Page - 16 - in Photovoltaic Materials and Electronic Devices

two of the three carboxylic functions, which proved to be a crucial feature for performances’optimization. N749 or Black Dye (BD), thanks to its panchromatic absorption (Figure 2, top) and represents a benchmark standard as tpy complex sensitizer. In this dye, ruthenium(II) is complexed by a tpy, the 4,4’,4’’-tricarboxy-2,2’:6’,2’-terpyridine (tctpy) and three isothiocyanate ancillary ligands. X-ray diffraction showed a slightly distorted octahedral coordination around the Ru atoms by the three nitrogen donors of tctpy and three nitrogen of isothiocyanate ligands. Very strong intermolecular bonds account for bidimensional arrays, in which the distance between the planes prevents π-stacking between the tpy rings (Figure 2, bottom) [21]. The final BD was prepared by titration with tetrabutylammonium hydroxide in order to deprotonate two of the three carboxylic functions, which proved to be a crucial feature for performances’ optimization. (a) (b) Materials 2016, 9, 137 3 of 37 (c) Figure 2. (a) Black Dye (BD) or N749 structure; (b) light absorption spectrum (red) and IPCE (black) [12] (Adapted from Ref 12 with permission of The Royal Society of Chemistry); and (c) crystal structure showing intermolecular hydrogen bonding [21] (Reprinted with permission from Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J. Am. Chem. Soc 2001, 123, 1613–24. Copyright 2001 American Chemical Society) Comparing to bipyridine structures, terpyridines allow to achieve lower band gap for the metal to ligand transition (MLCT), thus providing a better absorption at lower energies and, therefore, broader solar harvesting. The conversion efficiency of BD was first reported as 10.4% (TiO2: 18 μm, dye: 0.2 mM ethanol + 20 mM sodium taurodeoxycholate, electrolyte: 0.6 M DMPII (1,2-dimethyl-3-propylimidazolium iodide), 0.1 M I2, 0.5 M t-bupy (t-butylpyridine), 0.1 M LiI in methoxyacetonitrile) [21], and after further structural tuning (see Section 3.2.5), it was improved up to 11.2% (TiO2: 15 + 7 μm; dye 0.3 mM ethanol / t-butanol 1:1 with 0.6 mM of tetra-butylammonium deoxycholate and 1 mM deoxycholic acid (DCA) as co-adsorbate; electrolyte: 0.6 M DMPII, 0.05 M I2, 0.5 M t-bupy, 0.1 M LiI, 0.1 M GuNCS (guanidinium thiocyanate) in CH3CN) [22]. Despite the wider absorption, performances of BD are not superior to N719 [23] (Figure 3) or other optimized bipyridines complexes [24]. This behavior has been attributed to a lower molar extinction coefficient (7640 M−1cm−1 in DMF) [21] and worse surface coverage of titania [25]. Figure 2. (a) Black Dye (BD) or N749 structure; (b) light absorption spectrum (red) and IPCE (black) [12] (Adapted from Ref 12 with permission of The Royal SocietyofChemistry); and(c) crystal structureshowingintermolecularhydrogen bonding [21] (Reprinted with permission from Nazeeruddin, M. K.; Péchy, P.; Renouard,T.;Zakeeruddin,S.M.;Humphry-Baker,R.;Comte,P.;Liska,P.;Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; D a o , G. B.; Bignozzi, C. A.; Grätzel, M. Engineering of efficient panchromatic sensitizers for nanocrystalline TiO2-based solar cells. J. Am. Chem. Soc. 2001, 123, 1613–1624. Copyright 2001 American ChemicalSociety). Comparingtobipyridinestructures, terpyridinesallowtoachieve lowerband gapfor themetal to ligand transition(MLCT), thus providingabetterabsorption at lower energies and, therefore, broader solar harvesting. The conversion efficiency of BDw sfirst reportedas10.4%(TiO2: 18µm,dye: 0.2mMethanol+20m sodium taurodeoxycholate, electrolyte: 0.6 M DMPII (1,2-dimethyl- -propylimidazolium iodide),0.1MI2,0.5Mt-bupy(t-butylpyridine),0.1MLiIinmethoxyacetonitrile)[21], and after further structural tuning (see Section 3.2.5), it was improved up to 16