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Frontiers of Optoelectronics

Front Optoelec    2013, Vol. 6 Issue (4) : 418-428     DOI: 10.1007/s12200-013-0343-9
RESEARCH ARTICLE |
Side chains and backbone structures influence on 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based low-bandgap conjugated copolymers for organic photovoltaics
Debin NI1, Dong YANG2, Shuying MA1, Guoli TU1(), Jian ZHANG2()
1. Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China; 2. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory of Clean Energy, Dalian 116023, China
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Abstract

Five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight were synthesized to explore their optical, energy level and photovoltaic properties. By tuning the positions of hexyl side chains on DTBT unit, the DTBT-fluorene copolymers exhibited very different aggregation properties, leading to 60 nm bathochromic shift in their absorptions and the corresponding power conversion efficiencies (PCEs) value of photovoltaic cells varied from 0.38%, 0.69% to 2.47%. Different copolymerization units, fluorene, carbazole and phenothiazine were also investigated. The polymer based on phenothiazine exhibited lower PCE value due to much lower molecular weight owing to its poor solubility, although phenothiazine units were expected to be a better electron donor. Compared with the fluorene-based polymer, the carbazole-DTBT copolymer showed higher short circuit current density (Jsc) and PCE value due to its better intermolecular stacking.

Keywords 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)      conjugated polymers      low-bandgap      organic photovoltaics     
Corresponding Authors: TU Guoli,Email:tgl@mail.hust.edu.cn; ZHANG Jian,Email:jianzhang@dicp.ac.cn   
Issue Date: 05 December 2013
 Cite this article:   
Debin NI,Dong YANG,Shuying MA, et al. Side chains and backbone structures influence on 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based low-bandgap conjugated copolymers for organic photovoltaics[J]. Front Optoelec, 2013, 6(4): 418-428.
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http://journal.hep.com.cn/foe/EN/10.1007/s12200-013-0343-9
http://journal.hep.com.cn/foe/EN/Y2013/V6/I4/418
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Debin NI
Dong YANG
Shuying MA
Guoli TU
Jian ZHANG
Fig.1  Molecular structures of five DTBT based polymer
Fig.2  Synthetic procedures of monomers and polymers
Fig.3  TGA curves of five DBTB based copolymer at a scan rate of 10°C/min under a nitrogen atmosphere
Mna)/(kg?mol-1)PDIrepeatunitTdb)/°Clmax/nmEgopt c)/eVHOMO d)/eVLUMO e)/eV
P126.11.6930438.9374/4862.10-5.53-3.43
P228.81.7133443.6372/5301.98-5.47-3.49
P318.81.4025444.7390/5501.93-5.44-3.51
P416.62.4922327.2400/5621.85-5.44-3.59
P58.61.9311339.3390/5641.81-5.18-3.37
Tab.1  Yield, molecular weight, PDI, and thermal properties, opticals and electrochemical properties of the polymers
Fig.4  UV-vis absorption spectra of the thin film of the polymers
Fig.5  Cyclic voltammetry of polymers film coated on a glass carbon electrode in Bu4NPF4/CH3CN solution
Fig.6  characteristics of photovoltaic cells of five polymers
Fig.7  External quantum efficiency (EQE) of the PSCs based on five polymers
polymer: PC60BM(weight ratio)annealing temperature/°Cthickness/nmJsc/(mA?cm-2)Voc/VFFPCE/%
P11:460741.650.820.280.38
P21:370713.510.870.290.69
P31:3130736.670.860.432.47
P41:280668.650.760.402.60
P51:2100562.980.570.320.54
Tab.2  Optimized device characteristics of five DTBT based copolymers
Fig.8  AFM (5 μm × 5 μm) topography and phase images of five polymers
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