Page - 36 - in Photovoltaic Materials and Electronic Devices

whole visible range extending up to 950 nm was obtained. Further substituted β-diketonate ligands were tested in 2011 [119] showing a great potential to tune the photochemicalproperties. complex with 1,1,1-trifluoropentane-2,4-dionato ligand showed efficient panchromatic sensitization of nanocrystalline TiO2 solar cells. Additionally, a longer alkyl chain (using 1,1,1-trifluoroeicosane-2,4-dionato ligand) [122] prevented surface aggregation of the sensitizer and allowed to avoid or reduce the use of chenodeoxycholic acid. The use of longer alkyl chains may protect the TiO2 surface, through steric hindrance and hydrophobic effect, preventing the access of electrons to the redox electrolyte, favouring a higher Voc. On the other hand, the bulky alkyl group may not only facilitate the ordered molecular arrangement on the TiO2 surface, but also keep dye molecules far away e ch ther, thus suppressing intermolecular dye interaction and increasing Jsc [126]. Figure 25. β-diketonates ligands by Islam et al. [119–125]. In 2006 [123] the same group further modified the β-diketonate ligand with a halogen p-chlorophenyl group. Aryl substituents with different electron-donating strength were allowed to control the shift of the low-energy MLCT band and Ru oxidation potential. A very efficient sensitization ( = 9.1%; TiO2: 20 μm, dye: 0.2 mM CH3CN / t-butanol 1:1 with 20 mM DCA, electrolyte: 0.6 M DMPII, 0.05 M I2, 0.1 M LiI, 0.07 M t-bupy in CH3CN), with an IPCE greater than 80% in the whole visible range extending up to 950 nm was obtained. Further substituted Figure25.β-diketonates ligandsbyIslam et al. [119–125]. 3.2.5. PyrazolylLigands Novel NˆN bidentate ligands, different from the bipyridines, were proposed by Chen et al. [127]. A series of 2-(pyrazol-3-yl)pyridine ligands were used as an alternative to thiocyanate in BD and tested in cells (32, Figure 26). These dyes overcome the efficiency of BD tested in the same conditions (η= 1005 vs 9.07% ; TiO2: 18 + 4µm, dye: 0.3 mM DMF / t-butanol 1:1 with 10 mM DCA, electrolyte: 0.6MDMPII, 0.1MI2, 0.1 MLiI, 0.5Mt-bupyin CH3CN)dueto theirhighermolar extinction coefficients between 400 and 550 nm and their extended absorption up to 850 nm, as a consequence of the HOMO destabilization by the pyrazole. The same group reported, in 2011 [128], a series of tridentate 2,6-bis(3-pyrazolyl)pyridine ligands bearing various substitutions in 4- position (33, Figure 26). The reported IPCEspectrashowedaworsesensitization intheNIRregionwithrespect toN749 but a better conversion in the visible range which accounts for efficiencies up to 10.7% (TiO2: 15 + 5 µm, dye: 0.3 mM ethanol / DMSO 4:1 with 1M CDCA, electrolyte: 0.6 M DMPII, 0.1 M I2, 0.5 M t-bupy, 0.1 M LiI in CH3CN). The results were explained by the bulky ligand effect, which may allow better packing of the dye molecules on the TiO2 surface and prevent interfacial charge recombination. Ontheotherhand, thecontributionof thepyridine in the ligand,which isneutral with respect to the negatively charged thiocyanates, might allow the negative dipole moment to be localized closer to the surface, thus affording a higher Voc. Further investigations on these complexes were carried out by replacing the tctpy with a dicarboxytpy ligand substituted in the 5- or 6- position of a terminal pyridyl unit with pi-conjugated thiophene pendant chains, obtaining good stability and 36