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 (JSC) of 21.28 mA cm−2, open-circuit voltage (VOC) 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 JSC value of 23.07 mA cm−2 with VOC of 0.68 V, and FF of 0.71, and owing to the higher JSC value of HIS-2, an improved η value of 11.1% is achieved.
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