A small-molecule zwitterionic electrolyte without a π-delocalized unit as a charge-injection layer for high-performance PLEDs

Article abstract of DOI:10.1002/anie.201209959

Down to the bare bones: Small-molecule zwitterionic materials were found to be more efficient as charge-injection materials in an organic electronic device than a previously described polymer (see structures). Furthermore, the superior device performance observed for 1 indicates that it is not necessary to focus only on π-delocalized systems and that solid ionic liquids may be promising alternative candidates for charge-injection materials.

Full text of DOI:10.1002/anie.201209959

Angewandte
Chemie
DOI: 10.1002/anie.201209959
Organic Electronic Devices
A Small-Molecule Zwitterionic Electrolyte without a p-Delocalized
Unit as a Charge-Injection Layer for High-Performance PLEDs**
Chao Min, Changsheng Shi, Wenjun Zhang, Tonggang Jiu, Jiangshan Chen,* Dongge Ma, and
Junfeng Fang*
Conjugated organic optoelectronic materials have attracted
significant attention owing to their application in light-
emitting diodes (LEDs), solar cells, thin-film transistors,
optically amplified biosensor assays, and other areas.[1] For
organic optoelectronic devices, interfacial modification
between the organic material and the electrode is a crucial
issue and becomes one of the most important research focal
points, since charge injection/extraction affect device perfor-
mance greatly.[2] Excellent charge injection leads to a lower
turn-on voltage and higher efficiency in organic LEDs.[3] In
the case of organic solar cells, good interface properties result
in a higher short-circuit current and a higher open-circuit
voltage, which are the most important factors for solar-cell
performance.[4]
Among the many kinds of solution-processable interfa-
cial-modification materials,[4a,b,5] conjugated polyelectrolytes
(CPEs) and oligoelectrolytes (OEs) containing a p-delocal-
ized backbone with pendant groups that can be ionized have
attracted great interest. Their solubility in polar solvents
enables the fabrication of multilayer polymer-based devices
by the solution-processing approach.[6a–g] The reported per-
formance of devices based on CPEs/OEs is comparable to
that of devices based on the conventional Ca/Ba electrode.
However, mobile counterions, such as Na+, BrÀ, and tetra-
substituted borates,[6a,b,7] which can migrate during device
operation, make the device mechanism more complicated and
seriously affect the device turn-on time.[1h,8]
materials owing to their unique chemical structure, in which
both the positive and the negative ions are combined. As the
ions are not mobile, the device response time is improved
significantly. Furthermore, the solubility of the materials in
polar solvents is good enough for multilayer-device fabrica-
tion as a result of the presence of the charged groups.[2a] In
2009, Bazan and co-workers reported small-molecule zwit-
terionic materials synthesized by the addition of iodoalkanes
to sodium tetrakis(1-imidazolyl)borate (NaBIm4); the device
response time and performance were comparable to those of
the Ba/Al electrode.[6d] In a previous study,[9] one of our
research groups reported a conjugated polyelectrolyte with
a fluorene-based zwitterionic sulfoammonium structure and
no free counterions. When this zwitterionic polymer known as
“F(NSO3)2” (CPE1) was used as the electron-injection layer,
devices based on poly(9,9’-dioctylfluorene-co-benzothiadia-
zole) (F8BT) as the emitter showed very fast response times;
moreover, both efficiency and brightness were considerably
improved (by a factor of more than 2) in comparison with
standard calcium-based devices. Huang and co-workers
independently reported a fluorene-based zwitterionic poly-
mer, “PF6NSO”,[3a] the luminance efficiency of which was as
high as 23.8 cdA with poly[2-(4-(3’,7’-dimethyloctyloxy)-
phenyl)-p-phenylenevinylene] (P-PPV) as the emitter. The
overall device performance was much better than that of the
neutral precursor and of conventional CPEs with mobile ions.
Recently, Huang, Bazan, and co-workers reported a series of
amine N-oxide functionalized CPEs. These materials showed
similar device performance to that of the Ba/Al electrode in
PLEDs. In the case of polymer solar cells, the Voc value and
the fill factor of the polymer solar cells were increased
significantly.[3b] Besides the conjugated zwitterionic electro-
lytes mentioned above, only few examples of conjugated
zwitterionic derivatives have been developed in the past, and
the research has not been extended to the investigation of
such materials as the injection layer in electronic devices.[10a,b]
At present, most of the reported electrolyte interfacial-
modification materials based on zwitterions or mobile ions
have been conjugated polymers/oligomers. Research results
on small molecules are limited.[6h] Small-molecular semi-
conductors offer several intrinsic advantages over conjugated
polymers/oligomers in organic electronic applications. They
In recent years, conjugated zwitterionic materials have
attracted great interest as a new kind of charge-injection
[*] C. Min, W. Zhang, T. Jiu, Prof. J. Fang
Ningbo Institute of Materials Technology and Engineering
Chinese Academy of Sciences
Ningbo, 315201 (China)
E-mail: fangjf@nimte.ac.cn
C. Shi, Prof. J. Chen, Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences
Changchun, 130022 (China)
E-mail: jschen@ciac.jl.cn
[**] This project (51273208) is supported by the National Natural
Science Foundation of China. The research was also supported by
the Hundred Talent Program of the Chinese Academy of Sciences;
the Starting Research Fund of Team Talent (Y10801A01) at NIMTE;
the Ningbo Natural Science Foundation of China (2012A610114),
and the Open Fund of the State Key Laboratory of Luminescent
Materials and Devices (South China University of Technology).
PLED=polymer light-emitting diode.
À
can be synthesized readily without complex C C coupling
and polymerization reactions, are monodisperse in nature
with well-defined chemical structures, and are synthetically
well-reproducible. For charge-injection materials, reported
studies mainly focused on p-delocalized structures, since such
structures are considered crucial for the conducting proper-
ties of conjugated materials. Herein, we report a series of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209959.
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small-molecule zwitterionic materials (SMZWs) with and
without p-delocalized groups (Scheme 1). These materials
have superior electron-injection characteristics; they outper-
form Ca electrodes. Furthermore, for the first time, we found
that a zwitterionic material without any p-delocalized unit
showed excellent device performance.
The SMZWs were synthesized by quaternization of the
neutral precursor with 1,4-butane sultone in a THF/methanol
Scheme 1. Structures of the small-molecule zwitterionic materials used
in this study and the device structure. ITO=indium tin oxide,
PEDOT=poly(3,4-ethylenedioxythiophene).
solvent mixture. The precursors used were 1,1’-(2,2’-(4,4’-
sulfonylbis(1,4-phenylene)bis(oxy))bis(ethane-1,2-diyl))-
dipiperidine (M1), 2,2’-(4,4’-sulfonylbis(1,4-phenylene)bis-
(oxy))bis(N,N-dimethylethanamine) (M2), and N1,N1,N6,N6-
tetramethylhexane-1,6-diamine (M3). The resulting zwitter-
ionic products S1, S2, and S3 were highly soluble in methanol
and dimethyl sulfoxide (see the Supporting Information). The
neutral precursors chosen for this study had a sulfonyl and
a tertiary amine group and were thus similar to sulfoammo-
nium zwitterions in their chemical structure but without any
charges. The materials were tested with the device structure:
ITO/PEDOT/F8BT/injection layer/Al (Scheme 1). The cor-
responding device with Ca/Al as the electrode was also
fabricated for comparison.
Figure 1. Luminance (Lv), current efficiency (CE), power efficiency (PE),
and current density (j) as a function of voltage for PLEDs with the
device structure: ITO/PEDOT:PSS/F8BT(100 nm)/SMZW or M1 or
Ca/Al. PSS=poly(4-vinylbenzenesulfonic acid).
The device characteristics of luminance (L) versus voltage
(V) and current efficiency (CE) versus voltage are shown in
Figure 1. The turn-on voltage of the devices (voltage at which
the brightness reached 1 cdmÀ2) for S1, S2, S3, M1, and Ca/Al
was 2.4, 2.5, 2.9, 3.1, and 3.1 V, respectively (Table 1). In the
case of M1, the chemical structure of which is similar to that of
a sulfoammonium zwitterion, the device showed poorer
overall performance, including its turn-on voltage (Von),
brightness, and efficiency. The quaternization of M1 to give
S1 led to an impressive improvement in performance: the
turn-on voltage decreased by 0.2 V, the brightness was
improved by a factor of 3.5, and the current efficiency was
improved by a factor of 9. The current efficiency of the device
with S1 were even twice as high as those for the Ca-based
device at all operation voltages, but the device current was
lower than that of the Ca-based device. This improvement in
performance can be ascribed to the insertion of a layer of
a zwitterionic material, which should also alleviate light-
emission quenching induced by metal electrodes. The max-
imum current efficiency reached 8.8 cdAÀ1 (at 6.0 V) and
Table 1: Summary of the performance of PLEDs with the device
structure: ITO/PEDOT/F8BT/cathode.
[a]
Cathode
Von
Maximum CE
Maximum Lv
Maximum PE
[cdAÀ1
]
[cdmÀ2
]
[lmWÀ1
]
Ca/Al
M1/Al
S1/Al
S2/Al
S3/Al
CPE1
3.1
3.1
2.9
2.5
2.4
2.4
4.0
1.0
8.8
9.9
10.4
9.9
22916
3248
11395
22819
35093
50382
1.9
0.4
4.7
7.0
7.5
6.6
[a] Von is the voltage at which the brightness reached 1 cdmÀ2
.
a power efficiency (PE) of 4.7 lmWÀ1 (at 5.8 V). This result
indicates that the formation of a zwitterionic structure is
crucial for the electron injection.
In the material S1, there are long alkyl groups on the
sulfobetaine zwitterionic structure. According to previous
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studies, dipole formation is induced by the insertion of
a zwitterionic layer between the Al electrode and the active
layer; the resulting shift in the vacuum level is the main
contributing factor for device improvement.[6d,9] The steric
bulk of the big alkyl groups on S1 may affect dipole formation
at the interface. We synthesized S2 with smaller alkyl groups
and observed a further improvement in device performance
relative to that of the S1-based device: the brightness was
improved by a factor of 2 and reached 22819 cdmÀ12, and the
power efficiency reached 7.0 lmWÀ1 (Table 1). Since the turn-
on voltage decreased significantly from 2.9 to 2.5 V, the
overall performance is superior to that of the S1-based device,
especially at a low operation voltage (2.5–6.0 V). These
results indicate better charge injection for the S2-based
device, the efficiency of which is similar to that found with our
previously reported zwitterionic polymer electrolyte CPE1.[9]
They imply that small molecules are also excellent and
competitive candidates for the electron-injection layer.
Our results based on M1, S1, and S2 indicate that the
zwitterionic part of the molecule plays an important role in
charge injection. We were interested to find out whether small
molecules without conjugated p-delocalized structures were
also suitable. The conjugated backbone of the polymer/
oligomer electrolyte undoubtedly affects the device perfor-
mance significantly, as observed previously.[7c] We wanted to
know whether conjugated p-delocalized units were indispen-
sable for the charge-injection function, since most reported
electrolyte interfacial-modification materials have a typical
conjugated p-delocalized unit, such as a fluorene, phenyl, or
imidazole group. We therefore synthesized the zwitterionic
material S3, which does not contain a conjugated unit
(Scheme 1). To our surprise, devices based on S3 as the
electron-injection layer showed superior performance even to
that of S2. The turn-on voltage was 2.4 V, which is the lowest
observed for these materials, the current density was much
higher than that of the Ca-based device, and the brightness
reached a maximum of 35093 cdmÀ12 at 6.8 V with a high
current efficiency of 7.4 cdAÀ1. The highest current efficiency
and power efficiency were 10.4 cdAÀ1 and 7.5 lmWÀ1, respec-
tively (Table 1). The brightness was 3365 cdmÀ12 at 4 V with
a corresponding current efficiency of 9.5 cdAÀ1 (7.5 lmWÀ1).
Scheme 2 describes the differences in the chemical
structures of the polymer CPE1 and the small molecules S2
and S3, and the efficiencies of the devices with these materials
as the electron-injection layer. Details of device performance
are also summarized in Table 1 for
ciency (from 9.9 to 10.4 cdAÀ1). Our results clearly show that,
at least for sulfoammonium zwitterionic interfacial materials,
p-delocalized structures are not absolutely necessary for the
electron-injection function. However, the p-delocalized struc-
tures may still be useful for the device performance. For
example, the presence of a conjugated polymer/oligomer
backbone results in a better film-forming ability; it could also
improve the dipole at the interface further or the optical
properties of the device.
We also found that the small-molecule zwitterionic
materials resulting from quaternization of the neutral pre-
cursors showed an improved film-forming ability. The
improvement could be ascribed to the interaction of zwitter-
ionic structures in the different molecules and the resulting
cross-linking or partial cross-linking.[10c] We also investigated
polymer solar cells fabricated with these zwitterionic materi-
als as the electron-extraction layer; however, we have
unfortunately not observed obvious device improvement in
the P3HT:PCBM-based polymer solar cells so far (P3HT=
poly(3-hexylthiophene), PCBM = phenyl-C61-butyric acid
methyl ester). A plausible reason for this lack of improvement
is that the nonpolar and crystalline properties of P3HT could
lead to a wetting problem. The use of a different active
polymer or device structure should overcome this problem.
We will come back to this issue in the future.
In summary, we have synthesized a series of novel small-
molecule zwitterionic materials with and without a p-delo-
calized structure. Device performance was considerably
improved in comparison with that of Ca-based devices when
these materials were used as the electron-injection layer in
PLEDs. Our results indicate that the zwitterionic nature of
the molecules is important for charge injection. They also
strongly suggest that p-delocalized structures are not neces-
sary for the electron-injection function. For the further
mechanistic study of zwitterionic materials, an effect of the
electrical and optical properties induced by the p-delocalized
structure on device performance could be ruled out; the
function of the p-delocalized part and the zwitterionic part of
the molecules could be identified separately. As S1, S2, and S3
are in fact typical ionic liquids in terms of their chemical
structure, solid ionic liquids may be promising alternative
candidates for charge-injection materials in organic electronic
devices. The results of this study will lead to broader
approaches to the design of charge-injection materials,
simpler synthetic procedures for efficient charge-injection
comparison. When the fluorene unit of
the functional part of polymer CPE1 was
substituted for a sulfonyldiphenol unit,
the device containing the resulting small
molecule S2 showed comparable effi-
ciency to that of the polymer-based
device. In the case of S3, in which the
fluorene or sulfonyldiphenol unit has
been completely removed and only the
charged group is left, the device showed
a 13.6% improvement in the power
efficiency (from 6.6 to 7.5 lmWÀ1) and
Scheme 2. Structures of the polymer CPE1 and the small-molecule zwitterionic materials S2
a 5% improvement in the current effi- and S3 with the corresponding device efficiencies.
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Received: December 13, 2012
Published online: && &&, &&&&
Keywords: charge-carrier injection · conducting materials ·
.
light-emitting diodes · polymers · zwitterions
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Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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Angewandte
Chemie
Communications
Organic Electronic Devices
Down to the bare bones: Small-molecule
zwitterionic materials were found to be
more efficient as charge-injection mate-
rials in an organic electronic device than
a previously described polymer (see
C. Min, C. Shi, W. Zhang, T. Jiu, J. Chen,*
D. Ma, J. Fang*
&&&& — &&&&
A Small-Molecule Zwitterionic Electrolyte
without a p-Delocalized Unit as a Charge-
Injection Layer for High-Performance
PLEDs
structures). Furthermore, the superior
device performance observed for 1 indi-
cates that it is not necessary to focus only
on p-delocalized systems and that solid
ionic liquids may be promising alternative
candidates for charge-injection materials.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!