490
Y. Ooyama
4.5% was obtained in the TiO 2 DSSC based on Mc[18, 1] dye. Since the distance
between the chromophore and the TiO 2 surface affected the IPCE values, the control
of configuration and aggregation of merocyanine dyes is significantly important to
improve photovoltaic performances of DSSCs based on merocyanine dyes.
14.2.12 Squaraine Dyes
A successful strategy for improvement of the light-harvesting efficiency of organic
dyes would be the use of NIR dye skeletons, which have good photoabsorption
properties in the red/NIR region of the solar spectrum because the red/NIR radiation
(600–1000 nm) accounts for about 25% of the total energy of sunlight reaching the
earth surface, while the visible radiation (350–700 nm) accounts for about 45%.
Squaraine dyes are known to absorb the red/NIR radiation well and therefore, have
been widely studied as dye sensitizers for DSSCs. However, these cationic squaraine
dyes have low LUMO energy levels and also reduced electron injection yields from
the excited dyes to the CB of the TiO 2 electrode due to the formation of H-aggregates
(Qin et al. 2013; Chen et al. 2005b; Li et al. 2005, 2010a; Tatay et al. 2007; Burke
et al. 2007, 2008; Alex et al. 2005; Yum et al. 2007; Park et al. 2012; Kuster et al.
2010; Choi et al. 2010; Shi et al. 2011; Paek et al. 2011; Bisht et al. 2017; Jradi et al.
2015; Alagumalai et al. 2016).
A series of symmetric (Sq1–4) and asymmetric (Sq5–7) squaraine dyes were
designed and synthesized, and the photovoltaic performances of the TiO 2 DSSCs
based on these dyes were investigated (Fig. 14.18) (Alex et al. 2005). The asymmetric squaraine dyes showed much higher performances as sensitizers than those
of the symmetric squaraine dyes. Considering the molecular design of D–π–A dye
sensitizers, an asymmetric squaraine dye SQ1 with a carboxylic acid group directly
attached to the chromophore was developed. SQ1 also showed much higher performances than those of symmetric squaraine dyes, as a result of unidirectional flow
of electrons toward the indoline moiety with the carboxylic acid anchoring group
in the photoexcited state and consequently enhanced charge separation (Yum et al.
2007; Park et al. 2012). The TiO 2 DSSC using SQ1 achieved an η value of up to
4.5%. Additionally, an asymmetric bis(indoline) squaraine dye YR6 showed good
photoabsorption property in the red/NIR region (~700 nm) of the solar spectrum and
unidirectional flow of electrons in the photoexcited state (Shi et al. 2011). The TiO 2
DSSC based on YR6 yielded a J sc value of 14.8 mA cm
−2 , a V oc value of 642 mV,
and a ff value of 0.71, corresponding to an η value of 6.74%, under standard AM 1.5
simulated sunlight.
Paek et al. developed a new series of stable asymmetric squaraine NIR sensitizers
JK-216 and JK-217 with both thiophenyl pyrrolyl and indolium groups. JK-216
and JK-217 exhibited an photoabsorption band at 669 nm (ε = 93400 M
−1 cm
−1 )
and 672 nm (ε = 77900 M
−1 cm
−1 ), respectively (Paek et al. 2011). The DSSC using
JK-216 yielded a J sc value of 13.92 mA cm
−2 , a V oc value of 610 mV, and a ff value
of 0.74, corresponding to an η value of 6.29%, while that using JK-217 yielded an
Y. Ooyama
4.5% was obtained in the TiO 2 DSSC based on Mc[18, 1] dye. Since the distance
between the chromophore and the TiO 2 surface affected the IPCE values, the control
of configuration and aggregation of merocyanine dyes is significantly important to
improve photovoltaic performances of DSSCs based on merocyanine dyes.
14.2.12 Squaraine Dyes
A successful strategy for improvement of the light-harvesting efficiency of organic
dyes would be the use of NIR dye skeletons, which have good photoabsorption
properties in the red/NIR region of the solar spectrum because the red/NIR radiation
(600–1000 nm) accounts for about 25% of the total energy of sunlight reaching the
earth surface, while the visible radiation (350–700 nm) accounts for about 45%.
Squaraine dyes are known to absorb the red/NIR radiation well and therefore, have
been widely studied as dye sensitizers for DSSCs. However, these cationic squaraine
dyes have low LUMO energy levels and also reduced electron injection yields from
the excited dyes to the CB of the TiO 2 electrode due to the formation of H-aggregates
(Qin et al. 2013; Chen et al. 2005b; Li et al. 2005, 2010a; Tatay et al. 2007; Burke
et al. 2007, 2008; Alex et al. 2005; Yum et al. 2007; Park et al. 2012; Kuster et al.
2010; Choi et al. 2010; Shi et al. 2011; Paek et al. 2011; Bisht et al. 2017; Jradi et al.
2015; Alagumalai et al. 2016).
A series of symmetric (Sq1–4) and asymmetric (Sq5–7) squaraine dyes were
designed and synthesized, and the photovoltaic performances of the TiO 2 DSSCs
based on these dyes were investigated (Fig. 14.18) (Alex et al. 2005). The asymmetric squaraine dyes showed much higher performances as sensitizers than those
of the symmetric squaraine dyes. Considering the molecular design of D–π–A dye
sensitizers, an asymmetric squaraine dye SQ1 with a carboxylic acid group directly
attached to the chromophore was developed. SQ1 also showed much higher performances than those of symmetric squaraine dyes, as a result of unidirectional flow
of electrons toward the indoline moiety with the carboxylic acid anchoring group
in the photoexcited state and consequently enhanced charge separation (Yum et al.
2007; Park et al. 2012). The TiO 2 DSSC using SQ1 achieved an η value of up to
4.5%. Additionally, an asymmetric bis(indoline) squaraine dye YR6 showed good
photoabsorption property in the red/NIR region (~700 nm) of the solar spectrum and
unidirectional flow of electrons in the photoexcited state (Shi et al. 2011). The TiO 2
DSSC based on YR6 yielded a J sc value of 14.8 mA cm
−2 , a V oc value of 642 mV,
and a ff value of 0.71, corresponding to an η value of 6.74%, under standard AM 1.5
simulated sunlight.
Paek et al. developed a new series of stable asymmetric squaraine NIR sensitizers
JK-216 and JK-217 with both thiophenyl pyrrolyl and indolium groups. JK-216
and JK-217 exhibited an photoabsorption band at 669 nm (ε = 93400 M
−1 cm
−1 )
and 672 nm (ε = 77900 M
−1 cm
−1 ), respectively (Paek et al. 2011). The DSSC using
JK-216 yielded a J sc value of 13.92 mA cm
−2 , a V oc value of 610 mV, and a ff value
of 0.74, corresponding to an η value of 6.29%, while that using JK-217 yielded an