Page - 25 - in Photovoltaic Materials and Electronic Devices

these modifications did not allow to achieve better results respect to the BD in terms of efficiency, they gave an insight into the structure-property relationships, as well as fundamental issues about charge transfer, polarization, or binding. Thienyl-substituted analogues showed better performances with respect to triphenylamino donors, giving an efficiency of 5.57% (TiO2: 14 + 3 μm, dye: 0.5 mM ethanol / t-butanol + 10 mM CDCA, electrolyte: 0.5 M DMPII, 0.5 M t-bupy, 0.1 M LiI, 0.05 M I2 in CH3CN). Figure 10. 4’ substituted Black Dye analogs [80]. Ozawa et al. proposed a series of tpy having anchoring groups either in the classical 4-, 4’- and 4’’- positions or 3’-, 4’- positions, obtaining mono, bis, tri, and tetra-anchored complexes (Figure 11) [82,83]. Substitution with hexylthiophene in 3- or 4- positions was also investigated by impedance spectroscopy (EIS) and open circuit voltage decay (OCVD), revealing that charge recombination with electrolyte solution is largely promoted when compared to the carboxylic-modified one (Figure 10) [84,85]. Efficiencies close to the BD reference were recorded for the tetra-anchored complex 13, and for the 4’’-thienyl dicarboxy substituted complexes 9. The symmetric substitution with two hexyltiophene groups was also taken into consideration [86,87]. Figure10. 4’ substitutedBlackDyeanalogs [80]. Ozawa et al. proposed a series of tpy having anchoring groups either in the classical 4-, 4’- and 4”-positions or 3’-, 4’-positions, obtaining mono, bis, tri, andtetra-anchoredcomplexes (Figure 11) [82,83]. Substituti n withhexylthiophene in 3- or 4-positions was also investigated by impedance spectroscopy (EIS) and open circuit voltage decay (OCVD), revealing that charge recombination with electrolyte solution is largely promoted when compared to the carboxylic-modified one(Figure10) [84,85]. Efficienciesclose to theBDreferencewererecordedfor the tetra-anchoredcomplex13, andfor the4”-thienyldicarboxysubstitutedcomplexes 9. Thesymmetricsubstitutionwith twohexyltiophenegroupswasalso takeninto consideration[86,87]. QuaterpyridineLigand Tpy modification included the design of tetrapyridines as tetradentate ligands, that were proposed in order to avoid the geometrical isomerism of bipyridine complexes that leads to cis and trans conformers, showing different optical properties [88]. In fact, trans isomers of bipyridines complexes show better photophysical properties, but they are converted by thermal and photoinduced isomerization to the more stable cis isomers that, unfortunately, show worse panchromatic absorption. Tetradentate ligands, owing to their planar structure, coordinate the ruthenium in the plane and only leave apical position available for ancillary ligands, thus avoiding the isomerization and ensuring better solar harvesting features. The first example of a tetradentate ligand for DSCs applications was proposed in 2001 by Renouard et al. [89] who synthesized a 6,6’-bis-benzimidazol-2-yl-2,2’-bipyridine and a 2,2’:6’,2”:6”,2”’-quaterpyridine bearing ethyl ester functionalities. The qtpy ligand wasthen characterized for DSCs applications as a complex with Ruthenium (15, Figure 12) [90]. The ester moieties 25