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.