Page - 35 - in Photovoltaic Materials and Electronic Devices

dye: 0.4 mM ethanolwith 40 mMDCA, electrolyte: 0.6M DMPII, 0.05M I2, 0.1 M LiI, 0.5Mt-bupyinCH3CN).These ligandswere further investigated in2013[117] bycomputational studies. Materials 2016, 9, 137 17 of 37 group [116] extended the C^N ligands series to 2-phenylpyrimidines, substituted on the phenyl ring with trifluoromethyl groups. The CF3 group further reduces the electron donor behavior of the ligand and stabilizes the HOMO level. In this way a 10.7% efficiency was obtained, with respect to 10.1% of BD tested in the same conditions (TiO2: 25 + 6 μm, dye: 0.4 mM ethanol with 40 mM DCA, electrolyte: 0.6 M DMPII, 0.05 M I2, 0.1 M LiI, 0.5 M t-bupy in CH3CN). These ligands were further investigated in 2013 [117] by computational studies. Figure 24. C^N bidentate ligands proposed by Funaki et al. for Ru(II)-complexes [116–118]. 3.2.4. β Diketonate Ligands A series of β-diketonate ligands (31 in Figure 25) was investigated by Islam et al. [119–125] as ancillary ligands alternative to thiocyanates in the BD structure. The strong σ-donating nature of the negatively-charged oxygen donor atom destabilizes the ground-state energy level of the dye compared to BD, leading to a shift of the MLCT transitions to lower energies. In 2002 [125] a Ru(II) 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 each other, 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, Figure 24. CˆN bidentate ligands proposed by Funaki et al. for Ru(II)-complexes [116–118]. 3.2.4.βDiketonateLigands A series of β-diketonate ligands (31 in Figure 25) was investigated by Islam et al. [119–125] as ancillary ligands alternative to thiocyanates in the BD structure. The strongσ-donating nature of the negatively-charged oxygen donor atomdestabilizestheground-stateenergylevelofthedyecomparedtoBD,leadingto ashiftof theMLCTtransitions to lowerenergies. In2002[125]aRu(II) complexwith 1,1,1-trifluoropentane-2,4-dionatoligandshowedefficientpanchromaticsensitization 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 andhydrophobiceffect,preventingtheaccessofelectrons to theredoxelectrolyte, favouring a higher Voc. On the other hand, the bulky alkyl group may not only facilitate theordered molecular arrangementon the TiO2 surface, butalso keep dye molecules far away each other, thus suppressing intermolecular dye interaction and increasing Jsc [126]. In2006[123] thesamegroupfurthermodifiedtheβ-diketonate ligandwitha halogenp-chlorophenylgroup. Arylsubstituentswithdifferentelectron-donating strength were allowed to control the shift of the low-energy MLCT band and Ru oxidation potential. A very efficient sensitization (η = 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 35