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Journal Name
Journal of Materials Chemistry A
DOI: 10.1039/C4TA01629G
indicating that in both cases charge transfer from TPA-Th-TPA was
obtained (Supplementary Figure S18).
In conclusion, two conjugated small-molecule azomethines were
synthesized via Schiff base condensation chemistry. The small
molecules both exhibit reversible oxidation behaviour, a fairly deep
lying HOMO energy level, and a bandgap between 1.9 to 2.2 eV.
Photovoltaic devices based on MoOx as the hole-transporting layer
performed better when compared to PEDOT:PSS, which is mainly
attributed to an increased voltage. Both small molecules showed
efficiencies around 1.2% with [70]PCBM as the electron acceptor.
TPA-Th-TPA performed slightly better (PCE of 1.21% with a Jsc of 3.7
mA cm-2, Voc of 0.83 V, and FF of 39%) after post annealing. Both
azomethine-based devices outperform their vinyl analogues, indicating
that azomethines are promising candidates for organic photovoltaic
applications. We also demonstrated that this chemistry enables the
fabrication of OPV devices directly from the reaction mixture without
any product workup. These devices showed a PCE of 0.6%. Because
azomethine chemistry is easy, clean and proceeds under near ambient
conditions we believe that this approach has the ability to reduce
materials and production costs of organic photovoltaic devices.
Figure 3. Device performance of small molecule BHJ solar cells. a, J–V
characteristics and , EQE spectra of small molecule bulk heterojunction solar
b
cells based on TPA-X-TPA:[70]PCBM using MoOx as hole blocking layer after
annealing.
As expected from the absorption spectra, TPA-TBT-TPA, which has
the smallest bandgap and thus the best overlap with the solar spectrum,
shows a higher current. However, because of the slightly higher open-
circuit voltage and fill factor TPA-Th-TPA shows the best power
conversion efficiency (PCE). The high open-circuit voltages up to 0.83
V are in agreement with the fairly low-lying HOMO energy levels. All
devices show a strong voltage dependence, i.e. significantly higher
currents were obtained under reverse bias. This indicates that charge
carriers are formed, but are not extracted under working conditions,
which can be explained by a low charge carrier mobility and/or a poor
morphology. The poor fill factors also hint towards unbalanced charge
transport, which is likely attributed to the limited mobility of
azomethines when processed under these conditions. The vinyl
analogues of TPA-Th-TPA and TPA-TBT-TPA have been published
by other groups and show PCEs of 0.34% and 0.26%, respectively after
optimization,12,15,16 which is significantly lower than 1.21% and 1.15%
found for the azomethine analogues published herein. Also a TPA-
TBT-TPA analogue where the TPA moieties are directly connected to
the TBT core without a vinyl or azomethine bond has been published
and reached a PCE of 1.3% after extensive device optimization using
[70]PCBM, which is only slightly higher compared to the azomethine
we report.
Acknowledgements
This research forms part of the research program of the Dutch Polymer
Institute (DPI), project #717. The work in Cambridge was supported by
the Engineering and Physical Sciences Research Council (grant number
EP/G060738/1). We acknowledge the EPSRC UK National Service for
Computational Chemistry Software (NSCCS) for access to the
Columbus Cluster. We acknowledge Prof. René Janssen, Dr. Martijn
Wienk, and Dr. Dhritiman Gupta of the Eindhoven University of
Technology for access to their device fabrication and characterization
facilities. We also acknowledge Prof. Laurens Siebbeles for access to
the photoluminescence equipment.
Notes and references
a
Delft University of Technology, Faculty of Aerospace Engineering,
Kluyverweg 1, 2629 HS Delft, The Netherlands.
E-mail: t.j.dingemans@tudelft.nl
The external quantum efficiency (EQE) spectra of the devices
coincide with the peaks in the absorption spectra of the small molecules
(Figure 3b), indicating that excitons created in the small molecules are
separated into charges at the interface with PCBM. Using [70]PCBM
instead of [60]PCBM resulted in slight broadening of the absorption
spectrum and an overall increase of the EQE to 30% (Supplementary
Figure S17). The absorption maximum around 310 nm was not
observed in the EQE spectrum, which is explained by the absorption of
the ITO substrate and MoOx.24
The film morphology of the small-molecule:[70]PCBM active
layers was investigated using transmission electron microscopy (TEM).
Both active layers show an amorphous film with PCBM islands on the
order of 2–3 nm (Supplementary Figure S16). These domains are
relatively small; the use of a different solvent or a co-solvent might
induce a stronger phase separation, which is expected to improve the
device performance.
b Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The
Netherlands.
c
University of Cambridge, Optoelectronics Group, Cavendish
Laboratory, J.J. Thomson Avenue, Cambridge, CB3 0HE, United
Kingdom.
†
Electronic Supplementary Information (ESI) available: Synthesis,
materials and methods, computational study and figures and tables as
referred to in the text, See DOI: 10.1039/b000000x/
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Condensation chemistry is very convenient for the synthesis of
conjugated small-molecule azomethines. Besides the molecular
components and a suitable solvent no other reagents or catalysts are
required and water is the only side product. To demonstrate the
simplicity and ease of this chemistry, we prepared a device by
spincoating the reaction mixture without any form of workup or
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purification. For this 2,5-thiophenedicarboxalde (2a, Th) and two
equivalents of 4-aminotriphenylamine ( TPA) were stirred overnight
1,
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in dry chloroform at 60 °C. The next day the resulting deep orange
solution was diluted to the required concentration and [70]PCBM was
added prior to spincoating the active layer. The device showed a PCE
of 0.60%, demonstrating that “one-pot” synthesized azomethines can be
used as donor material in organic photovoltaic devices (Table 2 and
Supplementary Figure S15). The lower efficiency might be explained
by the presence of unreacted functional groups, which are known to act
as charge traps, and/or protonation of the azomethine bond. The EQE
spectrum of the “one-pot” synthesized TPA-Th-TPA device coincides
with that of the devices prepared using the conventional method,
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C. Greenham, T. J. Dingemans, Polym. Chem. 2013, 4, 4182.
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