14 Photoenergy Conversion (Dye-Sensitized Solar Cells)
511
14.2.21 Co-Sensitized DSSC Employing Two Dyes
In order to further improve photovoltaic performances of DSSCs, co-sensitization
using two kinds of organic dyes, e.g., combinations of phthalocyanine and organic
dyes, porphyrin and organic dyes, and Ru-complex and organic dyes, has been
expected as an effective approach, because it can improve the surface dye coverage
on the TiO 2 electrode and the light-harvesting efficiency (LHE) and suppress dye
aggregation and charge recombination between the injected electrons in the TiO 2
electrode and the I 3
– ions in the electrolyte. In fact, Grätzel et al. reported a highperformance co-sensitized DSSC using two kinds of dyes with a carboxyl group
and a Co
II/III tris(bipyridyl)-based redox electrolyte. The co-sensitization of the D–
π–A zinc porphyrin dye YD2-o-C8 (Fig. 14.16) and an organic D–π–A dye Y123
(Fig. 14.24) yielded a J sc value of 17.66 mA cm
−2 , a V oc value of 935 mV, and a ff
value of 0.74, corresponding to an η value of 12.3% (Yella et al. 2011). However,
in such co-sensitization, both carboxyl-anchoring dyes often compete to adsorb at
the Brønsted acid sites on the TiO 2 surface, which decreases the amount of adsorbed
dyes on the TiO 2 electrode, lowers the LHE, and reduces the power conversion efficiency. Thus, co-sensitized DSSCs based on the combinations of the Ru-complex
dye (black dye, Fig. 14.5) with carboxyl groups and a D–π–A dye (NI5 or YNI-2,
Fig. 14.26) with one or two pyridyl groups were fabricated to achieve site-selective
dye adsorption on the TiO 2 surface and reduce the competitive adsorption between
the two dyes (Shibayama et al. 2014). Black dye with carboxyl groups can adsorb at
the Brønsted acid sites (surface-bound hydroxy groups, Ti–OH) on the TiO 2 surface,
while NI5 and YNI-2 with pyridyl groups can adsorb at the Lewis acid sites (exposed
Ti
n+ cations) on the TiO 2 surface. Therefore, the competitive adsorption is expected
not to occur between carboxyl-anchoring dyes and pyridine-anchoring dyes. Indeed,
the photovoltaic performances of the co-sensitized DSSCs using the combinations of
black dye and NI-5 (η = 10%) or black dye and YNI-2 (η = 9.8%) were higher than
that of the DSSC using only black dye (η = 9.5%), which successfully demonstrated
the effective co-sensitization of two dyes bearing different anchoring groups by their
site-selective adsorption without competitive adsorption.
Moreover, as an innovative approach that enables effective and convenient cosensitization with an enhanced dye coverage on the TiO 2 electrode, Ooyama et al.
developed a new co-sensitization method using two kinds of D–π–A dyes with
pyridyl groups capable of adsorbing at both Brønsted acid sites and Lewis acid sites
on the TiO 2 surface, i.e., one-step co-adsorption of SAT-1 and NI-6 or YNI-2 to
the TiO 2 electrode using their mixed solution (Fig. 14.26) (Ooyama et al. 2015d).
SAT-1 with benzo[4,5]thieno[2,3-c]pyridine can predominantly adsorb on the TiO 2
electrode through hydrogen bonding at the Brønsted acid sites on the TiO 2 surface,
while NI6 and YNI-2 adsorb at the Lewis acid sites. The co-sensitization of SAT1 and NI-6 or YNI-2 on the TiO 2 electrode successfully demonstrated their siteselective adsorption without competitive adsorption and resulting enhancement of
the dye coverage on the TiO 2 electrode. On the other hand, Ooyama et al. also reported
another co-sensitization method for the site-selective adsorption using a D–π–A dye
511
14.2.21 Co-Sensitized DSSC Employing Two Dyes
In order to further improve photovoltaic performances of DSSCs, co-sensitization
using two kinds of organic dyes, e.g., combinations of phthalocyanine and organic
dyes, porphyrin and organic dyes, and Ru-complex and organic dyes, has been
expected as an effective approach, because it can improve the surface dye coverage
on the TiO 2 electrode and the light-harvesting efficiency (LHE) and suppress dye
aggregation and charge recombination between the injected electrons in the TiO 2
electrode and the I 3
– ions in the electrolyte. In fact, Grätzel et al. reported a highperformance co-sensitized DSSC using two kinds of dyes with a carboxyl group
and a Co
II/III tris(bipyridyl)-based redox electrolyte. The co-sensitization of the D–
π–A zinc porphyrin dye YD2-o-C8 (Fig. 14.16) and an organic D–π–A dye Y123
(Fig. 14.24) yielded a J sc value of 17.66 mA cm
−2 , a V oc value of 935 mV, and a ff
value of 0.74, corresponding to an η value of 12.3% (Yella et al. 2011). However,
in such co-sensitization, both carboxyl-anchoring dyes often compete to adsorb at
the Brønsted acid sites on the TiO 2 surface, which decreases the amount of adsorbed
dyes on the TiO 2 electrode, lowers the LHE, and reduces the power conversion efficiency. Thus, co-sensitized DSSCs based on the combinations of the Ru-complex
dye (black dye, Fig. 14.5) with carboxyl groups and a D–π–A dye (NI5 or YNI-2,
Fig. 14.26) with one or two pyridyl groups were fabricated to achieve site-selective
dye adsorption on the TiO 2 surface and reduce the competitive adsorption between
the two dyes (Shibayama et al. 2014). Black dye with carboxyl groups can adsorb at
the Brønsted acid sites (surface-bound hydroxy groups, Ti–OH) on the TiO 2 surface,
while NI5 and YNI-2 with pyridyl groups can adsorb at the Lewis acid sites (exposed
Ti
n+ cations) on the TiO 2 surface. Therefore, the competitive adsorption is expected
not to occur between carboxyl-anchoring dyes and pyridine-anchoring dyes. Indeed,
the photovoltaic performances of the co-sensitized DSSCs using the combinations of
black dye and NI-5 (η = 10%) or black dye and YNI-2 (η = 9.8%) were higher than
that of the DSSC using only black dye (η = 9.5%), which successfully demonstrated
the effective co-sensitization of two dyes bearing different anchoring groups by their
site-selective adsorption without competitive adsorption.
Moreover, as an innovative approach that enables effective and convenient cosensitization with an enhanced dye coverage on the TiO 2 electrode, Ooyama et al.
developed a new co-sensitization method using two kinds of D–π–A dyes with
pyridyl groups capable of adsorbing at both Brønsted acid sites and Lewis acid sites
on the TiO 2 surface, i.e., one-step co-adsorption of SAT-1 and NI-6 or YNI-2 to
the TiO 2 electrode using their mixed solution (Fig. 14.26) (Ooyama et al. 2015d).
SAT-1 with benzo[4,5]thieno[2,3-c]pyridine can predominantly adsorb on the TiO 2
electrode through hydrogen bonding at the Brønsted acid sites on the TiO 2 surface,
while NI6 and YNI-2 adsorb at the Lewis acid sites. The co-sensitization of SAT1 and NI-6 or YNI-2 on the TiO 2 electrode successfully demonstrated their siteselective adsorption without competitive adsorption and resulting enhancement of
the dye coverage on the TiO 2 electrode. On the other hand, Ooyama et al. also reported
another co-sensitization method for the site-selective adsorption using a D–π–A dye