Synthesis and characterizations of red/near-IR absorbing A–D–A–D–A-type oligothiophenes containing thienothiadiazole and thienopyrazine central units

A series of p-conjugated A-D-A-D-A-type oligothiophenes (1-3) comprising heterocyclic thieno[3,4-c][1,2,5] thiadiazole (TTDA) or thieno[3,4-b] pyrazine (TP) as the core and dicyanovinyl (DCV) or trifluoroacetyl (TFA) as terminal acceptor groups have been developed as red/near-IR absorbers. The resulting oligomers were characterized by UV-Vis spectroscopy, cyclic voltammetry and thermal analysis. The low energy absorption band of these oligomers was located at 620-707 nm in solution and at 695-830 nm in thin films. Theoretical studies reveal that the insertion of the bicyclic nonclassical thiophenes to the conjugated backbone imposed partial quinoidal character to the resulting oligomers and thus reduced the band gap. HOMO-LUMO energies of 1-3 derived from electrochemical measurements were found to be suitable for the use as donor material in combination with fullerene-C-60 as acceptor in planar heterojunction solar cells prepared by vacuum deposition. Such devices incorporating these low band gap oligomers exhibited high FF values up to 0.60, and power conversion efficiencies of up to 1.3% under air mass (AM) 1.5G illumination. External quantum efficiency (EQE) spectra clearly showed the contribution of the low energy absorption to the overall photocurrent.