14 Photoenergy Conversion (Dye-Sensitized Solar Cells)
503
14.2.19 D–π–a Dyes with Pyridine, Pyrazine, and Triazine
as Anchoring Group
In the above-mentioned TiO 2 DSSCs, –COOH, –SO 3 H,–H 2 PO 3 , or -Si(OR) 3 have
been generally incorporated into conventional dye sensitizers as anchoring groups,
by which the dyes are adsorbed on the TiO 2 surface to form a bidentate bridge
(dye-O-Ti) at Brønsted acid sites of surface-bound hydroxyl groups (Ti–OH) on the
TiO 2 surface (Fig. 14.3c). Particularly, various kinds of D–π–A dye sensitizers with
a carboxyl group (–COOH) as an electron-withdrawing and anchoring group have
been reported to establish good electronic communication between the dyes and TiO 2
by the formation of the strong bidentate bridge, as described above.
On the other hand, Ooyama et al. designed and developed a new type of D–π–A
carbazole-pyridine dye sensitizers NI3–NI6 containing a pyridine ring as an electronwithdrawing, electron-injecting, and anchoring groups (Fig. 14.26) (Ooyama et al.
2011b, c). In addition, functionally separated D–π–A dye sensitizers NI7 and NI8
having a carboxyl group as an anchoring group and a pyridine ring as an electronaccepting group were also synthesized. The FTIR spectra of NI3–NI8 adsorbed on
the TiO 2 nanoparticles indicate that NI3–NI6 formed the coordination bond [dye(nitrogen of pyridine)-Ti] between their pyridine rings and Lewis acid sites (exposed
Ti
n+ cations) on the TiO 2 surface (Fig. 14.3d) and NI7 and NI8 formed both bidentate
bridge and coordination bond. The TiO 2 DSSCs using the new-type D–π–A dye
sensitizers NI3–NI6 showed higher J sc and η values than those of conventional D–
π–A dye sensitizers NI1 and NI2 with a carboxyl group as an electron-withdrawing
and anchoring group. Consequently, the formation of the coordination bond between
the pyridine ring of NI3–NI6 and the Lewis acid sites on the TiO 2 surface enabled
efficient electron injection through the good electron communication between them,
rather than the formation of the bidentate bridge between the carboxyl group of NI1
and NI2 and the Brønsted acid sites of the TiO 2 surface.
High-performance DSSCs based on porphyrin dyes or D–π–A dyes bearing
pyridyl groups have been reported (Fig. 14.27) (Zhang et al. 2013; Lu et al. 2013;
Daphnomili et al. 2012; Stangel et al. 2014; Mai et al. 2015; Ooyama et al. 2013b,
c; Daphnomili et al. 2013; Wang et al. 2013; Sakurada et al. 2014; Mao et al. 2014;
Ikeuchi et al. 2015; Bolisetty et al. 2015; Franchi et al. 2014; Verbitskiy et al.
2014; Jia et al. 2016; Angaridis et al. 2016; Meinhardt et al. 2016). Zhou et al.
reported that a DSSC using a D–π–A dye TTC105 with a pyridyl group showed
an η value of 1.88% (Zhang et al. 2013). Wang et al. reported that a DSSC using
a D–π–A porphyrin dye LW11 with a pyridyl group showed an η value of 3.96%
(Lu et al. 2013). Goutsolelos et al. developed porphyrin dyes P1–3 with pyridyl
groups, and the DSSC using P2 with four pyridyl groups yielded an η value of 3.9%
(Daphnomili et al. 2012). However, the amounts (<5.0 × 10
16 molecules per cm
2 ) of
these porphyrin dye sensitizers with pyridyl groups adsorbed on the TiO 2 electrode
were much lower than those of porphyrin dye sensitizers with carboxyl groups.
The low loading of the dye sensitizers led directly to poor surface coverage of the
503
14.2.19 D–π–a Dyes with Pyridine, Pyrazine, and Triazine
as Anchoring Group
In the above-mentioned TiO 2 DSSCs, –COOH, –SO 3 H,–H 2 PO 3 , or -Si(OR) 3 have
been generally incorporated into conventional dye sensitizers as anchoring groups,
by which the dyes are adsorbed on the TiO 2 surface to form a bidentate bridge
(dye-O-Ti) at Brønsted acid sites of surface-bound hydroxyl groups (Ti–OH) on the
TiO 2 surface (Fig. 14.3c). Particularly, various kinds of D–π–A dye sensitizers with
a carboxyl group (–COOH) as an electron-withdrawing and anchoring group have
been reported to establish good electronic communication between the dyes and TiO 2
by the formation of the strong bidentate bridge, as described above.
On the other hand, Ooyama et al. designed and developed a new type of D–π–A
carbazole-pyridine dye sensitizers NI3–NI6 containing a pyridine ring as an electronwithdrawing, electron-injecting, and anchoring groups (Fig. 14.26) (Ooyama et al.
2011b, c). In addition, functionally separated D–π–A dye sensitizers NI7 and NI8
having a carboxyl group as an anchoring group and a pyridine ring as an electronaccepting group were also synthesized. The FTIR spectra of NI3–NI8 adsorbed on
the TiO 2 nanoparticles indicate that NI3–NI6 formed the coordination bond [dye(nitrogen of pyridine)-Ti] between their pyridine rings and Lewis acid sites (exposed
Ti
n+ cations) on the TiO 2 surface (Fig. 14.3d) and NI7 and NI8 formed both bidentate
bridge and coordination bond. The TiO 2 DSSCs using the new-type D–π–A dye
sensitizers NI3–NI6 showed higher J sc and η values than those of conventional D–
π–A dye sensitizers NI1 and NI2 with a carboxyl group as an electron-withdrawing
and anchoring group. Consequently, the formation of the coordination bond between
the pyridine ring of NI3–NI6 and the Lewis acid sites on the TiO 2 surface enabled
efficient electron injection through the good electron communication between them,
rather than the formation of the bidentate bridge between the carboxyl group of NI1
and NI2 and the Brønsted acid sites of the TiO 2 surface.
High-performance DSSCs based on porphyrin dyes or D–π–A dyes bearing
pyridyl groups have been reported (Fig. 14.27) (Zhang et al. 2013; Lu et al. 2013;
Daphnomili et al. 2012; Stangel et al. 2014; Mai et al. 2015; Ooyama et al. 2013b,
c; Daphnomili et al. 2013; Wang et al. 2013; Sakurada et al. 2014; Mao et al. 2014;
Ikeuchi et al. 2015; Bolisetty et al. 2015; Franchi et al. 2014; Verbitskiy et al.
2014; Jia et al. 2016; Angaridis et al. 2016; Meinhardt et al. 2016). Zhou et al.
reported that a DSSC using a D–π–A dye TTC105 with a pyridyl group showed
an η value of 1.88% (Zhang et al. 2013). Wang et al. reported that a DSSC using
a D–π–A porphyrin dye LW11 with a pyridyl group showed an η value of 3.96%
(Lu et al. 2013). Goutsolelos et al. developed porphyrin dyes P1–3 with pyridyl
groups, and the DSSC using P2 with four pyridyl groups yielded an η value of 3.9%
(Daphnomili et al. 2012). However, the amounts (<5.0 × 10
16 molecules per cm
2 ) of
these porphyrin dye sensitizers with pyridyl groups adsorbed on the TiO 2 electrode
were much lower than those of porphyrin dye sensitizers with carboxyl groups.
The low loading of the dye sensitizers led directly to poor surface coverage of the