Cyanomethylbenzoic acid: An acceptor for donor-π-acceptor chromophores used in dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201200636

Sensing the sun: Incorporation of a cyanomethyl benzoic acid electron acceptor into donor-π-acceptor sensitizers for dye-sensitized-solar cell is shown to lead to devices with improved conversion efficiency when compared with more widely used cyanoacetic acid acceptor. Copyright

Full text of DOI:10.1002/cssc.201200636

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DOI: 10.1002/cssc.201200636
Cyanomethylbenzoic Acid: An Acceptor for Donor–p–
Acceptor Chromophores Used in Dye-Sensitized Solar Cells
Wanchun Xiang,[a] Akhil Gupta,[b, c] Muhammad Kalim Kashif,[a] Noel Duffy,[c] Ante Bilic,[d]
Richard A. Evans,*[c] Leone Spiccia,*[a] and Udo Bach*[b, e, f]
During the past two decades, dye-sensitized solar cells (DSCs)
have received great attention due to their low fabrication cost
compared to conventional silicon solar cells.[1] Energy conver-
sion efficiencies of up to 11% have been reported for DSCs
based on iodide/iodine liquid electrolytes and ruthenium(II)
dyes.[2] However, in terms of practical applications, these high-
performance iodine-based electrolytes are afflicted with
a number of potential disadvantages, such as their high volatil-
ity, significant coloration, and corrosive nature. The latter gives
rise to major incompatibility issues with a number of metals
and sealing materials, limiting the use of metal substrates or
charge-collecting grids in the construction of DSC modules.
Iodine-free, one-electron outer-sphere redox couples, such
as cobalt(II)/(III) polypyridyl complexes, are promising alterna-
tive redox mediators due to their weak coloration and their
compatibility with a wide range of metal substrates.[3] DSCs
based on these mediators can reach high conversion efficien-
cies, especially when used in conjunction with organic sensitiz-
ers, featuring high molar extinction coefficients and TiO2 elec-
trodes of only a few microns in thickness.[4] Yella et al. recently
reported a DSC with a new benchmark efficiency of 12.3%,
which used [Co(bpy)3]2+/3+ (bpy=2,2’-bipyridine) as redox me-
diator, causing a paradigm shift in dye-sensitized solar cells.[5]
Thus, the best-performing DSC features neither a ruthenium(II)
polypyridyl complex as sensitizer, nor an iodide-based electro-
lyte. Previous studies suggested that the performance of
cobalt redox mediators in DSCs is hindered by rapid recombi-
nation of electrons in the TiO2 conduction band with the
cobalt(III) species and, furthermore, by slow dye regeneration.[6]
These issues can be overcome by: (1) matching sensitizers with
cobalt polypyridyl complexes, such that a sufficient driving
force is available to ensure efficient dye regeneration;[6] and
(2) applying sensitizers with high molar extinction coefficients
to ensure excellent light harvesting even when using thin TiO2
films.[7]
A number of organic sensitizers have been tested in con-
junction with cobalt electrolytes. Most of these dyes are
donor–p-bridge–acceptor (D–p–A) dyes. Among donor groups,
triphenylamine and its derivatives have shown promise in the
development of DSCs due to their nonplanar structure sup-
pressing the aggregation of dye molecules.[8] Oligothiophenes,
and their derivatives, have been widely used as conjugated p-
bridges, due to their high polarizability as well as their tunable
spectroscopic and electrochemical properties.[7,9] The focus of
electron-acceptor groups has been largely on carboxyl acid, cy-
anoacrylic acid, and the rhodanine-3-acetic-acid moiety as they
bind strongly to the TiO2 semiconductor surface.[5,10] In particu-
lar, much attention has been paid to cyanoacrylic acid because
of its strong electron-withdrawing ability, which may result in
an efficient electron–hole separation within the dye molecule.
In contrast, few studies have ever been reported that examine
whether the structural alternatives of this particular acceptor
improve the performance of DSCs. In this work, cyanomethyl-
benzoic acid is introduced as a new acceptor moiety for DSC
sensitizers and is compared to the more established cyanoace-
tic acid moiety. It was thought that the cyanomethylbenzoic
acid may provide better performance as it will cause a redshift
of the absorption spectrum. For this purpose, two new dyes
were synthesized. K6 (see Figure 1) is a structural analogue of
dye C240,[11] differing slightly in the degree and type of alkoxy-
substitution on the phenyl rings of the triarylamine unit. K7 is
a structural analogue of K6, in which cyanoacrylic acid was re-
placed with the new cyanomethylbenzoic-acid acceptor. Mo-
lecular orbital calculations show similar highest occupied and
lowest unoccupied molecular orbital (HOMO/LUMO) electron
distributions for the two dyes (Figure 1).
[a] Dr. W. Xiang,+ M. K. Kashif, Prof. Dr. L. Spiccia
School of Chemistry
Monash University
Victoria 3800 (Australia)
Fax: (+61)3-9905-4597
E-mail: leone.spiccia@monash.edu
[b] A. Gupta,+ Prof. Dr. U. Bach
Department of Materials Engineering
Monash University
Victoria 3800 (Australia)
Fax: (+61)3-9905-4940
E-mail: udo.bach@monash.edu
[c] A. Gupta,+ Dr. N. Duffy, Prof. Dr. R. A. Evans
CSIRO Materials Science and Engineering
Flexible Electronics Theme
Bayview Avenue, Clayton South, 3169, Victoria (Australia)
Fax: (+61)395452446
E-mail: richard.evans@csiro.au
[d] Dr. A. Bilic
CSIRO Mathematics Informatics and Statistics
Bayview Avenue, Clayton 3169, Victoria (Australia)
[e] Prof. Dr. U. Bach
Commonwealth Scientific and Industrial Research Organization
Materials Science and Engineering
Flexible Electronics Theme
Clayton South, Victoria 3169 (Australia)
[f] Prof. Dr. U. Bach
Melbourne Centre for Nanofabrication
151 Wellington Road, Clayton, VIC 3168 (Australia)
[+] These authors contributed equally to this work.
The K6 and K7 dyes were synthesized by reacting the alde-
hyde precursor, 6-[4-(bis{4-[(2-ethylhexyl)oxy]phenyl}amino)-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200636.
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characterization details are pro-
vided in the Supporting Informa-
tion.
The UV/Vis spectra of K6 and
K7 measured in chlorobenzene
solution, and when absorbed on
a
TiO2 film, are shown in
Figure 2. In solution, the absorp-
tion maximum of K7 is slightly
red-shifted by 25 nm relative to
the cyanoacrylic acid dye, K6,
whereas the molar extinction co-
efficient
at
lmax
of
K7
(43700 Lmolꢀ1 cmꢀ1@521 nm) is
slightly higher than that of K6
(40800 Lmolꢀ1 cmꢀ1@506 nm).
When adsorbed onto TiO2 films,
the absorption maxima for both
K6 and K7 shift by about 30 nm
to shorter wavelengths when
compared to the solution spec-
trum, which may be due to the
deprotonation of the carboxylic
acid group on the surface of
Figure 1. a) Chemical structures of sensitizers K6 and K7. b) Electron-density distribution for the LUMOs and
HOMOs of K6 and K7. Density functional theory (DFT) calculations were performed on all the materials using the
Gaussian 03 suite of programs and B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level of theory.
phenyl]-4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b’]dithiophene-
2-carbaldehyde, at reflux with cyanoacetic acid in a 1:1 (v/v)
acetonitrile/chloroform mixture and 4-(cyanomethyl)benzoic
acid in a 9:1 (v/v) acetonitrile/chloroform solvent mixture, re-
spectively, in the presence of piperidine as base. Synthetic and
TiO2. K7 shows superior light absorption at wavelengths longer
than 460 nm.
The dye anchoring in DSCs is critical in achieving high cell
efficiencies. K6 contains a cyanoacrylic acid group as anchoring
moiety, which is typically introduced into most organic dyes to
chemically attach to the TiO2 surface.[12] To compare the bind-
ing ability of K7, which has a cyanomethylbenzoic acid accept-
or, with that of K6, the absorption of the two dyes on TiO2 was
investigated. In Figure 3, the absorption of the dye adsorbed
on the mesoporous TiO2 film is plotted as a function of the
dye’s equilibrium concentration (cequ) in the dye bath. The two
dyes show a very similar concentration-dependent absorption
behavior with the dye absorption reaching a plateau at cequ
ꢁ0.1mm. According to classical Langmuir absorption theory[13]
the dye-binding constant can be extracted from a plot of the
ꢀ1
inverse of the surface coverage Gꢀ1 as a function of cequ
[Eq. (1)].[13] The inset of Figure 3 shows the corresponding plot,
confirming Langmuir-type adsorption behavior. The calculated
binding constants, K, for the two dyes are identical [(4.42ꢂ
0.07)ꢁ105 and (4.46ꢂ0.03)ꢁ105 Lmolꢀ1 for K6 and K7, respec-
tively], suggesting that the new acceptor moiety attaches to
the TiO2 surface just as strongly as the traditional cyanoacrylic
acid group. In keeping with this finding, the dye loading for K7
[6.87 (ꢂ0.03)ꢁ10ꢀ8 mol cmꢀ2] and K6 [6.91 (ꢂ0.02)
ꢁ10ꢀ8 molcmꢀ2], under the conditions used to dye films for
DSC construction, is identical.
ꢀ ꢁ
ꢀ
ꢁ
1
G
1
cequ
1
K
¼
þ 1
ð1Þ
Figure 2. a) Molar extinction coefficients (e) of K6 (black) and K7 (red) in
chlorobenzene. b) Absorptivity spectra of 1.2 mm thick TiO2 films sensitized
with K6 (black) and K7 (red) recorded in air. Sensitization of the films was
performed using a 1.5mm dye solutions in a chlorobenzene and ethanol mix-
ture (v/v=1:1) for 8 h.
Cyclic voltammetry was carried out to determine the redox
properties of these two dyes (see Figure S1). The oxidation of
K7 occurs at a half-wave potential of E1/2(K7/K7+)=1.02 V
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Table 1. Photovoltaic performance of DSCs under simulated AM1.5G
solar irradiation (100 mWcmꢀ2).[a]
[b]
[c]
Dye
Electrolyte
Voc
Jsc
[mAcmꢀ2
ff[d]
h[e]
[mV]
]
[%]
K6
K7
K6
K7
846ꢂ3
832ꢂ1
745ꢂ8
738ꢂ7
10.0ꢂ0.1
12.5ꢂ0.2
10.7ꢂ0.3
11.5ꢂ0.3
0.76ꢂ0.01
0.73ꢂ0.01
0.73ꢂ0.01
0.71ꢂ0.01
6.4ꢂ0.2
7.6ꢂ0.1
5.8ꢂ0.2
6.1ꢂ0.2
2+/3+
Co(bpy)3
ꢀ
Iꢀ/I3
[a] DSCs were constructed with the K6 and K7 dyes and tested with two
electrolyte systems the compositions of which were as follows: (1) cobalt
electrolyte: 0.20m tris(2,2’-bipyridyl)cobalt(II) bis(trifluoromethanesulfony-
l)imide ([Co(bpy)3](TFSI)2), 0.060m [Co(bpy)3](TFSI)3, 0.050m LiTFSI, 1.00m
tert-butylpyridine (tBP) in acetonitrile; (2) iodide electrolyte: 0.20m LiI,
0.060m I2, 0.050m LiTFSI, 1.00m tBP in acetonitrile; both K6 and K7 were
adsorbed from a 0.3 mm dye solution in a mixture of chlorobenzene and
ethanol (v/v=1:1) for 12 h. [b] Voc is the open circuit voltage. [c] Jsc is the
short circuit current density. [d] ff is the fill factor. [e] h is the energy con-
version efficiency.
Figure 3. Langmuir isotherms. Dye absorption on 1.2 mm thick mesoporous
TiO2 films at the wavelength of maximum absorption (K6 black and K7 red)
as a function of the equilibrium concentration of the dye (cequ) in the solu-
tion (chlorobenzene/ethanol=1:1 v/v). The films were kept in the dye bath
for 20 h. Inset: The inverse of TiO2 surface coverage Gꢀ1 plotted against the
ꢀ1
inverse of the dye equilibrium concentration (cequ
)
with G=Abs(lmax)/Abs0-
(lmax) where Abs0(lmax) corresponds to Abs(lmax) measured at cequ =0.5mm.
versus normal hydrogen electrode (NHE). The corresponding
value for K6 is 170 mV higher [E1/2(K6/K6+)=1.19 V versus
NHE]. The spectral properties of the dyes, shown in Figure 2,
were combined with the electrochemical data of the dyes in
solution to calculate the E1/2(D*/D+) energy levels shown in
Figure 4.[14] Based on the electrochemical properties, the dye-
scattering layer (400 nm particle size). To facilitate the compari-
son of DSC performance, the concentrations of the oxidized
and reduced redox mediators were chosen to be identical for
both electrolyte systems. Other additives, such as tBP and
LiTFSI, were also maintained at the same concentration and
acetonitrile was used as solvent.
DSCs sensitized with K7 clearly outperform devices based on
the K6 sensitizer in both electrolyte systems. The improvement
is mostly due to an increase in the short-circuit current density
(JSC). The analysis of the incident-photon-to-current conversion
efficiency (IPCE) measurements of these DSCs (see Figure 5a)
reveals that the IPCE for K7-sensitized DSCs levels out at values
of 82–86% at around 500 nm for the [Co(bpy)3]2+/3+- and
iodide-based electrolytes, whereas the values for K6-sensitized
cells are somewhat lower reaching maxima of approximately
67% for both electrolyte systems. Only the DSC based on K6
and iodide/triiodide show a weak redshift in the IPCE spec-
trum. The two dyes show up to 80% light absorption when
adsorbed onto a 1.2 mm thick TiO2 film (see Figure 2b). Light
harvesting by the sensitized TiO2 layers used for the construc-
tion of these DSCs (4 mm thick+scattering layer) will, there-
fore, be close to quantitative over a significant range of the
spectral region around the absorption maxima of the sensitiz-
ers. To further rationalize the observed IPCE spectra of K6 and
K7 in [Co(bpy)3]2+/3+-based DSCs, absorbed-photon-to-electron
(APCE) spectra were calculated based on the absorption prop-
erties of the sensitized films (see Figure 5b). The average APCE
values across the spectral range of 450–550 nm for K6 and K7
are 63 and 70%, respectively, indicating a significantly in-
creased charge conversion efficiency per adsorbed photon for
K7-sensitized [Co(bpy)3]2+/3+-based DSCs. This increase can be
caused by: (1) improved charge injection; (2) improved dye-re-
generation; and/or (3) reduced recombination. To check
whether the observed photocurrents during the IPCE and
APCE measurements were limited by charge collection, prelimi-
nary front/back-IPCE measurements were performed (Figure S3
in the Supporting Information). The results show negligible
charge recombination during electron diffusion through TiO2
Figure 4. Energy diagram of the sensitizers K6, K7, [Co(bpy)3]2+/3+ and
a iodide/triiodide electrolyte based on the results of cyclic voltammetry
measurements (see the Supporting Information).
regeneration driving forces for K6 and K7 can be estimated to
be 620 and 460 mV, respectively, when these dyes are used in
conjunction with a [Co(bpy)3]2+/3+ (bpy=2,2’-bipyridine) elec-
trolyte. Both driving forces are in excess of 390 mV, which was
previously found to be sufficient for efficient dye regeneration
by the same redox mediator.[6]
The photovoltaic performance of K6 and K7 was evaluated
in DSCs constructed with [Co(bpy)3]2+/3+ as well as a conven-
tional triiodide/iodide-based electrolyte (see Table 1). The TiO2
electrode structures were optimized to maximize the perfor-
mance of [Co(bpy)3]2+/3+-based DSCs and consisted of a 4 mm
thick transparent layer (30 nm particle size) and a 6 mm thick
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Figure 6. IMVS and IMPS measurements performed on K6- (black) and K7-
(red) sensitized DSCs based on [Co(bpy)3]2+/3+ (solid line) and Iꢀ/I3ꢀ (dashed
line) electrolytes (same devices as reported in Table 1). a) Mean charge tran-
sient time versus short circuit current. b) Electron lifetime versus electron
density.
Figure 5. a) Incident-photon-to-electron conversion efficiencies (IPCEs) of
DSCs sensitized with K6 (black) and K7 (red) based on [Co(bpy)3]2+/3+ (solid
line) and Iꢀ/I3ꢀ (dashed line) electrolytes (same devices as reported in
Table 1). b) Absorbed-photon-to-current generation efficiencies (APCEs) as
a function of wavelength for K6 (black) and K7 (red) in conjunction with the
[Co(bpy)3]2+/3+ electrolyte described in Table 1. DSCs based on 4 mm thick
transparent TiO2 films were used for this analysis.
sitized DSCs. The somewhat longer electron lifetime observed
for K6-sensitized devices is also consistent with the 14 mV in-
crease in VOC and slightly higher ff for K6, relative to K7-sensi-
tized [Co(bpy)3]2+/3+ DSCs (see Table 1). This rules out both in-
creased charge recombination as well as inefficient dye-regen-
eration as a cause for the inferior performance of K6-sensitized
DSCs as both effects should result in a reduction, rather than
an increase, in electron lifetime. Consequently, superior charge
injection properties for K7, relative to K6, are the most likely
origin of the observed performance difference. This is also con-
sistent with a 170 mV more negative E1/2(K7+/K7*) level com-
pared to E1/2(K6+/K6*) (see Figure 4).
film, suggesting that the electron diffusion length is well
below the thickness of the TiO2 film.
To further elucidate the origin of the observed differences in
photovoltaic performance, the electron lifetime and mean
transit time of photo-injected charge carriers were studied by
means of intensity modulated photovoltage and photocurrent
spectroscopy (IMVS and IMPS)[15] in combination with charge
extraction experiments. The IMPS results, shown in Figure 6a,
indicate that similar mean electron transit times were observed
for both dyes and both electrolytes and suggest comparable
electron transport within the TiO2 layer in all cases. Charge ex-
traction measurements according to Duffy et al[16] were used
to determine the amount of charge stored in DSCs as a func-
tion of VOC (see Figure S3). The results obtained through IMVS
and charge extraction experiments were then combined to
yield the electron lifetime as a function of charge stored in the
DSC, as shown in Figure 6b. The results for the [Co(bpy)3]2+/3+
electrolytes clearly show an increase in electron lifetime by
about a factor of 1.6 for K6-sensitized DSCs over devices sensi-
In summary, we have evaluated a new cyanomethylbenzoic
acid as acceptor for d–p–A DSC sensitizers by comparing its
performance to the more widely used cyanoacrylic acid accept-
or unit in a pair of analogous dyes. These dyes were modeled
on previously reported sensitizers that yielded high DSC per-
formances when combined with [Co(bpy)3]2+/3+ electrolytes
(C240).[11] The dye featuring the cyanomethylbenzoic acid ac-
ceptor (K7) outperformed the model sensitizer comprising the
cyanoacrylic acid group (K6) in terms of its photovoltaic perfor-
mance. Superior charge injection properties of K7 compared to
K6 were identified as the most likely reasons for the observed
increases in Jsc, efficiency; and IPCE. The performance increase
could be realized in spite of a slight electron lifetime decrease
for [Co(bpy)3]2+/3+-based DSCs when the cyanoacrylic acid
group is replaced with cyanomethylbenzoic acid.
ꢀ
tized with K7. When Iꢀ/I3 is used as redox mediator, there is
no apparent difference in electron lifetime for K6- and K7-sen-
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The authors would like to acknowledge the ARC for providing
equipment (LE0883019) and fellowship (U.B., DP110105312) sup-
port. This work was further supported by the Australian Solar In-
stitute, the Victorian State Government (DBI-VSA and DPI-ETIS),
and was performed in part at the Melbourne Centre for Nano-
fabrication, an initiative partly funded by the Commonwealth of
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Keywords: benzoic acid · cobalt complexes · iodine free ·
solar cells · sensitizers
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Received: August 28, 2012
Published online on January 23, 2013
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