Syntheses of pyrimidine-based polymers containing electron-withdrawing substituent with high open circuit voltage and applications for polymer solar cells

Abstract

Polymers using new electron-deficient units, 2-pyriminecarbonitrile and 2-fluoropyrimidine, were synthesized and utilized for the photovoltaics. Donor-acceptor (D-A) types of conjugated polymers (PBDTCN, PBDTTCN, PBDTF, and PBDTTF) containing 4,8-bis(2-octyldodecyloxy)benzo[1,2-b;3,4-b′]dithiophene (BDT) or 4,8-bis(5-(2-octyldodecyloxy)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) as electron rich unit and 2-pyriminecarbonitrile or 2-fluoropyrimidine as electron deficient unit were synthesized. We designed pyrimidine derivatives in which strong electron-withdrawing group (C-N or fluorine) was introduced to the C2 position for the generation of strong electron-deficient property. By the combination with the electron-rich unit, the pyrimidines will provide low band gap polymers with low highest occupied molecular orbital (HOMO) energy levels for higher open-circuit voltages (VOC). For the syntheses of the polymers, the electron-rich and the electron-deficient units were combined by Stille coupling reaction with Pd(0)-catalyst. Absorption spectra of the thin films of PBDTTCN and PBDTTF with BDTT unit show shift to a longer wavelength region than PBDTCN and PBDTF with BDT unit. Four synthesized polymers provided low electrochemical bandgaps of 1.56 to 1.96 eV and deep HOMO energy levels between -5.67 and -5.14 eV.