A facile approach to synthesize vinylene-linked donor-acceptor conjugated polymers is reported and can now be considered for general use. This approach led to a low-bandgap, structurally defect-free vinylene-linked benzothiadiazole-thiophene (PTVBT) polymer and its corresponding model compound bisTVBT using a set of three reactions: Heck coupling, Hunsdiecker reaction, and Suzuki coupling reaction. The polymer and the model compound were fully characterized by 1H and 13C NMR, IR, and elemental analysis. GPC revealed that the number-average molecular weights of PTVBT ranged from 20000 to 31000 Da and polydispersity indices from 1.7 to 2.4. Thermal analysis demonstrated that the polymer was stable up to 380 °C under nitrogen without decomposition. Spectroelectrochemical results showed the PTVBT had an optical bandgap of 1.5 eV and was both p- and n-type dopable. The energetic positions of the band edges were determined by cyclic voltammetry and differential pulse voltammetry and suggested that the polymer had a HOMO level at 5.2-5.4 eV, as well as a LUMO level at 3.5-3.6 eV, which renders a strong tendency for photoinduced charge transfer to fullerene acceptors. The polymer has been investigated as an electron donor in photovoltaic devices in blends with PCBM ([6,6]-phenyl C61-butyric acid methyl ester) as an electron acceptor. Power conversion efficiencies ~0.2-0.3% have been obtained with an open-circuit voltage (Voc) of 0.61 V and a short-circuit current density (Isc) of 1.6 mA/cm2. The relatively low conversion efficiencies may result from low content of polymer in the blends and phase separation as revealed by AFM studies.
Mei, J., N. C. Heston, S. V. Vasilyeva, J. R. Reynolds