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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 2
BKCSDE 35(2)
February 20, 2014 

Synthesis and Photovoltaic Properties of Alternating Conjugated Polymers Derived from Thiophene-Benzothiadiazole Block and Fluorene/Indenofluorene Units
Jianfeng Li*, Junfeng Tong, Peng Zhang, Chunyan Yang, Dejia Chen, Yuancheng Zhu, Yangjun Xia*, Duowang Fan
Fluorene, Narrow band gap, Indeno[1,2-b]fluorene, Benzothiadiazole, Synthesis
A new donor-accepter-donor-accepter-donor (D-A-D-A-D) type 2,1,3-benzothiadiazole-thiophene-based acceptor unit 2,5-di(4-(5-bromo-4-octylthiophen-2-yl)-2,1,3-benzothiadiazol-7-yl)thiophene (DTBTTBr2) was synthesized. Copolymerized with fluorene and indeno[1,2-b]fluorene electron-rich moieties, two alternating narrow band gap (NBG) copolymers PF-DTBTT and PIF-DTBTT were prepared. And two copolymers exhibit broad and strong absorption in the range of 300-700 nm with optical band gap of about 1.75 eV. The highest occupied molecular orbital (HOMO) energy levels vary between –5.43 and –5.52 eV and the lowest unoccupied molecular orbital (LUMO) energy levels range from –3.64 to –3.77 eV. Potential applications of the copolymers as electron donor material and PC71BM ([6,6]-phenyl-C71 butyric acid methyl ester) as electron acceptors were investigated for photovoltaic solar cells (PSCs). Photovoltaic performances based on the blend of PF-DTBTT/PC71BM (w:w; 1:2) and PIF-DTBTT/PC71BM (w:w; 1:2) with devices configuration as ITO/ PEDOT: PSS/blend/Ca/Al, show an incident photon-to-current conversion efficiency (IPCE) of 2.34% and 2.56% with the open circuit voltage (Voc) of 0.87 V and 0.90 V, short circuit current density (Jsc) of 6.02 mA/ cm2 and 6.12 mA/cm2 under an AM1.5 simulator (100 mW/cm2). The photocurrent responses exhibit the onset wavelength extending up to 720 nm. These results indicate that the resulted narrow band gap copolymers are viable electron donor materials for polymer solar cells.
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