Conjugated polymers containing pyrimidine with electron withdrawing substituents for organic photovoltaics with high open-circuit voltage

Polymers using 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) with high planarity were synthesized and utilized for the photovoltaics. Push-pull types of conjugated polymers (PTTICN, PTTICNR and PTTIFR) containing TTI as electron pushing unit and 2-pyriminecarbonitrile or 2-fluoropyrimidine as electron pulling unit were synthesized. We designed pyrimidine derivatives with strong electron-withdrawing group (CN or fluorine) for the generation of strong electron pulling property. By the combination with the electron pushing unit, the pyrimidines with strong electron pulling units will provide low highest occupied molecular orbital (HOMO) energy levels for higher open-circuit voltages (VOC). For the syntheses of the polymers, the electron pushing and the electron pulling units were combined by Stille coupling reaction with Pd(0)-catalyst. The polymers of PTTICN and PTTICNR with CN unit show higher VOC than the polymer with fluorine unit. The device comprising PTTICNR and PCBM (1:4) with diiodooctane (DIO) additive showed a VOC of 0.82 V, a JSC of 6.38 mA/cm2, and a fill factor (FF) of 0.53, giving a power conversion efficiency of 2.81%. [Display omitted] •The series of pyrimidine-based polymers were synthesized.•The polymers exhibited good thermal stability.•The polymers had high open circuit voltage.•The optimal device performance reached a power conversion efficiency of 2.81%.