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NEW DEVELOPMENTS 687
photosensitizers has not been attained. This is believed to be because the HOMO level of
Ru complexes derived from the d orbitals of the Ru metal center (i.e. the redox potential of
Ru(II)/Ru(III)) is best matched to the iodine redox potential to accept electrons effectively.
Porphyrin [131–134] and phthalocyanine [135] derivatives have also been
employed as photosensitizers in the DSSC. A nanocrystalline TiO 2 solar cell sensitized
by Cu chlorophyllin produced 2.6% efficiency (J SC = 9.4 mAcm −2 and V OC = 0.52 V)
under 100 mW cm −2 [131]. To develop new efficient metal complex photosensitizers, an
increase in the absorption coefficient of the metal complex as well as an increase in the
red shift of the absorption region is needed because the absorption coefficient decreases
as the red shift increases.
15.3.2.2 Organic and natural dye photosensitizers
Organic dyes whose HOMO and LUMO levels match the conduction-band level of
the semiconductor and the iodine redox potential can also be utilized as photosensitizers.
As described in Section 15.1.1, organic dyes such as 9-phenylxanthene dyes
were used as photosensitizers in early researches. Organic dyes have several advantages
as photosensitizers: (1) they have a variety of structures for molecular design,
(2) they are cheaper than metal complexes, and (3) they have large absorption coefficients
attributed to intermolecular π –π ∗ transition. Construction of nanocrystalline
DSSCs using organic dye photosensitizers has been reported, and some structures are
shown in Figure 15.13 [90, 91, 136–145].
Br
NaO
COONa
Br
O O
Br Br
Eosin yellow [137]
S
S S
N
N
O CH 2 COOH
C 18 H 37
Merocyanine dye [141]
CH 3
S
N
CH
N
3
(CH 2 ) 3 SO −
3
Cyanine dye [139, 140]
H 3 C CH 3
CN
N O O
COOH
CH 3
CH 3
Coumarin dye [143, 145]
HOOC
O
N
O
N
O
Perylene dye [136]
O
COOH
Figure 15.13
Molecular structures of organic dye photosensitizers