Synthesis and properties of the conjugated polymers with indenoindene and benzimidazole units for organic photovoltaics

A new electron deficient unit, dimethyl-2H-benzimidazole (MBI), and dihydroindeno[2,1-a]indene (ININE) moiety as electron-rich unit were coupled to synthesize the conjugated polymers containing electron donor-acceptor pair for organic photovoltaics. ININE, MBI, and thiophene (or bithiophene) units were incorporated using Stille and Suzuki polymerization to generate poly(2,7-(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydro- indeno[2,1-a]indene)- alt-5,5-(4′,7′-di-2-thienyl-2,2-dimethyl-2H-benzimidazole)) (PININEDTMBIs) (or PININEBBTMBIs). In MBI, the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was replaced with dialkyl-substituted carbon, whereas keeping the 1,2-quinoid form, to improve the solubility of the polymers. The field-effect hole mobility of PININEBBTMBI was 3.2 × 10^-4 cm²/Vs which was improved as compared to that of PININEDTMBI (2.7 × 10^-5 cm²/Vs) caused by the introduction of bithiophene units. In case of the most efficient polymer, PININEBBTMBI, the device with the configuration of indium tin oxide (ITO)/poly(3,4- ethylenedioxythiophene) (PEDOT):polystyrene sulfonate (PSS)/polymer:PC ₇₁BM(1:4 w/w)/Al, annealed at 100 C for 10 min demonstrated a open circuit voltage of 0.78 V, a short-circuit current density of 6.66 mA/cm ², and a fill factor of 0.41, leading to the power conversion efficiency of 2.11%, under white-light illumination (AM 1.5 G, 100 mW/cm ²).