Page - 34 - in Photovoltaic Materials and Electronic Devices

Caramori et al. [115], using an heteroleptic thienylterpyridine Ru complex, improved the electron collection efficiency owing to an electrolyte based on the combination of cobaltandironpolypyridinecomplexes. Very recently, Koyyada [100] replaced all thiocyanates in the BD structure with a tris (t-butyl) tpy, thusmaintainingtctpyas theanchoringmoiety (29 inFigure23). Thecomplexshowedgoodopticalproperties,withahypsochromicshift inthevisible rangeof thespectrumandahighermolarextinctioncoefficient respect toBD,but the overallperformanceswerequite low. 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-fluo ene units wer exploit d in order to incorporate the resulting Ru complexes in the polymer chains; carboxyl acid functi alization of the bipyridine moieties resulte in improve efficiency. Caramori et al. [115], using an heteroleptic thienylterpyridine Ru complex, improved the electron c llection efficiency owing to an electrolyte based on the combination of cobalt and iron polypyridine complexes. Very r cently, Koyyada [100] replaced all thiocyanates in the BD structure with a tris (t-butyl) tpy, thus maintaining tctpy as the anchoring moiety (29 i 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 23. Modification of the BD structure with tris (t-butyl) tpy [100]. 3.2.3. Phenylpyridine and Pyrimidine Funaki investigated the possibility to maintain the same terpyridine ligand of Black Dye, tctpy, substituting two thiocyanates with a series of C^N bidentate ligands (30 in Figure 24) [116–118]. These complexes were designed in order to utilize ancillary ligands with stronger donor properties with respect to thiocyanates in order to destabilize the t2g HOMO orbital, to reduce the band gap and to harness lower energy regions of the solar spectrum. 2-Phenylpyridines as such, and those substituted in 4’ position with a phenyl ethynyl group [118], were used to obtain cyclometalated ruthenium(II) complexes. The wider π-extension allowed to obtain higher molar extinction coefficients and a higher charge injection with an IPCE value of 10% at 900 nm. The main drawback of these complexes was a low oxidation potential that reduced the driving force for dye regeneration. In order to raise the HOMO level and ease the dye regeneration by iodine, the same Figure23. Modificationof theBDstructurewith tris (t-butyl) tpy[100]. 3.2.3. PhenylpyridineandPyrimidine Funaki investigated the possibility to maintain the same terpyridine ligand of Black Dye, tctpy, substituting two thiocyanates with a series of CˆN bidentate ligands (30 in Figure 24) [116–118]. These complexes were designed in order to utilize ancillary ligands with stronger donor properties with respect to thiocyanates in order to destabilize the t2g HOMO orbital, to reduce the band gap and to harness lower energy regions of the solar spectrum. 2-Phenylpyridines as such, and those substituted in 4’ position with a phenyl ethynyl group [118], were used to obtain cyclometalatedruthenium(II) complexes. Thewiderpi-extensionallowedtoobtain higher molar extinction coefficients and a higher charge injection with an IPCE value of 10% at 900 nm. The main drawback of these complexes was a low oxidation potential that reduced the driving force for dye regeneration. In order to raise the HOMOlevelandeasethedyeregenerationbyiodine, thesamegroup[116]extended theCˆNligandsseries to2-phenylpyrimidines, substitutedonthephenyl ringwith 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 + 6µm, 34