8-Hydroxylquinoline as a strong alternative anchoring group for porphyrin-sensitized solar cells

Article abstract of DOI:10.1039/c2cc31440a

8-Hydroxylquinoline (OQ) is demonstrated for the first time as a strong alternative anchoring group porphyrin dyes to improve the long-term stability of solar cells.

Full text of DOI:10.1039/c2cc31440a

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8-Hydroxylquinoline as a strong alternative anchoring group for
porphyrin-sensitized solar cellsw
Hongshan He,*ab Ashim Gurunga and Liping Sia
Received 27th February 2012, Accepted 17th April 2012
DOI: 10.1039/c2cc31440a
8-Hydroxylquinoline (OQ) is demonstrated for the first time as
a strong alternative anchoring group porphyrin dyes to improve
the long-term stability of solar cells.
to increase their binding capability. Imahori et al.6 found that
energy conversion efficiency decreased when a second COOH
was introduced into a quinoxaline-fused porphyrin. Rochford
et al.9 found that introduction of four benzoic groups into
the porphyrin ring would eliminate aggregation and binding
capability and electron injection could be enhanced; however,
this design limits further structural modifications. Campbell
et al.10 and Park et al.11 also introduced two COOH groups
into a porphyrin through a long 2,4-pentadienonic acid group
through b-position and increased efficiency was observed;
sophisticated synthetic procedure and lack of functionality
also impede the application of this strategy. It is therefore
imperative to find a simple alternative anchoring group with
comparable or superior TiO2 binding capability to benzoic
acid group.
Dye-sensitized solar cells (DSCs) are alternative technologies
recognized as being effective in lowering the cost of converting
sunlight to electricity.1 In the modern configuration of DSCs,
reported by Gratzel in 1991,2 randomly packed interconnected
¨
dye-coated titanium dioxide nanoparticle (TiO2 NP) electrodes,
platinum-coated counter-electrodes, and iodide/triiodide redox
couple solution are ‘‘sandwiched’’. To maintain their capability
of converting photons to electrons, the dye molecules have to
bind to the surface of TiO2 nanoparticles strongly without
significant dissociation during cell operation, which is achieved
by coordination of carboxylic acid (COOH) groups of dyes to
titanium atoms. While the extensively studied ruthenium-based
dyes showed promise,3 the limited supply and potential
environmental impact of this heavy metal ion led to the search
for other alternatives. Of the organic dyes considered, porphyrin
dyes stand out as attractive candidates due to their easy structural
modification, abundant supply, and appealing color.4 A unique
donor–p–acceptor structure has been identified as a premium
model for the construction of high performance porphyrin
dyes.5,6 In 2010, Bessho et al.7 integrated a porphyrin ring into
this model and obtained an efficiency as high as 11% with the use
of an iodide-based electrolyte. In 2011, Yalla et al.8 introduced
long alkoxyl groups into this porphyrin and achieved an
unprecedented 12.3% energy conversion efficiency in combination
with a cobalt-based electrolyte.
8-Hydroxylquinoline (OQ) is a promising candidate. The
OQ has been widely used for quantitative analysis of various
metal ions. The N and O atoms of OQ can chelate to metal
ions and form very stable complexes. It is expected that such
chelation can enforce the binding of porphyrin dyes on the
surface of TiO2 nanoparticles, and decrease their dissociation
from the TiO2 surface. In this communication, we report for the
first time that 8-hydroxylquinoline is an alternative anchoring
group to benzoic acid in porphyrin dyes. To demonstrate its
potential, a comparative investigation of an 8-hydroxylquinoline
modified porphyrin, TPPZn-OQ, and a benzoic acid modified
porphyrin, TPPZn-COOH, was carried out. The two porphyrins
have similar structures, absorption and fluorescence spectra.
Single-crystal X-ray diffraction analysis of TPPH2-OQ showed
that the OQ group aligned at B901 to the porphyrin ring, which
is quite similar to TPPZn-COOH (ESIw). The structures of these
two porphyrins are shown in Fig. 1.
However, the use of a single benzoic acid anchoring group
to immobilize porphyrin dyes on TiO2 nanoparticles (there are
two to four in ruthenium-based dyes) has been of concern in
the case of porphyrin-sensitized solar cells. Porphyrin dyes
have a high tendency to dissociate gradually from the TiO2
surface, lose their capability to convert photons to electrons,
and reduce the long-term stability of solar cells. Multiple
carboxylic acid groups were introduced into porphyrin dyes
First, we examined the photovoltaic performance of
TPPZn-OQ and TPPZn-COOH in TiO2 NP-based solar cells.
A mixed CH2Cl2–CH3OH solvent (volume ratio: 1 : 9) was
used to prepare the dye solution due to the poor solubility of
porphyrins in CH3OH. The concentration of the dye was fixed
a Center for Advanced Photovoltaics, South Dakota State University,
Brookings, SD 57007, USA. E-mail: Hongshan.he@sdstate.edu;
Fax: +1 605 688 4401; Tel: +1 605 688 6962
b Department for Chemistry and Biochemistry, South Dakota State
University, Brookings, SD 57007, USA
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31440a
Fig. 1 Structures of TPPZn-OQ, TPPZn-COOH, and BET for this study.
c
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Fig. 3 The J–V curves of TPPZn-OQ, TPPZn-OQ–BET, and
Fig. 2 The J–V curves of porphyrin-sensitized solar cells. The electrolyte
composition: 0.6 M 1-propyl-2,3-dimethylimidazolium iodide, 0.05 M I2,
0.1 M LiI, and 0.5 M tert-butylpyridine. The active area was 0.16 cm2 and
TPPZn-OQ–CDCA sensitized solar cells.
light intensity was 100 mW cmÀ2
.
at 0.3 mM. In all experiments, TiO2 NP films with a thickness
of B6 mm were immersed in dye solution at room temperature
for 4 hours as immersing porphyrin dyes for a longer time
decreases the energy conversion efficiency. The cells were
fabricated as we described previously.12 Typical J–V curves
of TPPZn-OQ and TPPZn-COOH sensitized solar cells are
shown in Fig. 2. Detailed photovoltaic parameters are listed in
Table 1. The TPPZn-OQ and TPPZn-COOH gave 1.55% and
1.82% energy conversion efficiency, respectively, indicating
that TPPZn-OQ functions successfully as an operational dye
in the devices. Compared to TPPZn-COOH-sensitized solar
cells, those of TPPZn-OQ exhibited a drop of 0.38 mA cmÀ2
and 40 mV in JSC and VOC, respectively.
Fig. 4 The IPCE spectra of TPPZn-OQ, TPPZn-OQ–BET, and
TPPZn-OQ–CDCA sensitized solar cells.
To verify if such differences arose from dye aggregation in the
two porphyrins, we measured the cell performance when a mixture
of TPPZn-OQ and CDCA was used. CDCA (chenodeoxycholic
acid) has been widely used as an effective co-adsorber for the
reduction of aggregation, and can anchor on TiO2 nanoparticles
between the two porphyrin dyes to break the short contact
between the dyes.13 Fig. 3 and 4 show the J–V curves and IPCE
spectra of TPPZn-OQ in the presence of CDCA. The addition of
CDCA increased the VOC and JSC in both porphyrins, indicating
that aggregation does exist in the TPPZn-OQ porphyrin. A
more pronounced enhancement was observed in TPPZn-COOH
sensitized solar cells. To explain this, we performed density
functional theory (DFT) calculations (B3LYP/6-31g(d)) of the
two porphyrins on a dinuclear model compound [Ti2O2
(OH)2(H2O)4]2+ and found the O1 and N atoms of TPPZn-
OQ to chelate with the Ti1 atom, forming a tilted geometry as
shown in Fig. 5. This could make CDCA more difficult to bind
between the two porphyrins to reduce aggregation, as shown
in Fig. 5. However, it is also possible that aggregation in
TPPZn-OQ is less than that in TPPZn-COOH due to its non-
vertical orientation.
We then studied electron recombination between TPPZn-OQ
and TiO2 nanoparticles using electrochemical impedance spectro-
scopy (EIS).14 The experiments were performed between 5 Hz and
100 KHz and a Randles model was used to fit the data. In this
model, Rs is the series resistance accounting for transport resistance
of TCO; Rct is the combination of charge transfer resistance for
electron recombination at the TCO–TiO2–electrolyte interface;
while CPE1 is the constant phase element representing capacitance
at the TiO2–electrolyte interface. The Nyquist plots at an applied
bias of À0.60 V are shown in Fig. 6. The extracted data
are listed in Table 2. It was found that Rs, Rct and CPE1 of
Table 1 Photovoltaic parameters of TPPZn-OQ, TPPZn-COOH and
BET sensitized solar cells
JSC/mA cmÀ2
VOC/V
FF
Z (%)
TPPZn-OQ
3.86
5.29
6.00
4.24
5.40
5.16
0.63
0.56
0.59
0.58
0.60
0.65
0.59
0.47
0.72
0.72
0.72
0.72
0.73
0.72
0.70
1.56
2.25
2.49
1.82
2.57
2.21
0.21
TPPZn-OQ–CDCA
TPPZn-OQ–BET
TPPZn-COOH
TPPZn-COOH–CDCA
TPPZn-COOH–BET
BET
Fig. 5 Geometry-optimized structure of TPPZn-OQ that is adsorbed
on a [Ti2O2 (OH)2(H2O)4]2+ model compound and its electron density
distribution of LUMO (lowest unoccupied molecular orbital) and
HOMO (highest occupied molecular orbital).
c
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shown in Fig. 4. The VOC also increased from 0.56 to 0.58 V, which
may come from light absorption of BET. A similar enhancement
was also observed in TPPZn-COOH sensitized solar cells (ESIw).
This further confirmed that BET had similar functionality to
CDCA, though the enhancement was not as strong as CDCA.
Finally, the TPPZn-OQ and TPPZn-COOH coated TiO2
nanoparticle films were immersed in an acetic acid solution
(28 mM) in acetonitrile to test the stability. It was found that
TPPZn-COOH dissociated from the TiO2 nanoparticle film
after 3 h; whereas no obvious dissociation of TPPZn-OQ from
the TiO2 nanoparticle film was observed. The film remained
almost the same after being immersed in this solution for three
weeks. The optical images of these films are shown in Fig. 7.
This experiment demonstrated clearly that OQ is a much
stronger anchoring group compared to the benzoic acid group
that has been widely used in porphyrin dyes.
Fig. 6 Nyquist plots of TPPZn-OQ and TPPZn-COOH sensitized
cells under a bias of À0.60 V. Inset is equivalent Randles circuit
impedance model. The dots are experimental data; the solid lines are
fitting results.
Table 2 EIS parameters obtained by fitting experimental data with
Randle’s model
In summary, we demonstrated for the first time that
8-hydroxylquinoline (OQ) is a promising alternative anchoring
group to benzoic acid for porphyrin dyes. The resulting porphyrin
dye adsorbs on the surface of TiO2 nanoparticles, readily exhibiting
comparable photovoltaic performance to its analogues with
benzoic acid as the anchoring group. A significant increase in
energy conversion efficiency was observed when a complementary
dye boron dipyrromethene, BET, was used.
Rs/O
Rct/O
CPE1/F
t/ms
TPPZn-COOH
TPPZn-OQ
TPPZn-OQ–CDCA
TPPZn-OQ–BET
12.25
15.36
17.11
16.12
688.90
615.27
804.16
628.01
3.07 Â 10À5
2.42 Â 10À5
8.40 Â 10À5
3.20 Â 10À5
21.17
14.83
67.55
20.13
This material is based upon work supported by the National
Science Foundation/EPSCoR Grant No. 0903804 and by the
State of South Dakota.
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TPPZn-COOH. Addition of CDCA increased all of these
parameters significantly, indicating that CDCA reduced the
electron recombination, but also increased the sheet resistance.
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c
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Chem. Commun.