NIR-absorbing poly(thieno[3,4-b]thiophene-2-carboxylic acid) as a polymer dye for dye-sensitized solar cells

Thieno[3,4-b]thiophene-2-carboxylic acid (TTHC) was newly synthesized and poly(thieno[3,4-b]thiophene-2-carboxylic acid) (PTTHC), showing NIR absorption with a low band gap of 1.0 eV, was prepared by chemical or electrochemical polymerization. Vis–NIR spectroscopic studies on PTTHC adsorption revealed that the presence of pendant carboxylic groups on a polymer chain is important to obtain highly sensitized TiO2. The extent of PTTHC adsorption showed proportional increases as raising the adsorption temperatures, reaching saturation at the amount equivalent to a monolayer formation (this does not mean PTTHC monolayer coverage on TiO2). Sequential treatments of TiO2 revealed that the polymer can be adsorbed on N3-adsorbed TiO2 without desorbing N3. The amounts of PTTHC adsorbed on N3-adsorbed TiO2 with increasing adsorption temperatures implied that only a small fraction of polymer chains is involved in anchoring with most chain segments remaining unbound to TiO2. The dye adsorption behaviors were further studied by FTIR, EDX, and TGA. Finally, PTTHC was applied for the preparation of panchromatic dye-sensitized solar cells. The power conversion efficiency (η) of 1.2% for PTTHC-adsorbed TiO2 was increased to 4.3% after N3 adsorption on PTTHC-adsorbed TiO2. Similar enhancement of η was observed after PTTHC treatment on N3-adsorbed TiO2.