A "cyanine-cyanine" salt exhibiting photovoltaic Properties

Article abstract of DOI:10.1021/ol901764t

Association between cationic and anionic heptamethine dyes led to the formation of a new organic salt (3) displaying exceptional lightharvesting properties In the NIR spectral range and having amphoteric redox character. Preliminary results of molecular b

Full text of DOI:10.1021/ol901764t

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4806-4809
A “Cyanine-Cyanine” Salt Exhibiting
Photovoltaic Properties
Pierre-Antoine Bouit,†,‡ Daniel Rauh,§ Stefan Neugebauer,| Juan Luis Delgado,‡,¶
Emmanuel Di Piazza,⊥ Ste´phane Rigaut,⊥ Olivier Maury,† Chantal Andraud,*,†
Vladimir Dyakonov,*,§,| and Nazario Martin*,‡,¶
UniVersity of Lyon, Laboratoire de Chimie, UMR 5182 CNRS - Ecole Normale
Supe´rieure de Lyon, 46 alle´e d’Italie, 69007 Lyon, France, IMDEA-Nanociencia,
Facultad de Ciencias, Módulo C-IX, 3a planta, Ciudad UniVersitaria de Cantoblanco,
28049 Madrid, BaVarian Centre for Applied Energy Research (ZAE Bayern), Am
Hubland, 97074 Würzburg, Germany, Experimental Physics VI, Julius-Maximilians
UniVersity of Würzburg, Am Hubland 97074 Würzburg, Germany, UMR 6226 CNRS
UniVersité de Rennes 1, Campus de Beaulieu, 35042 Rennes, France, Departamento de
Química Orgánica, Facultad de Ciencias Químicas, UniVersidad Complutense de
Madrid, 28040, Madrid, Spain
nazmar@quim.ucm.es; dyakonoV@physik.uni-wuerzburg.de; chantal.andraud@ens-lyon.fr
Received July 31, 2009
ABSTRACT
Association between cationic and anionic heptamethine dyes led to the formation of a new organic salt (3) displaying exceptional light-
harvesting properties in the NIR spectral range and having amphoteric redox character. Preliminary results of molecular bulk heterojunction
solar cells based on the title compound 3 and [60]PCBM as the only active layer reveal this new dye as a promising light-harvesting material
for photovoltaics.
Organic dyes displaying strong absorption/emission in the
near-infrared (NIR) have been the subject of great interest
over the past decade. On one hand, their fluorescence
properties in the “biological window” allowed design of
efficient bioprobes.1 On the other hand, their exceptional
absorption properties have been used in the field of materials
science (nonlinear absorbing materials,2 hybrid solar cells,3
(1) Ghorroghchian, P. P.; Frail, P. R.; Susumu, K.; Blessington, D.;
Brannan, A.; Bates, F. S.; Chance, B.; Hammer, D. A.; Therien, M. J. Proc.
Nat. Ac. Sci. U.S.A. 2005, 102, 2922–2927.
† University of Lyon.
‡ IMDEA-Nanociencia.
(2) (a) Beverina, L.; Fu, J.; Leclercq, A.; Zojer, E.; Pacher, P.; Barlow,
S.; Van Stryland, E. W.; Hagan, D. J.; Bre´das, J.-L.; Marder, S. R. J. Am.
Chem. Soc. 2005, 127, 7282–7283. (b) Bouit, P.-A.; Wetzel, G.; Berginc,
G.; Toupet, L.; Feneyrou, P.; Bretonnie`re, Y.; Maury, O.; Andraud, C. Chem.
Mater. 2007, 19, 5325–5335.
§ Bavarian Centre for Applied Energy Research.
| Julius-Maximilians University of Würzburg.
⊥ Université de Rennes 1.
¶ Universidad Complutense de Madrid.
10.1021/ol901764t CCC: $40.75
Published on Web 10/01/2009
 2009 American Chemical Society

etc.). More recently, organic photovoltaic has appeared as a
new field of application for these light-harvesting molecules.4
After 16 years of intensive studies, bulk heterojunction (BHJ)
organic solar cells based on poly(3-hexylthiophene) (P3HT)
and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have
reached around 5% of power conversion efficiency (PCE).5
However, the low absorbing properties of most of conjugated
polymers such as P3HT, especially in the NIR range where
almost 50% of the sunlight intensity is displayed, remains
the main drawback of these devices.5a To overcome this
trouble, a first strategy has been the use of low-band gap
polymers.6 However, another appealing strategy is the
replacement of the polymer by organic dyes. Hence, in
addition to their good absorption properties, organic dyes
are monodisperse and are easily and reproducibly synthe-
sizable. In this context, PCEs in the range of 1.5% have been
described in the last few months using squaraine dyes,7
push-pull chromophores,8 or Bodipy.9,10
carbon skeleton (for example, 1[Br],2b Figure 1). Some of
us previously reported an unusual anionic heptamethine
cyanine dye ([Na]2, Figure 1) in which the delocalized charge
is negative.12 This chromophore displayed the typical pho-
tophysical properties of heptamethine dyes.
At the same time, we emphasize that the lowering of the
band gap of the donor material well below 2 eV (as in P3HT)
alone is not sufficient in the case of single-junction solar
cells, as other parameters, such as LUMO level of the donor,
should comply accordingly to keep the open circuit voltage
maximum.11 Very low band gap materials with a gap of 1.2
eV could, however, find potential in the low-band-gap part
of organic multijunction cells. In this paper, we describe the
properties of a new organic salt composed of two NIR
absorbing cyanine dyes. In this salt, both the cationic and
the anionic moiety are chromophoric units. The outstanding
optical properties of this new dye are presented as well as
its electrochemical properties. As an example of its applica-
tions, preliminary report of photovoltaic performance dis-
playing PCE of about 0.4% are presented.
Figure 1. Synthesis and structure of the new dye 3.
By mixing, the easily synthesizable 1[Br] and Na[2] in
dichloromethane at room temperature, spontaneous ion
metathesis occurred between the two chromophores, as
evidenced by thin-layer chromatography (TLC). Further
washing with water and chromatography on silica gel allowed
to remove the inorganic salt and to afford quantitatively the
new organic salt 3, composed by the two chromophores 1+
and 2- (Figure 1). NMR analyses clearly showed the presence
of both dyes in the complex 3 and elemental analysis
confirmed the removal of the inorganic counterions. Interest-
ingly, 3 presents good solubility in organic solvents, while
Na[2] was only soluble in highly polar solvents. Zwitterionic
cyanines composed by cationic dye with tethered sulfonate
have already been described,13 but to our knowledge, this is
the first example of such a compound in which both ions
are cyanines.
Optical properties of 3 were studied by means of UV-vis
spectroscopy. The absorption spectrum of 3 in dichlo-
romethane consists of two absorption bands in the NIR with
high extinction coefficients [λ1 ) 795 nm (ε ) 420000
L·mol-1·cm-1), λ2 ) 902 nm (ε ) 300000 L·mol-1·cm-1)]
(Figure 2). Moreover, the compound displays an extinction
coefficient superior to 50000 L·mol-1·cm-1 between 700 and
940 nm. The two transitions are assigned to the two
chromophoric units of 3 (1+ and 2-), and the obtained
spectrum appears as a superposition of the spectra of 1[Br]
and [NBu4]2 measured under the same experimental condi-
tions (Figure 2).2b,12 From these observations, we may
conclude that there is no electronic ground-state interaction
between the two chromophores. Nevertheless, it is important
to point out that they both conserve their particular optical
properties. The absorption spectra of solid films of 3, 2, and
1 made from chloroform solution (Figure S2, Supporting
Information) confirm that the spectrum of 3 is a superposition
of those of 2 and 1. It is evident that, in the solid state,
absorption up to 1050 nm with the chromophore 3 is possible,
Cyanine dyes are generally cationic organic colorant in
which a positive charge is delocalized on an odd-number
(3) Yum, J.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.; Nu¨esch, F.;
De Angelis, F.; Gra¨tzel, M.; Nazeeruddin, M. K. J. Am. Chem. Soc. 2007,
129, 10320–10321.
(4) (a) Castro, F. A.; Faes, A.; Geiger, T.; Graeff, C. F. O.; Nu¨esch, F.;
Hany, R. Synth. Met. 2006, 156, 973–978. (b) Fan, B.; Hany, R.; Moser,
J. E.; Nu¨esch, F. Org. Elec. 2008, 9, 85–94.
(5) (a) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58–77. (b) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV.
Funct. Mater. 2005, 15, 1617–1622. (c) Li, G.; Shrotriya, V.; Huang, J.;
Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864–865.
For other fullerenes derivatives, see: (d) Riedel, I.; Von Hauff, E.; Parigi,
J.; Martin, N.; Giacalone, F.; Dyakonov, V. AdV. Func. Mater. 2005, 15,
1979–1987. (e) Backer, S. A.; Sivula, K.; Kavulak, D. F.; Fre´chet, J. M. J
Chem. Mater. 2007, 19 (12), 2927–2929.
(6) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L.
J. Am. Chem. Soc. 2009, 131, 56–57.
(7) Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.;
Marks, T. J. J. Am. Chem. Soc. 2009, 130, 17640–17641.
(8) (a) Roquet, S.; Cravino, A.; Leriche, P.; Ale´veˆque, O.; Fre`re, P.;
Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459–3466. (b) Kroneneberg,
N. M.; Deppish, M.; Wu¨rthner, F.; Ledemann, H. V. A.; Deing, K.;
Meerholz, K. Chem. Commun. 2008, 6489–6491.
(9) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.; Roncali,
J. Chem. Commun. 2009, 1673–1675.
(10) Roncali, J. Acc. Chem. Res. 2009 ( DOI: 10.1021/ar900041b).
(11) Scharber, M. C.; Mu¨hlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
C.; Heeger, A. J.; Brabec, Ch. J. AdV. Mater. 2006, 18, 789–794.
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Guennic, B.; Andraud, C.; Maury, O. Org. Lett. 2008, 10 (19), 4159–4162.
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Org. Lett., Vol. 11, No. 21, 2009
4807

To justify our assumption on the applicability of the
chromophores 1-3 for photovoltaic energy conversion, we
fabricated solar cells in a configuration glass/ITO/PEDOT:
PSS/chromophore:[60]PCBM/Ca/Al. The details on device
fabrication are available in the Supporting Information. The
external quantum efficiency (EQE) and the current-voltage
(J-V) characteristics were measured for chromophores 3 and
2 based devices (no operating devices with 1 could be made
so far). The EQE determines the number of generated charge
carriers extracted into the outer circuit relative to the number
of incident photons. Additionally, the short circuit current
density of the solar cell can be calculated for any illumination
spectrum at given intensity, in our case, for the AM1.5G
solar spectrum at 100 mW/cm2. The J-V measurements, and
hence the PCE determination, were performed with a solar
simulator. Generally, the spectrum of the latter differs from
the AM 1.5G; therefore, a so-called mismatch factor should
be calculated by using a reference cell with the known
spectral response.14 In a next step, the light intensity of the
solar simulator should be adjusted to yield the expected short
circuit current of the reference cell, i.e., the short-circuit
current of the reference cell under AM 1.5G divided by the
mismatch factor. By using this intensity, the organic solar
cell can finally be measured. This procedure is a common
way for an accurate PCE determination. This is an indirect
method to accurately determine the main solar cell param-
eters, such as the short circuit current density (JSC), open-
circuit voltage (VOC), and fill factor (FF) of the solar cell. In
Figure 4, the EQE spectra of BHJ solar cells with 3 and 2,
Figure 2. Absorption spectra in dichloromethane of 1[Br] (red),
[NBu4]2 (green), and 3 (blue) at room temperature.
thus making it an excellent candidate for NIR absorption.
Cyclic voltammetry (CV) was used to investigate the redox
properties of complex 3, as shown in Figure 3 (CH2Cl2, 0.2
Figure 3. CV trace obtained for 3 (CH2Cl2, 0.2 M Bu4NPF6, V )
100 mV·s-1). Inset shows the reversibility of the first two oxidation
processes.
M Bu4NPF6, V ) 100 mV·s-1). This compound undergoes
two reversible monoelectronic oxidations at E°ox1 ) 0.39 V
vs SCE (∆Ep ) 60 mV, Ipc/Ipa ≈ 1) and E°ox2 ) 0.70 V vs
SCE (∆Ep ) 60 mV, Ipc/Ipa ≈ 1), followed by a third
irreversible oxidation at Epa ) 1.19 V. Comparison of the
CV traces with those of 1[Br] and Na[2] (Figures S2 and
S3, Supporting Information) suggests that the first and second
oxidation processes correspond, respectively, to the oxidation
of 2- (E°ox ) 0.39 V, ∆Ep ) 60 mV, Ipc/Ipa ≈ 1) and 1+
(E°ox ) 0.69 V, Ep ) 90 mV, Ipc/Ipa ≈ 0.5), while the
irreversible oxidation has been assigned to the second
oxidation of 2- (E°ox ) 1.22 V).12 On the other hand, the
Figure 4. External quantum efficiency of BHJ solar cells fabricated
with chromophore 3 (blue, red) and 2 (green, yellow) blended with
[60]PCBM in a 2:1 weight ratio. For comparison, the EQE spectra
measured with and without white light bias are shown.
broad signal observed at negative potential for 3 (Epc
)
-0.69 V and Epa ) -0.58 V) is ascribed to the overlapping
of their partially reversible reduction processes (E°red
-0.62 V, ∆Ep ) 70 mV, Ipc/Ipa ≈ 0.65 for 1+ and E°red
)
)
as electron donor, and [60]PCBM, as acceptor, are shown.
The best EQE values of 11% were obtained at a wavelength
of 785 nm for the chromophore 3. At a closer look at the
-0.69 V ∆Ep ) 90 mV, Ipc/Ipa ≈ 0.85 for 2-). From these
experiments, it appears that each chromophoric unit con-
serves its own redox properties in the complex 3 but with a
notable increase in the reversibility of the second oxidation
corresponding to oxidation of the 1+ moiety.
(14) Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.
AdV. Funct. Mater. 2006, 16, 2016–2023.
4808
Org. Lett., Vol. 11, No. 21, 2009

EQE, one can see that there are two regions, where light is
absorbed and charge carriers are generated: the first one
around 400 nm (related to the absorption of PCBM, see
Figure S6, Supporting Information) and the second one
between 700 to 1000 nm (related to the NIR absorption of
the dye).
3.78 mA/cm2 was achieved with 3:[60]PCBM blends. For
the 3:[60]PCBM blend system we found the optimal blend
ratio of 1:2. For this device, the PCE was found to be 0.4%,
which is, to our knowledge, one of the best results obtained
for a cyanine-based photovoltaic device.4
In summary, a readily available new dye has been
prepared, by association of two NIR absorbing cyanine dyes,
which exhibits a good solubility in organic solvents and
outstanding light-harvesting properties in the NIR region with
giant extinction coefficients. The cyclic voltammetry mea-
surements of 3 reveal an amphoteric redox character,
preserving the typical features of their components (1 and
2). Furthermore, in preliminary experiments, we were able
to demonstrate photovoltaic activity for chromophores 3 and
2 on BHJ solar cells using [60]PCBM as acceptor. The sun
to power conversion efficiency (0.4%) obtained for
3:[60]PCBM reveals that the new dye is an appealing system
for further studies as NIR light harvester for photovoltaics,
for example, in conjunctions with other materials or in
tandem solar cells.
Acknowledgment. We thank the De´le´gation Generale des
Arme´e (DGA) and IMDEA-Nanociencia for grants to P.-
A.B. D.R.’s work is funded by the DEPHOTEX project
within the seventh framework programme of the EU. V.D.’s
work at the ZAE Bayern is financed by the Bavarian Ministry
of Economic Affairs, Infrastructure, Transport and Technology.
This work has been supported by the European Science
Foundation (05-SONS-FP-021 SOHYD), the MEC of Spain
(CT2008-00795/BQU, and Consolider-Ingenio 2010C-07-
25200, Nanociencia Molecular), and Comunidad de Madrid (P-
PPQ-000225-0505). J.L.D. thanks the MICINN for a Ramo´n
y Cajal Fellowship, cofinanced by the EU Social Funds.
Figure 5. Current-voltage characteristics of BHJ solar cells
fabricated with chromophores 3 (blue) and 2 (green) blended with
[60]PCBM in a 2:1 weight ratio under illumination. PEC (3) )
0.37%; PCE (2) ) 0.0 7%.
By applying the optical bias we are able to assess the
role of traps as limiting factors. It can also be seen that
steady-state white light bias affects the photocurrent
measurement of a solar cell. It should be mentioned that
this white light does not contribute to the photocurrent
directly, as the lock-in technique was used. Therefore, we
concluded that the effect of the light bias is directly related to
the trap filling and therefore hindering the transport of charge
carriers.
Supporting Information Available: Synthetic procedure,
complete characterisations, and device preparation. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The corresponding J-V curves are shown in Figure 5.
As can be seen, the highest short circuit current density of
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Org. Lett., Vol. 11, No. 21, 2009
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