Page - 21 - in Photovoltaic Materials and Electronic Devices

with the purpose of overcoming the slow desorption of the carboxyl anchoring group from the semiconductor surface in presence of water. Waser [69] proposed a tpy bearing a phosphonic acid functionality, coupled with TiO2 for DSCs and water splitting applications, while Anthonysamy et al. [70] proposed a 4’-methacryloyloxymethylphenylmoietyasananchoringgroup. As far as the carboxyl anchoring group is concerned, in 2002 Wang et al. [71] tested a 4’-carboxyphenyl substitution (Figure 4), obtaining an appreciable bathochromic shift with respect to N3 (cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)), but a sensible loss in short circuit current in comparison with BD occurred, which canbeexplainedbythe fewergraftingpointsonthestructure. to increase the molar extinction coefficient and further stabilize the LUMO level. In this way more photons can be harnessed and converted thanks to a simultaneous hyperchromic effect and bathochromic shift in the absorption spectra, respectively. Other common structural modifications are the substitution of one of the three pyridines with either a donor group (such as triphenyl amine), in order to enhance the push-pull system character, or a hydrophobic group, in order to reduce recombination with the electrolyte. Particularly interesting are the structural variations related to the anchoring moieties. The tctpy used in BD offers three possible anchoring points, allowing a proper sensitizer-semic nd ctor coupling and improving the stability of the device. Moreover, alter ative anchoring groups, with respect to the carboxylic acid functionality, have been tested. Zakeeruddin [25] proposed a terpyridine functionalized with a phosphonic acid group on 4’- position with the purpose of overcoming the slow desorption of the carboxyl anchoring group from the semiconductor surface in presence of water. Waser [69] proposed a tpy bearing a phosphonic acid functionality, coupled with TiO2 for DSCs and water splitting applications, while Anthonysamy et al. [70] proposed a 4’-methacryloyloxymethylphenyl moiety as an anchoring group. As far as the carboxyl anchoring group is concerned, in 2002 Wang et al. [71] tested a 4’-carboxyphenyl substitution (Figure 4), obtaining an appreciable bathochromic shift with respect to N3 (cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II)), but a sensible loss in short circuit current in comparison with BD occurred, which can be explained by the fewer grafting points on the structure. Figure 4. Structure proposed by Wang et al. and N3 dye [71]. Figure4. StructureproposedbyWang et al. andN3dye[71]. Funaki et al. [72]proposedasimilarsubstitution, inwhichphenyleneethylene moieties (3a inFigure5)were introducedbetweentheCOOHfunctionalityandthe 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 (36µm vs. 10µm) and higher light intensity (100mW/cm´2 vs. 78mW/cm´2)wereused. The injectionefficiencyprovedtobe lowerwithrespect toBD(16.7mA/cm2), tested in thesameconditions. 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, butasignificantly lower Jsc valuewasobserved(5.7mA/cm2)whichwasascribedto an increaseddyeaggregation. 21