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 (VOC). 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 VOC of 0.62 V, a JSC of 1.69 mA/cm2, and a fill factor (FF) of 0.27, giving a power conversion efficiency of 0.28%.