Table 1 Photophysical and electrochemical properties of 1 and 2
Absorption (solution) lmaxa/onset/nm
[e/(Mꢀ1 cmꢀ1)]
Absorption (film)
lmaxb/onset/nm
Emission (solution)
lmaxa/nm
Dye
EHOMO eVc
Ebandgap eVd
ELUMO eVe
1
2
590/722 [61 028]
514/636 [41 249]
584/790
544/728
800
716
ꢀ5.40
ꢀ5.60
1.6
1.7
ꢀ3.8
ꢀ3.9
a
b
Absorption and emission spectra were measured in chloroform solution. Absorption spectra of thin solid films spin-cast from chloroform
c
solutions. HOMO levels of the dyes were measured using Photo Electron Spectroscopy in Air (PESA) on thin solid films on glass. Energy band
e
gaps were estimated from the absorption onset in thin solid films. LUMO levels were calculated from the optical band gaps and HOMO levels
(ELUMO = Ebandgap + EHOMO).
d
In conclusion, we have demonstrated the use of cyano-
pyridone to improve the performance of donor–acceptor small
molecules in OPVs. In a direct comparison we have shown that
this acceptor group lowers the band gap of an oligothiophene
dye and, in devices, the cyanopyridone-functionalised com-
pound shows higher device power conversion efficiency com-
pared with an analogue containing a dicyanovinyl acceptor
group. We have also shown that the solubilising alkyl group
on the cyanopyridone fragment delivers a material that can be
easily solution processed, that does not crystallise and that
affords devices that can be fabricated with simple processes.
This research was funded through the Flexible Electronics
Theme of the CSIRO Future Manufacturing Flagship and was
also supported by the Victorian Organic Solar Cell Consortium
(Victorian Department of Primary Industries, Sustainable
Energy Research and Development Grant and Victorian
Department of Business and Innovation, Victoria’s Science
Agenda Grant).
Fig. 3 Current–voltage curves for optimised devices based on 1 and 2
in blends with PC61BM (1 : 1 wt.) under simulated sunlight (AM1.5,
1000 W/m2). Device Structure is: ITO/PEDOT:PSS (38 nm)/Active
layer/Ca (20 nm)/Al (100 nm). For 1 the active layer was 46 nm thick,
for 2 the active layer was 56 nm thick.
C60-based devices (see Section 7.2 in ESIw). Improvements
seen in using C70 are usually attributed to enhanced absorp-
tion in the film. Our results suggest that other factors, such as
the low charge mobility or the film morphology, are the
limiting factors in devices based on compound 1. In particular,
the modest fill factors observed for all devices suggest that the
pathways for charges to the electrodes are not optimised, a
problem that could be addressed by using donor fragments
that encourage stronger intermolecular interactions.
Notes and references
1 (a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger,
Science, 1995, 270, 1789–1791; (b) J. J. M. Halls, C. A. Walsh,
N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti and
A. B. Holmes, Nature, 1995, 376, 498–500.
With regards to the differences in processing conditions, it is
noteworthy that, in contrast to compound 1, devices based on
compound 2 showed a significant drop in current when
fabricated using a high boiling solvent. We13 and others2 have
shown that this is a common issue for small molecule semi-
conductors where there is a need to limit the formation of
large-scale crystals through the use of low boiling solvents.
However, the use of low boiling solvents in printing processes
is extremely problematic so the finding that compound 1
performs best when used with high boiling solvents is signifi-
cant. AFM images of the active layer of devices show the finest
morphology for the as-deposited films of compound 1 spun
from chlorobenzene (see Fig. S7 and S8 in ESIw). Finally, with
a view to printing devices under ambient conditions, we also
conducted side-by-side comparisons of optimised devices fabri-
cated in air and under inert conditions. While the differences
were only minor, it was surprising that the best performing
devices with compound 1 (PCE = 2.62%) were prepared in air
with no annealing of either the PEDOT:PSS or the active layer
(see section 7.5 in ESIw). Taken as a whole, these results
strongly suggest that the solubilising group on the cyano-
pyridone acceptor reduces the tendency of the material to
crystallise making it less dependent on processing conditions
to deliver optimised devices. The coupling of this acceptor
group with higher oligothiophenes is the subject of on-going
work in our laboratories.
2 B. Walker, C. Kim and T.-Q. Nguyen, Chem. Mater., 2011, 23,
470–482.
´
3 J. L. Delgado, P.-A. Bouit, S. Filippone, M. A. Herranza and
´
N. Martın, Chem. Commun., 2010, 46, 4853–4865.
4 (a) G. D. Wei, S. Y. Wang, K. Sun, M. E. Thompson and
S. R. Forrest, Adv. Energy Mater., 2011, 1, 184–187; (b) Y. Liu,
X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang and
Y. Chen, Adv. Energy Mater., 2011, 1, 771–775.
5 J. E. Anthony, Angew. Chem., Int. Ed., 2008, 47, 452–483.
6 N. M. Kronenberg, M. Deppisch, F. Wurthner, H. W. A. Lademann,
K. Deing and K. Meerholz, Chem. Commun., 2008, 6489–6491.
7 N. M. Kronenberg, V. Steinmann, H. Burckstummer, J. Hwang,
¨
¨
D. Hertel, F. Wurthner and K. Meerholz, Adv. Mater., 2010, 22,
4193–4197.
8 (a) A. B. Tamayo, X.-D. Dang, B. Walker, J. Seo, T. Kent and
T.-Q. Nguyen, Appl. Phys. Lett., 2009, 94, 103301; (b) B. Walker,
A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia,
M. Tantiwiwat and T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19,
1–7.
9 S. Steinberger, A. Mishra, E. Reinold, C. M. Muller, C. Uhrich,
¨
¨
M. Pfeiffer and P. Bauerle, Org. Lett., 2011, 13, 90–93.
10 Y. Li, Q. Guo, Z. Li, J. Pei and W. Tian, Energy Environ. Sci.,
2010, 3, 1427–1436.
11 (a) P. F. Xia, X. J. Feng, J. Lu, S.-W. Tsang, R. Movileanu, Y. Tao
and M. S. Wong, Adv. Mater., 2008, 20, 4810–4815; (b) P. F. Xia,
X. J. Feng, J. Lu, R. Movileanu, Y. Tao, J.-M. Baribeau and
M. S. Wong, J. Phys. Chem. C, 2008, 112, 16714–16720.
12 M. J. Frisch, et al., Gaussian 03, revision C.02, Gaussian, Inc.,
Wallingford CT, 2004.
13 K. N. Winzenberg, P. Kemppinen, G. Fanchini, M. Bown,
G. E. Collis, C. M. Forsyth, K. Hegedus, Th. B. Singh and
S. E. Watkins, Chem. Mater., 2009, 21, 5701–5703.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1889–1891 1891