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Heteroleptic ruthenium(II) bipyridyl complex, cis-Ru(II)(4,4'-bis(4-tert-butylstyryl)-2,2'-bipyridyl) (4,4'-dicarboxy-2,2'-bipyridyl) (NCS 2 ) (H112) was synthesized and characterized by 1 H-NMR, MASS, Spectrofluorometer and UV-Vis spectroscopes. The photo-voltaic performance of the sensitizer was evaluated in Dye Sensitized Solar Cell (DSSC) under irradiation of AM 1·5 G solar light and the photovoltaic characteristics were compared with those of reference cells of HRS1 and N719 fabricated under comparable conditions. Compared to N719, H112 sensitizer showed enhanced molar extinction coefficient and relatively better monochromatic incident photon-to-current conversion efficiency (IPCE) across the spectral range of 400 to 800 nm with solar energy-to-electrical conversion efficiency (η) of 2·43% [open circuit photovoltage (V OC ) = 0·631 V, short-circuit photocurrent density (J SC ) = 8·96 mA/cm 2 , fill factor (ff) = 0·430], while η values of 2·51% (V OC = 0·651 V, J SC = 9·41mA/cm 2 , ff = 0·410) and 2·74% (V OC = 0·705 V, J SC = 8·62 mA/cm 2 , ff = 0·455) were obtained for HRS1 and N719 sensitized solar cells respectively. The introduction of 4,4'-bis(4-tert-butylstyryl) moieties on one of the bipyridine moieties of N719 complex shows higher light absorption abilities, IPCE and J SC .

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A new high molar extinction coefficient ruthenium(II) bipyridyl complex 'cis-Ru(L1)(2,2 -bipyridine-4,4 -dicarboxylic acid) (NCS) 2 , BDF', where L1 = 4,4-bis(9,9-dibutyl-9H-fluorene-2-yl)-[2,2] bipyridine, has been synthesized and characterized by Fourier transform infrared (FTIR), hydrogen nuclear magnetic resonance ( 1 H-NMR) and electrospray ionization mass (ESI-MASS) spectroscopes. The dye, upon anchoring onto mesoporous nano-crystalline TiO 2 solar cells, exhibited a broader photocurrent action spectrum, with a solar-to-electric energy conversion efficiency (η) of 6.58% (J SC = 14.66 mA cm −2 , V OC = 640 mV, fill factor = 0.71) under sunlight at air mass (AM) 1.5, larger than the reference Z907 sensitized solar cell fabricated under similar conditions, which exhibited an η-value of 4.65% (J SC = 11.52 mA cm −2 , V OC = 566 mV, fill factor = 0.72). Absorption measurements and time-dependent density functional theory (TDDFT) calculations show that the increased conjugation length by introducing 9,9-dibutyl-9H-fluorene moiety relatively enhances the spectral response of the ancillary ligand and the corresponding BDF complex. The calculated dipole moments for BDF and Z907 are 17.71 and 16.34 Debye, respectively. The first three highest occupied molecular orbitals (HOMOs) of BDF have a t 2g character, as observed in Z907, while HOMO-4 and HOMO-5 have considerable sizable mixing from Ru-NCS with π-orbitals of L1.

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