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

Abstract

Abstract Newly designed 2 H ‐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)‐9 H ‐carbazole)‐ alt ‐5,5‐(4′,7′‐di(thien‐2‐yl)‐2 H ‐benzimidazole‐2′‐spirocyclohexane)] (PCDTCHBI) and poly[2,7‐(9‐(1′‐octylnonyl)‐9 H ‐carbazole)‐ alt ‐5,5‐(4′,7′‐di(thien‐2‐yl)‐2 H ‐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 2 H ‐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 (PC 71 BM) are 1.03 and 1.15%, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011