Triphenylamine/tetracyanobutadiene-based1 D-A-D π-conjugated systems as molecular donors for organic solar cells

Article abstract of DOI:10.1021/ol201002j

Thiophene-based D-A-D π-conjugated systems containing triphenylamine end groups connected to a 1,1,4,4-tetracyanobuta-1,3-diene acceptor by oligothiophene chains of variable length have been synthesized. These compounds show interesting light-harvesting p

Full text of DOI:10.1021/ol201002j

ORGANIC
LETTERS
2011
Vol. 13, No. 12
3098–3101
Triphenylamine/Tetracyanobutadiene-
Based D-A-D π-Conjugated Systems as
Molecular Donors for Organic Solar Cells
ꢀ
Antoine Leliege, Philippe Blanchard,* Theodulf Rousseau, and Jean Roncali
ꢁ
University of Angers, CNRS, MOLTECH-Anjou, Linear Conjugated Systems Group,
2 Bd Lavoisier, 49045 Angers, France
philippe.blanchard@univ-angers.fr
Received April 15, 2011
ABSTRACT
Thiophene-based D-A-D π-conjugated systems containing triphenylamine end groups connected to a 1,1,4,4-tetracyanobuta-1,3-diene acceptor
by oligothiophene chains of variable length have been synthesized. These compounds show interesting light-harvesting properties and low-lying
HOMO levels. Preliminary results on bilayer heterojunction solar cells with C60 as acceptor show power conversion efficiency higher than 1.0%.
Small π-conjugated molecules have represented a major
class of active materials in organic solar cells (OSCs)
during the early developments of the field.1
Although the past decade has been marked by the
tremendous development of bulk heterojunction (BHJ)
cells based on conjugated polymers, molecular donors
have remained a focus of high interest not only for the
fabrication of vacuum-deposited bilayer heterojunctions2
but more recently as donor materials for the realization of
solution-processed BHJ solar cells.3
In this context, an interesting approach consists of the
design of molecular donors in which an internal charge
transfer is created by the introduction of electron acceptor
(A) groups in the structure of the donor.4
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r
10.1021/ol201002j
Published on Web 05/19/2011
2011 American Chemical Society

In recent years, this approach has led to the synthesis of
different classes of active molecules.3ꢀ17 In particular,
symmetrical D-A-D or A-D-A systems have been shown
to lead to interesting photovoltaic performances in both
vacuum-deposited planar heterojunction and solution-
processed bulk heterojunction OSCs.
Thus, a large variety of donor materials have been
designed by combining donor blocks D, essentially derived
from oligothiophenes or triphenylamine (TPA), with
electron-acceptor moieties A such as dicyanovinyl,4,9
dicyanomethylenepyrane,10 fluorenone,11 squaraine,12
diketopyrrolopyrrole,13 indigo,14 benzothiadiazole,15,16
or borondipyrromethene.17
donor blocks together with preliminary results on the
potentialities of these compounds in OSCs.
The synthesis of the target compounds 1 is shown in
Scheme 1. Regioselective monobromination of TPA-deri-
vatized-thiophene and 2,20-bithiophene 2 and 319 by NBS,
gave the bromo compounds 4 and 5, respectively. Sonoga-
shira reaction of trimethylsilylacetylene with 4 and 5
followed by elimination of the trimethylsilyl group gave
the acetylenic compounds 6 and 7. These latter compounds
were subjected to another Sonogashira coupling with
bromo compounds 4 or 5 in the presence of CuI and
Pd(PPh3)4 as catalyst to give the symmetrical (8a, 8c) and
unsymmetrical (8b) compounds. Reaction of compounds
8 with tetracyanoethylene (TCNE) led to the target
molecules 1aꢀc in 68ꢀ90% yield after purification by
chromatography.
Scheme 1. Synthesis of Molecules 1
This latter reaction is highly efficient for introducing a
strong electron acceptor such as TCBD in place of an
acetylenic bond20 as illustrated by the synthesis of TPA21
and oligothiophene22 derivatives.
Figure 1. CVs of 0.5 mM solutions of compounds 1 in 0.1 M
Bu4NPF6/CH2Cl2, Pt electrode, scan rate = 100 mV/s.
1,1,4,4-Tetracyanobuta-1,3-diene (TCBD) has been de-
veloped as an electron-accepting group for the synthesis of
D-π-A pushꢀpull chromophores with enhanced NLO
properties and improved thermal stability.18 We report
here on the synthesis and the electronic properties of
symmetrical and unsymmetrical D-π-A-π-D molecules 1
using TCBD as acceptor and oligothienyl-TPA chains as
The electronic properties of compounds 1 have been
analyzed by cyclic voltammetry (CV) and UVꢀvis absorp-
tion spectroscopy. The CV of 1a (Figure 1) exhibits a two-
electron reversible oxidation wave with anodic peak po-
tential Epa1 at 1.05 V assigned to the simultaneous oxida-
tion of the two thienyl-TPA branches. The CV shows also
€
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Org. Lett., Vol. 13, No. 12, 2011
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Table 1. Electrochemical, Optical, and Thermal Data for Compounds 1
compd
Epr2/Va
Epr1/Va
Epa1/Va
Epa2/Va
Epa3/Va
λ
max (ICT)/nm (ε/Mꢀ1cmꢀ1 b
)
ΔEelec (eV)c
ΔEopt (eV)d
T/°Ce
1a
1b
1c
ꢀ0.71
ꢀ0.68
ꢀ0.67
ꢀ0.55
ꢀ0.53
ꢀ0.50
1.05
0.93
0.94
569 (46600)
576 (57400)
586 (61600)
1.53
1.38
1.37
2.18
2.15
2.12
375
165
208
1.05
1.44
1.44
a Performed with 0.5 mM of compound in 0.1 M Bu4NPF6/CH2Cl2, Pt electrode, SCE reference, scan rate = 100 mV/s. b In CH2Cl2. c Estimated from
the difference between standard potentials of the first electrochemical oxidation and reduction waves. d Estimated from λmax (ICT). e Temperature
corresponding to 5% weight loss obtained from TGA analysis under N2 with a heating rate of 10 °C/min.
two successive one-electron reduction waves at -0.55 V
(Epr1) and -0.71 V (Epr2) associated to the reduction of the
central TCBD. Epa1 is positively shifted by 220 mV com-
pared to 8a due to the ꢀM and ꢀI effects of the TCBD
block (Table S1, Supporting Information).
The UVꢀvis absorption spectra of compounds 1 in
CH2Cl2 show a broad ICT band extending from 450 to
700 nm with relatively high molar extinction coefficients ε
(Figure 2 and Table 1). The extension of the conjugated
branches from 1a to 1c leads to a 17 nm bathochromic shift
of λmax with an increase of ε up to 61600 Mꢀ1cmꢀ1. This
small red shift confirms, in agreement with electrochemical
data, a limited π-conjugation through the central TCBD
segment.
The lengthening of the oligothiophene chain in1b and 1c
induces a negative shift of Epa1 together with the appear-
ance of additional oxidation peaks at more positive
potential (Figure 1 and Table 1). Thus, the CV of the
unsymmetrical compound 1b shows three one-electron
reversible oxidation waves peaking at 0.93, 1.04, and 1.44
V. The first and the second processes are assigned to the
successive oxidation of the bithienyl- and thienyl-TPA
branches in cation radical whereas the third one can be
related to the oxidation of the bithienyl-TPA branch to the
dication state. This assignment is consistent with the CV of
1c, which shows two two-electron reversible oxidation
peaks at Epa1 = 0.94 V and Epa2 = 1.44 V corresponding
to the successive formation of the cation radical and
dication of the two bithienyl-TPA branches.
These Epa values show that the lateral branches are
rather electronically independent suggesting that the cen-
tral TCBD interrupts the conjugation. This interpretation
is further supported by the limited positive shift observed
in the first reduction potential between 1a and 1c.
Figure 3. Geometry of 1a optimized with Gaussian 09 at the
B3LYP/6-31G(d,p) level of theory.
The structure of 1aꢀc has been optimized using DFT
with Gaussian 09. In all cases, the geometry shows a
significant distortion at the center of the molecule with a
dihedral angle of 96.4°, 93.1°, and 94.8°, between the
planes containing the two dicyanovinyl groups (Figure 3).
This resultconfirms therestrictedπ-electron delocalization
in the TCBD core in agreement with CV and optical data.
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G.; Diederich, F. Chem.;Eur. J. 2008, 14, 7638. (b) Tang, X.; Liu, W.; Wu,
J.; Lee, C.-S.; You, J.; Wang, P. J. Org. Chem. 2010, 75, 7273.
Figure 2. Absorption spectra of compounds 1 in CH2Cl2.
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Navarrete, J. T.; Ruiz Delgado, M. C.; Zotti, G.; Vercelli, B.; Lovander,
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3100
Org. Lett., Vol. 13, No. 12, 2011

Such structural feature has been also observed in the
solid state for parent compounds.18a,20,23 As expected,
the HOMO is delocalized along the π-conjugated oli-
gothiophene-TPA arms with a prominent contribution of
TPA while the LUMO is essentially located on the TCBD
core. Thus the HOMO to LUMO electronic transition has
a strong ICT character (Figure S25, Supporting Informa-
tion, for 1a).
Compounds 1 are thermally stable and TGA curves
show a 5% weight loss at 375 °C, 165 and 208 °C respec-
tively for 1a, 1b and 1c (Figure S27ꢀ29, Supporting
Information, Table 1). The compounds are readily soluble
in common organic solvents such as CHCl3, THF or
even cyclohexane. The solubility can be related to the
non planar structure which may reduce chromophore
aggregation.18a,20
Thin-films of 1a have been prepared by thermal evapora-
tion under high vacuum or by spin-casting from CHCl3
solutions. Both kinds of films present the same absorption
spectrum with a broadening of the absorption band and a 23
nm red-shift of λmax compared to the solution spectrum
(Figure S22, Supporting Information). A band gap Eg of
1.70 eV was estimated from the low-energy absorption onset
of the optical spectrum of film of 1a on glass.
Bilayer heterojunction solar cells have been fabricated
by spin-casting films of 1a on ITO substrates precoated
with a 40 nm of PEDOT-PSS, from a 10 mM CHCl3
solution. A 20 nm thick film of C60 was then thermally
evaporated and the devices were completed by deposition
of 100 nm-thick aluminum electrode.
The power conversion efficiency of the various cells has
been analyzed under simulated solar illumination in AM
1.5 conditions at 90 mW/cm2. A cursory analysis of the
effect of the thickness of the donor layer showed that best
results were obtained with a 25 nm thick layer of 1a (Table
S5, Supporting Information). Average power conversion
efficiency (PCE) of 1.05% was obtained from analysis of
four different cells. The best device gave a short-circuit
current density of 3.06 mA cmꢀ2 and an open-circuit
voltage of 0.97 V. This high voltage can be related to the
low HOMO energy level of 1a. Combined with a fill-factor
FF = 0.33, these results led to a PCE of 1.08% (Figure 4).
Note that bilayer heterojunction solar cells prepared
from vacuum deposited films of 1a of 13 nm thickness,
gave an average PCE of 0.56% (Table S6, Supporting
Information). While these first results are encouraging, the
S shape of the J vs V curve suggests that the quality of the
interfaces needs to be improved and that better results
could be expected for optimized devices.
Figure 4. Current vs voltage curves of a bilayer heterojunction
1a:C60. Open circles: in the dark. Black circles: under AM 1.5
white light illumination at 90 mW/cm2.
In summary, symmetrical and unsymmetrical D-A-D
π-conjugated systems based on a TCBD acceptor have been
synthesized. These compounds exhibit strong absorption
in the visible spectrum, high oxidation potentials, high
stability and good solubility in common organic solvents.
Preliminary investigations of their potentialities as donor
material carried out on bilayer heterojunction solar cells
have given promising results with particularly high cell
voltage.
Further investigations of the photovoltaic properties of
these materials in bilayer and bulk heterojunction solar
cells as well as the synthesis of other members of this class
of D-A-D systems are now underway and will be reported
in future publications.
ꢀ
Acknowledgment. The Ministere de la Recherche is
acknowledged for the PhD grant to A. Leliege. We thank
ꢀ
V. Bonnin from the University of Angers for elemental
analysis and the PIAM of the University of Angers for
characterization of organic compounds.
Supporting Information Available. Synthetic proce-
dures and characterization for new compounds, CV and
UVꢀvis data for 8, geometry optimization for 1 and
OSCs characterization. This material is available free of
charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 12, 2011
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