A [3,2-b]thienothiophene-alt-benzothiadiazole copolymer for photovoltaic applications: design, synthesis, material characterization and device performances

The synthesis of a new alternating copolymer (PTBzT(2)-C12) based on a thiophene-benzothiadiazole-thiophene (TBzT) segment and a thieno[3,2-b]thiophene unit and its utilization as electron donor in photovoltaic bulk heterojunctions are reported. The copolymer has been obtained via a Stille cross-coupling reaction. The material's optical and electrochemical properties, in solution or in thin films, have been investigated using UV-visible absorption as well as photoluminescence spectroscopy and cyclic voltammetry. A significant red-shift of the absorption edge is observed during film formation, leading to an optical bandgap of 1.88 eV. The polymer ionisation potential is as high as 5.3 eV and makes the material particularly interesting for photovoltaic applications since it is a prerequisite for high open circuit voltages and better chemical stability. Bulk heterojunction solar cells using blends of the copolymer, as the electron donor, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as the electron transporting material, have been elaborated. A power conversion efficiency of 1% is achieved with a 1:4 PTBzT(2)-C12:PCBM weight ratio and a 30 nm semi-transparent active layer. The open-circuit voltage is equal to 0.74 V and agrees with the expected value. It is anticipated from the copolymer optoelectronic properties, that this novel material should allow power conversion efficiencies above 5% after further optimization of the processing steps.