Immobilization of Rhodamine B Isothiocyanate on TiO
2
for Light Harvesting
inZinc Phthalocyanine Dye-sensitized Solar Cells
Yuya Takekuma,
1,2
Tsuyoshi Ochiai,
2,3,4
and Morio Nagata*
1,5
1
Graduate School of Chemical Sciences and Technology, Tokyo University of Science,
12-1 Ichigaya-funagawara, Shinjuku-ku, Tokyo 162-0826, Japan
2
Photocatalyst Group, Local Independent Administrative Agency, Kanagawa Institute ofindustrial Science and TEChnology (KISTEC),
407 East Wing, Innovation Center Building, KSP, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan
3
Materials Analysis Group, Kawasaki Technical Support Division, KISTEC, Ground Floor East Wing, Innovation Center Building,
3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan
4
Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
5
Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science,
12-1 Ichigaya-funagawara, Shinjuku-ku, Tokyo 162-0826, Japan
E-mail: nagata@ci.kagu.tus.ac.jp
Dye-sensitized solar cells (DSSCs) were fabricated with
immobilized Rhodamine B isothiocyanate (RB-ITC) on TiO
2
as energy relay dye (ERD), by reaction of the isothiocyanate
group of RB-ITC and the amino group of 6-aminohexanoic
acid (6AHA) adsorbed on TiO
2
. The resulting DSSCs showed
improved performance in the spectral range of 450600 nm and
the power conversion efficiency via Förster resonant energy
transfer (FRET).
Keywords: Dye-sensitized solar cell (DSSC) | FRET |
Light-harvesting
Dye-sensitized solar cells (DSSCs) have attracted consid-
erable attention as low-cost, lightweight, and flexible devices.
1
The photocurrent in such devices is generated by photons
absorbed according to the spectral response of the sensitizing
dye. Therefore, increasing the dye’ s spectral absorption range
plays a key role in improving the power conversion efficiency of
DSSCs. For this purpose, energy relay dyes (ERDs) have been
studied for enhanced light harvesting and broadened the spectral
response.
25
The photons absorbed by ERDs are transferred to
the sensitizing dye via Förster resonant energy transfer (FRET),
and are subsequently converted to electrons by the sensitizing
dye. ERDs usually use fluorescent molecules that are introduced
into the cell by dissolution in the electrolyte
3
or dispersion in the
hole transporter.
4
A few recent studies immobilized ERD on
mesoporous TiO
2
by a functionalization approach
5
to reduce
the distances between the ERD and sensitizing dye. This ERD
functionalization method bypasses the issue of limited solubility
of ERDs in the electrolyte.
5a
In addition, ERD functionalized on
TiO
2
plays the roleof co-adsorbent, reducing the aggregation of
dyes on the TiO
2
surface and increasing surface passivation,
which represses the recombination ofinjected electrons with I
3
¹
in the electrolyte.
5a
Herein, we report a novelimmobilization of an ERD on
nanoporous TiO
2
in DSSCs. We used Rhodamine B isothiocya-
nate (RB-ITC) as the ERD (Figure 1). RB-ITC is a commer-
cially available dye that is used as a fluorescent tag in a variety
of biological applications,
6
and the labeling is known to occur
via relatively fast reactions without the need for catalysts. To
immobilize RB-ITC on TiO
2
, 6-aminohexanoic acid (6AHA)
with amino and carboxy groups was introduced. The amino
group reacts with the isothiocyanate group of RB-ITC, and the
carboxy group anchors 6AHA on TiO
2
, as shown Figure 1. The
immobilization allowed RB-ITC to remain in the proximity of
the sensitizing dye. This design is analogous to light harvesting
in photosynthesis, in which the light-harvesting (LH) antenna
complexes, for example light-harvesting antenna core complex
(LHI-RC)
7
or light-harvesting antenna complex II (LHCII),
8
are
held in the proximity of the reaction centers (RCs).
9
These
factors contribute to an increase in the device efficiency of up to
0.99%.
The immobilization of RB-ITC on TiO
2
electrodes was
simply carried out by sequentialimmersion in two solutions (for
details, see the Supporting Information (SI)). The first solution
contained PcS2
10
as the sensitizing dye and 6AHA. During
immersion, these molecules were co-adsorbed on mesoporous
TiO
2
. In the second step, immersion in a solution of RB-ITC
caused the ERD to attach by the aforementioned reaction,
as shown inFigure 1. To verify the adsorption of 6AHA,
ninhydrin, which is used in an amino group (NH
2
) detection
method, was sprayed onto the TiO
2
with adsorbed 6AHA. The
colored TiO
2
(Figure S1) indicated the presence of amino
groups on TiO
2
with absorbed 6AHA.
The absorption and emission spectra of RB-ITC-6AHA on
the TiO
2
film were measured. The emissions profile overlaps
with PcS2-TiO
2
absorption, as shown inFigure 2. RB-ITC-
6AHA on TiO
2
also exhibited emission due to the alkyl chain
of 6AHA, which avoided quenching of photoluminescence of
RB-ITC by electron injection into TiO
2
. This indicates that the
alkyl chain enables energy transfer to PcS2. The light absorption
TiO
2
PcS2
Rhodamine B
isothiocyanate
(RB-ITC)
6-aminohexanoic
acid (6AHA)
Immobilization
FRET
Electron
injection
Figure 1. Schematic representation of RB-ITC immobilization
on TiO
2
surface. Photons absorbed by RB-ITC transfer to PcS2
and are subsequently converted into electrons.
Received: October 31, 2017 | Accepted: November 26, 2017 | Web Released: January 19, 2018 CL-171024
Chem. Lett. 2018, 47, 225–227 | doi:10.1246/cl.171024 © 2018 The Chemical Society of Japan | 225