We report the synthesis,
characterization, and photovoltaic properties of four ruthenium complexes
(<b>CI101</b>, <b>CBTR</b>, <b>CB111</b>, and <b>CB108</b>) having various N-heterocyclic carbene ancillary ligands,
pyridine-imidazole, -benzimidazole, -dithienobenzimidazole, and -phenanthroimidazole,
respectively. These complexes were designed to investigate the effect
of extended conjugation ordained from ring fusion on the power conversion
efficiencies of the solar cells. The device sensitized by <b>CB108</b>, the pyridine-phenanthroimidazole conjugated complex, showed an improved
efficiency (9.89%) compared to those of pyridine-benzimidazole conjugated
system (<b>CBTR</b>, 9.72%) and the parent unfused ring system
(<b>CI101</b>, 6.24%). Surprisingly, the sulfur-incorporated
pyridine-dithienobenzimidazole system (<b>CB111</b>, 9.24%)
exhibited a little lower efficiency than that of <b>N719</b> (9.41%). The enhanced photovoltaic performance of <b>CB108</b> was mainly attributed to the increase in electron lifetime and diffusion
length confirmed by the electrochemical impedance spectroscopy
