Trifluoromethyl benzimidazole-based conjugated polymers with deep HOMO levels for organic photovoltaics

• The series of trifluoromethyl benzimidazole-based polymers were synthesized.
• The polymers exhibited good thermal stability.
• The polymers had deep HOMO levels.
• The optimal device performance reached a power conversion efficiency of 0.55%.

We synthesized new alternating conjugated polymers, PCDTFMMBI and PBDTDTFMMBI, which consist of trifluoromethyl substituted dimethyl-2H-benzimidazole (FMMBI) as new electron deficient unit, by palladium(0)-catalyzed Suzuki or Stille polymerization. Trifluoromethyl group has attractive features such as high solubility, chemical and thermal stability and strong electron withdrawing ability to adjust redox potentials. In addition to this, the introduction of trifluoromethyl group into benzimidazole unit will expand the absorption range into the longer wavelength region. Poly(4,8-bis(2-octyldodecyloxy) benzo[1,2-b:3,4-b]dithiophene-alt-5,5-(2-methyl-4′,7′-di-(2-thienyl)-2-(trifluoromethyl)-2H-benzimidazole)) (PBDTDTFMMBI) film, in which benzodithiophene unit was introduced as the electron rich unit, showed an absorption onset at 932 nm, corresponding to a low band gap of 1.33 eV. Poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(2-methyl-4′,7′-di-(2-thienyl)-2-(trifluoromethyl)-2H-benzimidazole)) (PCDTFMMBI), with carbazole as the electron rich unit, showed low lying HOMO energy level of −5.69 eV.