Page - 26 - in Photovoltaic Materials and Electronic Devices

showed poor adsorption on TiO2; thus, a further hydrolysis step proved mandatory in order to anchor the dye to the semiconductor surface. Thiocyanate ancillary ligands resulted in blue shifted absorption with respect to chlorine ones due to the strongerσ-acceptor properties of SCN. Remarkable conversion efficiency was recorded, up to 940 nm with 75% IPCE in the plateau region and 18 mA/cm2 Jsc (TiO2: 12µm,dye: 0.3 mMethanol/ DMSO95:5, electrolyte: 0.6M DMPII,0.1M I2, 0.5Mt-bupy,0.1MLiI inmethoxyacetonitrile). Materials 2016, 9, 137 10 of 37 Figure 11. Structures proposed by Ozawa et al. [82–87]. Quaterpyridine Ligand 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 was then characterized for DSCs applications as a complex with Ruthenium (15, Figure 12) [90]. The ester moieties showed poor adsorption on TiO2; thus, a further hydrolysis step proved mandatory in order to anchor the dye to the semiconductor surface. Thiocyanate ancillary ligands resulted in blue shifted absorption with respect to chlorine ones due to the stronger σ-acceptor properties of SCN. Remarkable conversion efficiency was recorded, up to 940 nm with 75% IPCE in the plateau region and 18 mA/cm2 Jsc (TiO2: 12 μm, dye: 0.3 mM ethanol / DMSO 95:5, electrolyte: 0.6 M DMPII, 0.1 M I2, 0.5 M t-bupy, 0.1 M LiI in methoxyacetonitrile). Figure11. StructuresproposedbyOzawa et al. [82–87]. Materials 2016, 9, 137 11 of 37 F gure 12. Thef rst qtpy complex ppl ed in DSCs by Renouard et al. [90]. A further investigation was reported by Barolo et al. [91], in 2006, with the lateral functionalization of the quaterpyridines with t-butyl moieties as electron-releasing, bulky groups (16, Figure 13). The proposed dye, n med N886, showed remarkable differences between protonated and non-protonated forms. Wider absorption with res ect to N719 was reported, together with a lower molar extinction coefficient and unfavourable alignment of its excited state (as demonstrated by DFT calculations). With the purpose of overcoming these drawbacks, in 2011 the same research Figure12. Thefirstqtpycomplexapplied inDSCsbyRenouard et al. [90]. 26