Page - 33 - in Photovoltaic Materials and Electronic Devices

performances of this class were attributed to an undesired localisation of the LUMO orbital on thiophenes and, as a consequence, to a difficult charge injection into the TiO2. In order to improve the performances, the same group in 2007 introduced an unconjugated bridge between the tpy and the polythiophene moiety [108]. Figure 21. A first series of bis-tpy complexes proposed by Houarner et al. [107]. Further improvements to the Houarner series were reported in 2007 [109] by introducing a thiophene π-conjugated bridge between the terpyridine and the phosphonate anchoring group, improving the photoconversion efficiency (28, Figure 22). The thiophene spacer proved to be an Figure21. first seriesofbis-tpyco plexesproposedby ouarner et al. [107]. Further improvements to the Houarner series were reported in 2007 [109] by introducing a thiophene pi-conjugated bridge between the terpyridine and the phosphonate anchoring group, improving the photoconversion efficiency (28, Figure 22). The thiophene spacer proved to be an interesting and efficient relay in the molecular design; however, overall low efficiencies were obtained, owing to a lowerdrivingforce forcharge injection. Materials 2016, 9, 137 16 of 37 interesting and efficient relay in the molecular design; however, overall low efficiencies were obtained, owing to a lower driving force for charge injection. Figure 22. A structural variation of bis-py Ru complex proposed by Houarner et al. [109]. Krebs and his research group [110] further investigated bis-tpy Ru complexes using bromophenyl, carboxyphenyl, carboxyl acid [111], and ester moieties in order to compare their anchoring properties. Ester moieties showed weaker absorption to TiO2 with respect to carboxylic acid and non-symmetric complexes reported efficiencies three times higher with respect to symmetric ones. The same group [112,113] and Chan [114] studied bis-tpy Ru-complexes in conjugated polymers, and their application to polymeric solar cells [112,113]. Tpy-bearing polyphenylene-vinylene and thienyl-fluorene units were exploited in order to incorporate the resulting Ru complexes in the polymer chains; carboxyl acid functionalization of the bipyridine moieties resulted in improved efficiency. Caramori et al. [115], using an heteroleptic thienylterpyridine Ru complex, improved the electron collection efficiency owing to an electrolyte based on the combination of cobalt and iron polypyridine complexes. Very recently, Koyyada [100] replaced all thiocyanates in the BD structure with a tris (t-butyl) tpy, thus maintaining tctpy as the anchoring moiety (29 in Figure 23). The complex showed good optical properties, with a hypsochromic shift in the visible range of the spectrum and a higher molar extinction coefficient respect to BD, but the overall performances were quite low. Figure 22. A structural variation of bis-py Ru complex proposed by Houarner et al. [109]. Krebsandhisresearchgroup[110] further investigatedbis-tpyRucomplexes using bromophenyl, carboxyphenyl, carboxyl acid [111], and ester moieties in order to compare their anchoring properties. Ester moieties showed weaker absorption to TiO2 with respect to carboxylic acid and non-symmetric complexes reported efficiencies three times higher with respect to symmetric ones. The same group[112,113]andChan[114] studied bis-tpy Ru-complexes in conjugated polymers, and their application to polymeric solar cells [112,113]. Tpy-bearing polyphenylene-vinylene and thienyl-fluorene units were exploited in order to incorporate the resulting Ru complexes in the polymer chains; carboxyl acid functionalization of the bipyridine moieties resulted in improved efficiency. 33