Synthesis and photovoltaic properties of conjugated copolymers containing cyclopentadithiophene and two different electron-deficient moieties in the polymer backbone

By using cyclopentadithiophene (CPDT) as an electron-rich unit, difluro-2,1,3-benzothiadiazole (DFBT), diketopyrrolopyrrole (DPP) and thienopyrroledione (TPD) as electron deficient units, two novel alternating conjugated copolymers with a D-A1-D-A2 structure, PCPDT-DFBT-TPD and PCPDT-DFBT-DPP, were designed and synthesized. The incorporation of two electron-deficient units in the structure of terpolymer could enhance solubility, adjust band gap, optimize stack property, and change the electron distribution in the system. Both the polymers PCPDT-DFBT-TPD and PCPDT-DFBT-DPP show highly solubility in o-dichlorobenzene, chlorobenzene, and o-xylene. Optical bandgaps of these polymers can be estimated by extrapolating from the absorption edge of these films to be 1.56 eV for PCPDT-DFBT-TDP and 1.34 eV for PCPDT-DFBT-DPP. The power conversion efficiencies of 3.15 % for PCPDT-DFBT-TPD and 3.11 % for PCPDT-DFBT-DPP were achieved in a bulk heterojunction (BHJ) photovoltaic device with a configuration of ITO/PEDOT: PSS/Polymers:PC71BM/C60-bis surfactant/Ag under the illumination of AM 1.5G at 100 mA cm−2.