University of Groningen
Efficient electron injection from solution-processed cesium stearate interlayers in
organic light-emitting diodes
Wetzelaer, G. A. H.; Najafi, A.; Kist, R. J. P.; Kuik, M.; Blom, P. W. M.
Published in:
Applied Physics Letters
DOI:
10.1063/1.4790592
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to
cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2013
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Wetzelaer, G. A. H., Najafi, A., Kist, R. J. P., Kuik, M., & Blom, P. W. M. (2013). Efficient electron injection
from solution-processed cesium stearate interlayers in organic light-emitting diodes. Applied Physics
Letters, 102(5), 053301-1-053301-4. [053301]. DOI: 10.1063/1.4790592
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 15-06-2017
APPLIED PHYSICS LETTERS 102, 053301 (2013)
Efficient electron injection from solution-processed cesium stearate
interlayers in organic light-emitting diodes
G. A. H. Wetzelaer,1,2 A. Najafi,1 R. J. P. Kist,1 M. Kuik,1 and P. W. M. Blom1,3
1
Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
2
Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, The Netherlands
3
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
(Received 5 December 2012; accepted 21 January 2013; published online 4 February 2013)
The electron-injection capability of solution-processed cesium stearate films in organic light-emitting
diodes is investigated. Cesium stearate, which is expected to exhibit good solubility and film
formation due to its long hydrocarbon chain, is synthesized using a straightforward procedure. Lightemitting diodes are fabricated using orange-, yellow-, and blue-emitting conjugated polymers,
comprising a cesium stearate electron-injection layer deposited from ethanol solution. It is
demonstrated that these devices perform as well as benchmark devices using vacuum-deposited
C 2013 American Institute of Physics.
barium as electron-injection layer, without loss of color purity. V
[http://dx.doi.org/10.1063/1.4790592]
Organic light-emitting diodes (OLEDs)1,2 have attracted
considerable attention because of their potential use in displays and solid-state lighting.3,4 The main advantage of using
solution-processed organic semiconductors is the capability of
producing large-area and flexible light sources that can be
produced in a roll-to-roll process at a relatively low cost. The
solubility of organic materials conveniently renders them suitable for solution deposition by means of inkjet printing, spray
coating, or doctor blading.5 However, although these OLEDs
contain solution-processable organic active layers, suitable
electrode materials are also required in order to ensure proper
device operation. Deposition of these, frequently inorganic,
electrode materials from solution is less straightforward. Especially for electron-injection layers this appears to be troublesome. Usually, reactive metals with a low work function, such
as barium or calcium, are applied on top of the active layer by
thermal evaporation in high vacuum.3 Their low work function is required for efficient electron injection into the lowest
occupied molecular orbital (LUMO) of the organic semiconductor, generally situated at an energy of 2–3 eV below the
vacuum level.6 However, an immediate side effect of the use
of low-work function metals is their sensitivity towards oxygen and humidity, making them incompatible with wet deposition techniques.
To solve this issue, semiconducting transition metal
oxides such as ZnO and TiOx have been investigated as
electron-injection layers in organic-semiconductor devices.7,8
These metal-oxide layers can be formed by solutiondeposition techniques, for instance, using nanoparticle dispersions or via a precursor route.9 In spite of their ease of
processing and their stability towards air and moisture, electron injection from these metal oxides is not very efficient.10
This can be ascribed to the large offset between the electron
affinity of the metal oxide (4 eV) and the organic semiconductor (2–3 eV). Since organic semiconductors are generally
intrinsic (i.e., undoped), even a small injection barrier can
give rise to a substantial decrease in injection efficiency,
stressing the importance of well-aligned electron injection
layers.
0003-6951/2013/102(5)/053301/4/$30.00
Another approach to enhance electron injection is the use
of conjugated polyelectrolytes (CPEs), consisting of a conjugated backbone that has pendant groups with ionic functionalities.11,12 Inserting a layer of such a CPE between the emissive
layer and cathode can result in efficient electron injection even
from high-work function metals. Such increase in injection
efficiency in OLEDs has also been observed recently for a thin
layer of the naturally occurring polyelectrolyte DNA, ascribed
to the formation of an interfacial dipole layer.13 Tailor-made
CPEs, however, frequently require a time-consuming synthesis, which might be a drawback of their usage.14 Furthermore,
the presence of ionic species often leads to unwanted hysteresis
effects in the electrical characteristics. Recently, it was
reported that a thin layer of polyethylenimine could also
induce a dipole layer, effectively reducing the work function
of conducting surfaces.15
A very well-known method to achieve efficient electron
injection in organic semiconductors is the use of thin interfacial layers of alkali-metal salts. In particular, lithium fluoride
(LiF)16 and cesium carbonate (Cs2CO3)17,18 have received
considerable attention. These salts are frequently deposited
by means of thermal evaporation, but it has been shown
that Cs2CO3 can even serve as an electron-injection layer
when applied by a spin-coating process.19 However, in
order to achieve proper film formation, the toxic solvent
2-ethoxyethanol was used as casting solvent. In this report,
we show that an alternative cesium salt, cesium stearate
(CsSt), can serve as an efficient, solution-processed electroninjection layer in OLEDs. In contrast to Cs2CO3, this salt
can be conveniently spin cast from ethanol solution.
Cesium stearate was synthesized according to the
method described in Ref. 20. In short, Cs2CO3 and stearic
acid in a 1:2 molar ratio were added to n-butanol. This mixture was reacted at reflux temperature, under formation of
carbon dioxide and water. After gas formation had stopped,
butanol was separated on the rotary evaporator. The residue
was mixed with diethyl ether and aspirated. The afforded
white cesium stearate powder was obtained in a high yield
(>90%).
102, 053301-1
C 2013 American Institute of Physics
V
Downloaded 05 Feb 2013 to 129.125.63.113. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
053301-2
Wetzelaer et al.
In this study, OLEDs were prepared according to the following method. Glass substrates patterned with tin-doped
indium oxide (ITO) were cleaned using detergent solution, demineralized water, ultrasonication in acetone and isopropyl
alcohol, followed by UV-ozone treatment. The cleaned
substrates were coated with a 60 nm thick film of poly(3,4ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:
PSS; VP AI4083, H.C. Starck) in ambient air and annealed
subsequently at 140 C for 10 min. Single emissive layers of
conjugated polymers were spin cast on top of the PEDOT:PSS
hole-injection layer under controlled nitrogen atmosphere.
Subsequently, thin layers of cesium stearate were spun on
top from a dilute ethanol solution (2 mg/ml). Without further
processing of these layers, aluminum top electrodes (100 nm)
were thermally evaporated under vacuum (base pressure
1 10ÿ6 mbar). For the reference devices, the layer of
cesium stearate was omitted and replaced by a thermally
evaporated barium layer with a thickness of 5 nm as electroninjection layer. The electron-injection capability of cesium
stearate was investigated for a variety of conjugated polymers with different energy gaps, including orange-emitting
poly[{2 -(4-(30 ,70 -dimethyloctyloxyphenyl))}-co-{2-methoxy5 -(30 ,70 -dimethyloctyloxy)}-1,4-phenylenevinylene] (NRSPPV),21 yellow-emitting Super Yellow (SY-PPV; Merck
KGaA), and a blue-emitting polyalkoxyspirobifluoreneN,N,N0 ,N0 -tetraaryldiamino biphenyl (PSF-TAD) copolymer.22
Electrical measurements were conducted under controlled N2 atmosphere, using a computer-controlled Keithley
2400 source meter. Light emission was recorded simultaneously using a Hamamatsu S1336 silicon photodiode. An
Ocean Optics spectrometer (USB2000) was used to measure
the electroluminescence (EL) spectra.
Cesium stearate is expected to exhibit better solubility
and improved adhesion to the light-emitting polymer, as
compared to Cs2CO3, due to its long hydrocarbon chain.
Therefore, cesium stearate is a promising candidate as
electron-injection material in fully solution-processed
OLEDs. In order to investigate the performance of cesium
stearate interlayers, OLEDs comprising various conjugated
polymers were fabricated. Layers of cesium stearate were
spin-coated on top of these conjugated polymer films, where
the layer thickness of cesium stearate was estimated to be
less than a few nanometers. These thin layers of cesium stearate then form an interlayer between the emissive polymer
and the aluminum cathode. The OLEDs with cesium stearate
interlayer were compared to reference devices with a Ba/Al
cathode, which is known to provide Ohmic electron injection
for most polymers.
Figure 1(a) shows the current-density (J) and luminance
(L) of orange-emitting NRS-PPV OLEDs as a function of the
applied voltage (V). Depicted are the characteristics for a device with a CsSt/Al cathode and a reference device with a
Ba/Al cathode, which is an Ohmic electron contact. It is
clear that the characteristics of the device comprising a CsSt/
Al cathode are almost identical to the reference device.
Moreover, as can been seen in Fig. 1(b), the luminous efficacy as a function of voltage is also very similar for both
devices. Here, the luminous efficacy is the ratio of the photon
flux detected by the Si photodiode and the current density
flowing through the device. Since both devices consist of the
Appl. Phys. Lett. 102, 053301 (2013)
FIG. 1. (a) J-L-V characteristics and (b) luminous efficacy of NRS-PPV
OLEDs with Ba and CsSt electron-injection layers. The inset in (b) shows
the chemical structure of CsSt.
same (Ohmic) hole contact, hole injection into the highest
occupied molecular orbital (HOMO) of the polymer is
equally efficient for both devices. As a consequence, light
output is directly limited by the injection of electrons that
give rise to subsequent recombination with holes in the
HOMO of the polymer. The luminous efficacy, therefore, is
a sensitive measure of the injection efficiency of electrons
from the top electrode into the polymer LUMO. Since barium- and CsSt-based devices exhibit equal luminous efficacies (Fig. 1(b)), and considering that Ba/Al is an Ohmic
electron contact, it can be concluded that an interlayer
of CsSt can provide Ohmic electron injection into the
NRS-PPV LUMO (ÿ2.7 eV (Ref. 23)).
To investigate whether CsSt can be used as an efficient
electron-injection layer for other polymers, the poly(p-phenylene vinylene) (PPV) derivative Super Yellow (SY-PPV)
was selected as emitter material. SY-PPV is an efficient
yellow-emitting polymer, comprising a wider energy gap
than NRS-PPV. In spite of the wider energy gap of the emitter material, CsSt still functions as an efficient electron injection layer, as can be observed in Fig. 2. The luminous
efficacy of the CsSt-based device again matches the luminous efficacy of the device with a vacuum deposited Ba/Al
cathode. This shows that electron injection into SY-PPV
from a CsSt interlayer is as efficient as injection from a barium interlayer.
That CsSt greatly reduces the injection barrier that is
present when using a pure aluminum cathode is also shown
Downloaded 05 Feb 2013 to 129.125.63.113. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
053301-3
Wetzelaer et al.
Appl. Phys. Lett. 102, 053301 (2013)
FIG. 2. Luminous efficacy vs voltage characteristics of SY-PPV OLEDs
with Ba, CsSt, and aluminum electron-injection layers.
Fig. 2. When omitting the CsSt interlayer, the maximum
luminous efficacy is reduced by a factor of 30 as compared
to the device with CsSt injection layer. This once more
shows that the presence of an injection barrier at one of the
contacts is detrimental for the device performance.
Achieving efficient electron injection is the most challenging for blue-emitting organics, since the necessary wide
energy gap of these materials frequently involves a high
LUMO, which is not easily accessible. To investigate the
electron-injection capabilities of CsSt in combination with
blue-emitting polymers, a PSF-TAD copolymer22 was used.
PSF-TAD is a blue-emitting polyspirobifluorene derivative
containing triarylamine hole-transporting units in the backbone. Electron transport in such a polymer is due to hopping
between the fluorene segments. Although the exact energetic
position of the LUMO of PSF-TAD is not known, the measured electron affinity of a very similar polymer gave an estimated LUMO energy of 2.1 6 0.1 eV below vacuum,24
clearly higher than the previously discussed PPV derivatives
(2.7 eV below vacuum23). Nevertheless, even though this
imposes a large offset between the electron affinity of the
polymer and the work function of the aluminum cathode, efficient electron injection could be achieved with a CsSt interlayer. The obtained J-L-V characteristics (Fig. 3(a)) for the
CsSt and reference devices again show close resemblance.
The luminous efficacy characteristics as a function of voltage
(Fig. 3(b)) confirm that also in this case electron injection
from the CsSt layer is efficient.
The effect of the CsSt interlayer is even more apparent
when the luminous efficacy of the device without injection
layer between the polymer and aluminum cathode is considered, as is presented in Fig. 3(b). In this case, the inclusion of
a CsSt layer gives rise to a 300-fold enhancement of the conversion efficiency of electrical charges into photons. This efficiency increase is clearly more pronounced for PSF-TAD than
for SY-PPV (Fig. 2). This indicates that the barrier for electron injection from the aluminum cathode is larger for
PSF-TAD, which can be ascribed to the higher LUMO of this
polymer. Since the injection efficiency of CsSt and barium
interlayers relative to plain aluminum increases with increasing LUMO, the use of CsSt and barium causes electrode
FIG. 3. (a) J-L-V characteristics and (b) luminous efficacy of PSF-TAD
OLEDs with Ba and CsSt electron-injection layers. The luminous efficacy
for a plain aluminum cathode is also shown in (b).
pinning to the LUMO of the polymer, resulting in the formation of an Ohmic electron contact.
In order to compare the performance and processability
of CsSt with respect to spin-coated Cs2CO3, the latter salt
was used in PPV and PSF-TAD OLEDs as discussed in the
supplementary information.25 The best PPV-based devices
yielded a reasonable, twofold lower luminous efficacy than
the barium control devices, while for PSF-TAD no significant electron-injection improvement could be achieved with
Cs2CO3 interlayers as compared to the aluminum control
devices.
Taking into account the above results, it can be concluded that the electrical performance of devices comprising
a solution-processed CsSt electron-injection layer can rival
the performance of barium-based devices for various lightemitting polymers. To investigate whether the use of CsSt
induces any spectral changes, EL spectra were recorded. In
Fig. 4, the EL spectra for the blue-emitting polymer and
SY-PPV are displayed. The spectra for devices comprising
CsSt and barium are identical, showing that CsSt has no
influence on the emission color. As a result, also in this
respect solution-processed CsSt proves to be a good candidate to replace barium in an OLED, opening a route towards
fully solution-processed OLEDs.
In conclusion, we have demonstrated that solutionprocessed cesium stearate films are efficient electron-injection
layers for organic diodes. OLEDs with a CsSt electron-injection
Downloaded 05 Feb 2013 to 129.125.63.113. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
053301-4
Wetzelaer et al.
Appl. Phys. Lett. 102, 053301 (2013)
3
FIG. 4. Normalized EL spectra for OLEDs based on PSF-TAD and SY-PPV
emissive layers in combination with Ba and CsSt electron-injection layers.
layer are shown to be as efficient as devices using vacuum deposited barium, which is an Ohmic electron contact. Conjugated
polymers with consecutively increasing energy gaps were used
as emitters, to obtain orange-, yellow-, and blue-emitting
OLEDs comprising an efficient solution-processed CsSt
electron-injection layer. In contrast to previously reported
solution-processable salts, CsSt can conveniently be deposited
from ethanol solution, without the need of additional surfactants
to enhance film formation.
This work was supported by the Dutch Polymer Institute
(DPI), Project No. 678. The authors would like to acknowledge the technical assistance of Jan Harkema. We also thank
E. H. Valkenier-Van Dijk and A. J. Kronemeijer for assistance in the synthesis of cesium stearate.
1
2
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987).
J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539
(1990).
R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks,
C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. L€
ogdlund,
and W. R. Salaneck, Nature 397, 121 (1999).
4
S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. L€
ussem,
and K. Leo, Nature 459, 234 (2009).
5
F. C. Krebs, Sol. Energy Mater. Sol. Cells 93, 394 (2009).
6
L. L. Chua, J. Zaumseil, J. F. Chang, E. C. W. Ou, P. K. H. Ho, H.
Sirringhaus, and R. H. Friend, Nature 434, 194 (2005).
7
K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, M. K.
Nazeeruddin, and M. Graetzel, Appl. Phys. Lett. 89, 183510 (2006).
8
H. J. Bolink, E. Coronado, D. Repetto, and M. Sessolo, Appl. Phys. Lett.
91, 223501 (2007).
9
P. de Bruyn, D. J. D. Moet, and P. W. M. Blom, Org. Electron. 11, 1419
(2010).
10
M. Lu, P. de Bruyn, H. T. Nicolai, G. A. H. Wetzelaer, and P. W. M.
Blom, Org. Electron. 13, 1693 (2012).
11
C. V. Hoven, A. Garcia, G. C. Bazan, and T.-Q. Nguyen, Adv. Mater. 20,
3793 (2008).
12
H. Jiang, P. Taranekar, J. R. Reynolds, and K. S. Schanze, Angew. Chem.,
Int. Ed. 48, 4300 (2009).
13
P. Zalar, D. Kamkar, R. Naik, F. Ouchen, J. G. Grote, G. C. Bazan, and
T.-Q. Nguyen, J. Am. Chem. Soc. 133, 11010 (2011).
14
M. R. Pinto and K. S. Schanze, Synthesis 9, 1293 (2002).
15
Y. Zhou, C. Fuentes-Hernandez, J. Shim, J. Meyer, A. J. Giordano, H. Li,
P. Winget, T. Papadopoulos, H. Cheun, J. Kim, M. Fenoll, A. Dindar, W.
Haske, E. Najafabadi, T. M. Khan, H. Sojoudi, S. Barlow, S. Graham,
J.-L. Bredas, S. R. Marder, A. Kahn, and B. Kippelen, Science 336, 327
(2012).
16
L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152
(1997).
17
T. Hasegawa, S. Miura, T. Moriyama, T. Kimura, I. Takaya, Y. Osato, and
H. Mizutani, SID Int. Symp. Digest. Tech. Papers 35, 154 (2004).
18
R. Suhonen, R. Krause, F. Kozlowski, W. Sarfert, R. P€atzold, and A. Winnacker, Org. Electron. 10, 280 (2009).
19
J. Huang, G. Li, E. Wu, Q. Xu, and Y. Yang, Adv. Mater. 18, 114 (2006).
20
A. Kanitz, R. Patzold, W. Sarfert, and R. Suhonen, U.S. patent 0085472
(2009).
21
C. Tanase, J. Wildeman, and P. W. M. Blom, Adv. Funct. Mater. 15, 2011
(2005).
22
H. T. Nicolai, A. Hof, J. L. M. Oosthoek, and P. W. M. Blom, Adv. Funct.
Mater. 21, 1505 (2011).
23
H. T. Nicolai, M. Kuik, G. A. H. Wetzelaer, B. de Boer, C. Campbell, C.
Risko, J. L. Bredas, and P. W. M. Blom, Nature Mater. 11, 882 (2012).
24
S. Harkema, R. A. H. J. Kicken, B. M. W. Langeveld-Voss, S. L. M. van
Mensfoort, M. M. de Kok, and R. Coehoorn, Org. Electron. 11, 755
(2010).
25
See supplementary material at http://dx.doi.org/10.1063/1.4790592 for the
results with cesium carbonate interlayers.
Downloaded 05 Feb 2013 to 129.125.63.113. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions