to the bang gap between the HOMO of donor materials (conjugated polymers) and the lowest unoccupied molecular orbital (LUMO) energies of the acceptor (fullerene). The “weak donor-strong acceptor” strategy which is widely used in the synthesis of donor materials in OPVs can maintain low-lying HOMO energy levels to promote the Voc and effectively reduce band gap through intramolecular charge transfer (ICT) (Zhou et al, 2010). For the acceptor fragment, [1,2,5]selenadiazolo[3,4-c]pyridine (PSe) and 5fluorobenzo[c][1,2,5]selenadiazole (FBSe) are selected due to the similar property to the S-containing analog, which demonstrates asymmetric reactivity between ortho- and meta-positions (Sun et al, 2012)
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