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group to increase the hole injection achieving efficiencies in pDSCs between 0.07 and 0.09 (49, Figure 40). Both bis-tpy and phenylbipy-tpy complexes were investigated showing better performances with iodine electrolyte with respect to the Co-based one, which was ascribed to high charge recombination with NiO. Figure 40. “K1” structure proposed by Wood et al. [164]. 5. Co-Sensitization The Black Dye has also been used in cocktail with other sensitizers characterized by higher molar extinction coefficient in the high energy regions of the spectrum, in order to increase the IPCE at lower wavelengths. Ogura et al. [165] used BD in combination with the push-pull indoline dye D131 (50, Figure 41), reaching a conversion efficiency of 11.0% (working electrode was made with different layers of TiO2 mixtures with increasing amounts of polystyrene; 0.19 mM D131 and 0.56 mM BD in CH3CN / t-butanol 1:1, electrolyte: 0.15 M NaI, 0.075 M I2, 1.4 M DMPII, CH3CN / methoxyacetonitrile 9:1). Ozawa et al. [166] optimized this system using 20 mM chenodeoxycholic acid achieving a 11.6% efficiency with a TiO2 film with 45 μm thickness (0.14 mM D131 and 0.2 mM BD in 1-propanol, electrolyte: 0.05 M I2, 0.1 M LiI, 0.6 M DMPII, 0.3 M t-bupy in CH3CN). Figure40. “K1”structureproposedbyWood et al. [164]. 5. Co-Sensitization TheBlackDyehasalsobeenusedincocktailwithothersensitizerscharacterized by higher molar extinction coefficient in the high energy regions of the spectrum, in order to increase the IPCE at lower wavelengths. Ogura et al. [165] used BD in combination with the push-pull indoline dye D131 (50, Figure 41), reaching a conversion efficiency of 11.0% (working electrode was made with different layers of TiO2 mixtures with increasing amounts of polystyrene; 0.19 mM D131 and 0.56 mM BD in CH3CN / t-butanol 1:1, electrolyte: 0.15 M NaI, 0.075 M I2, 1.4 M DMPII, CH3CN/methoxyacetonitrile9:1). Ozawa et al. [166]optimizedthissystemusing 20 mM chenodeoxycholic acid achieving a 11.6% efficiency with a TiO2 film with 45µm thickness (0.14 mM D131 and 0.2 mM BD in 1-propanol, electrolyte: 0.05 M I2, 0.1MLiI, 0.6MDMPII,0.3Mt-bupyinCH3CN).Materials 2016, 9, 137 26 of 37 Figure 41. D131 structure used in cosensitization [165,166]. Sharma [167] proposed the cosensitization of a modified BD complex with a Zn porphyrin, with a recorded efficiency of 8.15%. Bahreman [168] synthesized a Ru complex in which a tpy was covalently bound to rhodamine B through an ethanolamine spacer, thus pursuing an energy transfer by “reverse“ FRET. 6. Summary and Outlook The literature offers multiple choices in order to tune the photoelectrochemical properties of terpyridine-based complexes such as the Black Dye, ranging from the modification of the donor and acceptor ligands to the exchange of the metal center with other cations. The increase of the molar extinction coefficient has been commonly pursued by extending the π-conjugation on the ligands. Figure41. D131structureusedincosensitization[165,166]. Sharma [167] proposed the cosensitization of a modified BD complex with a Zn porphyrin, with a recorded efficiency of 8.15%. Bahreman [168] synthesized 47