14 Photoenergy Conversion (Dye-Sensitized Solar Cells)
487
ZnD1–6 (Campbell et al. 2007), meso- and β-naphthalene-fused porphyrincarboxylic
acid fused-Zn-1 (Tanaka et al. 2007; Hayashi et al. 2008), and β,β’-edge-fused
ZnQMA with a quinoxaline moiety (Eu et al. 2008) were designed and developed
as highly efficient push–pull-type Zn-porphyrin dye sensitizers for TiO 2 DSSCs. In
these dyes, the aryl groups act as electron donors, and the carboxyl and malonic
acid anchoring groups act as electron acceptors. The fused naphthyl moiety in the
porphyrin ring of fused-Zn-1 can shift the photoabsorption band (the Soret and Q
bands) bathochromically. A carboxyl group is attached to the fused naphthyl moiety
to facilitate electron injection from the photoexcited singlet state of fused-Zn-1 to
the CB of the TiO 2 electrode. Bulky mesityl groups are also introduced at the three
meso positions of the porphyrin ring to suppress the aggregation of the fused-Zn-1
molecules on the TiO 2 surface. The TiO 2 DSSC based on fused-Zn-1 exhibited an
η value of 4.1%, while those based on ZnD1–6 showed η values of >5%. Therefore,
it has been shown that the porphyrin π-system strongly interacts with the olefinbridged electron acceptor substituted at the β-pyrrolic position of the porphyrin ring
and that the malonic acid group binds strongly to the TiO 2 surface with a consequent
improvement in the electronic coupling of the dye.
A new series of D–π–A porphyrin dyes with good photoabsorption in the red/nearinfrared (NIR) region have been successfully designed and developed to improve
the performances of DSSCs. A series of D–π–A Zn-porphyrin dyes YD2, YD6,
and YD7, which possess a diarylamino group as an electron-donating part and an
ethynylbenzoic acid moiety as an electron-accepting part linked through a porphyrin
chromophore as a π-bridge, showed good photoabsorption properties (bathochromic
shift and broadening of the Soret and Q bands) in the red/NIR region (~700 nm) of
the solar spectrum and unidirectional flow of electrons in the photoexcited states
(Fig. 14.16) (Higashino 2015; Brogdon et al. 2018; Ji et al. 2018; Mathew et al.
2014; Yella et al. 2014; Wang et al. 2005a; Tanaka et al. 2007; Hayashi et al. 2008;
Eu et al. 2008; Campbell et al. 2007; Lee et al. 2009; Lu et al. 2009; Hsieh et al. 2010;
Bessho et al. 2010). The TiO 2 DSSC based on YD2 exhibited an η value of 11%
with a J sc value of 18.6 mA cm
−2 , a V oc value of 770 mV, a ff value of 0.764, and a
maximum IPCE value of 90% at 675 nm under standard AM 1.5 simulated sunlight
(100 mW cm
−2 ), which are comparable to those of ruthenium complexes (Bessho
et al. 2010). Moreover, a D–π–A Zn porphyrin dye YD2-o-C8 was designed and
developed as a judiciously tailored variant of YD2 by modification with long alkyl
chains to attenuate interfacial back electron transfer reaction and dye aggregation
(Yella et al. 2011). YD2-o-C8 absorbed light over the whole visible range. The
DSSC based on YD2-o-C8 with the Co
II/III tris(2,2
-bipyridine) electrolyte exhibited
impressively high IPCE values over the whole visible wavelength range, exceeding
80% from 450 to 680 nm except a narrow dip around 530 nm. High η values of nearly
11.9% have been achieved in the DSSC, which showed a V oc value of 965 mV, a J sc
value of 17.3 mA cm
−2 , and a ff value of 0.71 under standard AM 1.5 sunlight at
100 mW cm
−2 intensity. This result is attributed to the fact that the molecular design
of YD2-o-C8, particularly, the presence of the four octyloxy groups, greatly retards
the rate of interfacial back electron transfer from the CB of the TiO 2 electrode to
487
ZnD1–6 (Campbell et al. 2007), meso- and β-naphthalene-fused porphyrincarboxylic
acid fused-Zn-1 (Tanaka et al. 2007; Hayashi et al. 2008), and β,β’-edge-fused
ZnQMA with a quinoxaline moiety (Eu et al. 2008) were designed and developed
as highly efficient push–pull-type Zn-porphyrin dye sensitizers for TiO 2 DSSCs. In
these dyes, the aryl groups act as electron donors, and the carboxyl and malonic
acid anchoring groups act as electron acceptors. The fused naphthyl moiety in the
porphyrin ring of fused-Zn-1 can shift the photoabsorption band (the Soret and Q
bands) bathochromically. A carboxyl group is attached to the fused naphthyl moiety
to facilitate electron injection from the photoexcited singlet state of fused-Zn-1 to
the CB of the TiO 2 electrode. Bulky mesityl groups are also introduced at the three
meso positions of the porphyrin ring to suppress the aggregation of the fused-Zn-1
molecules on the TiO 2 surface. The TiO 2 DSSC based on fused-Zn-1 exhibited an
η value of 4.1%, while those based on ZnD1–6 showed η values of >5%. Therefore,
it has been shown that the porphyrin π-system strongly interacts with the olefinbridged electron acceptor substituted at the β-pyrrolic position of the porphyrin ring
and that the malonic acid group binds strongly to the TiO 2 surface with a consequent
improvement in the electronic coupling of the dye.
A new series of D–π–A porphyrin dyes with good photoabsorption in the red/nearinfrared (NIR) region have been successfully designed and developed to improve
the performances of DSSCs. A series of D–π–A Zn-porphyrin dyes YD2, YD6,
and YD7, which possess a diarylamino group as an electron-donating part and an
ethynylbenzoic acid moiety as an electron-accepting part linked through a porphyrin
chromophore as a π-bridge, showed good photoabsorption properties (bathochromic
shift and broadening of the Soret and Q bands) in the red/NIR region (~700 nm) of
the solar spectrum and unidirectional flow of electrons in the photoexcited states
(Fig. 14.16) (Higashino 2015; Brogdon et al. 2018; Ji et al. 2018; Mathew et al.
2014; Yella et al. 2014; Wang et al. 2005a; Tanaka et al. 2007; Hayashi et al. 2008;
Eu et al. 2008; Campbell et al. 2007; Lee et al. 2009; Lu et al. 2009; Hsieh et al. 2010;
Bessho et al. 2010). The TiO 2 DSSC based on YD2 exhibited an η value of 11%
with a J sc value of 18.6 mA cm
−2 , a V oc value of 770 mV, a ff value of 0.764, and a
maximum IPCE value of 90% at 675 nm under standard AM 1.5 simulated sunlight
(100 mW cm
−2 ), which are comparable to those of ruthenium complexes (Bessho
et al. 2010). Moreover, a D–π–A Zn porphyrin dye YD2-o-C8 was designed and
developed as a judiciously tailored variant of YD2 by modification with long alkyl
chains to attenuate interfacial back electron transfer reaction and dye aggregation
(Yella et al. 2011). YD2-o-C8 absorbed light over the whole visible range. The
DSSC based on YD2-o-C8 with the Co
II/III tris(2,2
-bipyridine) electrolyte exhibited
impressively high IPCE values over the whole visible wavelength range, exceeding
80% from 450 to 680 nm except a narrow dip around 530 nm. High η values of nearly
11.9% have been achieved in the DSSC, which showed a V oc value of 965 mV, a J sc
value of 17.3 mA cm
−2 , and a ff value of 0.71 under standard AM 1.5 sunlight at
100 mW cm
−2 intensity. This result is attributed to the fact that the molecular design
of YD2-o-C8, particularly, the presence of the four octyloxy groups, greatly retards
the rate of interfacial back electron transfer from the CB of the TiO 2 electrode to