Fine-tuning of triarylamine-based photosensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201100031

The synthesis of a series of novel organic photosensitizers based on modified triarylamine motives has been achieved. The new "push-pull" dyes, bearing one to three naphthyl units in place of phenyl rings, have been successfully obtained following original synthetic routes. The structural, optical and electrochemical properties of these chromophores were studied by combining crystallographic, experimental and theoretical data. We further present a systematic study of this series of dyes. When embedded in dye-sensitized solar cell devices, the new photosensitizers showed good spectral response with IPCE greater than 85 % from 470 to 580 nm and overall power conversion efficiencies reaching 6.6 %. One of these new sensitizers, containing one naphthyl attached to the π-conjugated linker and having phenyl rings on the outside, led to the highest molar extinction coefficient and energy conversion efficiency of the series while exhibiting higher electron lifetime. Copyright

Full text of DOI:10.1002/cssc.201100031

DOI: 10.1002/cssc.201100031
Fine-Tuning of Triarylamine-Based Photosensitizers for
Dye-Sensitized Solar Cells
Cꢀline Olivier,*[a] Frꢀdꢀric Sauvage,[b] Laurent Ducasse,[a] Frꢀdꢀric Castet,[a] Michael Grꢁtzel,[b]
and Thierry Toupance*[a]
The synthesis of a series of novel organic photosensitizers
based on modified triarylamine motives has been achieved.
The new “push-pull” dyes, bearing one to three naphthyl units
in place of phenyl rings, have been successfully obtained fol-
lowing original synthetic routes. The structural, optical and
electrochemical properties of these chromophores were stud-
ied by combining crystallographic, experimental and theoreti-
cal data. We further present a systematic study of this series of
dyes. When embedded in dye-sensitized solar cell devices, the
new photosensitizers showed good spectral response with
IPCE greater than 85% from 470 to 580 nm and overall power
conversion efficiencies reaching 6.6%. One of these new sensi-
tizers, containing one naphthyl attached to the p-conjugated
linker and having phenyl rings on the outside, led to the high-
est molar extinction coefficient and energy conversion efficien-
cy of the series while exhibiting higher electron lifetime.
Introduction
The development of renewable energy sources is promoted by
the increasing public awareness that the Earth’s fossil reserves
will run out during this century, while the amount of energy
needed will at least double within the next 50 years. In the
field of renewable energies, solar energy appears as a key re-
source to fulfill the rising worldwide demand for clean and sus-
tainable power sources. In this context, one attractive ap-
proach is the development of systems that mimic natural pho-
tosynthesis, similar to how a leaf uses solar flux to produce
fuels. Following this concept, dye-sensitized solar cells (DSCs)
have attracted great interest since a 7.1% conversion efficiency
was reported by Grꢁtzel and co-workers.[1] Because of their ver-
satility, low cost, and easy manufacturing, DSCs are a highly
promising technology and represent a viable alternative to tra-
ditional p–n silicon solar cells.[2] The heart of DSC devices com-
prises a light harvester (the photosensitizer), covalently anch-
ored to a high-surface-area mesoporous layer of a few micro-
meters thick, composed of anatase TiO2 nanocrystals. The
progress realized in dye chemistry, in particular that of poly-
pyridyl ruthenium complexes, and light containment in the
photo-anode has afforded certified power conversion efficien-
cies (PCEs) of over 11%.[3] However, the scarcity and high cost
of ruthenium have motivated the synthesis of novel dyes, for
example dyes using inexpensive metal complexes, such as zinc
porphyrins and phthalocyanines,[4] or fully organic dyes,[5] such
as indoline-, coumarin-, perylene-, phenoxazine-,[6] or triaryla-
mine-based dyes.[7,8] The latter have important advantages
compared to metal-complex-based sensitizers, including,
among others, higher molar extinction coefficients and larger
structural versatility to judiciously modulate the optical proper-
ties of the dye while maintaining a low production cost and
long-term stability.[8] In particular, p-conjugated molecules,
finely tuned for effective charge transfer processes, exhibit
valuable properties for DSCs applications with optimized D-p-
A structural designs reaching over 9% PCE.[9]
Herein, we report on a methodical synthetic approach for a
series of organic “push-pull” dyes that contain naphthyl units
at various positions, featuring the triarylamine core as electron
donor part and the common cyanoacrylic acid moiety acting
as charge acceptor and anchoring group. These two parts are
bridged by a bi-thiophenyl p-conjugated spacer to ensure effi-
cient charge transfer from the ground state to the excited
state. This conjugated spacer is endowed with an additional
cyanovinyl unit to increase the absorption wavelength of the
dyes for better light harvesting. Our systematic approach ex-
plores the effect of the insertion of naphthyl units instead of
the phenyl groups. A particular emphasis is placed on examin-
ing the relationship between their position and the spectral re-
sponse, energy level of the HOMO–LUMO orbitals, and the
photovoltaic properties of these new organic chromophores in
comparison to that of C212, reported first by Wang’s group.[10]
The motivation for making use of the electron-rich 2,6-substi-
tuted naphthyl units is that they increase the donor character
[a] Dr. C. Olivier, Dr. L. Ducasse, Dr. F. Castet, Prof. T. Toupance
UMR-CNRS 5255 Institut des Sciences Molꢀculaires
Universitꢀ de Bordeaux
351 cours de la Libꢀration, 33405 Talence (France)
Fax: (+33)5 40 00 69 94
E-mail: c.olivier@ism.u-bordeaux1.fr
t.toupance@ism.u-bordeaux1.fr
[b] Dr. F. Sauvage, Prof. M. Grꢁtzel
Laboratoire de Photonique et Interfaces
Institut des Sciences et Ingꢀnierie Chimique
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100031.
ChemSusChem 2011, 4, 731 – 736
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
731

C. Olivier. T. Toupance et al.
of triarylamine, thereby enhancing the intramolecular charge-
transfer processes.[11]
ing two donating methoxy groups located on the remote part
of the dye and one methyl ester function in a suitable position
4. Further reduction of the ester, via a two-step procedure of
reduction to the alcohol 5 followed by mild oxidation, led to 6
bearing the aldehyde function, proper to graft the p-conjugat-
ed linker. The introduction of two thiophene units was succes-
sively achieved via Knoevenagel condensation leading to the
bromo-derivative 7, and Suzuki coupling reaction yielding 8. Fi-
nally, the commonly applied Knoevenagel condensation
method, using cyanoacetic acid and piperidine, afforded the
cyanoacrylic acid moiety of dye 1. The same procedure was ap-
plied to achieve the synthesis of dyes 2 and 3. The triphenyla-
mine derivative (C212) was re-synthesized following the proce-
dure reported by Wang[10] and used as reference compound
for this study.
Results and Discussion
Following appropriate synthetic procedures (vide infra), we
achieved the synthesis of three novel organic dyes which con-
tain selectively either one (1), two (2), or three (3) naphthyl
units.
Single crystals of 1 were grown from a dichloromethane/
methanol mixture. The crystal structure was solved by X-ray
diffraction analysis (Figure 1). Interestingly, the two nitrile func-
Figure 1. Crystal structure of 1.
tions are alternately in trans and cis configuration relative to
the sulphur atom of the thiophene ring. Moreover, the p-con-
jugated pathway in 1 displays an almost planar structure with,
firstly, a torsion angle between the two thiophene rings of less
than 28 and secondly, a dihedral angle between the median
plane of the bi-thiophenyl unit and the one of the naphthyl of
only 198, which does not hamper intramolecular charge trans-
fer. This structure provides a linear, undisrupted, and highly
conjugated pathway of more than 20 ꢃ for electron flow from
the donating amine moiety to the terminal anchoring group.
To do so, an original synthetic route was applied to get the
target dyes; classical procedures being accordingly modified
and adapted to the presence of naphthyl moieties. The syn-
thetic route to dye 1 is depicted in Scheme 1 as an example.
The key step was a palladium-catalyzed Buchwald–Hartwig
cross-coupling reaction to generate the triarylamine core bear-
Scheme 1. Synthetic route to dye 1: i) Pd(OAc)2, P(tBu)3, Cs2CO3, toluene; ii) LiAlH4, THF; iii) MnO2, CH2Cl2; iv) 5-bromo-2-thiopheneacetonitrile, NaOH, EtOHabs.
v) 5-formylthiophen-2-boronic acid, PdCl2(dppf)2, K2CO3, DMSO; vi) cyanoacetic acid, piperidine, CHCl3.
;
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Triarylamine-Based Photosensitizers for Dye-Sensitized Solar Cells
slight anodic shift of the ground state energy when compared
to the phenyl rings (1 and C212). However, comparing 1 with
C212, and 2 with 3, reveals that a bridging naphthyl unit does
facilitate the oxidation process. In each case, the energetic po-
sition of I3ꢀ/Iꢀ redox couple (ca. +0.4 V vs. NHE)[12] is signifi-
cantly more negative than the HOMO level of the dyes giving
sufficient driving force for iodide to regenerate the oxidized
form of the dye. The LUMO level was obtained from the
HOMO position and the zero-zero energy (E0ꢀ0) estimated from
the intercept between absorption and emission spectra (Fig-
ure S2). As a result, the LUMO level of these dyes, namely
EoxꢀE0ꢀ0, is sufficiently more negative than the conduction
band edge position of TiO2 (ca. ꢀ0.5 V vs. NHE)[12] which en-
sures an effective rate for electron injection from the sensitizer
Figure 2. Absorption spectra of the photosensitizers in CH2Cl2.
UV/vis absorption spectra of the dyes were recorded in to the semiconductor.
CH2Cl2 solution (Figure 2). The corresponding data are listed in
DFT calculations show good spatial separation of the frontier
Table 1. The dyes show strong absorption bands in the visible molecular orbitals. Isodensity plots of the HOMO and LUMO
region with a maximum centered at around 500 nm and attrib- energy levels of 1 are depicted in Figure 3 (see Supporting In-
uted to the HOMO!LUMO transition. Compared to the triphe- formation for further details). At the ground state, the electron
density is uniformly distributed
along the donating triarylamine
Table 1. Photophysical, electrochemical, and photovoltaic performance data.
unit while at the LUMO level ex-
cited electrons are transferred
towards the cyanoacrylic acid
unit, due to intramolecular
charge transfer along the p-con-
jugated skeleton. As a result of
this excellent directionality in
the charge transfer process, elec-
trons from the excited state are
expected to be injected effec-
tively into the conduction band
of TiO2. Concomitantly, the locali-
zation of the positive charge
onto the remote triarylamine
unit impairs the charge back-re-
action with tri-iodide while pro-
moting the kinetics of the dye-
regeneration process.
Absorption[a] Emission[a] Absorption[b] Potentials and energy levels
Photovoltaic data[e]
[c]
[d]
Dye lmax [nm]
lmax
lmax
Eox
E0ꢀ0
[V]
EoxꢀE0ꢀ0
Jsc
Voc
ff
h
(e [Mꢀ1 cmꢀ1]) [nm]
[nm]
[V]
[V]
[mAcmꢀ2
]
[mV] [%] [%]
1
2
3
517 (42700) 650
500 (36100) 631
512 (33400) 655
482
490
488
485
0.93
1.05
1.01
0.99
2.16
2.17
2.13
2.13
ꢀ1.23
ꢀ1.12
ꢀ1.12
ꢀ1.14
14.0
13.9
13.9
13.9
627 74.5 6.6
618 71.7 6.2
625 73.2 6.4
631 74.2 6.5[f]
C212 518 (30000) 650
[a] Absorption and emission spectra measured in CH2Cl2. [b] Absorption of the dyes grafted on TiO2 film after
immersion in CH2Cl2 solutions. [c] Oxidation potential measured in CH2Cl2 including 0.1m of Bu4NPF6 as salt
support at a scan rate of 100 mVsꢀ1 (working electrode: Pt disc; reference electrode: Ag/AgCl, calibrated with
ferrocenium/ferrocene as internal reference, potentials were referred to NHE reference by addition of
630 mV;[13] counter electrode: Pt). [d] E0ꢀ0 estimated from the intersection between absorption and emission
spectra. [e] Performances of DSCs measured with a 0.159 cm2 aperture area (see Experimental section for de-
tails). [f] Under different conditions C212 allowed conversion efficiency up to 7.5%, as reported by Wang
et al.[10]
nylamine derivative, the introduction of naphthyl units induces
a slight blue-shift of the absorption maximum and a noticeable
enhancement of the molar extinction coefficients, reaching up
to e=42700mꢀ1 cmꢀ1 for dye 1. Absorption spectra were also
recorded after the dye’s adsorption onto 3 mm thick TiO2 films
(Supporting Information Figure S1). The attachment of the
photosensitizers onto the surface of TiO2 led to slightly blue-
shifted absorption maxima compared to those in solution. This
phenomenon is partly due to the smaller electron-withdrawing
effect of the TiO2-carboxylate unit compared to the free car-
boxylic acid function, but this also indicates that these sensitiz-
ers may form H-aggregates on the surface. This stresses the
importance of using a co-adsorbent, for example cheno-deoxy-
cholic acid, along with the dye to hinder aggregation
phenomena.[8a–c,9b]
Cyclic voltammetry (CV) analyses were performed in CH2Cl2
to measure the first oxidation potential (Eox) relative to the en-
ergetic position of the HOMO level (Table 1). The introduction
of naphthyl units at the terminal positions (2 and 3) induces a
Figure 3. Molecular frontier orbitals of 1: HOMO (top) and LUMO (bottom).
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C. Olivier. T. Toupance et al.
The procedures for the DSC fabrication and photovoltaic
characterization are described in the Experimental section. Be-
cause of the higher solubility of these dyes in halogenated sol-
vents, TiO2 electrodes were stained in CH2Cl2 solutions contain-
ing cheno-deoxycholic acid as a co-adsorbent over 5 h. All
photovoltaic characteristics obtained for this new series of
dyes are gathered in Table 1. The best results were obtained
for dye 1, whose incident photon-to-electron conversion effi-
ciency (IPCE) and photocurrent-voltage (J–V) curves are shown
in Figure 4 (see Figure S4 for the other dyes).
the positive effect of having replaced the phenyl ring by a
naphthyl unit endowed to the p-conjugated linker.
Further insight on the effect of the naphthyl position on the
electron recombination and trap state distribution in TiO2 was
gained by combining charge extraction measurements with
transient photovoltage decay.[14] Figure 5a shows the corre-
spondence between the Voc and the distribution of traps in
Figure 4. a) IPCE, and b) current–voltage profiles of a DSC device using dye
1 under 100 mWcmꢀ2 illumination (plain line) and in the dark (dotted line).
Figure 5. a) Energetic level as a function of density of trap states (DOS).
b) Evolution of apparent electron lifetime as a function of density of trap
states (DOS).
For this series of new dyes, the overall power conversion ef-
ficiencies were relatively close to each other, with values lying
between 6.2 to 6.6%. Regardless of the dye, the IPCE curve ex-
hibits a large plateau over 85% from 470 nm to 580 nm with a
maximum of 90% around 495 nm. This is consistent with the
absorption maxima measured onto TiO2 photoanode in air. In-
terestingly, the naphtyl unit attached to the bi-thiophenyl p-
conjugated spacer (dye 1) allows increasing the red-response
at the expense of the IPCE maximum. Nonetheless, the latter,
which also exhibits the highest molar absorption coefficient,
achieved the best cell performance among the four, with
short-circuit photocurrent density Jsc =14.0 mAcmꢀ2, open-cir-
cuit voltage Voc =627 mV, and fill factor ff=74.5%, yielding an
overall power conversion efficiency h=6.6%. Notably, the dyes
containing naphthyl groups at terminal positions (dyes 2 and
3) showed lower Voc, ff, and therefore h values. The decrease of
these parameters especially occurs when the p-conjugated
bridge starts with a phenyl ring (2), but is attenuated when
the phenyl is replaced by a naphthyl unit (3). This highlights
TiO2 films. The incorporation of naphthyl units slightly modifies
the distribution of traps with a slight down-shift in energy. This
is particularly more pronounced in the case where the phenyl
has been replaced by one naphthyl in the conjugated bridge
(dye 1). Derived from the photovoltage rate decay, Figure 5b
shows the evolution of electron lifetime as a function of the
density of states. The results suggest that naphthyl units in ter-
minal position have no noticeable influence on the electron
lifetime. By contrast, when the naphthyl group is linked to the
bridge, a perceptible increase of electron lifetime is observed
(dye 1). Together with the deeper distribution of traps, these
data could explain to some extent the lower Voc values ob-
served for naphthyl-containing dyes compared to the triphe-
nylamine derivative.
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Triarylamine-Based Photosensitizers for Dye-Sensitized Solar Cells
The spectral output of the lamp was filtered using Schott K113
Tempax sunlight filter (Prꢁzisions Glas & Optik GmbH, Germany) to
reduce light mismatch between real solar illumination and the si-
mulated one to less than 2%. Light intensities were regulated with
wire mesh attenuators. J–V measurements were performed using a
Keithley model 2400 digital source meter (Keithley, USA) by inde-
pendently applying an external voltage to the cell and by measur-
ing the photo-generated current out from the cell. IPCE measure-
ments were realized using a 300 W xenon light source (ILC Tech-
nology, USA). A Gemini-180 double monochromator Jobin Yvon
Ltd. (UK) was used to select and increment wavelength irradiation
to the cell. A white light bias corresponding to about 10% sun was
superimposed to the monochromatic light. This was coupled to a
shutter with 1 Hz frequency. The photovoltaic performances of the
cells were measured by using a metal mask with an aperture area
of 0.159 cm2. Photovoltage transient measurements were carried
out using a 200 ms light pulse generated by a set of red-light emit-
ting diodes (LEDs). A similar white LED array enabled adjustment
of the Voc of the cell to the desired value. Typically, the light inten-
Conclusions
Three novel triarylamine-based organic photosensitizers, bear-
ing one, two, or three naphthyl units, are successfully synthe-
sized. The crystal structure of dye 1 is confirmed by X-ray dif-
fraction analysis while the photo-physical properties of the
series are determined by combining experimental and theoret-
ical data. Furthermore, a systematic study highlights the rela-
tionship between the position of the naphthyl group and the
opto-electronic properties and photovoltaic characteristics of
the new chromophores. This systematic survey enables to
stress the positive effect of the substitution of the phenyl ring
by a naphthyl unit in the p-conjugated bridge, as it allows to
simultaneously increase the molar extinction coefficient and
the electron lifetime. As a result, dye 1 achieves a PCE of 6.6%
under standard AM1.5 G conditions with a maximum IPCE of
90% at 495 nm. Based on this first ground state survey, a new
systematic study is underway in order to expand our under-
standing of this family of dyes and to further improve the char-
acteristics of the donor part of these photosensitizers.
sities of these LEDs are adjusted from 150 to 0.1 mWcmꢀ2
.
Acknowledgements
Experimental Section
This work was supported by the CNRS (PIE-Nanodisflex). F.S. and
M.G. are also indebted to the EU for their financial support on
FP7 “RobustDSC” project (agreement n8 212792). The authors
thank Dr. Brice Kauffmann for crystal structure resolution and Dr.
Dario Bassani for kind help with emission measurements.
Electrodes preparation and devices fabrication
FTO-coated conducting glass substrates (NSG10, 10 ohmsquareꢀ1
,
glass thickness 4 mm) were cleaned in ethanol followed by an ul-
trasonic treatment in an alkaline detergent solution. The remaining
carbon residues were removed by heat-treating the FTO in air at
5008C for 30 min. To increase photo-anode adhesion while reduc-
Keywords: dye-sensitized solar cells
· electron transfer ·
ꢀ
ing recombination between TCO and I3 at this interface, the con-
naphthylamines · photovoltaic conversion · sensitizers
ducting glass was treated by a 40 mm TiCl4 aqueous solution at
708C for 30 min. The photo-anode, constituted of two types of
layers, was prepared by screen-printing.
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The first layer of 8 mm was composed of acidic anatase TiO2 nano-
particles (ca. 20 nm). This transparent layer was sheltered by using
a 5 mm thick scattering layer made of larger TiO2 particles (ca. 400
nm; JGCC) to backscatter the unabsorbed photons towards the
transparent layer. Ethyl-cellulose and terpineol were removed by
gradual thermal treatment under air flow at 3258C (5 min), 3758C
(5 min), 4508C (15 min), and 5008C (15 min). The as-obtained films
were again treated with 40 mm TiCl4 aqueous solution at 708C for
30 min. Prior to sensitization, the film was again heated at 5008C
for 30 min. After cooling to ca. 608C, the electrodes were im-
mersed in 0.3 mm dye solutions (CH2Cl2) containing an optimized
2 mm concentration of cheno-deoxycholic acid playing both the
role of co-adsorbent and de-aggregating agent. The sensitization
time was also optimized to 5 h in dark. Counter-electrode was pre-
pared by coating FTO glass (TEC15, 15 ohmsquareꢀ1, thickness
2.3 mm) with a drop of H2PtCl6 solution (5 mm in ethanol). This
complex was thermally decomposed in air at 4208C for 15 min to
leave Pt nanoparticles. The photo-anode and counter-electrode
were assembled using a hot-melt Surlyn polymer gasket (25 mm,
Dupont). The electrolyte used was composed of 1.0m DMII, 0.03m
I2, 0.5m TBP, 0.1m GNCS, 0.05m LiI in 85:15 (v/v) acetonitrile/valero-
nitrile.
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A 450 W xenon light source (Oriel, USA) was used to provide an in-
cident irradiance of 100 mWcmꢀ2 at the surface of the solar cells.
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