Synthesis and characterization of low‐bandgap copolymers based on dihexyl‐2h‐benzimidazole and cyclopentadithiophene

Abstract

A series of new semiconducting polymers based on 4,4‐dihexyl‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene, 2,2‐dihexyl‐2H‐benzimidazole, and thiophene units was synthesized. The polymers show good solubility at room temperature in organic solvents owing to long alkyl chain in new acceptor, 2,2‐dihexyl‐2H‐benzimidazole. The advantage of dihexyl‐2H‐benzimidazole compared to the benzothiadiazole is to improve the solubility of the polymer. It was found that these polymers can finely be tuned for photovoltaic application by adjusting the contents ratio of the dihexyl‐2H‐benzimidazole unit. The spectra of the solid films show absorption bands with maximum peaks in the range of 421–577 nm and the absorption onsets at 588–683 nm, corresponding to band gaps of 2.11–1.82 eV. The devices with PCPDTDTHBI‐1:PC71BM showed an open‐circuit voltage (VOC) of 0.46 V, a short‐circuit current density (JSC) of 3.83 mA/cm2, and a fill factor of 0.36, giving a power conversion efficiency of 0.64%. Decrease of the dihexyl‐2H‐benzimidazole contents in the polymers induced red‐shift of the UV absorptions, and increased VOC and JSC values, to improve the efficiency of organic photovoltaics. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Dimethyl-2H-benzimidazole based small molecules as donor materials for organic photovoltaics

“…In our previous study, new electron-deficient unit, 2,2-dimethyl-2H-benzimidazole (MBI), has been designed and utilized for the intramolecular charge transfer for OPVs [17]. In the new acceptor, MBI, the sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to make a highly soluble electron-deficient moiety [18,19].…”

2,2-dimethyl-2H-benzimidazole based small molecules for organic solar cells

“…The reaction mixture was stirred for 2 days at 120 o C, concentrated under reduced pressure, and purified by flash column chromatography to give 400 mg of SM2 as dark navy solid; mp 124 o C; 1 19 which was prepared from phenanthroquinone (1) over four steps, were coupled to provide SM1 through Stille reaction with Pd(0)-catalyst. By using the same polymerization condition, compound 12 and 4,7-bis(5-bromo-2-thienyl)-2,2-dimethyl-2H-benzimidazole (9), 14,15 which was synthesized from 3, 6-dibromo-1, 2-phenylenediamine (5) over three steps, provided SM2. The structures and purities of all the compounds were confirmed by 1 H NMR, 13 C NMR, and HRMS.…”

“…MBI replaced the sulfur at 2-position of BT unit with dialkyl substituted carbon to maintain the 1,2-quinoid form and provide a high solubility. 14,15 With PT unit, it was intended to expend the planarity for the enhancement of π-stacking with a penetrating fullerene network in the blended system 16,17 for the generation of higher charge carrier mobility to improve the short circuit current (J SC ). 18 Benzo[1,2-b;3,4-b']dithiophene (BDT) including a rigid and coplanar fused ring system has highly extended conjugated system for the improvement of π-stacking.…”

Synthesis of a conjugated copolymer with benzodithiophene and benzimidazole units

“…20 In the dimethyl-2H-benzimidazole unit, the sulfur at the 2 position of the BT moiety is replaced with dialkyl substituted carbon while the 1,2-quinoid form is maintained, resulting in a highly soluble electron-deficient moiety for the efficient intramolecular charge transfer (ICT) to generate polymers with low bandgaps. 13,14 Polymers with an dimethyl-2H-benzimidazole unit absorb at a longer wavelength compared with those containing the same electron-rich and BT units. To achieve absorption at longer wavelengths, two thiophene units without alkyl groups can be incorporated between the electron-rich and electron-deficient units, resulting in low solubility of the polymers.…”

“…1,7 The interaction between the electron-rich and electron-deficient segments along the conjugated polymer backbone can effectively reduce the bandgaps (E g o1.8 eV) of the polymers in bulk heterojunction solar cells. 1,[8][9][10][11] Many of the low-bandgap (1.4-1.9 eV)-conjugated polymers with excellent efficiencies have electron-deficient heterocycles, such as benzothiadiazole (BT) 12 or benzimidazole, 13,14 and electron-rich moieties, such as carbazole, 15,16 fluorene, 17 cyclopentaphenanthrene 18 or thiophene. 19 The dialkoxybenzo[1,2-b:3,4-b 0 ]dithiophene unit has emerged as a promising electron-rich unit in conjugated polymers for OPVs and thin-film transistors to provide power conversion efficiencies 47%.…”