Chemically Altering the Solubility and Durability of Dyes for Sensitized Solar Cells

Article abstract of DOI:10.1021/acs.orglett.5b01921

By designing dyes with fluoroalkyl groups, the optical and electronic properties of the alkyl analogue were maintained while dramatically altering the solubility. Dyes, F-TABTA (8) and its masked derivative F-TABTSi (9), that enable them to be deposited under conventional organic solvent and scCO₂ conditions, respectively, were developed. In liquid DSSC devices, the fluoroalkyl dye (F-TABTA, 8) performs slightly better than its alkyl analogue (D21L6, 10), and interestingly, it was found that the former device showed better stability over time. Deploying the silyl-masked precursor F-TABTSi (9), this dye was deposited onto TiO₂ photoanodes from scCO₂ in very short contact times (2.5 h), and ECEs of 7.70% were obtained that exceed the performance of the alkyl dye when deposited by conventional methods.

Full text of DOI:10.1021/acs.orglett.5b01921

Letter
pubs.acs.org/OrgLett
Chemically Altering the Solubility and Durability of Dyes for
Sensitized Solar Cells
Subashani Maniam,† Andrew B. Holmes,†,‡ Gary A. Leeke,§ Ante Bilic,∥ and Gavin E. Collis*,†
†CSIRO Manufacturing Flagship, Advanced Fibers and Industrial Chemicals, Clayton South 3169, Victoria Australia
‡School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne 3010, Victoria Australia
§School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.
∥CSIRO Manufacturing Flagship, Virtual Nanoscience, Parkville 3052, Victoria Australia
S
* Supporting Information
ABSTRACT: By designing dyes with fluoroalkyl groups, the
optical and electronic properties of the alkyl analogue were
maintained while dramatically altering the solubility. Dyes, F-
TABTA (8) and its masked derivative F-TABTSi (9), that
enable them to be deposited under conventional organic
solvent and scCO2 conditions, respectively, were developed. In
liquid DSSC devices, the fluoroalkyl dye (F-TABTA, 8) performs slightly better than its alkyl analogue (D21L6, 10), and
interestingly, it was found that the former device showed better stability over time. Deploying the silyl-masked precursor F-
TABTSi (9), this dye was deposited onto TiO2 photoanodes from scCO2 in very short contact times (2.5 h), and ECEs of 7.70%
were obtained that exceed the performance of the alkyl dye when deposited by conventional methods.
substituents have improved power conversion efficiencies when
he use of computational modeling to direct synthetic
Tchemistry has resulted in “designed” semiconductor compared to analogous nonfluorous dyes.21
Here, we demonstrate how, by using fluoroalkyl groups, the
materials with specific chemical, optical, and electronic
properties for use in organic electronics.1−5 The emergence
of new dyes for DSSC applications has made significant
progress over the past decade because of the development and
use of key design rules.6,7 Chemists have used aromatic and
heteroaromatic semiconducting building blocks to access dyes
with desirable optoelectronic features, used functional groups
that have electron-withdrawing or -donating properties to fine
tune the HOMO and LUMO energy levels of the dye to align
with the energy levels of the photoanode and electrolyte, and
used the positioning of substituents around the dye structure
(i.e., linear and branched alkyl groups, polyether groups etc.) to
assist the charge-transfer processes at the metal oxide/dye/
electrolyte interfaces.6,8−13 However, less activity has focused
around understanding the impact that physical properties (e.g.,
dye solubility, dye adsorption/deadsorption, molecular assem-
bly, etc.) may have on device performance and stability.12,14−19
We have been interested in the use of fluoroalkyl groups to
modify the solubility and the electronic interaction between the
dye soaking solvent and electrolyte/dye interface, respectively.
Unlike the alkyl and polyether groups that are commonly used
in DSSC dye design, fluorous materials,20 which offer very
unique electronic, chemical, and physical properties, have found
very little application. As proof-of-concept, we showed that
replacement of the alkyl substituents with fluoroalkyl groups
enabled access to perylene diimide dyes that could be deposited
from supercritical carbon dioxide (scCO2) as the solvent.19
Encouragingly, Fungo, Pozzi, and co-workers recently have
shown that donor/acceptor dyes containing bulky fluoroalkoxy
solubility of an oligothiophene dye can be easily modified to
have dual function, whereby it can be deposited under
conventional dye soaking solvent protocols or from scCO2.
Comparison of the computational modeling and experimental
data indicate that the fluoroalkyl-dye, F-TABTA (8), and the
alkyl-analogue, D21L6 (10), show little difference in their
electronic and optical properties but dramatically differ in
solubility. The DSSC performance of dyes D21L6 (10) and F-
TABTA (8) deposited by conventional solvent methods show
that the fluorinated dye produces marginally better energy
conversion efficiencies (ECEs). Masking the carboxylic acid
group of the fluorinated dye F-TABTA (8) as the labile silyl
ester F-TABTSi (9) delivers a scCO2-compatible dye precursor.
DSSC devices fabricated using these precursors under a scCO2
deposition process (14 MPa, 50 °C), which requires only a few
hours, achieved ECEs as high as 7.70%. Most noticeable is the
enhanced shelf life stability of the fluoroalkyl dye in a device
when an organic redox electrolyte is used.
We chose to study three target compounds, F-TABTA (8),
F-TABTSi (9), and D21L6 (10)22 (Scheme 1). Initial DFT
modeling of F-TABTA (8) indicates the incorporation of the
fluorohexyl groups does not significantly alter the HOMO and
LUMO levels compared to D21L6 (10) and therefore was a
suitable target to study (see the Supporting Information, Table
S1). A convergent synthetic strategy was developed to access F-
Received: July 5, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01921
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Synthesis of F-TABTA (8) and F-TABSi (9) Dyes and Structure of D21L6 (10)
TABTA (8) and F-TABTSi (9) (see Scheme 1 and the
Supporting Information for full details). N,N-Bis(4′-methox-
yphenyl)-4-bromoaniline (1) was demethylated to afford the
bisphenolic compound 2, which was subsequently alkylated
with fluorohexyl methanesulfonate ester/K2CO3 to afford the
fluorinated ether 3. The boronate ester 5 was prepared by
metal−halogen exchange with the bromo-substituted bithio-
phene 4 and trapping with trisisopropylborate, followed by
transesterification with pinacol. The bithiophene pinacolatobor-
on 5 was coupled under Suzuki conditions23 with the
brominated triarylamine 3 to afford the product 6 in good
yield. Following the biphasic conditions developed previously,24
the dimethyl-1,3-dioxane acetal protecting group was removed
to liberate the aldehyde 7. Knoevenagel condensation of the
aldehyde 7 with cyanoacetic acid and piperidine gave F-TABTA
(8) in 60% yield. This material was further purified by reversed-
phase HPLC and determined to be a mixture of E/Z isomers
that could not be unambiguously assigned but consisted of
major and minor isomers in 1:0.05 ratio. Attempts to isomerize
the geometric isomers to one favorable form using light,24 heat,
Figure 1. Qualitative indication of solubility of (a) D21L6 (10) and F-
TABTA (8) in acetonitrile and (b) F-TABTA )8 and F-TABTSi (9) in
n-hexane.
groups on F-TABTA (8) dye did not appreciably improve the
solubility (Figure 1b); presumably, the properties of the dye are
heavily dictated by the presence of the polar carboxylic acid
functionality. However, the combination of the fluoroalkyl
group and silyl-masked carboxylic acid resulted in a dramatic
difference whereby dye precursor, F-TABTSi (9), was
completely dissolved in hexane (Figure 1b). This dramatic
variation in solubility between alkylated and fluoroalkyl-
substituted systems is consistent with the general behavior of
fluorinated compounds.20 Previously, we have found quantita-
tive solubility measurements of the dye in CO2 using the Static
Precise Mass Measuring method19 to be extremely useful for
getting an indication of how the dye will behave in the scCO2
deposition process (see the Supporting Information for details).
The masked fluoroalkyl dye F-TABTSi (9) was measured to
have a high solubility value of 156 mg/kg (14 MPa and 50 °C)
reflecting its solubility in hexane. However, solubility values for
D21L6 (10) and F-TABTA (8) could not be determined as
both showed no recordable values; this was attributed to the
presence of the polar carboxylic acid group.
25
or catalytic iodine in CH2Cl2 were surprisingly unsuccessful.
The transformation of F-TABTA (8) into a tert-butyldime-
thylsilyl-masked group was undertaken using a known literature
protocol. F-TABTSi (9) was found to be easy to prepare,
isolate, and use in scCO2 studies, once again consisting of a
mixture of E/Z isomers.
A comparison of the physical properties of the alkyl dye
D21L6 (10) and fluorinated dyes 8 and 9 in conventional
organic solvents and scCO2 was performed. Qualitative
solubility studies of these dyes were undertaken in common
organic solvents and showed dramatic differences imparted by
the fluoroalkyl groups. In acetonitrile, a common dye soaking
and electrolyte solvent, D21L6 (10) was found to be
moderately soluble, while F-TABTA (8) was sparingly soluble
with the majority of the dye suspended on the surface of the
liquid (Figure 1a). In hexane, a solvent commonly used to
emulate the solubility of materials in scCO2, the fluoroalkyl
The electronic properties of D21L6 (10) and F-TABTA (8)
were compared by spectroscopic and electrochemical methods.
Owing to the limited solubility of the fluorinated dye in
acetonitrile, we found a mixture of ethanol/chloroform (1:1 v/
B
DOI: 10.1021/acs.orglett.5b01921
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
v) to be more suitable for spectroscopic studies and as the dye-
soaking solvent system (see later). As expected, the UV−vis
absorption spectra for D21L6 (10) and F-TABTA (8) are very
similar. The introduction of the fluoroalkyl group results in a
small blue shift of the band from 460 nm (D21L6 (10)) to 452
nm (F-TABTA (8)) (see the Supporting Information, Figure
S1). We attribute this single absorption band to the
intramolecular charge transfer from the donor moiety to the
acceptor functionality found in both dyes. Furthermore, the
molar absorption coefficients of the fluoroalkyl dye F-TABTA
(8) (ε = 36925 M−1 cm−1) increases slightly over D21L6 (10)
(ε = 35225 M−1 cm−1). Encouragingly, the UV−vis spectra of
the D21L6 (10) and F-TABTA (8) adsorbed onto the
transparent TiO2 films (see the Supporting Information, Figure
S2), gave very similar UV/vis spectra intensity and absorption
maxima to those observed in solution.
Figure 2. Stability of D21L6 (10) and F-TABTA (8) in acetonitrile-
based electrolyte measure at AM 1.5, 100 mW cm−2 at defined time
intervals and without the use of an UV filter.
Cyclic voltammetry (CV) of D21L6 (10) and F-TABTA (8)
was performed in DMF (0.1 M tetrabutylammonium
hexafluorophosphate) with a scan rate of 100 mV/s and
internally calibrated with ferrocene/ferrocenium (Fc/Fc+) (see
the Supporting Information, Table S2). The HOMO of D21L6
(10) and F-TABTA (8) are −5.06 and −5.10 eV, while the
LUMO are −3.46 and −3.48 eV, respectively. The experimental
data show less deviation from the computational calculations,
which suggests that the presence of ethylene spacers on the
fluorohexyl groups minimizes the electronic interaction with
the conjugated dye core.
fluorinated self-assembled monolayers are well-known to be
hydrophobic,26 and recent device/dye degradation studies27
have shown that residual water in organic solvent based DSSCs
can lead to hydrolysis of the cyanoacrylic acid dye from the
TiO2/dye interface.
With these promising results, we directed our initial efforts
toward scCO2 dye deposition on commercial TiO2 photo-
anodes. DSSCs fabricated using the scCO2 method with either
D21L6 (10) and F-TABTA (8) were extremely poor, as
expected, because of their extremely low solubility. However,
the silyl-masked fluorohexyl dye F-TABTSi (9) produced very
good photovoltaic responses with an ECE of 4.39 0.01%, Voc
= 682 mV, Jsc = 10.96 mA/cm2, and a FF = 0.58. Not only do
these results compare well with data for the dyes deposited
from conventional organic solvents (Table 1), but the scCO2
method required a 80% shorter dye deposition time (3 h, 14
MPa and 50 °C) compared with the conventional solvent
method (15 h). Under the cell configuration employed here,
the incident photon-to-current conversion efficiency (IPCE)
from the dye precursor F-TABTSi (9) using the scCO2 method
and of F-TABTA (8) from the conventional method are similar
over 400−700 nm with a high plateau value above 60% (see the
Supporting Information, Figure S3).
D21L6 (10) and F-TABTA (8) were initially evaluated by
the conventional solvent soaking process and tested in liquid
electrolyte DSSCs (Table 1). We employed a commercially
Table 1. Cell Performance of D21L6 (10) and F-TABTA (8)
Sensitizers Deposited from Conventional Organic Solvent,
Ethanol/Chloroform (1:1 v/v)
dye
Jsc (mA/cm2)
Voc (mV)
FF
ECE (%)
D21L6
11.47
12.28
660
643
0.57
0.55
4.31 0.01
4.40 0.01
F-TABTA
available TiO2 photoanode fabricated with tetrapropylammo-
nium iodide (0.5 M), iodine (0.05 M), LiI (0.1 M), and 4-tert-
butylpyridine (0.5 M) in acetonitrile as the electrolyte. We
identified that a mixture of ethanol and chloroform (1:1 v/v),
over a period of 15 h, was a suitable solvent system for both
D21L6 (10) and F-TABTA (8). The current−voltage (J−V)
characteristics constructed from this photoanode−electrolyte
system are summarized in Table 1. D21L6 (10) gave an overall
ECE of 4.31%, while F-TABTA (8) showed a slight
improvement over D21L6 (10) with an ECE of 4.40%, and
the fill factors (FF) for both devices were comparable.
With these fabricated devices, we examined the changes in
ECEs over time (shelf life) by storing them in the dark at room
temperature. Interestingly, devices fabricated using F-TABTA
(8) showed better long-term durability than those made with
D21L6 (10) using this electrolyte (Figure 2). Although it has
been previously reported that D21L6 (10) is extremely
photostable over 1000 h at 1 sun22 with an ionic liquid
electrolyte, we suggest that the higher stability observed with F-
TABTA (8) under these electrolyte solvent conditions can be
attributed to the fluoroalkyl groups, which make the dye less
soluble in the acetonitrile electrolyte as shown previously in
Figure 1a, and thus unlikely to desorb from the TiO2 surface.
Another plausible reason for the increased durability is that
Having established that the silyl masked fluorohexyl dye F-
TABTSi (9) is compatible with scCO2 conditions we explored
how photoanode thickness might impact DSSC performance. A
multilayer photoanode with 20 and 400 nm particle layers (7 +
5 μm thickness) with TiCl4 treatment was used. In our hands
using the conventional solvent soaking method, D21L6 (10)
and F-TABTA (8) were deposited over 15 h and produced
ECEs of 7.02 and 7.12%, respectively (see the Supporting
Information, Table S3). Using our scCO2 protocols,19 F-
TABTSi (9) gave a significantly improved ECE of 7.70%. The
improvement in performance is mainly attributed to a higher
Voc and FF (Figure 3). This improvement in ECE using the
scCO2 method may be associated with the ease by which the
dye coated fluid can penetrate the mesoporous TiO2 achieving
better dye coverage.19
In summary, the use of fluoroalkyl substituents to decorate
dyes for DSSCs offers access to materials with different
solubility properties. We are able to use the unique physical
features of F-TABTA (8) to deposit the dye onto TiO2
photoanodes using conventional organic solvents (ethanol/
chloroform) or scCO2. The fluorinated dye performs slightly
better than the alkyl derivative D21L6 when using the
conventional dye solvent process. Even higher efficiencies
(7.70%) and shorter contact times can be achieved with the
C
DOI: 10.1021/acs.orglett.5b01921
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(5) Shu, Y.; Collis, G. E.; Dunn, C. J.; Kemppinen, P.; Winzenberg,
K. N.; Williamson, R. M.; Bilic, A.; Singh, T. B.; Bown, M.; McNeill, C.
R.; Thomsen, L. J. Mater. Chem. C 2013, 1, 6299−6307.
(6) Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed.
̈
2009, 48, 2474−2499.
(7) Kim, B.-G.; Chung, K.; Kim, J. Chem. - Eur. J. 2013, 19, 5220−
5230.
(8) Snaith, H. J. Adv. Funct. Mater. 2010, 20, 13−19.
(9) Bazzan, G.; Deneault, J. R.; Kang, T.-S.; Taylor, B. E.; Durstock,
M. F. Adv. Funct. Mater. 2011, 21, 3268−3274.
(10) Listorti, A.; O’Regan, B.; Durrant, J. R. Chem. Mater. 2011, 23,
3381−3399.
(11) Gratzel, M. Acc. Chem. Res. 2009, 42, 1788−1798.
̈
Figure 3. I/V curve of F-TABTSi (9) deposition in scCO2 (14 MPa,
50 °C) using a multilayer photoanode architecture. The inserted
images show an uncoated and dye-coated TiO2 photoanode and
scCO2 reactor with a view of the photoanode through the sapphire
window.
(12) Zhang, L.; Cole, J. M. ACS Appl. Mater. Interfaces 2015, 7,
3427−3455.
(13) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H.
Chem. Rev. 2010, 110, 6595−6663.
(14) O’Regan, B.; Xiaoe, L.; Ghaddar, T. Energy Environ. Sci. 2012, 5,
7203−7215.
(15) Wen, P.; Xue, M.; Ishikawa, Y.; Itoh, H.; Feng, Q. ACS Appl.
Mater. Interfaces 2012, 4, 1928−1934.
scCO2 process when compared with the alkyl analogue and
conventional organic dye soaking. A highly significant benefit of
the fluoroalkyl group is the enhanced dye durability in the
DSSC device. The use of fluoroalkyl groups may be beneficial
in the design of dyes in conventional and next generation redox
mediator DSSCs.28,29
(16) Chen, X.; Jia, C.; Wan, Z.; Feng, J.; Yao, X. Org. Electron. 2014,
15, 2240−2249.
(17) Stevens, L. A.; Goetz, K. P.; Fonari, A.; Shu, Y.; Williamson, R.
́
M.; Bredas, J.-L.; Coropceanu, V.; Jurchescu, O. D.; Collis, G. E. Chem.
Mater. 2015, 27, 112−118.
(18) Uam, H.-S.; Jung, Y.-S.; Jun, Y.; Kim, K.-J. J. Photochem.
Photobiol., A 2010, 212, 122−128.
ASSOCIATED CONTENT
* Supporting Information
(19) Maniam, S.; Holmes, A. B.; Krstina, J.; Leeke, G. A.; Collis, G. E.
Green Chem. 2011, 13, 3329.
■
S
́
(20) Gladysz, J. A.; Curran, D. P.; Horvath, I. Y. Handbook of
Fluorous Chemistry; Wiley-VCH: Weinheim, 2004.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b01921.
(21) Marzari, G.; Durantini, J.; Minudri, D.; Gervaldo, M.; Otero, L.;
Fungo, F.; Pozzi, G.; Cavazzini, M.; Orlandi, S.; Quici, S. J. Phys. Chem.
C 2012, 116, 21190−21200.
Computational calculations, experimental procedures,
NMR data, CO2 solubilities data, electrochemical data,
and DSSC device fabrication methods (PDF)
(22) Yum, J.-H.; Hagberg, D. P.; Moon, S.-J.; Karlsson, K. M.;
Marinado, T.; Sun, L.; Hagfeldt, A.; Nazeeruddin, M. K.; Gratzel, M.
Angew. Chem., Int. Ed. 2009, 48, 1576−1580.
(23) Moon, S.-J.; Yum, J.-H.; Humphry-Baker, R.; Karlsson, K. M.;
̈
AUTHOR INFORMATION
Corresponding Author
Hagberg, D. P.; Marinado, T.; Hagfeldt, A.; Sun, L.; Gratzel, M.;
̈
■
Nazeeruddin, M. K. J. Phys. Chem. C 2009, 113, 16816−16820.
(24) Collis, G. E.; Burrell, A. K.; Scott, S. M.; Officer, D. L. J. Org.
Chem. 2003, 68, 8974−8983.
*E-mail: gavin.collis@csiro.au.
Notes
(25) Collis, G. E.; Burrell, A. K.; Blandford, E. J.; Officer, D. L.
Tetrahedron 2007, 63, 11141−11152.
The authors declare no competing financial interest.
(26) Wang, H.; Xue, Y.; Ding, J.; Feng, L.; Wang, X.; Lin, T. Angew.
Chem., Int. Ed. 2011, 50, 11433−11436.
ACKNOWLEDGMENTS
■
(27) Chen, C.; Yang, X.; Cheng, M.; Zhang, F.; Sun, L.
ChemSusChem 2013, 6, 1270−1275.
This work was funded by the CSIRO Future Manufacturing
Flagship (Post-doctoral Fellowship for S.M.), CSIRO Fellow-
Office of Chief Executive (OCE) Science Team (G.E.C., A.B.,
and A.B.H.). We thank Mr. Muhammad Kashif and Dr. Udo
Bach (Department of Materials Engineering, Monash Uni-
versity) for use of the screen-printing equipment and helpful
discussions.
(28) Sauvage, F.; Chhor, S.; Marchioro, A.; Moser, J.-E.; Gratzel, M. J.
̈
Am. Chem. Soc. 2011, 133, 13103−13109.
(29) Wu, J.; Lan, Z.; Lin, J.; Huang, M.; Huang, Y.; Fan, L.; Luo, G.
Chem. Rev. 2015, 115, 2136−2173.
REFERENCES
■
593.
(1) Takimiya, K.; Osaka, I.; Nakano, M. Chem. Mater. 2014, 26, 587−
(2) Hachmann, J.; Olivares-Amaya, R.; Jinich, A.; Appleton, A. L.;
Blood-Forsythe, M. A.; Seress, L. R.; Roman-Salgado, C.; Trepte, K.;
́
Atahan-Evrenk, S.; Er, S.; Shrestha, S.; Mondal, R.; Sokolov, A.; Bao,
Z.; Aspuru-Guzik, A. Energy Environ. Sci. 2014, 7, 698−704.
(3) Shu, Y.; Mikosch, A.; Winzenberg, K. N.; Kemppinen, P.; Easton,
C. D.; Bilic, A.; Forsyth, C. M.; Dunn, C. J.; Singh, T. B.; Collis, G. E.
J. Mater. Chem. C 2014, 2, 3895−3899.
(4) Winzenberg, K. N.; Kemppinen, P.; Scholes, F. H.; Collis, G. E.;
Shu, Y.; Singh, T. B.; Bilic, A.; Forsyth, C. M.; Watkins, S. E. Chem.
Commun. 2013, 49, 6307−6309.
D
DOI: 10.1021/acs.orglett.5b01921
Org. Lett. XXXX, XXX, XXX−XXX