Substitution of Carbazole Modified Fluorenes as π-Extension in Ru(II) Complex-Influence on Performance of Dye-Sensitized Solar Cells

A new high molar extinction coefficient ruthenium(II) bipyridyl complex “cis-Ru(4,4′-bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2′-bipyridine)(2,2′-bipyridine-4,4′-dicarboxylic acid)(NCS)2, BPFC” has been synthesized and characterized by FT-IR, 1H-NMR, and ESI-MASS spectroscopes. The sensitizer showed molar extinction coefficient of 18.5×103 M-1cm-1, larger as compared to the reference N719, which showed 14.4×103 M-1cm-1. The test cells fabricated using BPFC sensitizer employing high performance volatile electrolyte, (E01) containing 0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide, 0.5 M 4-tert-butylpyridine in acetonitrile solvent, exhibited solar-to-electric energy conversion efficiency (η) of 4.65% (short-circuit current density (JSC) = 11.52 mA/cm2, open-circuit voltage (VOC) = 566 mV, fill factor = 0.72) under Air Mass 1.5 sunlight, lower as compared to the reference N719 sensitized solar cell, fabricated under similar conditions, which exhibited η-value of 6.5% (JSC = 14.3 mA/cm2, VOC = 640 mV, fill factor = 0.71). UV-Vis measurements conducted on TiO2 films showed decreased film absorption ratios for BPFC as compared to those of reference N719. Staining TiO2 electrodes immediately after sonication of dye solutions enhanced film absorption ratios of BPFC relative to those of N719. Time-dependent density functional theory (TD-DFT) calculations show higher oscillation strengths for 4,4′-bis(9,9-dibutyl-7-(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-2-yl)-2,2′-bipyridine relative to 2,2′-bipyridine-4,4′-dicarboxylic acid and increased spectral response for the corresponding BPFC complex.