Seite - 22 - in Photovoltaic Materials and Electronic Devices

10 μm) and higher light intensity (100 mW/cm-2 vs. 78 mW/cm-2) were used. The injection efficiency proved to be lower with respect to BD (16.7 mA/cm2), tested in the same conditions. Moreover, when the spacer was represented by two phenylene ethynylene units (3b in Figure 4) a higher molar extinction coefficient and slight bathochromic shift were obtained, but a significantly lower Jsc value was observed (5.7 mA/cm2) which was ascribed to an increased dye aggregation. Figure 5. Complexes reported by Funaki et al. [72]. McNamara et al. [73] reported a ligand similar to 2 bearing a hydroxamic acid instead of the carboxyl moiety. The dye showed promising properties but was not tested on any device. In 2010, Vougioukalakis et al. [74] synthesized a 4’-carboxyterpyridine acid Ru(II) complex (4a in Figure 5). With the purpose of increasing the chelating sites, the two outer pyridine rings were also substituted with pyrazine, which resulted in the coordination of a second Ru(II) atom (4b in Figure 6). Figure 6. Complexes with one (4a) or two (4b) metal centers [74]. The overall performances were worse with respect to BD, even if a better absorption on TiO2 was recorded, due to the greater flexibility of the dyes bearing only one anchoring group, which accounts for a higher number of molecules adsorbed on the surface. Complex 4a, whose structure is similar to dye 2, showed similar Jsc (6.19 mA/cm2), but its absorption was hypsochromically shifted with respect to BD. The 2,6-dipyrazinylpyridine ligand (complex 4b) led to overall lowest performances with 0.27 mA/cm2 charge injection and 0.02% efficiency (TiO2: 22 μm, dye 0.3 mM ethanol, electrolyte PMII Ionic Salt, Dyesol). Further improvements in the number of chelated Ru(II) atoms have been reported by Manriquez et al. [75] in the preparation of supramolecular structures. Very recently, Kaniyambatti [76] reported a tpy substituted in 4’- with a cyanoacrylic acid moiety via a thiophene bridge (5 in Figure 7). The modification leads again to a hypsochromic shift in the absorption spectrum coupled with a higher molar extinction coefficient owing to the extended π-conjugation and strong auxochrome resulting from the thiophene moiety. Figure5. ComplexesreportedbyFunaki et al. [72]. McNamara et al. [73] re rteda ligandsimilar to2b aringah roxamicacid insteadof thecarboxylmoiety. Thedyeshowedpromisingpropertiesbutwasnot testedonanydevice. In 2010, Vougioukalakis et al. [74] synthesized a 4’-carboxyterpyridine acid Ru(II) complex (4a in Figure 5). With the purpose of increasing the chelating sites, the two outer pyridineringswerealso substitutedwithpyrazine,which resulted in thecoordinationofasecondRu(II)atom(4b inFigure6). Materials 2016, 9, 137 7 of 37 Funaki et al. [72] proposed a similar substitution, in which phenylene ethylene moieties (3a in Figure 5) were introduced between the COOH functionality and the tpy core, obtaining a better charge injection (12.8 mA/cm2) with respect to dye 2 (6.1 mA/cm2), even if a thicker TiO2 (36 μm vs. 10 μm) and higher light intensity (100 mW/cm-2 vs. 78 mW/cm-2) were used. The injection efficiency proved to be lower with respect to BD (16.7 mA/cm2), tested in the same conditions. Moreover, when the spacer was represented by two phenylene ethynylene units (3b in Figure 4) a higher molar extinction coefficient and slight bathochromic shift were obtained, but a significantly lower Jsc value was observed (5.7 mA/cm2) which was ascribed to an increased dye aggregation. Figure 5. Complexes reported by Funaki et al. [72]. McNamara et al. [73] reported a ligand similar to 2 bearing a hydroxamic acid instead of the carboxyl moiety. The dye showed promising properties but was not tested on any device. In 2010, Vougioukalakis et al. [74] synthesized a 4’-carboxyterpyridine acid Ru(II) complex (4a in Figure 5). With the purpose of increasing the chelating sites, the two outer pyridine rings were also substituted with pyrazine, which resulted in the coordination of a second Ru(II) atom (4b in Figure 6). Figure 6. Complexes with one (4a) or two (4b) metal centers [74]. The overall performances were worse with respect to BD, even if a better absorption on TiO2 was recorded, due to the greater flexibility of the dyes bearing only one anchoring group, which accounts for a higher number of molecules adsorbed on the surface. Complex 4a, whose structure is similar to dye 2, showed similar Jsc (6.19 mA/cm2), but its absorption was hypsochromically shifted with respect to BD. The 2,6-dipyrazinylpyridine ligand (complex 4b) led to overall lowest performances with 0.27 mA/cm2 charge injection and 0.02% efficiency (TiO2: 22 μm, dye 0.3 mM ethanol, electrolyte PMII Ionic Salt, Dyesol). Further improvements in the number of chelated Ru(II) atoms have been reported by Manriquez et al. [75] in the preparation of supramolecular structures. Very recently, Kaniyambatti [76] reported a tpy substituted in 4’- with a cyanoacrylic acid moiety via a thiophene bridge (5 in Figure 7). The modification leads again to a hypsochromic shift in the absorption spectrum coupled with a higher molar extinction coefficient owing to the extended π-conjugation and strong auxochrome resulting from the thiophene moiety. Figure6. Complexeswithone(4a)or two(4b)metal centers [74]. The overall performanc s were worse with respect to BD, even if a better absorptiononTiO2 wasrecorded,dueto thegreaterflexibilityof thedyesbearing only one anchoring group, which accounts for a higher number of molecules adsorbedont surface. Complex 4a,whosestructur is sim lar todye 2, showed similar Jsc (6.19 mA/cm2), but its absorpti n was hyps chromically shift d with respect to BD. The 2,6-dipyrazinylpyridine ligand (complex4b) led to overall low t performances with 0.27 mA/cm2 charge injection and 0.02% efficiency (TiO2: 22µm, dye0.3mMethanol, electrolytePMII IonicSalt,Dyesol). Further improvements in thenumberofchelatedRu(II)atomshavebeenreportedbyManriquez et al. [75] in thepreparationofsupramolecularstructures. 22