NOVELTY - A ruthenium complex having a heterocyclic tridentate ligand and a bipyridine ligand derivative is new.    USE - The ruthenium complex is useful as a dye-sensitizer or a photosensitive dye for fabricating a dye-sensitized solar cell device (claimed).    ADVANTAGE - The ruthenium complex can effectively control the highest occupied molecular orbital (HOMO) energy level of the dye-sensitizer, thus reducing the band gap and promoting the metal-to-ligand charge transfer (MECT) effect, and thus enhancing the charge separation efficiency. Further, the dye sensitization is extended towards long wavelength, such that the absorption region for the solar spectrum is increased, and thus a high-efficient dye-sensitized solar cell device can be fabricated. The dye-sensitized solar cell device utilizing the ruthenium complex has a high efficiency.    DETAILED DESCRIPTION - A ruthenium complex having a chemical formula RuL1L2X comprising a structure of formula (I), is new.    L1=a heterocyclic tridentate ligand with a structural formula (II);    L2=a bipyridine ligand derivative with a structural formula (III);    R1, R2, R4, R5=alkyl, alkoxy, aminoalkyl, haloalkanes or substituted Ph, carboxylic acid or its acid radical salt, sulfonic acid or its acid radical salt, phosphoric acid or its acid radical salt, or H;    R3=perhalogenated alkyl, alkoxy, alkyl, amino, halogens, or H; and    X=a monodentate anion ligand.    An INDEPENDENT CLAIM is included for a dye-sensitized solar cell device, at least comprising a first electrode (100) comprising a transparent conductive substrate (102) and a porous titanium dioxide (TiO2) thin film (104), formed on a surface of the transparent conductive substrate, for carrying a dye-sensitizer (i.e., the ruthenium complex); a second electrode (110); and an electrolyte fluid (120) located between the surface of the transparent conductive substrate and the second electrode.    DESCRIPTION OF DRAWING(S) - The drawing is an exploded perspective view of an opened dye-sensitized solar cell device.    First electrode (100)    Transparent conductive substrate (102)    Porous titanium dioxide thin film (104)    Second electrode (110)    Electrolyte fluid (120)