Enhanced light-harvesting capability of a panchromatic Ru(II) sensitizer based on π-extended terpyridine with a 4-methylstylryl group for dye-sensitized solar cells

A novel Ru π-expanded terpyridyl sensitizer, referred to as HIS-2, is prepared based on the molecular design strategy of substitution with a moderately electron-donating 4-methylstyryl group onto the terpyridyl ligand. The HIS-2 dye exhibits a slightly increased metal-to-ligand charge transfer (MLCT) absorption at around 600 nm and an intense π-π* absorption in the UV region compared with a black dye. Density functional theory calculations reveal that the lowest unoccupied molecular orbital (LUMO) is distributed over the terpyridine and 4-methylstyryl moieties, which enhances the light-harvesting capability and is appropriate for smooth electron injection from the dye to the TiO₂ conduction band. The incident photon-to-electricity conversion efficiency spectrum of HIS-2 exhibits better photoresponse compared with black dye over the whole spectral region as a result of the extended π-conjugation. A DSC device based on black dye gives a short-circuit current (J_SC) of 21.28 mA cm^-2, open-circuit voltage (V_OC) of 0.69 V, and fill factor (FF) of 0.72, in an overall conversion efficiency (η) of 10.5%. In contrast, an HIS-2 based cell gives a higher J_SC value of 23.07 mA cm^-2 with V_OC of 0.68 V, and FF of 0.71, and owing to the higher J_SC value of HIS-2, an improved η value of 11.1% is achieved. Moderate electron-donating substituents are important for enhancement of the light harvesting capability of Ru(II) sensitizers. Substitution of a terpyridyl ligand with a 4-methylstyryl group increases the molecular absorption coefficient from the UV to the NIR region compared with black dye owing to the wider lowest unoccupied molecular orbital (LUMO) distributed over the terpyridyl and extended π-conjugation. The intense absorption gives a high J_SC of 23.07 mA cm^-2, and a resulting overall conversion efficiency of 11.1%.