Alcohol-processable organic amorphous electrolytes as an effective electron-injection layer for organic light-emitting diodes

Article abstract of DOI:10.1002/asia.201200299

A new series of monoammonium-based organic electrolytes with the tetrafluoroborate (BF₄^-) counteranion have been synthesized. Replacing the pendant ethyl groups in the fluorenyl unit with 4-ethoxyphenyl groups dramatically improves both solubility and morphological stability. The characterization of the alcohol-processable amorphous ionic compounds as an electron-injection layer in organic light-emitting diodes (OLEDs) reveals that the organic electrolyte that comprises a rigid linear-conjugated unit provides better device performance, with respect to its counterpart containing a branched bulky moiety. The capability of these compounds to facilitate electron injection from air-stable aluminum metal is preliminarily discussed on the basis of the investigations of the electron-only devices and photovoltaic experiments. Copyright

Full text of DOI:10.1002/asia.201200299

DOI: 10.1002/asia.201200299
Alcohol-Processable Organic Amorphous Electrolytes as an Effective
Electron-Injection Layer for Organic Light-Emitting Diodes
Gang Liu, Yan-Hu Li, Wan-Yi Tan, Zhi-Cai He, Xiao-Tie Wang, Chi Zhang, Yue-Qi Mo,
Xu-Hui Zhu,* Junbiao Peng, and Yong Cao[a]
Abstract: A new series of monoammo-
electron-injection layer in organic
light-emitting diodes (OLEDs) reveals
that the organic electrolyte that com-
prises a rigid linear-conjugated unit
provides better device performance,
with respect to its counterpart contain-
ing a branched bulky moiety. The capa-
bility of these compounds to facilitate
electron injection from air-stable alu-
minum metal is preliminarily discussed
on the basis of the investigations of the
electron-only devices and photovoltaic
experiments.
nium-based organic electrolytes with
À
the tetrafluoroborate (BF4 ) counter-
anion have been synthesized. Replac-
ing the pendant ethyl groups in the flu-
orenyl unit with 4-ethoxyphenyl groups
dramatically improves both solubility
and morphological stability. The char-
acterization of the alcohol-processable
amorphous ionic compounds as an
Keywords: amorphous materials ·
electrolytes · electron injection ·
molecular devices · solution proc-
essability
Introduction
better electrochemical stability. With respect to compound
3, compounds 1 and 2 consist of a rigid linear-conjugated
The quest for low-cost, large-area, organic light-emitting de-
vices under electrical excitation has stimulated considerable
research on solution-processed functional materials and
device technologies.[1,2] Among others, an intriguing devel-
opment was the finding that conjugated polymer electro-
lytes[3–8] and later their small-molecule counterparts[9–13] can
facilitate electron injection from air-stable cathode metals
such as aluminum. Furthermore, electrolyte materials that
show processability from alcoholic solvents allow their uti-
lization as an electron-injection layer (EIL), thereby provid-
ing the least perturbation over the bottom emissive layer,
which generally consists of hydrophobic luminophors.
Recently, we reported an alcohol-processable small-mole-
cule electrolyte based on a monoammonium salt that fea-
tured an amorphous morphology (Scheme 1a). Notably, the
spin-cast thin film affords effective electron injection from
the Al metal, thus leading to device efficiency that ap-
proaches that of the Ba/Al cathode.[11]
In this contribution, we describe further a comparative
study on a new series of small-molecule electrolytes with the
À
BF4 counteranion (Scheme 1b), which is known to have
[a] G. Liu, Y.-H. Li, W.-Y. Tan, Z.-C. He, X.-T. Wang, C. Zhang,
Prof. Y.-Q. Mo, Prof. X.-H. Zhu, Prof. J. Peng, Prof. Y. Cao
State Key Laboratory of Luminescent Materials and Devices
Institute of Polymer Optoelectronic Materials and Devices
South China University of Technology (SCUT)
Guangzhou (China)
Fax : (+86)20-87110606
E-mail: xuhuizhu@scut.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200299.
Scheme 1. a) The small-molecule electrolyte described in Ref. [11], and
b) the chemical structures of compounds 1–3.
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Scheme 2. Synthesis route to compounds 1 and 2. i) [Pd
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaboralane; iv) 1,3-dibromopropane, K2CO3, DMF, 808C; v) bis(pinacolato)diborane, KOAc, [PdCl
ane, 908C; vi) N,N-dimethylethylamine, THF, methanol; and vii) NaBF4, THF, methanol, H2O.
(PPh3)4], 2m Na2CO3 aqueous solution, toluene, ethanol, reflux; ii) nBuLi, THF, À788C; iii) 2-iso-
2ACHTUNGTRNEN(UNG PPh3)2], diox-
unit. The effects of the chemical structure on the resulting
material properties such as solubility and electron injection
in OLEDs are discussed.
Subsequently, coupling of Suzuki reagents 9/10 with com-
pound 13 gives compounds 14 and 15. Quaternization of
compounds 14 and 15 with ice-cooled N,N-dimethylethyla-
The synthesis of compounds 1–3 is straightforward and
outlined in Schemes 2 and 3, respectively. A mono-Suzuki
coupling of 4-methoxyphenylboronic ester (4) with 2,7-di-
bromo-9,9-diethylfluorene (5) and 2,7-dibromo-9,9-di(4-
ethoxylphenyl)fluorene (6) leads to compounds 7 and 8, re-
spectively, which undergo successive lithiation–boronation
reactions to give Suzuki reagents 9/10.
Treatment of 6-bromonaphth-2-ol with an excess amount
of 1,3-dibromopropane in the presence of K2CO3 produces
2-(3-bromopropoxy)-6-bromonaphthalene (11). A one-pot
catalytic boronation of compound 11 affords Suzuki reagent
12,[14] which further reacts with compound 5 to yield com-
pound 13.
Abstract in Chinese:
Scheme 3. Synthesis route to compound 3, analogue of compounds 1 and
2.[11]
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Organic Amorphous Electrolytes
mine affords the precursor BrÀ salts 16/17 in approximately
95% yield. The target compounds 1 and 2 are obtained by
adding an excess amount of aqueous NaBF4 solution to 16
and 17 in methanol. Compound 3 was prepared analogously
from 3,5-dibromophenol.[11]
Noticeably, replacement of the counteranion BrÀ with
À
BF4 results in a remarkable downshift of the proton signals
À
in the ammonium cation [ OCH2CH2CH2*N
A
(CH3*)2
(CH2*CH3)] in the 1H NMR spectra, which serves as an indi-
cator for the chemical conversion (see Figure S1 in the Sup-
porting Information).
The solubility of the electrolytes 1–3 in alcoholic solvents
was checked in an attempt to process from an orthogonal
solvent against the emissive layer. Replacement of the ethyl
groups in the fluorenyl unit by 4-ethoxyphenyl groups pro-
duced a considerable increase in solubility, consistent with
our recent observation.[15a] Thus, whereas 0.5 mg of com-
pound 1 was difficult to solubilize in 1 mL of methanol
under heating, 2 mg of 2 became soluble. On the other
hand, compound 3 had a similar solubility to 2.
Results and Discussion
Optical Properties
The UV/Vis absorption and photoluminescence (PL) spectra
of solutions of compounds 1–3 in CH2Cl2 (approximately
10À5 molLÀ1) and as films are shown in Figure 1, and the rel-
evant data are summarized in Table 1.
Compounds 1 and 2 show similar absorption and PL spec-
abs
max
em
max
tra in solution with l at approximately 356 nm and l at
400 and 420 nm, excited at the absorption maxima. In thin
films, these emission maxima are redshifted by approximate-
ly 10 nm to approximately 412 and 433 nm, respectively,
whereas
a well-resolved low-energy emission shoulder
emerges at approximately 460 nm.
In contrast, compound 3 possesses a shorter absorption
maximum (lambasx) at 285 nm and emission maximum (l ) at
em
max
Figure 1. UV/Vis and PL spectra of solutions of compounds 1–3 in
361 nm in solution due to the limited p conjugation. The ab-
sorption and emission maxima are shifted respectively to ap-
proximately 298 and 374 nm in thin solid films.
CH2Cl2 and as films on quartz.
Table 1. UV/Vis absorption, photoluminescence (PL), and thermal data for com-
pounds 1–3.
abs
max
em
max
UV/Vis (l [nm])[a]
PL (l [nm])[a]
Tm [8C] Td [8C][b] Tg [8C]
Thermal Properties
solution
solid
solution
solid
Thermal gravimetric analysis (TGA) shows that
compounds 1–3 decompose at approximately
3508C, which corresponds to about 5% weight loss
(see Figure S2 in the Supporting Information). Dif-
ferential scanning calorimetry (DSC) measurements
suggest that compound 1 is crystalline in the solid
1
2
3
356
357
285
344
358
298
400, 420 412, 433, 460 (sh)
400, 421 411, 433, 460 (sh)
265
–
–
351
353
345
–
152
143
361
375
[a] In CH2Cl2 solution (ꢀ10À5 molLÀ1). [b] Corresponding to 5% weight loss.
state; it melts at 2658C during the first heating run and re-
melts at 2608C after crystallization upon cooling.
cal stability. As expected from our previous study,[11] com-
pound 3 exhibits an inherent Tg of 1438C.
However, compound 2 is intrinsically amorphous and
shows a high glass transition temperature (Tg) of around
1528C for the first heating run. Thus, in addition to increas-
ing solubility, replacement of the pendent ethyl groups with
4-ethoxyphenyl moieties is greatly beneficial to morphologi-
Electron-Injecting Properties in OLEDs
On the basis of their amorphous morphology and processa-
bility in methanol, compounds 2 and 3 were evaluated as an
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Xu-Hui Zhu et al.
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electron-injecting layer (EIL) in OLEDs. In addition to a so-
lution-processable molecular red emitter,[15] the OLED char-
acterization was also conducted on a polymer emitter (P-
PPV) (Scheme 4). The electrolyte was processed from meth-
anol (2 mgmLÀ1) at a spin speed of 2000 rpm. Similar devi-
ces that involved an Al cathode devoid of the EIL were fab-
ricated for purposes of comparison.
Scheme 4. The chemical structures of the molecular and polymer emit-
ters.
The device characteristics of the red-light-emitting diodes
(ITO/PEDOT:PSS/PVK/red
emitter/cathode;
ITO=
indium–tin oxide; PEDOT:PSS=poly(3,4-ethylenedioxy-
thiophene)poly(styrenesulfonate); PVK=poly(vinyl carba-
zole)) are shown in Figure 2, and the relevant data are sum-
marized in Table 2. The Al device devoid of EIL shows the
highest turn-on voltage of 6.0 V, defined at a luminance of
1 cdmÀ2. In contrast, the turn-on voltage is considerably re-
duced to 3.2 V with a thin layer of the electrolyte between
the emitting layer and Al cathode. The enhanced electron
injection by the ionic compounds is further supported by the
presence of a higher current density when the devices are
turned on relative to the reference device (Figure 2a).
Figure 2. a) J–V–L and b) LE–J characteristics of the OLEDs: ITO/PE-
DOT:PSS/PVK/red emitter/cathode (Al, 2/Al, and 3/Al).
Table 2. The summary of OLED data concerning different cathodes.
EML
Cathodes LEmax
@ꢀ20 mAcmÀ2
Vturn-on
[cdAÀ1
]
[V][a]
V
L
LE
EXE
[%]
[V] [cdmÀ2
]
[cdAÀ1
]
2/Al
3/Al
Al
2/Al
3/Al
Al
1.92
1.70
0.19
16.6
13.0
0.95
6.8
7.6
8.4
6.2 2777
7.0 2100
236
202
32
1.25
0.93
0.16
14.0
10.4
0.95
1.62
1.21
0.21
4.59
3.41
0.31
3.2
3.2
6.0
2.4
2.6
4.8
red
emitter
The OLED that contained compound 2 produced the
highest performance: maximum luminance efficiency
P-PPV
max
(LEmax)=1.92 cdAÀ1 (h =2.48%, EXE=external device
EXE
8.6
199.8
efficiency) versus 1.70 cdAÀ1 (h =2.2%), for the 3/Al
max
EXE
[a] At a luminance of 1 cdmÀ2
.
device and 0.19 cdAÀ1 for the Al device at approximately
20 mAcmÀ2. Whereas the luminance (L)=202 cdmÀ2, LE=
0.93 cdAÀ1, and V=7.6 V for the 3/Al device; and L=
236 cdmÀ2, LE=1.25 cdAÀ1, and V=6.8 V for the 2/Al
device. The latter parameters are nearly comparable to
those reported for a device with a Ba/Al cathode.[15a] No EL
characteristics show that the electron-only device that in-
cludes 2 exhibits a constantly larger current density than the
3-based device (Figure 4), thus giving rise to improved EL
characteristics with higher luminous efficiency and lower
working voltage. The reference Al device has the least elec-
tron current density, particularly at high voltages, which sug-
gests that electron injection is hindered.
emission occurs from the electrolytes, thus yielding pure red
EL
emission with l at 622 nm (Figure 3).
max
To gain direct insight into the electron-injection proper-
ties of compounds 2 and 3, the electron-only devices (ITO/
Al/red emitter (100 nm)/cathode) were fabricated. The J–V
The characterization of the OLEDs that contain the poly-
mer emitter (ITO/PEDOT:PSS/P-PPV/cathode) further sup-
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Organic Amorphous Electrolytes
Figure 3. EL spectra of the OLEDs: ITO/PEDOT:PSS/PVK/red emitter/
cathode (2/Al and 3/Al).
Figure 4. J–V characteristics of the electron-only devices: ITO/Al/red
emitter (100 nm)/cathode (Al, 2/Al, and 3/Al).
Figure 5. a) J–V–L and b) LE–J characteristics of the OLEDs: ITO/PE-
DOT:PSS/P-PPV/cathode (Al, 2/Al, and 3/Al).
ports the notion that compound 2 is a more effective elec-
tron-injection material. Thus, at around 20 mAcmÀ2, L=
2777 cdmÀ2, LE=14.0 cdAÀ1, and V=6.2 V for the 2/Al
device, whereas L=2100 cdmÀ2, LE=10.4 cdAÀ1, and V=
7.0 V for the 3/Al device (Table 2 and Figure 5).
The reduction of the built-in potential of the OLED has
been considered to contribute to the enhanced electron in-
jection on account of the formation of a favorable dipole
provided by the thin interfacial layer.[16] Therefore, photo-
voltaic measurements were carried out to estimate the built-
in potentials of the devices (ITO/PEDOT:PSS/PVK/red
emitter/cathode).
Figure 6. Current density/voltage curves for the devices: ITO/PE-
DOT:PSS/PVK/red emitter/cathode (Al, 2/Al, and 3/Al), under white-
light irradiation.
Under white-light illumination, the devices that consisted
of 2/Al and 3/Al cathodes show an open-circuit voltage (Voc)
of 1.5 and 1.3 V, respectively (Figure 6). By contrast, the Voc
of the reference Al device was considerably lower than the
expected value of around 1.0 V.[4b] Nevertheless, the loss of
Voc suggests an existing large energy barrier for electron
transport across the interface between the neat Al cathode
and molecular red-emitter layer.[17]
CsF, a vacuum-deposited inorganic electron-injection mate-
rial. For instance, at approximately 20 mAcmÀ2, L=
156 cdmÀ2, and LE=0.80 cdAÀ1 in the OLED structure
(ITO/PEDOT:PSS/PVK/red emitter/CsF/Al; see Figure S3
in the Supporting Information).
Finally, it is interesting to observe that compound 2 pro-
vides largely improved device efficiency, with respect to
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9H), 2.05–2.16 (m, 4H), 2.16–2.32 (m, 2H), 3.11 (s, 6H), 3.32–3.49 (m,
2H), 3.50–3.59 (m, 2H), 3.84 (s, 3H), 3.91–4.06 (m, 4H), 4.14–4.26 (m,
2H), 6.76–6.79 (m, 4H), 6.95 (d, J=8.67 Hz, 2H), 7.12–7.19 (m, 2H),
7.21–7.26 (m, 4H), 7.49–7.59 (m, 6H), 7.64–7.67 (m, 4H), 7.70–7.80 (m,
7H), 8.02 ppm (s, 1H); ESI-MS: m/z calcd: 988.53; found: 988.80
(100%) [MÀBF4].
Conclusion
In summary, we have shown that replacement of pendant
aliphatic chains by appropriate rigid bulky groups leads to
a general increase in both the solubility and morphological
stability of the organic electrolytes. Furthermore, the chemi-
cal structures of the compounds exert a distinct effect on
the electron-injection properties. The choice of the emitting
materials can be extended to some other interesting molecu-
lar emitters that show high solid-state PL efficiency and bi-
polar charge transport.[15b,18] In light of the general applica-
tion potentialities in the field of organic optoelectronics in-
cluding organic field-effect transistors and organic solar
cells, this class of amorphous ionic molecular compounds
that combines concise synthesis, facile purification, and alco-
hol processability should merit further research.
Synthesis of Compound 3
Prepared from 21 according to the procedure described for 1. Yield:
96.4%. 1H NMR (300 MHz, CDCl3): d=1.33 (t, J=6.45 Hz, 3H), 2.18
(m, 2H), 3.06 (s, 6H), 3.35–3.50 (m, 4H), 4.14 (m, 2H), 7.18 (s, J=
1.20 Hz, 2H), 7.42–7.57 (m, 16H), 7.65 (m, 3H), 7.81–7.90 (m, 12H),
8.06 ppm (d, J=7.95 Hz, 4H); MALDI-TOF: m/z calcd: 864.42; found:
864.31 (100%) [MÀBF4].
OLED Fabrication and Characterizations
The potentiality of compounds 2 and 3 was studied in OLEDs [ITO/
PEDOT:PSS
the reference device with the Al cathode (EML=red emitter (45 nm),
x=40 nm; EML=P-PPV (80 nm), x=0 nm). Patterned (15 WsquareÀ1
ACHTUNGTRNE(UNGN 40 nm)/PVKACHTUNREGTG(NNUN x nm)/EML/2, 3/AlCAHTNUGTRENUN(GN 200 nm)], together with
)
indium–tin oxide (ITO)-coated glass substrates were cleaned with dis-
tilled water, acetone, detergent, distilled water, and 2-propanol succes-
sively in an ultrasonic bath. After treatment with oxygen plasma, PE-
DOT:PSS (Baytron P4083, Bayer AG) was spin-coated from water at
3000 rpm onto the ITO substrate and dried in a vacuum oven at 808C for
8 h. PVK (Aldrich, MW 1100000) was coated atop the PEDOT:PSS
layer and dried at 1008C for approximately 15 min.
Experimental Section
General
All manipulations involving air-sensitive reagents were performed under
an inert atmosphere of dry nitrogen. THF was dried over Na/benzophe-
none and distilled prior to use. All the intermediates were isolated and
analyzed by TLC on silica gel. The syntheses of precursor compounds 16
and 17 are presented in the Supporting Information, as well as compound
21, which was prepared by a modified procedure.[11] All other starting
materials were purchased commercially and were used as received,
unless otherwise specified.
Subsequently, a thin film of the red emitter was spin-cast from p-xylene
solution (25 mgmLÀ1) at 3000 rpm and dried at 1308C for around 10 min.
By contrast, the polymer emitting layer P-PPV was spin-coated from p-
xylene solution (6 mgmLÀ1) at 1000 rpm. Compounds 2 and 3 were spin-
cast atop the emitting layer from a methanol solution (2 mgmLÀ1) at
2000 rpm. The CsF layer and the Al cathode were thermally deposited
through a mask in vacuum (<3ꢁ10À4 Pa) with the deposition speed and
thickness monitored by a thickness/rate meter. All steps except the proc-
essing of PEDOT:PSS were performed in a glovebox. The active area of
the device was 0.19 cm2.
Physical Measurements
1H NMR spectra were recorded with a Bruker AV 300 spectrometer with
deuterated solvents as the internal reference. TOF-MS was performed
with a KOMPACT MALDI mass spectrometer (Shimadzu/Kratos) in the
positive-ion mode with a matrix of dithranol. ESI mass spectrometry was
conducted with an Esquire HCT PLUS LC/MSn System in the positive
electrospray ionization (ESI+) mode. UV/Vis absorption spectra were
obtained with an HP 8453 spectrophotometer. Photoluminescence spec-
tra were measured with a Jobin–Yvon spectrofluorometer JY Fluorolog-
The current density/luminance/voltage characteristics were measured
with a Keithley 236 source measurement unit and a calibrated silicon
photodiode. The EL spectra were collected with a PR-705 photometer.
The photovoltaic measurements were carried out under the white-light il-
lumination of a xenon lamp.
3
spectrofluorometer. TGA was conducted with a TG 209 F1
(NETZSCH) thermal analysis system under a heating rate of 208CminÀ1
from 30 to 5508C. DSC was run with a DSC 204 F1 (NETZSCH) thermal
analysis system. The sample was heated from À608C at
a rate of
Acknowledgements
208CminÀ1
.
G. Liu, Y.-H. Li, and W.-Y. Tan contributed equally to this work. Finan-
cial support from NSF and MOST of China is gratefully acknowledged
(grant nos. 2012ZZ0001, 51173051, 2009CB930604, and 2009CB623601).
Synthesis of Compound 1
An excess amount of sodium tetrafluoroborate (1 g, 9.11 mmol) in water
(5 mL) was added to a solution of compound 16 (0.23 g, 0.26 mmol) in
mixed solvents of THF (30 mL) and methanol (30 mL). After the mixture
was stirred at room temperature for 3 d, the solvents were removed
under vacuum. The residue was extracted with CH2Cl2. The organic layer
was dried over anhydrous MgSO4, filtered, and concentrated under re-
duced pressure. The resulting white solid was redissolved in a minimum
amount of CH2Cl2, filtered, and reprecipitated with heptane. Yield:
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1
0.22 g (93.6%). H NMR (300 MHz, CDCl3): d=0.41–0.52 (m, 12H), 1.39
(t, J=7.05 Hz, 3H), 2.06–2.35 (m, 10H), 3.12 (s, 6H), 3.32–3.44 (m, 2H),
3.48–3.51 (m, 2H), 3.88 (s, 3H), 4.12–4.25 (m, 2H), 6.98–7.05 (m, 2H),
7.12–7.19 (m, 2H), 7.51–7.58 (m, 2H), 7.61–7.69 (m, 8H), 7.74–7.86 (m,
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Prepared from 17 according to the procedure described for 1. Yield:
1
95.2%. H NMR (300 MHz, CDCl3): d=0.35–0.49 (m, 6H), 1.26–1.45 (m,
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Organic Amorphous Electrolytes
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Received: April 6, 2012
Published online: && &&, 0000
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
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These are not the final page numbers! ÞÞ

FULL PAPERS
Molecular Devices
Gang Liu, Yan-Hu Li, Wan-Yi Tan,
Zhi-Cai He, Xiao-Tie Wang,
Chi Zhang, Yue-Qi Mo, Xu-Hui Zhu,*
Junbiao Peng, Yong Cao
&&&& — &&&&
Alcohol-Processable Organic Amor-
phous Electrolytes as an Effective
Electron-Injection Layer for Organic
Light-Emitting Diodes
Light work: A thin layer of the alco-
hol-processable amorphous small-mol-
ecule electrolyte that features a linear-
conjugated unit (see scheme) provides
improved electron injection from the
Al metal and thus better organic light-
emitting diode performance, unlike its
counterpart that contains a rigid
branched bulky moiety.
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www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!