Seite - 29 - in Photovoltaic Materials and Electronic Devices

This research topic became of interest again when, in 2011, Chandrasekharam et al. [98] proposed to substitute two thiocyanate ancillary ligands with a bipyridine having electron donor styryl moieties in 4,4’- position (20a,b, Figure15). Worsepanchromaticbehaviorwasobservedwithrespect toBD, but also better performances in device, owing to an increased molar extinction coefficient in the visible region. A low value of fill factor led to a 3.36% best efficiency, higher with respect to that of BD evaluated in the same conditions (TiO2: 9 + 4.8 µm, ethanol solution, Z580 electrolyte: 0.2 M I2, 0.5 M GuSCN, 0.5 M N-methylbenzimidazole in [bmim] [I] / 1-ethyl-3-methylimidazolium tetracyanoborate 65:35). Similar bipyridines, slightly modified in the styryl substitution, were also tested by Giribabu et al. [99] (20c, Figure 15). A more positive oxidation potential with respect to BD under the same conditions has been reported (0.78Vvs. 0.60V)whichwasassociatedwithamorenegativereduction(´1.30Vvs. ´1.10V)explainingthe loss inpanchromaticabsorption. Materials 2016, 9, 137 13 of 37 Figure 15. Monothiocyanate complexes proposed by Chandrasekharam [98] (top) and Giribabu [99] (bottom). Very recently, Koyyada et al. [100] reported other bipyridines 4,4’- substituted with fluoren-2-yl (21a in Figure 16) or carbazol-3-yl (21b) groups, as ancillary ligands. Even if the proposed structures reported good molar extinction coefficients and favourable oxidation and reduction potentials, the overall performances were quite low, mainly due to the poor generated photocurrent that was possibly related to an unfavorable localization of LUMO, far from the anchoring sites on titania. Figure 15. MonothiocyanatecomplexesproposedbyChandrasekharam[98] (top) andGiribabu[99] (bottom). Veryrecently,Koyyada et al. [100] reportedotherbipyridines4,4’- substituted withfluoren-2-yl (21a inFigure16)orcarbazol-3-yl (21b)groups,asancillary ligands. Even if the proposed structures reported good molar extinction coefficients and 29