Efficient iodine-free dye-sensitized solar cells employing truxene-based organic dyes

Article abstract of DOI:10.1039/c2cc32926c

Two new truxene-based organic sensitizers (M15 and M16) featuring high extinction coefficients were synthesized for dye-sensitized solar cells employing cobalt electrolyte. The M16-sensitized device displays a 7.6% efficiency at an irradiation of AM1.5 full sunlight.

Full text of DOI:10.1039/c2cc32926c

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Efficient iodine-free dye-sensitized solar cells employing truxene–based
organic dyes
Xueping Zong, Mao Liang,* Tao Chen, Jiangnan Jia, Lina Wang, Zhe Sun and Song Xue*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
In our previous study, we have explored triarylamine
50 organic dyes containing functionalized–truxene unit for DSCs
based on iodine electrolyte.5 The hexapropyltruxene group on
the dyes retards the rate of interfacial back electron transfer
from the conduction band of the nanocrystalline titanium
dioxide film to the I3– ions, which enables attainment of high
55 photovoltages approaching 800 mV. In addition, these
truxene–based dyes pose high extinction coefficients apart
from their ability of retarding charge recombination. It is
expected that these truxene–based dyes could show interesting
performance in the cobalt redox couple system.
Two new truxene–based organic sensitizers (M15 and M16)
featuring high extinction coefficients were synthesized for
dye-sensitized solar cells in cobalt electrolyte. M16–sensitized
device displays a 7.6% efficiency at an irradiation of AM1.5
10 full sunlight.
Research on replacing the conventional iodide/triiodide
electrolyte system in dye-sensitized solar cells (DSCs) with
Co-complex based redox couples has recently received great
attention because of their low corrosiveness, low visible light
15 absorption, and especially high redox potential, which enables
attainment of high photovoltages.1–3 Recently studies have
shown that cobalt redox couples can give impressively high
efficiencies comparable to iodide/triiodide electrolyte by
using organic dyes. By employing a combination of an zinc
20 porphyrin dye (YD2–o–C8) and an organic dye (Y123) in
conjunction with tris(2,2′–bipyridine)cobalt(II/III) redox
couple, Grätzel et al. showed a 12.3%–efficiency DSC.1 The
efficiency exceeds those obtained with today’s best ruthenium
60
In this work, two new truxene–based organic sensitizers
(M15 and M16) bearing binary π–conjugated linker were
designed, synthesized and characterized. To tune both the
electronic structures and steric properties of linker, benzene
ring and 3–methylthieno[3,2–b]thiophene (MTT) were
65 introducted, bringing on an opportunity to understand the dye
structure–performance relationship of iodine–free dye–
sensitized solar cells. Syntheses of dyes M15 and M16 in
detail are described in the ESI (Scheme S1 and S2†).
sensitizers.
Wang
et
al.
explored
tris(1,10–
25 phenanthroline)cobalt(II/III) redox shuttles in conjunction
with the triphenylamine-based organic dyes to fabricate DSCs
displaying power conversion efficiency of 5.8~9.4%.2
Boschloo and Hagfeldt et al. reported triphenylamine–based
organic
dye–sensitized
DSCs
employing
tris(2,2′–
30 bipyridine)cobalt(II/III) redox shuttles displaying the best
efficiency of 6.7%.3 An advantage of the cobalt electrolyte
over the iodide/triiodide redox couple is that very high
opencircuit voltages can be realized but without sacrificing
short circuit photocurrent or fill factor. However, simply
35 replacing the iodide/triiodide couple in the DSCs by cobalt
redox couples may led to poorly performing devices with low
photovoltages and photocurrents, which has been attributed to
slow mass transport and increased recombination of
photoinjected electrons with oxidized redox species in the
40 electrolyte.3b,4 To overcome the mass transport limitation
associated with the polypyridyl cobalt redox shuttles, a
relatively thin titania film is always needed. Additionally, to
meet the requirements of effective light harvesting and retards
the rate of interfacial back electron transfer from the
45 conduction band of the nanocrystalline titanium dioxide film
to the Co(III) ions, developing new class of photosensitizers
with high extinction coefficients, suitable proper electronic
structures and steric properties is warranted.
70
Fig. 1 The molecular structures of M15 and M16.
Both the dyes have a relatively broad and strong absorption
in the ultraviolet and visible region (Fig. 2a). The absorption
spectrum of M15 shows an absorption maximum (λmax
)
centered at 498 nm (ε = 7.6 × 104 M−1 cm−1), which is
75 assigned to the π–π* transition. M16 shows a broad absorption
spectrum covering a wide range of the visible region with a
superior light-harvesting efficiency to M15 (λmax = 500 nm, ε
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= 8.1 × 104 M−1 cm−1) due to the stronger electron-donating
0.5 M 4–tertpyridine (TBP) and 0.1 M Lithium bis–
nature of MTT and relatively smaller torsion angles between
the plane of the donor and that of the acceptor (Fig. S11†),
which is desirable for harvesting more solar light. It is noted
(trifluoromethanesulfonyl)imide (LiTFSI) in acetonitrile. For
comparison, the iodine electrolyte consisting of 0.25 M 1,2–
dimethyl–3–n–propylimidazolium iodide (DMPImI), 0.1 M
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5
that both the dyes have high extinction coefficients compared 50 LiTFSI, 0.05 M I2, and 0.5 M TBP in acetonitrile was
to ruthenium–based dyes (e.g. N719,6 1.47 × 104 M−1 cm−1),
which is desirable for thin–layer DSCs.
formulated.
Figure 3. J–V characteristics of DSCs with the EL-Cobalt and EL-Iodide
Fig. 2 Absorption spectra in dichloromethane (a) and IPCEs action
spectra (b) of DSCs.with electrolytes containing 0.5 M TBP
electrolytes.
10
55
The photocurrent density–voltage (J–V) curves of the
devices measured under AM 1.5 irradiation (100 mW cm–2)
are shown in Fig. 3 and the detailed photovoltaic parameters
are summarized in Table 1. The short–circuit photocurrent
density (JSC), open–circuit photovoltage (VOC) and fill factor
To obtain and understand the molecular orbital energy
levels, cyclic voltammetry (CV) was carried out in a typical
three–electrode electrochemical cell with TiO2 film (5.5 μm)
stained with sensitizer as the working electrode (Fig. S4†).
15 The HOMO levels of M15 and M16 are calculated to be 0.91
and 0.90 V (vs. NHE), respectively, which are more positive
than the [Co(II/III)(phen)3]n+ (phen = 1,10–phenanthroline)
60 (FF) of a M16–sensitized cell with the cobalt electrolyte are
11.9 mA cm–2, 830 mV and 0.70, respectively, affording an
overall power conversion efficiency (PCE) of 6.9%. Whereas
the M16–sensitized cell with the iodide electrolyte exhibits an
slightly increased JSC of 12.2 mA cm–2 and a remarkably
65 decreased VOC of 675 mV, leading to a relatively lower PCE
of 5.6%. For the M15–sensitized solar cells based on the
cobalt electrolyte, we again observe an obviously
improvement of VOC to 810 mV, but significant decreased JSC
to 9.8 mA cm–2, leading to an attenuated PCE compared to
70 corresponding iodine-based cells. This discrepancy of JSC is
well consistent with the IPCEs measurements (Fig. 2b).
–
and I–/I3 redox couples (0.62 V3b and 0.4 V vs NHE,7
respectively). In this work, the driving forces for dye
20 regeneration of the dyes are around 0.5 eV for the iodine
control cells, indicating the driving forces for dye
regeneration are sufficient for the iodine control cells.8
Recently, Wenger et al. has proved that efficient regeneration
of sensitizers with as little as a 0.15 eV driving force between
25 the oxidation potential of the sensitizer and the redox
potential of the mediator is possible.9 In the cobalt control
cells, the calculated driving forces for dye regeneration of
M15 and M16 are 0.29 and 0.28 eV, respectively. Considering
the fact that cobalt control cells presented comparable JSC
30 values related to that of iodine control cells (see next section).
It can be said that the driving forces in the cobalt control cells
were sufficient to regenerate most of these dyes. On the other
hand, the LUMO of M15 and M16 (–1.28 and –1.26 V vs
NHE, respectively) are more negative than the conduction
35 band of TiO2 (–0.5 V vs NHE). Assuming that energy gap of
0.2 eV is necessary for efficient electron injection,10 these
driving forces are sufficiently large for effective electron
injection.
Table 1. Photovoltaic performance of DSCs using a 5.5 μm film
Dyes
JSC/ mA cm–2 VOC/mV FF PCE%
TBP
M15/Iodine
M15/Cobalt
M15/Cobalt
M16/Iodine
M16/Cobalt
M16/Cobalt
11.8
9.8
9.6
12.2
11.9
11.9
756
810
870
675
830
900
0.68
0.69
0.70
0.68
0.70
0.71
6.1
5.5
5.8
5.6
6.9
7.6
0.5 M
0.5 M
0.8 M
0.5 M
0.5 M
0.8 M
Measurements were performed under AM 1.5 irradiation on the DSC
devices with 0.16 cm2 active surface area defined by a metal mask. The
75 amount of the dyes adsorbed on the TiO2 surface for M15 and M16 were
determined to be 1.86 ×10–7 and 1.65 ×10–7 mol cm–2, respectively.
Using the dyes as sensitizers, we compared the photovoltaic
40 performances of DSCs employing a cobalt electrolyte with
those employing an iodine electrolyte. A relatively thin titania
film (5.5 μm) is employed so as to avoid the mass transport
limitation of cobalt redox couple and to minimize interfacial
charge recombination. The cobalt electrolyte is composed of
45 0.25 M [Co(II)(phen)3](PF6)2, 0.05 M [Co(III) (phen)3](PF6)3,
The concentration of TBP was found to be very crucial in the
cobalt cells. The cobalt electrolytes with different concentration
of TBP were prepared for DSCs application. The corresponding
80 experimental results are shown in the Fig. S5†. Under the
optimized condition, cells containing 0.8 M TBP gave the best
efficiency among all the cobalt electrolytes. As presented in Fig.
3, the lower rate of electron recapture by [Co(III) (phen)3] for
2
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M16–sensitized nanocrystalline TiO2 film allows very high VOC
To summarize, the feasibility of employing the truxene–based
organic dyes for high efficiency dye-sensitized solar cell free of a
corrosive iodine electrolyte has been demonstrated. The
to be realized, with this sensitizer reaching a value of 900 mV in
full sunlight without sacrificing short circuit photocurrent or fill
factor. The advantage in voltage of M16 over M15 is maintained
hexapropyltruxene group on the dyes retards the rate of
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5
at lower light levels, down to 10 mW cm-2 solar intensity (Fig. 50 interfacial back electron transfer from the conduction band of the
S8†). The cumulative increases of VOC and FF give rise to an
efficiency of 7.6% at AM 1.5 global full sun. M15–sensitized
cells also display similar features. This phenomenon has also
nanocrystalline titanium dioxide film to the Co(III) ions, which
enables attainment of high photovoltage up to 900 mV. The
photovoltaic performance of the cobalt redox couple was superior
–
been observed by Yella et al.1 These results confirm that for M16,
to that of I–/I3 redox couple for thin–film DSCs sensitized with
–
10 the Co(II/III)tris(phen) redox couple outperforms the I–/I3 redox 55 M16. Our results strongly indicate that the application of truxene-
couple for thin–film DSCs.
based organic dyes as photosensitizers in DSCs employing cobalt
electrolyte is promising.
We are grateful to the National Natural Science Foundation
of China (21003096, 21103123) for financial supports.
It is noteworthy that the dye alteration from M15 to M16 in the
iodine cells (with 0.5 M TBP) have caused a VOC attenuation of
81 mV (756 mV versus 675 mV), sharply contrasting a 20 mV
15 enhancement (810 mV versus 830 mV) in the cobalt cells (with
0.5 M TBP). The opposite VOC variation for the iodine and cobalt
cells from M15 to M16 is intriguing. To clarify the origin of our
observation, the charge recombination resistance at the
TiO2/electrolyte interface (RCT
20 electrochemical impedance spectroscopy (EIS) as a function of
EF,n level, as shown in Fig. 4.
60 Notes and references
a Department of Applied Chemistry, Tianjin University of Technology,
Tianjin, 300384, P.R.China. Fax: +86 22 60214252; Tel: +86 22
60214250; E-mail: liangmao717@126.com; xuesong@ustc.edu.cn
† Electronic Supplementary Information (ESI) available: See
65 DOI: 10.1039/b000000x/
)
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85
90
Fig. 4. Plots of Rct versus EF,n level of the cobalt cells and the iodine cells.
The charge recombination resistance is related to the charge
25 recombination rate, such that a smaller RCT means the larger
charge recombination rate. For the cobalt cells, RCT increases in
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30 and some electron–rich segments of dye molecules could cause a
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atom) prefers formation of dye–iodine complexes in comparison
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vicinity of the TiO2 decreases the electron lifetimes (τ, extracted
from the Cμ and RCT using τ = Cμ × RCT12, Fig. S10†), leading to a
40 lower VOC for M16–sensitized cells. In cobalt cells, the VOC of
M16–sensitized cells higher than those of M15 can be partially
ascribed to the absence of this non-covalent interaction between
the dyes and electrolytes. These results suggest that organic dyes
with thiophene derivates as linkers are suitable for iodine–free
45 DSCs.
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