Two new ruthenium(II) mixed ligand terpyridine complexes, “Ru(Htcterpy)(NCS)(L1) (N(C4H9)4), mLBD1” and Ru(Htcterpy)(NCS)(L2)(N(C4H9)4), mLBD2 were synthesized and fully characterized by UV-Vis, emission, cyclic voltammogram, and other spectroscopic means, and the structures of the compounds are confirmed by 1H-NMR, ESI-MASS, and FT-IR spectroscopes. The influence of the substitution of L1 and L2 on solar-to-electrical energy conversion efficiency (η) of dye-sensitized solar cells (DSSCs) was evaluated relative to reference black dye. The dyes showed molar extinction coefficients of 17600？M？1？cm？1 for mLBD1 and 21300？M？1？cm？1 for mLBD2 both at λ maximum of 512？nm, while black dye has shown 8660？M？1？cm？1 at λ maximum of 615？nm. The monochromatic incident photon-to-collected electron conversion efficiencies of 60.71% and 75.89% were obtained for mLBD1 and mLBD2 dyes, respectively. The energy conversion efficiencies of mLBD1 and mLBD2 dyes are 3.15% ( ？mA/cm2, ？mV, ) and 3.36% ( ？mA/cm2, ？mV, ), respectively, measured at the AM1.5G conditions, the reference black dye-sensitized solar cell, fabricated and evaluated under identical conditions exhibited η-value of 2.69% ( ？mA/cm2, ？mV, ). 1. Introduction Among various photo voltaic technologies, dye-sensitized solar cells (DSSCs) are known to be less expensive, easy to fabricate and very efficient at varied incident angle of light. Therefore, intense attention has been devoted in the last two decades to the synthesis of new materials as sensitizers, metal oxide semiconductors, counter electrode materials, electrolytes, and so forth, for applications in high-performance and long durable DSSCs [1–6]. Especially sensitizer plays important role in the DSSCs device for obtaining high-efficiency and long-term durability because of the possibility to chemically modify the sensitizer for better anchoring on TiO2, electron injection property and HOMO-LUMO tuning. To achieve this, many researchers around the world have been working either on modification of various reported sensitizers or designing new sensitizers. A variety of ruthenium(II) polypyridyl, terpyridyl and tetrapyridyl complexes, metal-free organic sensitizers, porphyrins, and phthalocyanines have been developed, since Graetzel introduced the first efficient nanocrystalline TiO2 solar cell sensitized with cis-bis(thiocyanato) bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) bis (tetrabutylammonium) (N719) [7–14]. Introduced in 1997, another ruthenium complex, black dye [tri(isothiocyanato)-2,2′,2′′-terpyridyl-4,4′,4′′-tricarboxylate ruthenium(II)]
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