492
Y. Ooyama
Nithyanandhan et al. developed asymmetric squaraine NIR sensitizers RSQ1
and RSQ2 with benzodithiophene (BDT) as a π-spacer and cyanoacrylic acid as an
electron acceptor. The BDT moieties of RSQ1 and RSQ2 were modified with methyl
and 2-ethylhexyl groups, respectively, in order to investigate the effect of alkylated
π-spacers on the dye aggregation on the TiO 2 surface and charge recombination
reactions at TiO 2 /dye/electrolyte interfaces (Bisht et al. 2017). The DSSC using
RSQ2 worked well with higher V oc and ff values than those of the DSSC using
RSQ1 in spite of a comparable J sc value. The DSSC using RSQ2 exhibited an η
value of 6.72% with a J sc value of 18.53 mA cm
−2 , a V oc value of 538 mV, and a ff
value of 0.67, without any coadsorbent. The higher V oc and ff values using RSQ2
were attributed to attenuation of the charge recombination.
El-Sayed et al. studied a series of squaraine NIR sensitizers with asymmetric
push–pull D–π–A structures composed of cyanoacetic acid or cyanophosphonic
acid as an electron acceptor and anchoring group and a π-bridge of 4-hexyl4H-dithieno[3,2-b:2
,3
-d]pyrrole (DTP), dithieno[3,2-b:2
,3
-d]thiophene (DTT), or
4,4-bis(2-ethylhexyl)-4Hsilolo[3,2-b:4,5-b
]dithiophene (DTS) (Jradi et al. 2015).
The squaraine dyes with a DTS bridge achieved the highest efficiencies, which
is attributed to high J sc and V oc values due to suppressed dye aggregation and
slow charge recombination, respectively; the η values were 5.0% and 8.9% in the
DSSCs based on DTS-CA with cyanoacetic acid and DTS-PA with cyanophosphonic acid, respectively. The charge injection efficiencies from the squaraine dyes
with cyanoacetic acid to the TiO 2 electrode were higher than those from the squaraine
dyes with cyanophosphonic acid. On the other hand, it was possible that the squaraine
dyes with cyanophosphonic acid enabled slower charge recombination between the
injected electrons into the TiO 2 electrode and the I 3
− ions in the electrolyte than the
squaraine dyes with cyanoacetic acid. Moreover, this study suggested the possibility
that the two out-of-plane bulky (2-ethylhexyl) substituents on the DTS π-bridge
decreased the dye aggregation and charge recombination, consequently, increased
the J sc and V oc values of the DSSCs and enhanced the DSSC performances.
14.2.13 Cyanine Dyes
Cyanine dyes have significantly high absorption extinction coefficients (ca. 10
5 M
−1
cm
−1 ), with intense and broad photoabsorption bands in the visible and NIR regions.
Cyanine dyes adsorbed on the TiO 2 surface can form J- or H-aggregates. To establish
a guidance for designing efficient cyanine dye sensitizers, the photovoltaic performances of the TiO 2 DSSCs based on various cyanine dyes Cn-D (n = 0–3), Cn-N
(n = 1–3), C1’, C1-D#, C2-Ns, and C2-ND have been systematically investigated
(Fig. 14.19) (Sayama et al. 2001). The IPCE values of the DSSCs based on these
cyanine dyes increased with decreasing the distance between the dye skeleton and
the TiO 2 surface because of efficient charge transfer between them. Additionally,
although the increase of the number of methine unit (n) shifted the photoabsorption
maxim to longer wavelength regions by ca. 100 nm per methine unit, it suppressed
Y. Ooyama
Nithyanandhan et al. developed asymmetric squaraine NIR sensitizers RSQ1
and RSQ2 with benzodithiophene (BDT) as a π-spacer and cyanoacrylic acid as an
electron acceptor. The BDT moieties of RSQ1 and RSQ2 were modified with methyl
and 2-ethylhexyl groups, respectively, in order to investigate the effect of alkylated
π-spacers on the dye aggregation on the TiO 2 surface and charge recombination
reactions at TiO 2 /dye/electrolyte interfaces (Bisht et al. 2017). The DSSC using
RSQ2 worked well with higher V oc and ff values than those of the DSSC using
RSQ1 in spite of a comparable J sc value. The DSSC using RSQ2 exhibited an η
value of 6.72% with a J sc value of 18.53 mA cm
−2 , a V oc value of 538 mV, and a ff
value of 0.67, without any coadsorbent. The higher V oc and ff values using RSQ2
were attributed to attenuation of the charge recombination.
El-Sayed et al. studied a series of squaraine NIR sensitizers with asymmetric
push–pull D–π–A structures composed of cyanoacetic acid or cyanophosphonic
acid as an electron acceptor and anchoring group and a π-bridge of 4-hexyl4H-dithieno[3,2-b:2
,3
-d]pyrrole (DTP), dithieno[3,2-b:2
,3
-d]thiophene (DTT), or
4,4-bis(2-ethylhexyl)-4Hsilolo[3,2-b:4,5-b
]dithiophene (DTS) (Jradi et al. 2015).
The squaraine dyes with a DTS bridge achieved the highest efficiencies, which
is attributed to high J sc and V oc values due to suppressed dye aggregation and
slow charge recombination, respectively; the η values were 5.0% and 8.9% in the
DSSCs based on DTS-CA with cyanoacetic acid and DTS-PA with cyanophosphonic acid, respectively. The charge injection efficiencies from the squaraine dyes
with cyanoacetic acid to the TiO 2 electrode were higher than those from the squaraine
dyes with cyanophosphonic acid. On the other hand, it was possible that the squaraine
dyes with cyanophosphonic acid enabled slower charge recombination between the
injected electrons into the TiO 2 electrode and the I 3
− ions in the electrolyte than the
squaraine dyes with cyanoacetic acid. Moreover, this study suggested the possibility
that the two out-of-plane bulky (2-ethylhexyl) substituents on the DTS π-bridge
decreased the dye aggregation and charge recombination, consequently, increased
the J sc and V oc values of the DSSCs and enhanced the DSSC performances.
14.2.13 Cyanine Dyes
Cyanine dyes have significantly high absorption extinction coefficients (ca. 10
5 M
−1
cm
−1 ), with intense and broad photoabsorption bands in the visible and NIR regions.
Cyanine dyes adsorbed on the TiO 2 surface can form J- or H-aggregates. To establish
a guidance for designing efficient cyanine dye sensitizers, the photovoltaic performances of the TiO 2 DSSCs based on various cyanine dyes Cn-D (n = 0–3), Cn-N
(n = 1–3), C1’, C1-D#, C2-Ns, and C2-ND have been systematically investigated
(Fig. 14.19) (Sayama et al. 2001). The IPCE values of the DSSCs based on these
cyanine dyes increased with decreasing the distance between the dye skeleton and
the TiO 2 surface because of efficient charge transfer between them. Additionally,
although the increase of the number of methine unit (n) shifted the photoabsorption
maxim to longer wavelength regions by ca. 100 nm per methine unit, it suppressed