Enhancement of Near-Infrared Singlet–Triplet Absorption of Ru(II) Sensitizers for Improving Conversion Efficiency of Solar Cells

Panchromatic photoenergy conversion is one of the most important challenges in the field of solar energy conversion. However, many materials that absorb near-infrared (NIR) light possess a short excitation lifetime, which is often disadvantageous for the energy conversion process. Although the utilization of spin-forbidden transitions is an effective solution to overcome this problem, its absorption coefficient is small. This study proposes a series of phosphine-coordinated Ru(II) sensitizers, denoted as DX4 and DX5, as a carboxy-thiophene anchoring functional group into wideband Ru(II) sensitizers to improve the spin-forbidden transition intensity in the NIR region and the characteristics of dye-sensitized solar cells (DSSCs) that utilize these sensitizers. Isomers DX4 and DX5 with two carboxy-anchoring groups at different positions are synthesized, and their optical properties are analyzed. The incorporation of DX4 and DX5 led to an increase of approximately 20% in the singlet–triplet absorption intensity in the NIR region compared to that obtained using DX6, which does not contain a carboxy-thiophene group. In addition, the difference in the adsorption behavior observed when using DX4 and DX5 on TiO2 surface is analyzed using UV–vis spectroscopy and density functional theory. The results showed that the molecular orientation on the TiO2 surface changed depending on the position of the anchoring groups. Even though the terpyridine ligand with a carboxy-thiophene group has a highly planar π-conjugated system, the carboxy-thiophene group protected by the methyl ester inhibited the aggregation of the molecule. The DSSCs using DX4 showed panchromatic photoelectric conversion up to 1000 nm, and DX4 achieved an energy conversion efficiency of 10.2%.