498
Y. Ooyama
a ff value of 0.59, resulting in an η value of 6.06%, which is the highest value in
the DSSCs using BODIPY dyes even though the dye sensitizer possesses no strong
electron-donating groups. On the other hand, the DSSC based on DB1 exhibited an
η value of 5.24% with a J sc value of 19.24 mA cm
−2 , a V oc value of 530 mV, and a
ff value of 0.514, and the DSSC based on DB3 exhibited an η value of 5.48% with a
J sc value of 17.21 mA cm
−2 , a V oc value of 550 mV, and a ff value of 0.579. These
results indicate that photovoltaic performance of a DSSC is affected by the position
of the thienyl-cyanoacrylic acid unit on the isoindole ring and by the resultant change
in the orientation of the dye on the TiO 2 surface.
Furthermore, Ooyama et al. designed and developed a nonalkylated BODIPY
dye sensitizer YHO-1 and a hexa-alkylated BODIPY dye sensitizer YHO-2, which
have a diphenylamino-carbazole moiety as an electron donor at the 8-position on
the BODIPY core and a carboxyhexyl group as an anchoring group on the carbazole
ring (Ooyama et al. 2013a). In terms of the molecular structures of YHO-1 and
YHO-2, it is assumed that the BODIPY core locates in close proximity to the TiO 2
surface, when the two dyes were adsorbed on the TiO 2 surface. The strong absorption
bands of YHO-1 and YHO-2 were observed at 501 nm (ε = 54800 M
−1 cm
−1 ) and
524 nm (ε = 50600 M
−1 cm
−1 ), respectively, which are attributed to the S 0 → S 1
transition of the BODIPY core. YHO-2 (λ em = 535 nm, Φ f = 0.62) exhibited a much
higher fluorescence quantum yield (Φ f ) than YHO-1 (λ em = 532 nm, Φ f = 0.06).
The J sc and η values of the DSSC using YHO-2 were 2.33 mA cm
−2 and 0.65%,
respectively, which are greater than those of the DSSC using YHO-1 (1.24 mA cm
−2
and 0.34%, respectively). The maximum IPCE value of YHO-2 exceeded 30% at
530 nm, while that of YHO-1 was 11% at 517 nm. These results indicate that YHO-2
can inject electrons efficiently from the BODIPY core to the CB of the TiO 2 electrode
in comparison with YHO-1. Based on the spectroscopic properties and photovoltaic
performances of YHO-1 and YHO-2, the relatively low photovoltaic performance
of the nonalkylated BODIPY dye YHO-1 is attributed to the radiationless relaxation
of the photoexcited state due to the free rotation of the phenyl group at the 8-position
on the BODIPY core and/or due to the fluorescence deactivation from the twisted
intramolecular charge transfer (TICT) state, and the resulting suppression of direct
electron injection from the BODIPY core to the CB of the TiO 2 electrode. In contrast,
the higher photovoltaic performance of the hexa-alkylated BODIPY dye YHO-2 is
attributed to the restricted free rotation of the phenyl group at the 8-position by the
hexa-alkylation onto the BODIPY core. Consequently, this study demonstrated that
fluorescent BODIPY dyes can inject electrons efficiently from the BODIPY core
to the CB of the TiO 2 electrode, whereas non-fluorescent BODIPY dyes generate
low photocurrents due to the radiationless relaxation of the photoexcited state. On
the other hand, the V oc values of BODIPY dye sensitizers are lower than those of
conventional organic dye sensitizers. The relatively low V oc values (400–430 mV) of
the DSSCs using YHO-1 or YHO-2 are attributed to the fast charge recombination
between the injected electrons in the CB of TiO 2 and the I 3
− ions in the electrolyte
due to the approach of the I 3
− ions to the TiO 2 surface by the electrostatic interactions
between the BODIPY core and the I 3
− ion.
Y. Ooyama
a ff value of 0.59, resulting in an η value of 6.06%, which is the highest value in
the DSSCs using BODIPY dyes even though the dye sensitizer possesses no strong
electron-donating groups. On the other hand, the DSSC based on DB1 exhibited an
η value of 5.24% with a J sc value of 19.24 mA cm
−2 , a V oc value of 530 mV, and a
ff value of 0.514, and the DSSC based on DB3 exhibited an η value of 5.48% with a
J sc value of 17.21 mA cm
−2 , a V oc value of 550 mV, and a ff value of 0.579. These
results indicate that photovoltaic performance of a DSSC is affected by the position
of the thienyl-cyanoacrylic acid unit on the isoindole ring and by the resultant change
in the orientation of the dye on the TiO 2 surface.
Furthermore, Ooyama et al. designed and developed a nonalkylated BODIPY
dye sensitizer YHO-1 and a hexa-alkylated BODIPY dye sensitizer YHO-2, which
have a diphenylamino-carbazole moiety as an electron donor at the 8-position on
the BODIPY core and a carboxyhexyl group as an anchoring group on the carbazole
ring (Ooyama et al. 2013a). In terms of the molecular structures of YHO-1 and
YHO-2, it is assumed that the BODIPY core locates in close proximity to the TiO 2
surface, when the two dyes were adsorbed on the TiO 2 surface. The strong absorption
bands of YHO-1 and YHO-2 were observed at 501 nm (ε = 54800 M
−1 cm
−1 ) and
524 nm (ε = 50600 M
−1 cm
−1 ), respectively, which are attributed to the S 0 → S 1
transition of the BODIPY core. YHO-2 (λ em = 535 nm, Φ f = 0.62) exhibited a much
higher fluorescence quantum yield (Φ f ) than YHO-1 (λ em = 532 nm, Φ f = 0.06).
The J sc and η values of the DSSC using YHO-2 were 2.33 mA cm
−2 and 0.65%,
respectively, which are greater than those of the DSSC using YHO-1 (1.24 mA cm
−2
and 0.34%, respectively). The maximum IPCE value of YHO-2 exceeded 30% at
530 nm, while that of YHO-1 was 11% at 517 nm. These results indicate that YHO-2
can inject electrons efficiently from the BODIPY core to the CB of the TiO 2 electrode
in comparison with YHO-1. Based on the spectroscopic properties and photovoltaic
performances of YHO-1 and YHO-2, the relatively low photovoltaic performance
of the nonalkylated BODIPY dye YHO-1 is attributed to the radiationless relaxation
of the photoexcited state due to the free rotation of the phenyl group at the 8-position
on the BODIPY core and/or due to the fluorescence deactivation from the twisted
intramolecular charge transfer (TICT) state, and the resulting suppression of direct
electron injection from the BODIPY core to the CB of the TiO 2 electrode. In contrast,
the higher photovoltaic performance of the hexa-alkylated BODIPY dye YHO-2 is
attributed to the restricted free rotation of the phenyl group at the 8-position by the
hexa-alkylation onto the BODIPY core. Consequently, this study demonstrated that
fluorescent BODIPY dyes can inject electrons efficiently from the BODIPY core
to the CB of the TiO 2 electrode, whereas non-fluorescent BODIPY dyes generate
low photocurrents due to the radiationless relaxation of the photoexcited state. On
the other hand, the V oc values of BODIPY dye sensitizers are lower than those of
conventional organic dye sensitizers. The relatively low V oc values (400–430 mV) of
the DSSCs using YHO-1 or YHO-2 are attributed to the fast charge recombination
between the injected electrons in the CB of TiO 2 and the I 3
− ions in the electrolyte
due to the approach of the I 3
− ions to the TiO 2 surface by the electrostatic interactions
between the BODIPY core and the I 3
− ion.