494
Y. Ooyama
cyanine dye unlike the symmetric cyanine dyes. In addition, asymmetric cyanine
dyes Cy1, Cy2, and Cy3 were developed (Wu et al. 2008). In the comparison between
Cy2 and Cy3, the J sc and V oc values increased with increasing the length of alkyl
chains between the naphthalimide and triazole groups; consequently, the η value also
increased. The long alkyl chain is considered to prevent the recombination processes
of injected electrons with the dye cations and I 3
– ions in the electrolyte. In the three
cyanine dyes, the TiO 2 DSSC using Cy3 yielded the highest η value of 4.80%.
Furthermore, a TiO 2 DSSC based on DYE9 with a triphenylamine moiety exhibited
an η value of 7.62% (Ma et al. 2008).
14.2.14 Phthalocyanine Dyes
Phthalocyanine (Pc) dyes are also one of the most promising classes of NIR dye
sensitizers because of their intense photoabsorption in the UV/blue (Soret band) and
red/NIR (Q band at around 700 nm) spectral regions, in addition to their electrochemical, photochemical, and thermal stabilities. However, the power conversion
efficiencies of TiO 2 DSSCs based on Pcs are much lower than those using other
organic dye sensitizers. The low efficiencies are attributed to aggregation of Pc
dyes, lack of electron transfer directionality in the excited states, and poor electronic coupling between their LUMO and the CB of TiO 2 . Therefore, to minimize
the formation of dye aggregates and also improve the solubility in organic solvents,
bulky substituents such as tert-butyl groups have been introduced into the phthalocyanine skeleton (He et al. 2001, 2002; Aranyos et al. 2001; Nazeeruddin et al.
1999b). An asymmetric phthalocyaninatozinc sensitizer (PCH001) with three tertbutyl and two carboxylic acid groups as “push” and “pull” groups, respectively,
achieved efficient electron transfer from the excited dye to the TiO 2 electrode because
of the good electronic coupling between the dye’s LUMO and the TiO 2 CB energy
levels and suppressed the formation of dye aggregates (Fig. 14.20) (Reddy et al.
2007a; Giribabu et al. 2007). In addition, other efficient push–pull-type ZnPc dye
sensitizers with bulky electron-donating groups have been developed, including PcS
series with 2,6-diphenylphenoxy or 2,6-diphenyl-4-methoxyphenoxy groups (Mori
et al. 2010; Kimura et al. 2012), TT series with tert-butyl or 2,6-diphenylphenoxy
groups (Reddy et al. 2007b; Cid et al. 2007, 2009; Yum et al. 2008; García-Iglesias
et al. 2011; Ragoussi et al. 2012), ZnPc1 with triarylamine-terminated bisthiophene
groups (Ince et al. 2012), and ZnPc2 with hexylbisthiophene groups (Yamamoto et al.
2016) (Fig. 14.20), and the photovoltaic performances of their DSSCs were evaluated. The TiO 2 DSSC based on TT1 with three tert-butyl and one carboxylic acid
groups directly attached to the Pc skeleton yielded an η value of 3.52%. In highly sterically hindered asymmetric ZnPcs (PcS6 and PcS15) with 2,6-diphenylphenoxy or
2,6-diphenyl-4-methoxyphenoxy groups, respectively, the formation of dye aggregates onto the TiO 2 surface was inhibited almost completely. The η values of the
TiO 2 DSSCs using PcS6 or PcS15 are 4.6% and 5.3%, respectively (Mori et al.
2010; Kimura et al. 2012). Furthermore, a TiO 2 DSSC based on a low-symmetry
Y. Ooyama
cyanine dye unlike the symmetric cyanine dyes. In addition, asymmetric cyanine
dyes Cy1, Cy2, and Cy3 were developed (Wu et al. 2008). In the comparison between
Cy2 and Cy3, the J sc and V oc values increased with increasing the length of alkyl
chains between the naphthalimide and triazole groups; consequently, the η value also
increased. The long alkyl chain is considered to prevent the recombination processes
of injected electrons with the dye cations and I 3
– ions in the electrolyte. In the three
cyanine dyes, the TiO 2 DSSC using Cy3 yielded the highest η value of 4.80%.
Furthermore, a TiO 2 DSSC based on DYE9 with a triphenylamine moiety exhibited
an η value of 7.62% (Ma et al. 2008).
14.2.14 Phthalocyanine Dyes
Phthalocyanine (Pc) dyes are also one of the most promising classes of NIR dye
sensitizers because of their intense photoabsorption in the UV/blue (Soret band) and
red/NIR (Q band at around 700 nm) spectral regions, in addition to their electrochemical, photochemical, and thermal stabilities. However, the power conversion
efficiencies of TiO 2 DSSCs based on Pcs are much lower than those using other
organic dye sensitizers. The low efficiencies are attributed to aggregation of Pc
dyes, lack of electron transfer directionality in the excited states, and poor electronic coupling between their LUMO and the CB of TiO 2 . Therefore, to minimize
the formation of dye aggregates and also improve the solubility in organic solvents,
bulky substituents such as tert-butyl groups have been introduced into the phthalocyanine skeleton (He et al. 2001, 2002; Aranyos et al. 2001; Nazeeruddin et al.
1999b). An asymmetric phthalocyaninatozinc sensitizer (PCH001) with three tertbutyl and two carboxylic acid groups as “push” and “pull” groups, respectively,
achieved efficient electron transfer from the excited dye to the TiO 2 electrode because
of the good electronic coupling between the dye’s LUMO and the TiO 2 CB energy
levels and suppressed the formation of dye aggregates (Fig. 14.20) (Reddy et al.
2007a; Giribabu et al. 2007). In addition, other efficient push–pull-type ZnPc dye
sensitizers with bulky electron-donating groups have been developed, including PcS
series with 2,6-diphenylphenoxy or 2,6-diphenyl-4-methoxyphenoxy groups (Mori
et al. 2010; Kimura et al. 2012), TT series with tert-butyl or 2,6-diphenylphenoxy
groups (Reddy et al. 2007b; Cid et al. 2007, 2009; Yum et al. 2008; García-Iglesias
et al. 2011; Ragoussi et al. 2012), ZnPc1 with triarylamine-terminated bisthiophene
groups (Ince et al. 2012), and ZnPc2 with hexylbisthiophene groups (Yamamoto et al.
2016) (Fig. 14.20), and the photovoltaic performances of their DSSCs were evaluated. The TiO 2 DSSC based on TT1 with three tert-butyl and one carboxylic acid
groups directly attached to the Pc skeleton yielded an η value of 3.52%. In highly sterically hindered asymmetric ZnPcs (PcS6 and PcS15) with 2,6-diphenylphenoxy or
2,6-diphenyl-4-methoxyphenoxy groups, respectively, the formation of dye aggregates onto the TiO 2 surface was inhibited almost completely. The η values of the
TiO 2 DSSCs using PcS6 or PcS15 are 4.6% and 5.3%, respectively (Mori et al.
2010; Kimura et al. 2012). Furthermore, a TiO 2 DSSC based on a low-symmetry