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

Abstract

We report the small molecules utilizing new electron-deficient unit, 2,2-dimethyl-2H-benzimidazole (MBI) and phenanthro[9,10-c][1,2,5]thiadiazole (PT), for the photovoltaic application. Donor-acceptor-donor (D-A-D) types of conjugated small molecules containing benzo[1,2-b;3,4-b']dithiophene (BDT) as electron rich unit and MBI and PT as electron deficient units (SM1 and SM2) were synthesized. D-A-D type of small molecules has the abilities of increasing intramolecular charge transfer (ICT) inducing long wavelength absorption and lowering the HOMO level for the improvement of open-circuit voltage (V OC ). SM1 was synthesized by coupling BDT and PT units by Stille coupling reaction with Pd(0)-catalyst. By using same type of Pd(0)-catalized reaction, BDT unit and MBI unit linked with two thiophenes were coupled to provide SM2. The spectrum of SM2 in the solid thin film shows absorption band with maximum peak at 601 nm and the absorption onset at 756 nm, corresponding to band gap of 1.64 eV. The device comprising SM2 with PCBM (3:7) showed a V OC of 0.62 V, a J SC of 1.69 mA/cm 2 , and a fill factor (FF) of 0.27, giving a power conversion efficiency of 0.28%.

High-Efficiency Organic Photovoltaics with Two-Dimensional Conjugated Benzodithiophene-Based Regioregular Polymers

“…Bulk-heterojunction (BHJ) organic photovoltaic (OPV) devices have been extensively studied over the past decade because of their unique advantages, some of which are their low cost of manufacturing, the fact that they are lightweight, their mechanical flexibility, their visible transparency, and the fact that they are aesthetically pleasing. − Great efforts have been devoted to improving the photovoltaic performance of BHJ OPV devices, including development of novel materials, device architecture, and the printing process, over the past two decades. The most representative approach to high-performance BHJ OPV devices was to develop new low-bandgap materials with a deep highest occupied molecular orbital (HOMO) level and high mobility. − Among many reported low-bandgap donor polymers, alternating copolymers based on thieno­[3,4- b ]­thiophene (TT) as an electron-accepting unit (A) and benzo­[1,2- b :4,5- b ′]­dithiophene (BDT) as an electron-donating unit (D) (called PTB polymer series) have attracted considerable attention because of their low bandgap properties, relatively high hole mobility, and promising photovoltaic properties. − These polymers exhibited low-bandgap characteristics through quinoid resonance structure stabilization, which could potentially enhance hole mobility because of the rigid backbone . A PTB7 polymer with a fluorine atom in a TT unit reached a power conversion efficiency (PCE) of 7.4% because of the improved open-circuit voltage ( V oc ) caused by the HOMO level being deeper than those of previous PTB series .…”

Syntheses and optical, electrochemical, and photovoltaic properties of polymers with 6‐(2‐thienyl)‐4H‐thieno[2,3‐b]indole with a variety of electron‐deficient units

“…Therefore, we synthesized conjugated polymers with TTI and BT units with thiophene linkages to control their electron‐withdrawing ability. In addition to this, we reported earlier on MBI units, from which more redshift of the absorption spectra originated, compared to the case of the BT unit . So, three types of MBI units were incorporated with 2‐TTI units for the generation of efficient ICT.…”

“…Carefully purified compound 6 (0.3150 g, 0.4224 mmol), 4,7‐bis(5‐bromo‐2‐thienyl)‐2,2‐dimethyl‐2 H ‐​benzimidazole ( 9 ) (0.1978 g, 0.4224 mmol), 2 M K 2 CO 3 (0.1382 g), and Pd 2 dba 3 (3 mol %) were dissolved in 4 mL of anhydrous toluene. With the same method used for PTTIDOCF3, the final product, 0.1 g of PTTIMBI with a 30% yield, was obtained after in vacuo drying at 60 °C.…”

Synthesis and properties of low band gap polymers based on thienyl thienoindole as a new electron-rich unit for organic photovoltaics

“…The resulting polymer was soluble in THF, CHCl 3 , ODCB and toluene. 6) (395 mg, 0.48 mmol), 4,7-bis(5-bromo-2-thienyl)-2,2dimethyl-2H-benzimidazole [27][28][29][30] (205 mg, 0.48 mmol), tri-(o-tolyl)phosphine (53.4 mg, 4 mol%) and Pd 2 (dba) 3 (3 mol%) were dissolved in dry chlorobenzene. After reflux for 36 h under argon atmosphere, the reaction mixture was treated with trimethyl( phenyl)tin.…”

“…By replacing the six-membered benzene ring of the carbazole with an electron-rich five-membered thiophene ring, increased planarity between the electron-rich group and the electron-deficient group, leading to the enhancement of the ICT effect, is expected to result in improved π-electron delocalization, low band gap and increased light harvesting ability of the OPVs. 23,24 When coupled with electron-deficient units, such as 4,7-bis(4-hexylthiophen-2-yl)-2,1,3-benzothiadiazole (DTBT-h), 25,26 2-dimethyl-4,7-di(2-thienyl)-2H-benzimidazole (DTMBI) [27][28][29][30] and 3,6-di(2-thienyl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione (DPP), [31][32][33][34][35][36] the TTI unit is expected to have a stronger electron-donating ability compared to the carbazole unit, which could lead to more efficient low band gap polymers. 24 In this paper, we report the synthesis of novel polymers combining new electron-rich TTI unit with three electron-deficient units, 4,7-bis(4-hexylthiophen-2-yl)-2,1,3benzothiadiazole (PTTIDTBT-h), 2-dimethyl-4,7-di(2-thienyl)-2H-benzimidazole (PTTIDTMBI) and 3,6-di(2-thienyl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione (PTTIDPP) (Scheme 1).…”