Triphenylamine-thienylenevinylene hybrid systems with internal charge transfer as donor materials for heterojunction solar cells

Article abstract of DOI:10.1021/ja058178e

Star-shaped molecules based on a triphenylamine core derivatized with various combinations of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups have been synthesized. UV-vis absorption and fluorescence emis

Full text of DOI:10.1021/ja058178e

Published on Web 02/18/2006
Triphenylamine-Thienylenevinylene Hybrid Systems with
Internal Charge Transfer as Donor Materials for
Heterojunction Solar Cells
Sophie Roquet, Antonio Cravino, Philippe Leriche, Olivier Al e´ v eˆ que,
Pierre Fr e` re, and Jean Roncali*
Contribution from the Groupe Syst e` mes Conjugu e´ s Lin e´ aires, CIMMA, UMR CNRS 6200,
UniVersit e´ d’Angers, 2 Bd LaVoisier F-49045 Angers, France
Received December 1, 2005; E-mail: jean.roncali@univ-angers.fr
Abstract: Star-shaped molecules based on a triphenylamine core derivatized with various combinations
of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups
have been synthesized. UV-vis absorption and fluorescence emission data show that the introduction of
the electron-acceptor groups induces an intramolecular charge transfer that results in a shift of the absorption
onset toward longer wavelengths and a quenching of photoluminescence. Cyclic voltammetry shows that
all compounds present a reversible first oxidation process whose potential increases with the number of
electron-withdrawing groups in the structure. Prototype bulk and bilayer heterojunction solar cells have
been realized using fullerene C60 derivatives as acceptor material. The results obtained with both kinds of
devices show that the introduction of electron-acceptor groups in the donor structure induces an extension
of the photoresponse in the visible spectral region, an increase of the maximum external quantum efficiency,
and an increase of the open-circuit voltage under white light illumination. These synergistic effects allow
reaching power conversion efficiencies of ∼1.20% under simulated AM 1.5 solar irradiation at 100 mW
-
2
cm
.
5
Introduction
and fullerene derivatives has generated considerable interest
for bulk heterojunction solar cells based on interpenetrating
networks of conjugated systems and C60 derivatives.6 During
the past decade, intensive research effort invested in the
understanding and technological optimization of these devices7
has generated a continuous improvement of performances which
Organic semiconductors based on π-conjugated systems are
intensively investigated as active materials in organic field-effect
transistors (OFETs) and light-emitting diodes.1 Combined with
substrates such as polymer films or paper, these materials allow
envisioning the development of lightweight flexible devices such
as intelligent paper, displays, tags, or RFID by rather simple
and low-energy-demanding technologies. In this context, organic
solar cells investigated for fundamental reasons or with the long-
term goal of developing low-cost renewable energy sources
could also find a niche as power supplies in the emerging area
of flexible (opto)electronics.
,7
,2
-9
9
now reach power conversion efficiencies of 4-5%. Whereas
a small number of donor and acceptor materials such as
4
,10
phthalocyanines,
poly(p-phenylenevinylenes), poly(thio-
phenes), perylene,4,10 or fullerene derivatives have led to
the fabrication of efficient devices, over the longer term the
development of a chemistry focused on the design of materials
specifically tailored for photovoltaic conversion can be expected
to significantly contribute to the progress of the field. Recent
work in this area has been focused on the synthesis of new donor
6
-9
6-9
While a donor-acceptor heterojunction with a power conver-
3
sion efficiency of 0.95% was reported in 1986 by Tang, it is
only recently that the performances of this type of solar cell
11
or acceptor compounds, molecular or supramolecular archi-
have been significantly improved to reach efficiencies in the
4
3
-4% range. The discovery of ultrafast and ultraefficient
(
5) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258,
1474.
photoinduced electron transfer between π-conjugated polymers
(6) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995,
270, 1789.
(
1) (a) Dimitrakopolous, C. D.; Malenfant, P. AdV. Mater. 2002, 14, 99. (b)
Rogers, J. A.; Bao, Z.; Dodabalapur, A.; Crone, B.; Raju, V. R.; Katz, H.
E.; Kuck, V.; Ammundson, K. J.; Drzaic, P. Proc. Natl. Acad. Eng. 2001,
(7) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001,
11, 15.
(8) (a) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromhertz,
T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (b) van Duren, J. K.
J.; Yang, X.; Loos, J.; Bulle-Lieuwma, C. W. T.; Sieval, A. B.; Hummelen,
J. C.; Janssen, R. A. J. AdV. Funct. Mater. 2004, 14, 425. (c) Schillinsky,
P.; Waldauf, C.; Haush, J.; Brabec, C. J. J. Appl. Phys. 2004, 95,
2816.
9
8, 4817. (c) Garnier, F. Acc. Chem. Res. 1999, 32, 209. (d) Horowitz, G.
AdV. Mater. 1998, 10, 3. (e) Katz, H. E. J. Mater. Chem. 1997, 7, 369.
(2) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed.
1
998, 37, 402. (b) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10,
471.
1
(
3) Tang, C. W. Appl. Phys. Lett. 1986, 48, 2.
(9) (a) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang,
Y. Nat. Mater. 2005, 4, 864. (b) Ma, W.; Yang, C.; Gong, X.; Lee, K.;
Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617.
(
4) (a) Peumans, P.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 126. (b) Peumans,
P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (c) Xue, J.;
Uchida, S.; Rand, B. P.; Forrest, S. R. Appl. Phys. Lett. 2004, 84, 3013.
(10) W o¨ hrle, D.; Meissner, D. AdV. Mater. 1991, 3, 129.
10.1021/ja058178e CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 3459-3466
9
3459

A R T I C L E S
Roquet et al.
tectures combining donor and acceptor blocks,12 or conjugated
polymers with reduced band gaps.13
isotropic optical and charge-transport properties. Star-shaped
molecules consisting of a TPA core substituted by oligophe-
nylenes, 2-phenylthiophene, or fluorene have been described,1
and the first examples of OFETs based on materials containing
8-20
Organic semiconductors based on low-dimensional π-con-
jugated systems such as oligothiophenes are known to present
1
20
highly anisotropic electronic properties. This point has been
the TPA core have been recently reported. However, the use
clearly demonstrated on OFETs based on sublimed thin films
of oligothiophenes for which the highest hole mobility is
observed when the molecules are oriented perpendicularly to
the substrate.1 However, such an orientation is detrimental
for solar cells because it strongly reduces the absorption cross
section for the incident light as well as the efficiency of charge
transport through the cell thickness.1 A first solution to this
problem consists of using as active materials two-dimensional
conjugated systems prone to adopt an horizontal orientation on
of TPA-based materials for photovoltaic conversion has been
scarcely considered. TPA-based starburst molecules containing
nitro or dimesitylboryl acceptor groups have been synthesized.18
When used as donors in bilayer heterojunctions, these com-
pounds led to conversion efficiencies of 0.4 and 0.1% under
monochromatic irradiation at 440 nm where the molecules
,14
5,16
18
mainly absorb.
We now report the synthesis of hybrid donor compounds
consisting of various combinations of a TPA core derivatized
with dithienylethylene π-conjugated chains and electron-accep-
tor groups (Chart 1). These systems have been designed in order
to (i) take advantage of the hole-transport properties of TPA
3,4,10
the surface, for example phthalocyanines,
or more recently
1
1a
hexabenzocoronenes,
or planarized star-shaped oligothio-
phenes.1
1b,16
1
8-20
21
Another possible solution would consist in the development
of organic semiconductors in which the anisotropy of electronic
properties (and hence the need to control molecular orientation)
would be reduced by an increase of the dimensionality of the
elemental unit. As a first step in this direction, we have recently
shown that three-dimensional (3D) conjugated architectures in
which four oligothiophene chains are attached to a silicon node
lead to promising results as donors in bulk heterojunction solar
derivatives
and oligothienylenevinylenes, (ii) extend the
absorption spectrum of the donor toward longer wavelengths
1
3
by an intramolecular charge transfer, and (iii) confer a high
oxidation potential on the donor and thus preserve a high open-
2
2
circuit voltage for the resulting solar cells.
The electronic properties of the new compounds have been
analyzed by UV-vis absorption spectroscopy, fluorescence
emission spectroscopy, and cyclic voltammetry. The potentiali-
ties of these materials for photovoltaic conversion have been
evaluated on prototype bulk and bilayer heterojunction solar
cells using fullerene C60 derivatives as the acceptor, and the
results are discussed in relation to the chemical structure of the
donor.
1
7
cells.
Organic glasses derived from triphenylamine (TPA) deriva-
1
8
tives have been widely investigated for almost two decades.
Considerable effort in synthetic chemistry, in particular by
Shirota and co-workers, has led to the development of many
classes of TPA-based compounds as hole-transporting or
electroluminescent materials.18 Owing to the noncoplanarity of
the three phenyl substituents, TPA derivatives can be view as
Results and Discussion
3
D systems. The combination of TPA with linear π-conjugated
Synthesis. The synthesis of the target molecules is depicted
in Scheme 1. All final compounds derive from the key
trialdehyde 8 obtained in two steps from commercially available
tribromotriphenylamine 12. A Stille coupling between tribu-
tyltinthiophene 11 and compound 12 gave tris[4-(2-thienyl)-
systems could be expected to lead to amorphous materials with
(
11) (a) Schmidt-Mende, L.; Fechtenk o¨ tter, A.; M u¨ llen, K.; Moons, E.; Friend,
R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Nicolas, Y.;
Blanchard, P.; Levillain, E.; Roncali, J. Org. Lett. 2004, 6, 273. (c) Mason,
C. R.; Skabara, P. J.; Cupertino, D.; Schofield, J.; Meghdadi, F.; Ebner,
B.; Sariciftci, N. S. J. Mater. Chem. 2005, 15, 1446. (d) Jørgensen, M.;
Krebs, F. C. J. Org. Chem. 2004, 69, 6688. (e) Zheng, L.; Zhou, Q.; Deng,
X.; Yuan, M.; Yu, G.; Cao, Y. J. Phys. Chem. B 2004, 108, 11921.
12) (a) Segura, J. L.; Martin, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31.
19b,23
phenyl]amine
9 in 85% yield. Vilsmeier-Hack formylation
of compound 9 led to tris[4-(5-formyl-2-thienyl)phenyl]amine
8 in 90% yield.
(
(
b) Roncali, J. Chem. Soc. ReV. 2005, 34, 483. (c) Cravino, A.; Sariciftci,
The use of different experimental conditions for the Wittig-
Horner olefination of trialdehyde 8 with phosphonate 1024 led
to compounds 7, 6, and 1 corresponding to the mono-, bis-,
and tris-olefination of trialdehyde 8. Thus, the use of 5 equiv
of phosphonate 10 gave after 1 h reaction compound 1 in 70%
yield. The use of 2.5 equiv of 10 and 12 h reaction led to
compounds 1 and 7 in 50 and 20% yield, respectively. Finally,
N. S. J. Mater. Chem. 2002, 12, 1931. (d) Cremer, J.; Mena-Osteritz, E.;
Pshierer, M. G.; M u¨ llen, K.; B a¨ uerle, P. Org. Biomol. Chem. 2005, 3, 985.
(
e) Cremer, J.; B a¨ uerle, P. Eur. J. Org. Chem. 2005, 3715. (f) Neuteboom,
E. E.; Meskers, S. C. J.; van Hal, P.; van Duren, J. K. J.; Meijer, E. W.;
Janssen, R. A. J.; Dupin, H.; Pourtois, G.; Cornil, J.; Lazzaroni, R.; Br e´ das,
J.-L.; Beljonne, D. J. Am. Chem. Soc. 2003, 125, 8625. (g) Huang, C.-H.;
MacClenaghan, N. D.; Kuhn, A.; Hofstraat, J.; Bassani, D. M. Org. Lett.
2
005, 7, 3409. (h) Apperloo, J. J.; Martineau, C.; van Hal, P.; Roncali, J.;
Janssen, R. A. J. J. Phys. Chem. A 2002, 106, 21. (i) van Hal, P.; Beckers,
E. H. A.; Meskers, S. C. J.; Janssen, R. A. J.; Jousselme, B.; Blanchard,
P.; Roncali, J. Chem.sEur. J. 2002, 8, 5415.
2
equiv of phosphonate 10 and 15 h reaction led to a mixture
(
13) (a) Roncali, J. Chem. ReV. 1997, 97, 173. (b) Winder, C.; Sariciftci, N. S.
J. Mater. Chem. 2004, 14, 1077. (c) Vangeneugden, D. L.; Vanderzande,
D. J. M.; Salbeck, J.; van Hal, P. A.; Janssen, R. A. J.; Hummelen, J. C.;
Brabec, C. J.; Shaheen, S. E.; Sariciftci, N. S. J. Phys. Chem. B 2001, 105,
(19) (a) Nishimura, K.; Kobata, T.; Inada, H.; Shirota, Y. J. Mater. Chem. 1991,
1, 897. (b) Ohishi, H.; Tanaka, M.; Kegeyama, H.; Shirota, Y. Chem. Lett.
2004, 33, 1266.
(20) (a) Saragi, T. P. I.; Fuhrmann-Lieker, T.; Salbeck, J. Synth. Met. 2005,
148, 267. (b) Sonntag, M.; Kreger, K.; Hanft, D.; Strohriegl, P.; Setayesh,
S.; de Leeuw, D. Chem. Mater. 2005, 17, 3031.
(21) (a) Roncali, J. Acc. Chem. Res. 2000, 33, 147. (b) Videlot, C.; Ackermann,
J.; Blanchard, P.; Raimundo, J.-M.; Fr e` re, P.; Allain, M.; de Bettignies,
R.; Levillain, E.; Roncali, J. AdV. Mater. 2003, 15, 306.
(22) (a) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromhertz,
T.; Rispens, M. T.; Sanchess, L.; Hummelen, J. C. AdV. Funct. Mater.
2001, 11, 374. (b) Gadisa, A.; Svensson, M.; Andersson, M. R.; Ingan a¨ s,
O. Appl. Phys. Lett. 2004, 84, 1609.
1
1106. (d) Brabec, C. J.; Winder, C.; Sariciftci, N. S. Hummelen, J. C.;
Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. AdV. Funct. Mater. 2002,
1
2, 709. (e) Zhang, F.; Perzon, E.; Wang, X.; Mamo, W.; Andersson, M.
R.; Ingan a¨ s, O. AdV. Funct. Mater. 2005, 15, 745.
(
(
(
(
(
14) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet,
B.; Ries, S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716.
15) Videlot, C.; El Kassmi, A.; Fichou, D. Sol. Energy Mater. Sol. Cells 2000,
6
3, 69.
16) de Bettignies, R.; Nicolas, Y.; Blanchard, P.; Levillain, E.; Nunzi, J.-M.;
Roncali, J. AdV. Mater. 2003, 15, 1939.
17) Roncali, J.; Fr e` re, P.; Blanchard, P.; de Bettignies, R.; Turbiez, M.; Roquet,
S.; Leriche, P.; Nicolas, Y. Thin Solid Films, in press.
(23) Yamamoto, K.; Higushi, M.; Ushida, K.; Kojima, Y. Macromolecules 2002,
35, 5782.
(24) Seeboth, H.; Andeae, S. Z. Chem. 1977, 16, 399.
18) (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1. (b) Shirota, Y. J. Mater. Chem.
2
005, 15, 75.
3460 J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006

Triphenylamine−Thienylenevinylene Hybrid Systems
A R T I C L E S
Chart 1
containing 37% compound 6, 19% compound 7, and 13%
compound 1. These various compounds were easily separated
by column chromatography thanks to the remaining aldehyde
groups on compounds 6 and 7. Finally, compounds 2 (43%), 3
the two building blocks considerably enhances the photolumi-
nescence efficiency as already observed for combinations of
26
TPA and oligothiophenes. All compounds containing electron-
acceptor groups present an emission maximum red shifted by
ca. 100 nm and a strong quenching of luminescence in
agreement with the occurrence of an ICT process, as reported
for bipolar compounds combining TPA and the benzothiadiazole
(19%), 4 (48%), and 5 (76%) were obtained by Knoevenagel
condensation of indanedione or malononitrile with the appropri-
ate aldehyde.
2
7
Optical Properties. Table 1 lists the UV-vis absorption and
fluorescence emission data for compounds 1-5. The spectrum
of the reference compound 1 shows a single absorption band
corresponding to a π-π* transition with λmax at 424 nm. As
shown in Figure 1, introduction of electron-withdrawing groups
in the structure produces the emergence of a new transition
around 500 nm assigned to an intramolecular charge transfer
acceptor group.
For compounds 3 and 4 the emission maximum was ob-
served only in apolar toluene, whereas in dichloromethane only
a weak emission originating from the π-π* transition was
observed (see Supporting Information). It is noteworthy that
the quenching of luminescence is of much less magnitude in
the case of compound 5 with three dicyanovinyl groups
(ICT) between the TPA-thienylenevinylene donor part of the
(φem ) 0.15). Although we have no definitive explanation for
molecule and the acceptor end groups. The spectrum of
compound 2 is quite similar to that of compound 3 (see
Supporting Information). Comparison of the spectra of com-
pounds 3, 4, and 5 shows that the progressive replacement of
the thienylenevinylene groups by the dicyanovinyl ones induces
a concomitant decrease of the intensity of the π-π* transition
and an increase of the intensity of the ICT band. As shown by
the data in Table 1, these changes are accompanied by an
hypsochromic shift of both the π-π* and ICT bands while the
energy difference between the maximum of the two transitions
this result, the C3 symmetry of the molecule probably plays a
major role.
The band gap (Eg) of compounds 1, 3, 4, and 5 has been
determined from the optical spectrum of thin films prepared on
glass by thermal evaporation under high vacuum (taking the
intercept of the tangent to the low-energy side of the optical
spectrum with the x-axis; see Supporting Information). As shown
in Table 1, increasing the number of acceptor groups in the
structure produces at the same time a blue shift of the λmax of
the ICT band and a decrease of Eg. This unexpected result
indicates that the strength of intermolecular interactions in the
(∆E) increases.
The fluorescence emission properties of compounds 1-5 have
been analyzed in methylene chloride using anthracene as a
standard. Compound 1 presents a fluorescence efficiency φem
close to 0.40 (Table 2). This value, much higher than that of
the two individual constitutive parts (dithienylethylene φem ∼
(
25) Aperloo, J. J.; Martineau, C.; van Hal, P.; Roncali, J.; Janssen, R. A. J. J.
Phys. Chem. A 2002, 106, 21.
(26) Noda, T.; Nogawa, N.; Noma, N.; Shirota, Y. J. Mater. Chem. 1999, 9,
2177.
(27) Thomas, J. T.; Velusamy, M.; Tao, Y.-T.; Chuen, C.-H. AdV. Funct. Mater.
2
5
26
0
.06) and TPA (φem ) 0.06), shows that the combination of
2004, 14, 83.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006 3461

A R T I C L E S
Scheme 1
Roquet et al.
a
Table 1. UV-Vis Absorption and Fluorescence Emission Data for Compounds 1-5 in CH2Cl2
compd
λ
max
(π−π*) (nm)
λ
max(ICT) (nm)
∆
Ec (eV)
E
g
(eV)
λem (nm)
φ
em
1
2
3
4
5
424 (435)b
419
415 (426)
390 (404)
368 (382)
2.38
506
610c
620c
632
636
0.39
0.01
0.01
0.01
0.15
536
0.65
0.59
0.76
0.93
516 (544)
513 (540)
509 (538)
1.91
1.84
1.78
a
Using anthracene as a standard. b Values in parentheses have been measured on films. c ∆E corresponds to the energy difference between the maxima
of the π-π* and ICT bands in solution.
solid state increases with the number of acceptor groups in the
molecule. This phenomenon could reflect the development of
intermolecular donor-acceptor interactions between the electron-
rich inner part of one molecule and the electron-deficient
peripheral part of neighbor molecules. Further work is needed
to test this hypothesis.
(CV) in methylene chloride in the presence of Bu4NPF6 as
supporting electrolyte. The CV of the reference compound 1
shows a reversible oxidation process associated with the
generation of the cation radical at a redox potential E° of 0.62
V (Figure 2). As shown by the data in Table 2, replacement of
a thienylenevinylene group by an electron-withdrawing group
in compounds 2 and 3 produces a positive shift of E° of 100
and 90 mV, respectively. Further increase of the number of
Cyclic Voltammetry. The electrochemical properties of
compounds 1-5 have been analyzed by cyclic voltammetry
3462 J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006

Triphenylamine−Thienylenevinylene Hybrid Systems
A R T I C L E S
Figure 1. UV-vis absorption spectra recorded in CH2Cl2. Top: left, 1; right, 3. Bottom: left, 4; right, 5.
Table 2. Cyclic Voltammetric Data for Compounds 1-5 in 0.10 M
-
1
Bu4NPF6/CH2Cl2, Scan Rate 100 mV s , and Reference
Electrode Ag|AgCl
compd
E
°
(V)
Epc (V)
1
2
3
4
5
0.62
0.72
0.71
0.87
1.06
n.d.
n.d.
-1.37
-1.25
-1.23
acceptor groups in the structure leads to a larger shift of E°
with an overall increase of 0.44 V between compounds 1 and
5
(Figure 2 and Table 2).
The CV of compounds 3-5 exhibits an irreversible cathodic
wave at a peak potential (Epc) which shifts positively from -1.37
to -1.23 V when the number of dicyanovinyl groups is
increased from 1 to 3. These Epc values are close to those
reported by Sun et al. for compounds combining TPA and
dicyano acceptor groups.28
Photovoltaic Properties. The potentialities of donors 1-5
for photovoltaic conversion have been evaluated on two series
of prototype heterojunction solar cells. Compounds 1 and 2 have
been used for the realization of bulk heterojunctions by spin-
casting composite films of the donor and [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM)29 as acceptor in a 1:3 ratio.
On the other hand, due to the lower solubility of compounds
containing dicyanovinyl groups and to problems related to the
compatibility with PCBM (formation of aggregates in solution),
the photovoltaic properties of compounds 3, 4, and 5 have been
analyzed on bilayer heterojunctions realized by successive
thermal evaporation of layers of the donor and fullerene C60
under high vacuum. The eventual crystallinity of compounds
Figure 2. Cyclic voltammograms recorded in 0.10 M Bu4NPF6/CH2Cl2;
scan rate 100 mV s . Top, 1; bottom, 5.
-1
1
-5 has been investigated by powder X-ray diffraction studies
on thin film evaporated on glass under high vacuum. For all
compounds, the absence of reflection peaks in the spectra
confirmed the expected amorphous character of the materials.
The photocurrent action spectra under monochromatic ir-
radiation of the cells based on donors 1 and 2 are shown in
Figure 3. For donor 1, the spectrum exhibits a broad plateau in
the 350-450 nm range with two maxima corresponding to an
external quantum efficiency (EQE) of ca. 20%. Comparison with
the optical spectrum of Figure 1 shows that two EQE maxima
coincide with the two absorption maxima of the donor. However,
(28) Sun, X.; Liu, Y.; Xu, X.; Yang, C.; Yu, G.; Chen, S.; Zhao, Z.; Qiu, W.;
Li, Y.; Zhu, D. J. Phys. Chem. B 2005, 109, 10786.
(29) Hummelen, J. C.; Knight, B. W.; Lepec, F.; Wudl, F.; Yao, J.; Wilkins, C.
L. J. Org. Chem. 1995, 60, 532.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006 3463

A R T I C L E S
Roquet et al.
Figure 3. Photocurrent action spectra of bulk heterojunctions of donors 1
(top) and 2 (bottom) and PCBM (ratio 1:3) under monochromatic irradiation.
the first band probably contains also a significant contribution
from PCBM. The action spectrum of the cell based on donor 2
exhibits two distinct bands with maxima at 410 and 550 nm.
Again, examination of Figure 1 shows that these two bands
correspond to the π-π* and ICT bands in the UV-vis
spectrum. Comparison of the action spectra of the two cells
shows that the introduction of the acceptor group in the donor
structure produces an enhancement of EQE around 400 nm with
a maximum of ∼33%. Interestingly, irradiation in the ICT band
generates also a large photocurrent with an EQE of ca. 25%
around 600 nm. This result shows that the ICT in compound 2
extends the photoresponse of the cell in the long wavelength
region with ca. 100 nm red shift of the photocurrent onset up
to ∼780 nm.
Figure 4. Current-density vs voltage curves of bulk heterojunctions of
donors 1 (top) and 2 (bottom) and PCBM (ratio 1:3). In the dark (fillled
circles) and under white light irradiation at 100 mW cm (empty circles).
-
2
Table 3. Photovoltaic Characteristics of Bulk and Bilayer
Heterojunctions Based on Donors 1-5 under AM 1.5 Simulated
-
2
Solar Irradiation at 100 mW cm Intensity
-2
compd
Voc (V)
Jsc (mA cm
)
FF
η (%)
1a
2
1
3
4
5
0.60
0.66
0.48
0.72
0.89
0.96
2.43
4.10
2.33
1.97
3.65
3.65
0.28
0.30
0.41
0.34
0.36
0.29
0.41
0.81
0.46
0.49
1.17
1.02
a
Figure 4 shows the current-density (Jsc) vs voltage (V) curves
of the two cells under AM 1.5 simulated solar illumination at
an intensity of 100 mW cm . For compound 1, the curve
a
Bulk heterojunctions.
-
2
recorded in the dark shows an onset of rectification beyond
efficiency of the resulting bulk heterojunction due to the
combined effects of extended photoresponse and increased Voc.
+0.70 V, while for compound 2 the curve is fully symmetrical.
For both donors the curve recorded under white light irradiation
exhibits a persistent slope at negative voltage. This effect can
result from various factors such as low shunt resistance, high
series resistance, electron-hole recombination, or photocon-
ductivity. Further work is required to clarify this point.
The photovoltaic properties of donors 1, 3, 4, and 5 have
been analyzed on bilayer heterojunction solar cells fabricated
by successive thermal evaporation of 25 nm thick layers of
donor and fullerene C60 followed by evaporation of a 60 nm
thick aluminum electrode. Figure 5 shows the photocurrent
action spectra of the cells based on the three compounds
containing dicyanovinyl groups. For donor 3 the spectrum shows
a first EQE maximum of 33% at 410 nm and a second maximum
of 17% at 550 nm. Introduction of a second dicyanovinyl group
in the donor structure (4) produces a decrease of EQE at 400
nm to 28% and an increase to ca. 20% around 550-570 nm.
This trend is confirmed for the cell based on donor 5 for which
EQE undergoes a further decrease around 400 nm and an
increase around 550-600 nm, leading to a broad maximum of
∼28% in the 470-530 nm region.
The cell based on donor 1 delivers a short-circuit current
-2
density (Jsc) of 2.43 mA cm and an open-circuit voltage (Voc)
of 0.60 V. Combined with a filling factor (FF) of 0.28, these
data lead to a power conversion efficiency (η) of 0.41%. The
J/V curve of the cell based on donor 2 reveals an increase of
-
2
Jsc to 4.10 mA cm and an increase of Voc to 0.66 V. These
data together with a slightly better FF of 0.30 lead to a power
conversion efficiency of 0.81% (Table 3).
These preliminary results thus clearly show that the creation
of an ICT by introduction of an electron-withdrawing group in
the donor structure considerably improves the power conversion
3464 J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006

Triphenylamine−Thienylenevinylene Hybrid Systems
A R T I C L E S
Figure 5. Photocurrent action spectra of bilayer heterojunctions of C60
and donors 3 (top) 4 (middle), and 5 (bottom) under monochromatic
irradiation.
As for bulk heterojunctions, comparison of the action spectra
of the bilayer cells with the corresponding optical spectra in
Figure 1 shows that the changes in the EQE spectrum are tightly
correlated with the intensity of the ICT band in the absorption
spectrum of the donor and hence with the number of acceptor
groups in the structure. These results thus confirm that the
creation of an ICT by introduction of acceptor groups in the
donor structure is indeed effective for extending the photore-
sponse of the heterojunction solar cells.
Figure 6. Current-density vs voltage curves of bilayer heterojunctions of
C60 and donors 3 (top), 4 (middle), and 5 (bottom). In the dark (filled circles)
and under simulated AM 1.5 solar irradiation at 100 mW cm (empty
-2
circles).
the highest value of 1.17% is obtained with donor 4 because of
a better filling factor than for the cell based on donor 5.
These results thus confirm that the introduction of electron-
withdrawing groups in the donor structure leads to a considerable
improvement of the power conversion efficiency of both bulk
and bilayer heterojunction solar cells. To the best of our
knowledge, except for the widely investigated phthalocya-
Figure 6 shows the J/V curves of the three bilayer cells in
the dark and under simulated AM 1.5 solar irradiation at a power
-
2
3,4,10
30
intensity of 100 mW cm . The curves recorded in the dark
show a rectification behavior with an onset at ca. 0.80 V for
compounds 3 and 4 and around 1.10 V for compound 5 (see
Supporting Information).
nines,
and pentacene, the new donors reported here
represent the first class of small molecules capable of leading
to power conversion efficiencies better than 1%. Furthermore,
having in mind that these first results have been obtained on
devices of rather rudimentary technology, it seems reasonable
to believe that they probably represent a minimum and that these
new materials still have a significant margin of progress.
The short-circuit current density increases from 1.97 mA cm-
for compound 3 to 3.65 mA cm-2 for compounds 4 and 5. The
Voc increases from 0.48 V for the reference donor 1 up to 0.963
V for compound 5. This 0.48 V increase of Voc, which nicely
agrees with the 0.44 V higher E° value for compound 5 (Table
2
Conclusion
Amorphous donor materials based on star-shaped systems
involving a TPA core derivatized with thienylenevinylene
2
), clearly confirms the major influence of the oxidation potential
2
2b
of the donor on the Voc of heterojunction solar cells. The
power conversion efficiency η follows the same trend, although
(30) Yoo, S.; Domerq, B.; Kippelen, B. Appl. Phys. Lett. 2004, 85, 5427.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006 3465

A R T I C L E S
Roquet et al.
conjugated branches and electron-withdrawing groups have been
synthesized by a combination of Wittig-Horner and Knoev-
enagel reactions. Optical and electrochemical data have shown
that introduction of electron-acceptor groups in the donor
structure leads at the same time to an ICT and to a large increase
of the oxidation potential.
physical investigations, this first analysis of the structure-
property relationships in a new family of donors provides a
strong incitement to develop further work focused on both the
design of donor systems based on the same approach and on
the technological optimization of the devices.
Prototype bulk and bilayer heterojunctions solar cells have
been fabricated using C60 derivatives as acceptor material. The
action spectra of these devices under monochromatic irradiation
show that the creation of an ICT by introduction of electron-
withdrawing groups in the donor structure leads to a large
increase of the maximum EQE to values exceeding 30% and
to a considerable extension of the photoresponse toward longer
wavelengths. The results obtained under AM 1.5 simulated solar
irradiation show that the synergistic effects of extended pho-
toresponse and increased open-circuit voltage lead to a signifi-
cant improvement of the power conversion efficiency up to
values close to 1.20%.
Acknowledgment. We acknowledge financial support from
the Region Pays de la Loire for the Ph.D. grant of S. R.,
SAMSUNG Company for financial support of part of this work,
and SCAS of the University of Angers for analytical charac-
terizations.
Supporting Information Available: Experimental procedure
for the synthesis and characterization of all compounds.
Experimental details for solar cell fabrication and characteriza-
tion. UV-vis spectra of thin films of compounds 1-5 on glass.
Photoluminescence emission spectra of compounds 3, 4, and 5
in toluene, dichorobenzene, and acetone. This material is
available free of charge via the Internet at http://pubs.acs.org.
Although a detailed understanding of the influence of the ICT
on the process of exciton diffusion and dissociation, charge
recombination, and transport clearly requires detailed photo-
JA058178E
3466 J. AM. CHEM. SOC.
9
VOL. 128, NO. 10, 2006