[1,2,5]Thiadiazolo[3,4-f]benzotriazole based narrow band gap conjugated polymers with photocurrent response up to 1.1 μm

• Three novel narrow band gap conjugated copolymers containing [1,2,5]thiadiazolo[3,4-f]benzotriazole were synthesized.
• The copolymers exhibited comparatively narrow band gap of ca. 1 eV.
• Broad photocurrent response up to 1.1 μm was realized.

Three novel donor–acceptor type of narrow band gap conjugated copolymers were synthesized through a palladium-catalyzed Stille copolymerization based on [1,2,5]thiadiazolo[3,4-f]benzotriazole (TBZ) derivatives as acceptor and 4,8-di(2,3-didecylthiophen-5-yl)-benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor. All resulted copolymers exhibited absorbance up to near-infrared region along with relatively narrow band gap in the range of 0.96–1.10 eV. Cyclic voltammetry measurements illustrated that the highest occupied molecular orbital energy levels of copolymers lay in the range of −5.04 to −5.13 eV, and lowest unoccupied molecular orbital (LUMO) energy levels were in the range of −4.03 to −4.16 eV. Photovoltaic performances were evaluated based on the resulted copolymers as donor and [6,6]-phenyl-C60 butyric acid methyl ester (PC61BM) as acceptor with optimized weight ratio of 1:2. All devices displayed comparatively low power conversion efficiencies in the range of 0.1–0.4% due to the low-lying LUMO energy levels. Broad photocurrent response up to near infrared region of 1.1 μm was realized for copolymer P2 that containing thiophene unit as the bridge between BDT and TBZ moieties, indicating that it can be potentially applied for near infrared photodetectors.

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