512
Y. Ooyama
sensitizer (NI-6 or YNI-2) with one or two pyridyl groups and a type-II D–π–Cat
dye sensitizer (YM-2), which can adsorb at the Lewis acid sites and the Brønsted
acid sites on the TiO 2 surface, respectively (Ooyama et al. 2015e). It was found that
the most convenient and rapid co-adsorption of two different dye sensitizers on the
TiO 2 electrode has been achieved by one-step co-adsorption using their mixed dye
solution. Therefore, the co-sensitization method using two kinds of D–π–A dyes
with different bonding abilities to the Lewis acid sites and the Brønsted acid sites
on the TiO 2 surface was demonstrated to be one of the most promising strategies to
enhance the dye coverage on the TiO 2 electrode and the light-harvesting efficiency
of DSSCs.
14.3 N-ZnO-Based DSSCs
Dye sensitizers for n-ZnO-based DSSCs have been developed under a strategy similar
to that in the molecular design for n-TiO 2 -based DSSCs. As organic dye sensitizers
for ZnO DSSCs, Ru complex dyes (Shin et al. 2007; Saito and Fujihara 2008; Nguyen
et al. 2009; Tennakone et al. 1999; Keis et al. 2000, 2002; Kakiuchi et al. 2006;
Quintana et al. 2007; Chou et al. 2007; Wu et al. 2007), xanthene dyes (Hara et al.
2000a, b; Yoshida et al. 2000, 2004; Hosono et al. 2004; Bahadur and Srivastava 2003;
Guillén et al. 2008), indoline dyes (Hosono et al. 2008; Dentani et al. 2009; Matsui
et al. 2009, 2010a, b, 2011; Sakuragi et al. 2010; Guillén et al. 2011; Ambade et al.
2011; Higashijima et al. 2011; Cheng and Hsieh 2010), perylene dyes (Dentani et al.
2007; Szarko et al. 2008), heptamethine–cyanine dyes (Mikroyannidis et al. 2009;
Matsui et al. 2005; Otsuka et al. 2008; Dentani et al. 2008; Funabiki et al. 2011),
and squaraine dyes (Otsuka et al. 2006; Matsui et al. 2006) have been employed.
However, the photovoltaic performances of their ZnO DSSCs were lower than those
of TiO 2 DSSCs because of the chemical instability of ZnO, which is considered to be
mainly related to the high acidity of the carboxyl-anchoring groups in the dyes. The
adsorption of acidic dyes onto the ZnO electrode can cause the dissolution of Zn
2+
followed by the formation of Zn
2+ /dye agglomerates, which lowers electron injection
efficiency and/or dye regeneration efficiency. In order to avoid forming Zn
2+ /dye
agglomerates, it would be effective to reduce the acidity of the dye sensitizers by
structural modification, including optimization (reduction) of the number of carboxyl
groups, and replacement of the anchoring groups by weakly acidic anchoring groups
such as carboxylate, carboxylic acid anhydride, and catechol groups. This hypothesis
may be supported by the fact that organic dyes with the weakly acidic aliphatic
carboxylic acid and rhodanine-3-acetic acid anchoring groups have been generally
used as dye sensitizers for ZnO DSSCs, instead of the strongly acidic cyanoacrylic
acid group. In fact, the ZnO DSSC using the indoline dye D102 with rhodanine3-acetic acid (Fig. 14.10) yielded higher J sc and η values than those of the ZnO
DSSC using the corresponding indoline dye D131 with cyanoacrylic acid (Fig. 14.32)
(Hosono et al. 2008; Dentani et al. 2009; Matsui et al. 2009, 2010a, b, 2011; Sakuragi
et al. 2010; Guillén et al. 2011; Ambade et al. 2011; Higashijima et al. 2011).
Y. Ooyama
sensitizer (NI-6 or YNI-2) with one or two pyridyl groups and a type-II D–π–Cat
dye sensitizer (YM-2), which can adsorb at the Lewis acid sites and the Brønsted
acid sites on the TiO 2 surface, respectively (Ooyama et al. 2015e). It was found that
the most convenient and rapid co-adsorption of two different dye sensitizers on the
TiO 2 electrode has been achieved by one-step co-adsorption using their mixed dye
solution. Therefore, the co-sensitization method using two kinds of D–π–A dyes
with different bonding abilities to the Lewis acid sites and the Brønsted acid sites
on the TiO 2 surface was demonstrated to be one of the most promising strategies to
enhance the dye coverage on the TiO 2 electrode and the light-harvesting efficiency
of DSSCs.
14.3 N-ZnO-Based DSSCs
Dye sensitizers for n-ZnO-based DSSCs have been developed under a strategy similar
to that in the molecular design for n-TiO 2 -based DSSCs. As organic dye sensitizers
for ZnO DSSCs, Ru complex dyes (Shin et al. 2007; Saito and Fujihara 2008; Nguyen
et al. 2009; Tennakone et al. 1999; Keis et al. 2000, 2002; Kakiuchi et al. 2006;
Quintana et al. 2007; Chou et al. 2007; Wu et al. 2007), xanthene dyes (Hara et al.
2000a, b; Yoshida et al. 2000, 2004; Hosono et al. 2004; Bahadur and Srivastava 2003;
Guillén et al. 2008), indoline dyes (Hosono et al. 2008; Dentani et al. 2009; Matsui
et al. 2009, 2010a, b, 2011; Sakuragi et al. 2010; Guillén et al. 2011; Ambade et al.
2011; Higashijima et al. 2011; Cheng and Hsieh 2010), perylene dyes (Dentani et al.
2007; Szarko et al. 2008), heptamethine–cyanine dyes (Mikroyannidis et al. 2009;
Matsui et al. 2005; Otsuka et al. 2008; Dentani et al. 2008; Funabiki et al. 2011),
and squaraine dyes (Otsuka et al. 2006; Matsui et al. 2006) have been employed.
However, the photovoltaic performances of their ZnO DSSCs were lower than those
of TiO 2 DSSCs because of the chemical instability of ZnO, which is considered to be
mainly related to the high acidity of the carboxyl-anchoring groups in the dyes. The
adsorption of acidic dyes onto the ZnO electrode can cause the dissolution of Zn
2+
followed by the formation of Zn
2+ /dye agglomerates, which lowers electron injection
efficiency and/or dye regeneration efficiency. In order to avoid forming Zn
2+ /dye
agglomerates, it would be effective to reduce the acidity of the dye sensitizers by
structural modification, including optimization (reduction) of the number of carboxyl
groups, and replacement of the anchoring groups by weakly acidic anchoring groups
such as carboxylate, carboxylic acid anhydride, and catechol groups. This hypothesis
may be supported by the fact that organic dyes with the weakly acidic aliphatic
carboxylic acid and rhodanine-3-acetic acid anchoring groups have been generally
used as dye sensitizers for ZnO DSSCs, instead of the strongly acidic cyanoacrylic
acid group. In fact, the ZnO DSSC using the indoline dye D102 with rhodanine3-acetic acid (Fig. 14.10) yielded higher J sc and η values than those of the ZnO
DSSC using the corresponding indoline dye D131 with cyanoacrylic acid (Fig. 14.32)
(Hosono et al. 2008; Dentani et al. 2009; Matsui et al. 2009, 2010a, b, 2011; Sakuragi
et al. 2010; Guillén et al. 2011; Ambade et al. 2011; Higashijima et al. 2011).