Syntheses and characterization of carbazole based new low-band gap copolymers containing highly soluble benzimidazole derivatives for solar cell application

Newly designed 2H-benzimidazole derivatives which have solubility groups at 2-position have been synthesized and incorporated into two highly soluble carbazole based alternating copolymers, poly[2,7-(9-(1′-octylnonyl)-9H- carbazole)-alt-5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′- spirocyclohexane)] (PCDTCHBI) and poly[2,7-(9-(1′-octylnonyl)-9H- carbazole)-alt-5,5-(4′,7′-di(thien-2-yl)-2H-benzimidazole-2′- spiro-4″-((2‴-ethylhexyl)oxy)-cyclohexane)] (PCDTEHOCHBI) for photovoltaic application. These alternating copolymers show low-band gap properties caused by internal charge transfer from an electron-rich unit to an electron-deficient moiety. HOMO and LUMO levels are -5.53 and -3.86 eV for PCDTCHBI, and -5.49 and -3.84 eV for PCDTEHOCHBI, respectively. Optical band gaps of PCDTCHBI and PCDTEHOCHBI are 1.67 and 1.65 eV, respectively. The new carbazole based the 2H-benzimidazole polymers show 0.11-0.13 eV lower values of band gaps as compared to that of carbazole based benzothiadiazole polymer, poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT), while keeping nearly the same deep HOMO levels. The power conversion efficiencies of PCDTCHBI and PCDTEHOCHBI blended with [6,6]phenyl-C ₇₁-butyric acid methyl ester (PC₇₁BM) are 1.03 and 1.15%, respectively.