Page - 24 - in Photovoltaic Materials and Electronic Devices

Substitution with hexyl-EDOT (7b, EDOT: 3,4-ethylenedioxythiophene) afforded even higher efficiency(η= 10.3% withTiO2: 15 + 5µm). Similar modifications have beentaken intoconsiderationbyKimura et al. [79] (7d-g inFigure9). In theseries, structures with hindered hexyloxy-substituted rings resulted in better performances, probably because of the hindrance of alkyl chains towards the electrolyte, thus avoiding the redox couple to interact with titania and considerably reducing the darkcurrent. Among these, the best resultswere obtainedwhen theelectron donor hexyloxygroupsonthephenyl ringare inorthoorparapositions (7f inFigure9). Materials 2016, 9, 137 9 of 37 Figure 9. Series of 5’’-substituted tpy proposed by Yang (7a-c) [78]; and Kimura (7d-g) [79]. Very recently, Dehaudt [80] and Koyyada [81] proposed a simple synthetic pathway to achieve 4’-substituted Black Dye analogs (Figure 10) using octylthiophene (8b) and hexyl bithiophene (8d), pyrrole (8c), triphenylamine (8e), t-butyl phenyl (8f), phenoxazine, and phenothiazine groups. While these modifications did not allow to achieve better results respect to the BD in terms of efficiency, they gave an insight into the structure-property relationships, as well as fundamental issues about charge transfer, polarization, or binding. Thienyl-substituted analogues showed better performances with respect to triphenylamino donors, giving an efficiency of 5.57% (TiO2: 14 + 3 μm, dye: 0.5 mM ethanol / t-butanol + 10 mM CDCA, electrolyte: 0.5 M DMPII, 0.5 M t-bupy, 0.1 M LiI, 0.05 M I2 in CH3CN). Figure 10. 4’ substituted Black Dye analogs [80]. Ozawa et al. proposed a series of tpy having anchoring groups either in the classical 4-, 4’- and 4’’- positions or 3’-, 4’- positions, obtaining mono, bis, tri, and tetra-anchored complexes (Figure 11) [82,83]. Substitution with hexylthiophene in 3- or 4- positions was also investigated by impedance spectroscopy (EIS) and open circuit voltage decay (OCVD), revealing that charge recombination Figure 9. Series of 5”-substituted tpy proposed by Yang (7a-c) [78]; and Kimura (7d-g) [79]. Very recently, Dehaudt [80] and Koyyada [81] proposed a simple synthetic pathw ytoach eve4’-sub itu edBlackDy analogs(Figure10)usingoctylthiophene (8b) and hexyl bithiophene (8d), pyrrole (8c), triphenylamine (8e), t-butyl phenyl (8f), phenoxazine, and phenothiazine groups. While these modifications did not allowtoachievebetter results respect to theBDintermsofefficiency, theygavean insight into the structure-property relationships, as well as fundamental issues about charge transfer, polarization, or binding. Thienyl-substituted analogues showed betterperformanceswithrespect to triphenylaminodonors,givinganefficiencyof 5.57%(TiO2: 14+3µm,dye: 0.5mMethanol/ t-butanol+10mMCDCA,electrolyte: 0.5MDMPII,0.5Mt-bupy,0.1MLiI, 0.05MI2 inCH3CN). 24