Absorption tuning of monosubstituted triazatruxenes for bulk heterojunction solar cells

Article abstract of DOI:10.1021/ol202443g

A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption

Full text of DOI:10.1021/ol202443g

ORGANIC
LETTERS
2011
Vol. 13, No. 22
6030–6033
Absorption Tuning of Monosubstituted
Triazatruxenes for Bulk Heterojunction
Solar Cells
†
‡
§
§
§
ꢀ ^
Thomas Bura, Nicolas Leclerc, Sadiara Fall, Patrick Leveque, Thomas Heiser,
and Raymond Ziessel*,†
ꢀ
Laboratoire de Chimie Organique et Spectroscopies Avancees (LCOSA), CNRS,
Ecole de Chimie, Polymeres, Materiaux de Strasbourg (ECPM), 25 rue Becquerel,
ꢁ
ꢀ
ꢀ
67087 Strasbourg, Cedex 02, France, Laboratoire d’Ingenierie des Polymeres pour les
Hautes Technologies (LIPHT), Universite de Strasbourg, Ecole Europeenne de Chimie,
ꢁ
ꢀ
ꢀ
ꢁ
ꢀ
Polymeres et Materiaux, 25 rue Becquerel, 67087 Strasbourg, France, and Institut
ꢁ
ꢀ
d’Electronique du Solide et des Systemes (InESS), Universite de Strasbourg-CNRS,
23 rue du Loess, 67037 Strasbourg, France
ziessel@unistra.fr
Received September 15, 2011
ABSTRACT
A series of triazatruxene (TAT)-functionalized Bodipy dyes were prepared by a sequence of reactions involving either cross-coupling reactions
promoted by Pd complexes or a Knoevenagel reaction leading to a vinyl linker. The new dyes show large absorption coefficients and fluorescence
quantum yields as well as interesting electrochemical properties. The blue dyes of this series exhibit interesting photovoltaic effects (VOC = 0.83 V,
J
SC = 3.6 mA/cm2, efficiency 0.9%) in bulk heterojunction solar cells, due to the good hole mobility imported by the TAT entity.
Flat π-conjugated platformssuchastriphenylene, benzo-
from the point of view of linking various photoactive
modules to promote directional energy transfer.2 The
replacement of the three methylene groups at the 5, 10,
and 15 positions of truxene by three nitrogen atoms
endows the resultant triazatruxene (TAT) core, which
consists of a C3 symmetry of fused carbazole trimer and
also presents a flat aromatic surface.
The three indolic NH centers can be functionalized with
flexible side chains and the core by bromo groups at the
periphery.3 Star-shaped octupolar TAT derivatives are known
to be efficient in two-photon absorption spectroscopy,4
hexacoronenes, perylene, and other rylenes have been
extensively explored in organic electronics for applications
including energy conversion devices.1 In this context, we
became interested in the preorganized truxene platform
†
ꢁ
ꢀ
Ecole de Chimie, Polymeres, Materiaux de Strasbourg (ECPM).
‡
ꢀ
ꢀ
ꢁ
Universite de Strasbourg, Ecole Europeenne de Chimie, Polymeres et
ꢀ
Materiaux.
§
ꢀ
Universite de Strasbourg-CNRS.
€
(1) (a) Mullen, K. ; Wegner, G. In Electronic Materials: The Oligomer
Approach, Wiley-VCH, Weiheim, 1998. (b) Martin, R. E.; Diederich, F.
Angew. Chem., Int. Ed. 1999, 38, 1530. (c) Hadziioannou, G., van
Hutten, P. F. In Semiconducting Polymers: Chemistry, Physics, and
Engineering; Wiley-VCH: Weinheim, 2000.
(2) (a) Diring, S.; Ziessel, R. Tetrahedron Lett. 2009, 50, 1203.
(b) Diring, S.; Puntoriero, F.; Nastasi, F.; Campagna, S.; Ziessel, R.
J. Am. Chem. Soc. 2009, 131, 6108.
(3) (a) Garcia-Frutos, E. M.; Gomez-Lor, B. J. Am. Chem. Soc. 2008,
190, 9173. (b) Franceschin, M.; Ginnari-Satriani, L.; Alvino, A.; Ortaggi,
G.; Bianco, A. Eur. J. Org. Chem. 2010, 134.
r
10.1021/ol202443g
Published on Web 10/27/2011
2011 American Chemical Society

Scheme 1. Synthesis of the Triazatruxene-Linked Bodipy Derivatives
and organic-light-emitting diodes5 and self-assembled hexa-
functionalized TAT frameworks can be used to produce
electroactive discotic liquidꢀcrystalline materials with
high hole mobility.6 Surprisingly, despite these various
applications, the chemistry of TAT is limited largely to
that of homohexa- and homotrisubstituted derivatives,
and applications in solar cells have not been demonstrated
up to now, probably due to the limited absorption in the
visible portion of the solar spectrum. Our choice of TAT
was motivated by its high charge carrier mobilities in thin
films and its semiconductor character, attractive redox prop-
erties, and strong fluorescence.7 Here, we report that mono-
substituted TAT derivatives can be prepared in a controlled
manner and cross-linked to boronꢀdifluorodipyrromethene
(Bodipy) dyes to produce photoactive layers in bulk
heterojunction (BHJ) solar cells. As simple Bodipy deri-
vatives themselves are active materials in dye-sensitized
solar cells (DSSC)8 and BHJ solar cells,9 it was tempting to
try to link the photosensitizer (Bodipy) to TAT while
conserving the charge carrier mobility. Two strategies were
devised for the preparation of 3-bromo-5,10,15-triazatrux-
ene. The first requires the preparation of 3,8,15-tribromo-
5,10,15-triazatruxene and subsequent dehalogenation, and
the second necessitates monohalogenation of 5,10,15-tria-
zatruxene (Scheme 1). Asa first synthetic approach, a four-
step protocol starting from 2-oxindole, followed by alkyla-
tion with bromododecane giving 1, followed by a selective
bromination in the 5-position affording 2 was developed.
A final trimerization in neat POCl3 provided the target
compound 3. Such cyclocondensation is the most common
route to triazatruxene derivatives.10,5 The use of reductive
dehalogenation was based on previous work4b and was
performed in refluxing THF using formate salts as the
reducing agent and Pd supported on charcoal as catalyst.
The reaction was not selective but provided the target
monobromo derivative 5 and the dibromo compound in
27 and 20% yields, respectively. Cross-coupling of 5 with
(4) (a) Ji, L.; Fang, Q.; Yuan, M.-S.; Liu, Z.-Q.; Shen, Y. X.; Chen,
H.-F. Org. Lett. 2010, 12, 5192. (b) Shao, J.; Guan, Z.; Yan, Y.; Jiao, C.;
Xu, Q.-H.; Chi, C. J. Org. Chem. 2011, 76, 780.
(5) Lai, W.-Y.; He, Q.-Y.; Zhu, R.; Chen, Q.-Q.; Huang, W. Adv.
Funct. Mater. 2008, 18, 265.
(6) (a) Gomez-Lor, B.; Alonso, B.; Omenat, A.; Serrano, J. L. Chem.
Commun. 2006, 5012. (b) Luo, J.; Zhao, B.; Shao, J.; Lim, K. A.; Chan,
H. S. S. O.; Chi, C. J. Mater. Chem. 2009, 19, 8327. (c) Zhao, B.; Liu, B.;
Png, R. Q.; Zhang, K.; Lim, K. A.; Luo, J.; Shao, J.; Ho, P. K. H.; Chi,
C.; Wu, J. Chem. Mater. 2010, 22, 435.
(7) Talarico, M.; Termine, R.; Garcia-Frutos, E. M.; Omenat, A.;
Serrano, J. L.; Gomez-Lor, B.; Golemme, A. Chem. Mater. 2008, 20,
6589.
(8) (a) Erten-Ela, S.; Yilmaz, D.; Icli, B.; Dede, Y.; Icli, S.; Akkaya,
U. E. Org. Lett. 2008, 10, 3299. (b) Kolemen, S.; Cakmak, Y.; Erten-Ela,
S.; Altay, Y.; Brendel, J.; Thelakkat, M.; Akkaya, E. U. Org. Lett. 2010,
12, 3812. (c) Kolemen, S.; Bozdemir, O. A.; Cakmak, Y.; Barin, G.;
Erten-Ela, S.; Marszalek, M.; Yum, J.-H.; Zakeeruddin, S. M.; Nazeer-
(9) (a) Rousseau, T.; Cravino, A.; Roncali, J.; Bura, T.; Ulrich, G.;
Ziessel, R. Chem. Commun. 2009, 1673–1675. (b) Rousseau, T.; Cravino,
A.; Roncali, J.; Bura, T.; Ulrich, G.; Ziessel, R. J. Mater. Chem. 2009, 16,
2298–2300. (c) Rousseau, T.; Cravino, A.; Ripaud, E.; Leriche, P.; Rihn,
S.; De Nicola, A.; Ziessel, R.; Roncali, J. Chem. Commun. 2010, 46, 5082.
(10) Hiyoshi, H.;Kumagai, H.;Ooi, H. Pat. Appl. Publ. WO2005077956,
2005.
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uddin, M. K.; Gratzel, M.; Akkaya, E. U. Chem. Sci. 2011, 2, 949.
Org. Lett., Vol. 13, No. 22, 2011
6031

the alkyne-Bodipy 6 (Bodipy = borondipyrromethene)
afforded the truxene derivative 7 in modest yield
(Scheme 1).
The electrochemical properties of the dyes were evalu-
ated by cyclic voltammetry, and compounds 5, 6, and 8
were used as reference. The TAT substrates exhibit two
reversible monoelectronic oxidations about þ0.80 and
þ1.36 V, in keeping with literature data (Figure 2).6c The
simple Bodipy 6 exhibits two reversible monoelectronic
processes at þ1.16 and ꢀ1.24 V, assigned to the π-radical
cation and π-radical anion, respectively.15 Linking cova-
lently both subunits together, providing dye 7, gives a
cyclic voltammogram reflecting a combination of the
electroactivity of the TAT and Bodipy units (Figure 2a
and Table S2, Supporting Information).
For the second set of dyes 10 and 11, the first oxidation
potential around þ0.62 V is assigned to the Bodipy oxidation
while the second and third reversible one electron oxidation
waves define successive TAT oxidations. The reversible reduc-
tion wave is most reasonably assigned to the reduction of the
Bodipy to the radical anion. Note that for 11 the reduction
potential is cathodically shifted by 140 mV, a situation
expected based on the increase of the electron density imported
by the alkyne moieties. In contrast, the oxidation potential of
11 is shifted by 50 mV with respect to that of dye 10.
The hole mobilities in thin films of the new Bodipy dyes
were extracted from the transfer characteristics in the
saturation regime ofbottom contact field effect transistors.
Commercial, highly doped oxidized silicon was used as
substrate and gate electrode, while a hexamethyldisilazane
(HMDS)-treated, 200 nm thick, SiO2 layer was used as
gate dielectric. Gold source and drain electrodes were
obtained by photolithography. Bulk heterojunction solar
cells were composed of a standard ITO/PEDOT:PSS/
active layer/Al structure.
In a second synthetic approach, the monobromo deri-
vative 5 was prepared in two steps from N-dodecane-2-
oxindole 1 using the protocol described above for 3,
followed by a relatively selective (66%) bromination at
the 3-position. Then, a carboformylation reaction pro-
moted by a Pd-catalyst was performed in hot DMF under
a flux of CO at atmospheric pressure. The stable formyl
derivative 8 was allowed to react with the simple Bodipy 9
inthe presenceofpiperidine and trace amountsof p-TsOH.
The proton NMR spectrum is diagnostic for the mono-
substituted compound due to the presence of two β-pyrrolic
proton peaks at the expected chemical shifts (δ 6.06 and
6.73 ppm).11,12 Finally, substitution of the boron fluoro
groups using the Grignard reagent of 2,5-dioxaoct-7-yne13
led to dye 11 in excellent yield.
Data defining the photophysical properties of the new
monobromo and monoaldehyde triazatruxene and Bodipy
dyes are gathered in Table S1, Supporting Information.
The absorption spectra of 5 and 8 exhibit a major
absorption peak around 317 nm characteristic of triaza-
truxene derivatives (Figure S1, Supporting Information).4
Moderately efficient fluorescence of both truxene deriva-
tives was observed. The absorption spectra of the mixed
Bodipy/truxene dyes 7, 10, and 11 are typical of boradia-
zaidacene π systems, showing peaks at the expected wave-
length for unsubstituted dyes (for 7, λabs 501 nm, ε 87000
M
ꢀ1cmꢀ1) and monostyryl dyes (for 10, λabs 609 nm, ε
112000 Mꢀ1cmꢀ1, for11, λabs 607 nm, ε125000 Mꢀ1cmꢀ1).14
These absorptions are safely assigned, in light of pre-
vious accounts, to S0fS1 transitions typical of singlet
excited states. Interestingly, excitation in the truxene
module result in quantitative energy transfer to the
Bodipy fragment, which is the unique emitting species
(Figure 1). The Bodipy fluorescence quantum yields lie
in the range of 37ꢀ46% for dyes 7, 10, and 11 (Table S1,
Supporting Information).
Figure 2. (a) Cyclic voltammetry of dye 7 and added ferrocene in
CH2Cl2 and TBAPF6. The red line corresponds to the redox
processes associated with the Bodipy part. (b) Cyclic voltam-
metry of TAT derivative 8 (blue trace) and dye 10 (red trace), in
both cases with added ferrocene. *Residual water present in the
sample.
Figure 1. Absorption and emission spectra of 7 in THF (orange
and green lines) and of 10 in benzene (blue and red lines), at rt,
concentration ca. 1 ꢁ 10ꢀ6 M, excitation wavelength 460 nm for
7 and 550 nm for 10.
6032
Org. Lett., Vol. 13, No. 22, 2011

Table 1. Measured Hole Mobility for Different Bodipy Dyes and the Best Photovoltaic Parameters Obtained for Bodipy/PC61BM
Blend Photovoltaic Cellsa
bodipy/PCBM ratio
cathode
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
thick-ness (nm)
μ
h (cm2/V s)
3
7
1:2
1:1
1:1
1:1
Al
0.57
0.71
0.71
0.83
0.5
2.9
2.0
3.6
28
29
29
29
0.08
0.66
0.40
0.90
10
11
10
Al
55
50
55
1 ꢁ 10ꢀ4
4 ꢁ 10ꢀ3
1 ꢁ 10ꢀ4
Al
Ca/Al
a JSC for short-circuit current densities; FF for fill factors; PCE for power conversion efficiencies; μh for hole mobility; and VOC for open circuit
voltage.
The elaboration procedure (except for the active layer
0.9%. However, the (JꢀV) characteristics under standard
AM1.5 (100 mW/cm2) illumination conditions shown in
Figure 3 (characterized by a low fill factor and a low
parallel resistance at 0 V) indicate that the charge carrier
extraction is not efficient enough, even with the acceptable
hole mobility measured in the pure material.
deposition) has been described elsewhere.16 The active
layer was obtained by spin-coating chloroform solutions
ofcompounds7, 10, and 11withPC61BMin various ratios.
The best photovoltaic parameters for each compound are
listed in Table 1 together with the hole mobility of the pure
material.
Compound 7 with the triazatruxene linked in the meso
position gave negligible PCE, and the hole mobility could
not be measured due to the absence of a transistor effect.
The presence of a marked tilt angle between the Bodipy
and the TAT engendered by the phenyl ring is probably
detrimental for the π-stacking ability of the molecule.
In strong contrast, all other molecules have a reasonable
hole mobility. Photovoltaic devices exhibit short-circuit
current densities (Jsc) above 2.0 mA/cm2, fill factors (FF)
around 30%, and power conversion efficiencies (PCE)
around 0.5%. This significant increase in Jsc and PCE, in
comparison to compound 7, is mainly due to the accep-
table hole mobility values even if a small contribution of
the enhanced absorption ability of the monostyryl com-
pounds can not be completely neglected. The open circuit
voltage (Voc) increases also for compounds 10 and 11 to
reach a value around 0.7 V (when Al is used as cathode) in
good agreement with the HOMO levels determined by CV
measurements.17 For compound 10, we also performed mea-
surements on cells with a bilayer Ca(20 nm)/Al(120 nm)
cathode, characterized by a lower work function. This
time, the Voc reaches 0.83 V and the PCE a promising
Figure 3. JꢀV characteristics measured in a photovoltaic device
using compound 10 blended with PC61BM (1:1 wt ratio) as
active layer. The solar cell structure is IT0/PEDOT:PSS/active
layer/Ca/Al, and the measurements were performed in the dark
(closed triangles) and under standard AM1.5 (100 mW/cm2)
illumination conditions (open squares).
In summary, a series of TAT-functionalized Bodipys
have been prepared and their optical and electrochemical
properties have been investigated and processed in BHJ
solar cells. Further work is in progress to improve the
active layer morphology as well as the charge carrier
mobilities in the blend.
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Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (c) Ulrich, G.;
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Eur. J. 2009, 15, 10369.
Acknowledgment. We acknowledge the CNRS for pro-
viding research facilities and financial support, and Pro-
fessor Jack Harrowfield (ISIS in Strasbourg) for helpful
comments.
(16) Biniek, L.; Fall, S.; Chochos, C. L.; Anokhin, D. V.; Ivanov, D.
ꢀ ^
A.; Leclerc, N.; Leveque, P.; Heiser, T. Macromolecules 2010, 43, 2010,
9779-9786.
Supporting Information Available. Synthetic procedures
and analytical data reported herein. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
€
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