Page - 27 - in Photovoltaic Materials and Electronic Devices

Afurther investigationwasreportedbyBaroloetal. [91], in2006,withthelateral functionalizationof thequaterpyridineswith t-butylmoietiesaselectron-releasing, bulkygroups(16, Figure13). Theproposeddye,namedN886, showedremarkable differencesbetweenprotonatedandnon-protonatedforms. Widerabsorptionwith respect to N719 was reported, together with a lower molar extinction coefficient and unfavourablealignmentof itsexcitedstate (asdemonstratedbyDFTcalculations). With thepurposeofovercomingthesedrawbacks, in2011 thesameresearchgroup proposed to substitute t-butyls with EDOT-vinylene groups, to further extend the pi-conjugation (N1033, Figure 13) [92]. This complex showed a lower energy gap and a broad IPCE curve having still 33% conversion at 800 nm. The poorer efficiency with respect to N886 was ascribed to a lower driving force for electron injection, that limits the open circuit potential. The same drawback was also reported for a qtpy substituted with four COOH anchoring moieties (18, Figure 13) [68] but its high charge injection and an optimization of the electrolyte composition led to a record efficiency for qtpy Ru-complexes of 6.53% (TiO2: 12 + 5µm, dye: 0.18 mM t-butanol / CH3CN 1:1 with 10% DMF, electrolyte: 1.0 M dimethylimidazolium iodide, 0.03 M I2, 0.1M CDCA, 0.1M GuSCN, 0.23 M LiI in valeronitrile / CH3CN 15:85). Co-sensitization with D35, in order to enhance conversion at higher frequencies,wasalsoreported. Materials 2016, 9, 137 11 of 37 Figure 12. The first qtpy complex applied 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, named N886, showed remarkable differences between protonated and non-protonated forms. Wider absorption with respect 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 group proposed to substitute t-butyls with EDOT-vinylene groups, to further extend the π-conj gation (N1033, Figure 13) [92]. This complex showed a lower energy gap and a broad IPCE curve having still 33% conversion at 800 nm. The poorer efficiency with respect to N886 was ascribed to a lower driving force for electron injection, that limits the open circuit potential. The same drawback was also reported for a qtpy substituted with four COOH anchoring moieties (18, Figure 13) [68] but its high charge injection and an optimization of the electrolyte composition led to a record efficiency for qtpy Ru-complexes of 6.53% (TiO2: 12 + 5 μm, dye: 0.18 mM t-butanol / C 3CN 1:1 with 10% DMF, electrolyte: 1.0 M dimethylimidazolium iodide, 0.03 M I2, 0.1M CDCA, 0.1M GuSCN, 0.23 M LiI in valeronitrile / CH3CN 15:85). Co-sensitization with D35, in order to enhance conversion at higher frequencies, was also reported. Figure 13. Qtpy complexes investigated by Barolo et al. [68,91,92]. 3.2. Substitution of Ancillary Ligands: Heteroleptic and Cyclometalated Complexes A further modification on terpyridine complexes involved the substitution of commonly used thiocyanate ligands with other ancillary ligands. The monodentate thiocyanate ligand has the role to tune the spectral and redox properties of the sensitizers acting on the destabilization of the metal t2g orbital [93]. By exchanging these ligands with σ-donor groups, it was possible to tune the Figure13. Qtpycomplexes investigatedbyBarolo et al. [68,91,92]. 3.2. S bstitutionofAncillaryLigands: Heteroleptic andCyclo etalatedComplexes furthermodificationonterpyridinecomplexes involvedthesubstitutionof commonly used thiocyanate ligands with other ancillary ligands. The monodentate 27