A vinylene-linked benzo[1,2-b:4,5-b']dithiophene-2,1,3-benzothiadiazole low-bandgap polymer

A new heteroarylene-vinylene donor-acceptor polymer P(BDT-V-BTD) with reduced bandgap has been synthesized and its photophysical, electronic and photovoltaic properties investigated both experimentally and theoretically. The structure of the polymer comprises an unprecedented combination of a strong donor (4,8-dialkoxy-benzo[1,2-b:4,5-b']dithiophene, BDT), a strong acceptor (2,1,3-benzothiadiazole, BTD) and a vinylene spacer. The new polymer was obtained by a metal-catalyzed cross-coupling Stille reaction and fully characterized by NMR, UV-vis absorption, GPC, TGA, DSC and electrochemistry. Detailed ab initio computations with solvation effects have been performed for the monomer and model oligomers. The electrochemical investigation has ascertained the ambipolar character of the polymer and energetic values of HOMO, LUMO and bandgap matching materials-design rules for optimized organic photovoltaic devices. The HOMO and LUMO energies are consistently lower than those of previous heteroarylene-vinylene polymer while the introduction of the vinylene spacer afforded lower bandgaps compared to the analogous system P(BDT-BTD) with no spacer between the aromatic rings. These superior properties should allow for enhanced photovoltages and photocurrents in photovoltaic devices in combination with PCBM. Preliminary photovoltaic investigation afforded relatively modest power conversion efficiencies of 0.74% (AM 1.5G, 100 mW/cm 2), albeit higher than that of previous heteroarylene-vinylene polymers and comparable to that of P(BDT-BTD). © 2012 Wiley Periodicals, Inc.