COMMUNICATION
A star-shaped triphenylamine p-conjugated system with internal
charge-transfer as donor material for hetero-junction solar cells{
Antonio Cravino, Sophie Roquet, Philippe Leriche, Olivier Ale´veˆque, Pierre Fre`re and Jean Roncali*
Received (in Cambridge, UK) 28th November 2005, Accepted 6th February 2006
First published as an Advance Article on the web 21st February 2006
DOI: 10.1039/b516781g
The synthesis of compound 1 is depicted in Scheme 1. A Stille
coupling of the commercially available tribromotriphenylamine 4
with the stannyl derivative of thiophene gave tris[4-(2-thienyl)phe-
nyl]amine 318 in 85% yield. Vilsmeier formylation of compound 3
gave tricarboxaldehyde 2 in 90% yield. Knoevenagel condensation
of compound 2 with malononitrile gave the target molecule tris(4-
(5-dicyanomethylidenemethyl-2-thienyl)phenyl]amine 1 in 76%
yield.{
The cyclic voltammogram of compound 1 shows a reversible
anodic wave peaking at 1.09 V vs Ag.AgCl (Fig. 1). This high
value shows that the electron-withdrawing dicyanovinyl groups
induce a large increase of the oxidation potential compared to
compound 3 (Epa 5 0.51 V vs Ag.Ag+).18 Furthermore, the
stability of the cation radical indicated by the reversibility of
the oxidation process appears as a positive fact for the use of the
corresponding material for hole transport.
Introduction of dicyanovinyl groups on a triphenylamine-based
conjugated system leads to an intramolecular charge transfer
which extends the spectral response and raises the open-circuit
voltage of the resulting hetero-junction solar cells.
Organic solar cells are a focus of considerable research effort
motivated by the perspective of achieving low-cost, lightweight
and flexible power sources.1–8 Bulk hetero-junctions based on
interpenetrated networks of p-conjugated polymers and soluble
C60 derivatives have been intensively investigated in the past
decade and power conversion efficiencies in the 4–5% range have
been recently reported.5,6 In parallel, significant progress has been
also accomplished in multi-layer hetero-junctions based on small
molecules7–11 and efficiencies in the 3–4% range have been
published.9,10
Low dimensional p-conjugated systems such as e.g. oligothio-
phenes lead to organic semi-conductors with highly anisotropic
electrical and optical properties,11–15 which poses specific problem
for device fabrication. Thus, whereas a vertical orientation of the
conjugated chains on the substrate is known to improve mobility
in organic field-effect transistors,12–14 such an orientation is
detrimental for solar cells as it strongly reduces the absorption of
the incident light as well as hole transport to the electrodes.11,15,16
Organic glasses derived from triphenylamine (TPA)-based
compounds have been widely investigated as active material for
hole transport and electroluminescence.17 Whereas the amorphous
character of these materials offers possibilities to develop active
materials for solar cells with isotropic optical and charge-transport
properties, the use of TPA-based materials for photovoltaic
conversion has been scarcely considered. Shirota and co-workers
have used TPA-based starburst molecules containing nitro or
dimesitylboryl acceptor groups as donor in bilayer hetero-
junctions. However, the conversion efficiency of these cells was
limited to 0.4 and 0.1% respectively under monochromatic
irradiation in the absorption band of the donor (y440 nm).17b
We now report preliminary results on a star-shaped tris[4-(2-
thienyl)phenyl]amine core modified by peripheral electron-with-
drawing dicyanovinyl groups (1) and its use as donor material
for the realization of hetero-junction solar cells. We show that
the creation of an intramolecular charge-transfer (ICT) leads
simultaneously to an extension of the spectral response and to an
increase of the open-circuit voltage of the resulting solar cells.
The UV-Vis spectrum of compound 1 in CH2Cl2 shows a first
absorption band with a lmax at 368 nm and a second more intense
band with lmax at 510 nm (Fig. 2). Based of the lmax of compound
3 (366 nm),18 the first absorption band can be assigned to a p2p*
transition and the main absorption band at 510 nm to an ICT
between the donor and acceptor parts of the molecule. The optical
spectrum of a thin film of 1 thermally evaporated on glass exhibits
a lmax at 545 nm and a red shift of the absorption onset to
y680 nm. (Fig. 3 and ESI{). These large shifts suggest the
occurrence of strong intermolecular interactions in the solid state.
The low energy absorption edge of the spectrum leads to a band
gap Eg of y1.80 eV. Powder X-ray diffraction spectra of the films
did not show any peak, confirming that compound 1 is amorphous
similarly to many TPA derivatives.17
Hetero-junction solar cells of 6 mm diameter have been realized
on ITO substrates spin-coated with a 60 nm film of Baytron P1.
Layers of donor 1 and C60 fullerene as acceptor of y25 nm
thickness were successively grown by thermal evaporation under
vacuum and the devices were completed by deposition of a 60 nm
thick aluminium top electrode.
As shown in Fig. 3, the UV-Vis absorption spectrum of the
bilayer is more or less the sum of the spectra of the two
components. The spectrum shows a first intense band with lmax at
ca 350 nm corresponding to the absorption of C60 and to the first
absorption band of compound 1 and a second broad band with
lmax at 520 nm. The apparent blue shift of the ICT band of 1 in the
spectrum of the bilayer is due to the convolution with the C60
absorption shoulder around 450 nm. The photo-current action
spectrum recorded under monochromatic irradiation shows that
the incident photon conversion efficiency (IPCE) presents a first
shoulder around 350 nm and a broad maximum of ca 28% in the
Groupe Syste`mes Conjugue´s Line´aires, CIMMA, UMR CNRS 6200,
Universite´ d’Angers, 2 Bd Lavoisier F-49045 Angers
{ Electronic supplementary information (ESI) available: current voltage
curve of the cell ITO/1( 25nm)/C60 (25 nm)/Al (60 nm) and electronic
absorption spectrum of a film of 1 evaporated on glass. See DOI: 10.1039/
b516781g
1416 | Chem. Commun., 2006, 1416–1418
This journal is ß The Royal Society of Chemistry 2006
Cravino, Antonio
