Page - 17 - in Photovoltaic Materials and Electronic Devices

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(guanidiniumthiocyanate) inCH3CN)[22]. Despite thewiderabsorption, 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 extinctioncoefficient (7640M´1¨cm´1 inDMF)[21]andworsesurfacecoverageof titania [25]. 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 S ction 3.2.5), it was impr ved 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 3. N719 structure. With the aim to further improve BD performance, several modifications have been carried out concerning each component of the complex. In order to increase the molar extinction coefficient and other features ruthenium was substituted with other metals; thiocyanates were replaced with different pinchers in order to obtain cyclometalated or heteroleptic complexes; and the terpyridine ligand was substituted with a quatertpyridine in order to extend the π-conjugation. Figure3. N719structure. Withtheaimtofurther improveBDperformance, severalmodificationshave been carried out concerning each component of the complex. In order to increase themolarextinctioncoefficientandother featuresrutheniumw ssubstitutedwith othermetals; thiocyanateswerereplacedwithdifferentpinchers inorder toobtain cyclometalatedorheterolepticcomplexes;andtheterpyridineligandwassubstituted withaquatertpyridine inorder toextendthepi-conjugation. Thestateof theartofpolypyridinestructuresdesignedto further improveBD performances is summarized in the next sections. After a survey on the synthetic pathways to obtain tpy and qtpy structures, the three main types of changes underlined before (metal centre, ancillary, and tpy ligands) and their effect on DSCs performances will be taken into account in order to outline a structure-property relationship. Moreover,weremindthatDSCsareacomplexmultivariatesystem[26], withdifferentcomponentsandvariables,andthatadirectcorrelationbetweenthe photosensitizers’molecularstructuresandrelatedefficienciescansometimes lead to inaccurate conclusions. For this reason, we selected literature examples where an internal standard reference (BD, 719 or N3) is reported in order to compare the characteristicsof thenovelstructures. Moreover,specificconditionshavebeenadded toselectedreferences. 17