Modification of an ITO anode with a hole-transporting SAM for improved OLED device characteristics

Article abstract of DOI:10.1039/b206939c

Modification of inorganic electrodes has attracted much attention in the study of organic semiconductor devices. An ethoxysilane functionalized hole-transporting triphenylamine (TPA-CONH-silane) was synthesized and was self-assembled to form a monolayer o

Full text of DOI:10.1039/b206939c

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Modification of an ITO anode with a hole-transporting SAM for
improved OLED device characteristics{
Jaemin Lee,a Byung-Jun Jung,a Jeong-Ik Lee,*b Hye Yong Chu,b Lee-Mi Dob and
Hong-Ku Shim*a
aDepartment of Chemistry and School of Molecular Science (BK21), Center for Advanced
Functional Polymers (CAFPoly), Korea Advanced Institute of Science and Technology
(KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea.
E-mail: hkshim@mail.kaist.ac.kr
bBasic Research Laboratory, Electronics and Telecommunications Research Institute
(ETRI), 161 Gajeong-dong, Yuseong-gu, Daejeon 305-350, Republic of Korea.
E-mail: jiklee@etri.re.kr
Received 16th July 2002, Accepted 2nd September 2002
First published as an Advance Article on the web 4th October 2002
Modification of inorganic electrodes has attracted much attention in the study of organic semiconductor
devices. An ethoxysilane functionalized hole-transporting triphenylamine (TPA–CONH–silane) was synthesized
and was self-assembled to form a monolayer on an indium tin oxide (ITO) anode. The modified surface was
characterized by water contact angle, X-ray photoelectron spectroscopy (XPS), ellipsometry and atomic force
microscopy (AFM). The increase in surface work function is expected to facilitate hole injection from the
ITO anode. To investigate the effect of a self-assembled monolayer (SAM) on the characteristics an organic
light-emitting diode (OLED), a typical OLED device [SAM-modified ITO/TPD (50 nm)/Alq3 (60 nm)/LiF
(1 nm)/Al (70 nm)] was fabricated. The SAM-modified device could endure a higher current and showed a
much higher luminance (6300 cd m22) than the bare ITO device (2700 cd m22). The external quantum
efficiency was also shown to improve as a result of the presence of the SAM. In addition to these device
characteristics, a study of the TPD film morphology revealed an enhanced thermal stability for the
SAM-modified device. The variation of the terminal group of the SAM and possible further optimization
of SAM-modified OLEDs is under investigation.
report application of a SAM as either a current blocking layer12
Introduction
or a moisture penetration blocking layer.13 Enhanced charge
injection as a result of the surface dipolar layer of a SAM has
also been reported.11,14 Schwartz et al. reported a similar
phenomenon, that variation of the molecular dipole adsorbed
on the ITO surface can change the work function of the ITO.15
Layer-by-layer self-assembly of hole-transporting polymers
has been reported to increase the quantum efficiency of the
device,16 and the use of a polyaniline modified ITO electrode
for polymer light-emitting diodes (PLEDs) has also been
reported.17 In addition to their use in OLED devices, SAMs
with various kinds of surface properties can be applied to
molecular electronic devices such as chemical- and bio-
sensors,18 field-effect transistors (FETs),19,20 liquid crystal dis-
plays (LCDs)21 and microelectromechanical systems (MEMS).22
In this paper we report on the application of a SAM to an
ITO anode and report the characteristics of an OLED that
makes use of this modified anode. We have introduced a
triethoxysilane functionality into the hole-transporting triphe-
nylamine moiety both as constituents of a SAM for application
to the hydroxy surfaces. Introduction of a hole-transporting
moiety at the end of the SAM is expected to change the surface
work function as well as to improve the adhesion between the
inorganic ITO and the hole-transporting organic layers.
Expertise in electroluminescent organic1 and polymeric2
materials and devices has grown progressively during the last
decade, notably in the development of high performance
organic semiconductors and the optimization of device struc-
tures. In particular, from the standpoint of device optimization,
it has now been found that the organic/inorganic interface
cannot be neglected; although the importance of this interface
was missed in the early stages of organic light-emitting diode
(OLED) development. Poor adhesion between organic and
inorganic electrodes is known to cause device failure3 and the
crystallization of organic films.4 In the search for improved
interfacial contact, a sol–gel process forming hybrid organic–
inorganic materials at an indium tin oxide (ITO) anode has
been tried.5–7 Similarly, the use of a high work function oxide
anode, which facilitates hole injection, instead of the more
commonly used ITO has been reported recently.8 Therefore
modification and development of the inorganic electrode is an
active research area.
Self-assembled monolayers (SAMs) are in the limelight
as new tools for enhancing the macroscopic bulk properties
and microscopic molecular properties of functional materials.
SAMs are being studied extensively because of their ease of
formation and their wide applicability.9 It is well known that
surface hydroxy groups on the ITO can support a SAM with
various kinds of functionalities, such as carboxylic acids,10
phosphonic acids11 and silanes.12,13 Some previous studies
Experimental
[4-(Diphenylamino)phenyl]methanol (TPA–CH2OH) was pre-
pared by formylation of triphenylamine through the Vielsmeier
reaction23 followed by reduction of the aldehyde. {[4-
(Diphenylamino)phenyl]methoxy}-N-(4,4,4-triethoxy-4-silabutyl)-
formamide (TPA–CONH–silane) was then obtained by letting
{Electronic supplementary information (ESI) available: 3D struc-
ture of TPA–CONH–silane. See http://www.rsc.org/suppdata/jm/b2/
b206939c/
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This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b206939c

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TPA–CH2OH react with 3-(isocyanatopropyl)triethoxysilane.
The crude product was purified by column chromatography,
yielding a white powder. This kind of silane synthesis is widely
used,6,18,21 because it is easy to prepare and contains no metal
residue, which is seldom able to be removed. In addition, the
amide linkage in TPA–CONH–silane is known to improve the
ordering of the SAM through interchain hydrogen bonding.24
Details of the syntheses, surface modification and device
experiments are described in the following paragraphs.
in the case of the ITO). Then they were treated with a 1 : 1 : 5
solution of NH4OH, 35% H2O2 and H2O at 80 uC for 20 min.
The substrates were then placed in water and finally dried
under a stream of dry nitrogen. The cleaned substrates were
used immediately upon preparation.
Monolayer characterization
Static water contact angles were measured using a sessile drop
goniometer. XPS measurements were made using an ESCA-
LAB 200R X-ray photoelectron spectrometer with a mono-
chromatic Al Ka source. The thickness of the SAM was
measured by ellipsometry using an automatic ellipsometer
(Rudolph AutoEL-II) equipped with a HeNe laser (623.8 nm).
Both the imaginary and the real refractive indices of the silicon
wafer were measured prior to self-assembly. The morphologies
of the substrates were evaluated by atomic force microscopy
(AFM) using an AutoProbe CP (Park Scientific Instruments,
Inc.). The measurement was performed in contact mode using
triangle Si3N4 cantilevers in air at room temperature. Surface
work functions were measured using atmospheric photoelec-
tron spectroscopy (RIKEN Keiki AC-2).
Materials
Triphenylamine, phosphorus oxychloride, (3-isocyanatopro-
pyl)triethoxysilane and lithium fluoride were purchased from
Aldrich and used without further purification. TPD [N’-bis(3-
methylphenyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine] and
Alq3 [tris-(8-hydroxyquinoline)aluminium] were purchased from
TCI and purified by gradient sublimation. Sodium acetate
trihydrate, sodium borohydride, sodium hydroxide, triethyl-
amine, pyridine and magnesium sulfate were purchased from
Junsei and used without further purification. All other solvents
and reagents were analytical-grade quality, purchased commer-
cially and used as received unless otherwise indicated. Anhy-
drous grade solvents were used during the SAM formation steps.
Device fabrication
The bare ITO and SAM-modified ITO were loaded into a bell
jar deposition chamber housed in a nitrogen gas glove box. At
1026 Torr, a 50 nm layer of TPD was first deposited, followed
by 60 nm of Alq3 and 1 nm of LiF. Finally, a 70 nm thick
aluminium cathode was deposited.
[4-(Diphenylamino)phenyl]methanol (TPA–CH2OH)
TPA–CH2OH was synthesized by reduction of 4-(diphenyl-
amino)benzaldehyde (TPA–CHO) with sodium borohydride
(NaBH4). TPA–CHO was prepared using the well-known
Vielsmeier reaction according to a literature procedure.23 0.290 g
(7.60 mmol) of NaBH4 dissolved in 15.0 mL of aqueous 0.1 M
NaOH solution was added dropwise into 4.01 g (14.7 mmol) of
TPA–CHO in 50.0 mL of ethanol. The mixture was reacted at
room temperature for 4 h. The solution was extracted with
CH2Cl2–H2O, dried with magnesium sulfate (MgSO4) and then
rotary evaporated. Recrystallization with dichloromethane–
Device characterization
Electroluminescence spectra were obtained with a Minolta
CS-1000. The current–voltage and luminance–voltage char-
acteristics were taken with a current–voltage source (Keithley
238) and a Minolta LS-100.
1
hexane gave a white solid. The yield was 3.72 g (91.9%). H
Results and discussion
NMR (DMSO-d6, ppm) d: 7.21–7.26 (m, 6H), 6.92–6.99
(m, 8H), 5.17 (t, 1H), 4.42 (d, 2H). 13C NMR (DMSO-d6, ppm)
d: 147.369, 145.858, 137.413, 129.316, 127.822, 123.316,
122.484, 62.658. FT-IR(KBr, cm21) nmax: 3244.0 (nO–H).
For the formation of the SAM (Fig. 1), both the Si wafer
and the ITO were cleaned by a conventional wet cleaning
method.25,26 The pre-cleaned oxide substrates were immersed
in 1 mM anhydrous toluene solution of TPA–CONH–silane
for 1 h, followed by thorough rinsing and sonicating in toluene
for 5 min. They were then transferred to a 100 uC oven under
vacuum for 30 min, after which they were sonicated in toluene
again and finally dried under a stream of nitrogen.
{[4-(Diphenylamino)phenyl]methoxy}-N-(4,4,4-triethoxy-4-
silabutyl)formamide (TPA–CONH–silane)
Within a 250 mL two-necked round-bottomed flask, 10.0 g
(36.3 mmol) of TPA–CH2OH was dissolved in 30.0 mL of
DMF. 10.3 mL (72.6 mmol) of triethylamine was added to
the flask and stirred for a while. Then 8.50 mL (33.0 mmol) of
(3-isocyanatopropyl)triethoxysilane was added into the reaction
mixture and stirred at 70 uC for 48 h. After cooling to room
temperature, the reaction mixture was vacuum concentrated
and the product was purified by column chromatography (ethyl
acetate : hexane ~ 1 : 2). Recrystallization with dichloro-
methane–hexane gave a white solid. The yield was 7.59 g
Si wafers with native oxide were sometimes used instead of
ITO substrates for the convenience of monolayer characteriza-
tion; because ITO has a rough surface morphology and its
transparency is not suitable for surface characterization with,
for example, ellipsometry. It has already been reported that
both Si wafer and ITO have a similar surface density of reactive
sites, about 1026 mol m22 27
First, we measured the water
.
contact angles before and after self-assembly. Because the
hydroxy groups of the oxide layer generate a hydrophilic
surface, both bare Si wafer and bare ITO showed very low
water contact angle values (h % 15u), but after self-assembly,
relatively high values (h ~ 77u for Si wafer and h ~ 75u for
ITO) were observed, indicating that the surfaces of the
substrates had become hydrophobic. As well as these contact
angle measurements, an XPS study was also performed to
investigate the surface atomic composition. Fig. 2 shows the
XPS spectra of Si wafers before (lower) and after (upper) self-
assembly. Each spectrum was normalized with respect to the
peak intensities of abundant elements, that is, Si 2p for Si
wafers. Increases in the C 1s (284 eV) and N 1s (410 eV)
peak intensities confirmed the formation of a SAM com-
posed of carbon, nitrogen, hydrogen and oxygen. In addition
to the above two experiments, the thickness of the SAM
1
(44.4%). mp 80 uC. H NMR (CDCl3, ppm) d: 7.19–7.25 (m,
6H), 6.99–7.07 (m, 8H), 5.01 (s, 2H), 4.94 (s, 1H), 3.78 (q, 4H),
3.17 (q, 2H), 1.58–1.66 (m, 2H), 1.20 (t, 9H), 0.61 (t, 2H). 13C
NMR (CDCl3, ppm) d: 156.405, 147.701, 147.590, 130.497,
129.303, 129.134, 124.326, 124.019, 123.504, 122.875, 66.254,
58.389, 43.422, 23.255; 18.229, 7.578. FT-IR (KBr, cm21) nmax
:
3340.1 (nN–H), 1710.0 (nC~O). EI-MS calcd. for C29H38N2O5Si
522.26, found 522.25 (M1). Anal. (calcd.) C 66.73 (66.64), H
7.78 (7.33), N 5.45 (5.36).
Substrate preparation
Substrates were first cleaned with chloroform, water and then
treated with a 7 : 3 solution of concentrated H2SO4 and 35%
H2O2 at 120 uC for 10 min (This acid-treatment step is omitted
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Fig. 1 SAM formation.
Fig. 2 XPS spectra of Si wafers: upper, TPA–CONH–silane SAM;
lower, bare.
was measured by optical ellipsometry. The thickness of the
˚
TPA–CONH–silane SAM on the Si wafer was 17 A, and the
distance between the Si atom and a proton of the benzene ring
in a fully extended TPA–CONH–silane molecule was calcu-
28
lated to be 18.35 A. This implies that a layer of mono-
˚
molecular scale had successfully formed. The surface roughness
was confirmed by an AFM study of the Si wafers. Fig. 3 shows
the AFM images of a bare Si wafer and a SAM-modified
Si wafer. The observed RMS roughness before and after
Fig. 3 AFM images of: (a) bare Si wafer; and (b) SAM-modified Si
wafer.
˚
self-assembly was nearly identical, 1.03 A for the bare Si
˚
wafer (Fig. 3(a)) and 1.21 A for the SAM-modified Si wafer
(Fig. 3(b)), indicating that a uniform and flat monolayer is
formed during the self-assembly process. We also measured
the surface work function values of the ITO substrates using
atmospheric photoelectron spectroscopy. The bare ITO
exhibited a surface work function of 4.80 eV, but the SAM-
modified ITO exhibited a value of 5.30 eV, which indicates that
the surface work function of the anode had changed as a result
of the SAM formation. This measured value coincides with
the HOMO level of TPA–CONH–silane, 5.33 eV, determined
electrochemically using cyclic voltammetry (CV).29 The surface
characterization results are summarized in Table 1.
Table 1 Monolayer characterization results
Bare
SAM
Water contact angle/u
%15
%15a
—
1.03
4.8
77
75a
17
˚
Ellipsometry/A
b
RMS roughness /A
˚
1.21
5.3
Surface work functiona/eV
aValues are for ITO rather than Si wafers. bScanning region is a
square of 1 mm 6 1 mm.
To investigate the effect of the SAM on the OLED device
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Fig. 4 (a) OLED device configuration. (b) Energy band diagram of the
device.
Fig. 5 Device characteristics ($, SAM-modified device; &, control
device): (a) luminance–voltage diagram (inset: luminance–current
density diagram); (b) quantum efficiency–voltage diagram.
characteristics, a typical OLED device (SAM-modified ITO/
TPD (50 nm)/Alq3 (60 nm)/LiF (1 nm)/Al (70 nm)) was
fabricated, where TPD is the hole-transporting material and
Alq3 the emitting and electron-transporting material (Fig. 4(a)).
For comparison, a control device using bare ITO was fabri-
cated at the same time. Both devices showed Alq3 emission
of 525 nm. Fig. 5(a) shows the luminance–voltage diagram of
the devices. Although the devices exhibit similar threshold
turn-on voltages, the SAM-modified device shows much higher
brightness, 6300 cd m22, compared with that of the control
device, 2700 cd m22. The inset of Fig. 5(a) clearly shows the
difference between the two devices. The current limit of the
control device is 150 mA cm22, while the SAM-modified device
and the deposited TPD films was further studied. TPD is
the most widely used hole-transporting material in OLED
fabrication, but the relatively low glass-transition temperature
(Tg) of TPD (59–60 uC) is a disadvantage. Because the passage
of current during device operation leads to a temperature
rise, low Tg organic films are subject to morphology change.
Fig. 6(a) and (b) show the SEM images of TPD films deposited
on bare ITO substrates before and after heating at 80 uC for 1 h,
respectively. Fig. 6(c) is for a SAM-modified ITO substrate
after heating at 80 uC for 1 h. As can be seen, bare ITO shows
dewetting of the TPD molecules on heating whereas the SAM-
modified ITO maintains perfect wetting of the TPD and a flat
surface morphology. The dewetting of the TPD means that
when the LED device is operating, pinhole formation is
accelerated with increased dewetting of the TPD. The perfect
wetting of the SAM-modified ITO is due to the structural
similarity of the terminal groups of the SAM and of the
deposited TPD molecules. Therefore it has been shown that
interface compatibility problems can be solved through organic
SAM formation.
Although an improvement in the performance of the OLED
device was proved as a result of a SAM, the constant device
turn-on voltage is still open to further discussion. There are
two kinds of effect of SAM-modification on the device char-
acteristics. One is the change of device turn-on voltage, by
lowering the threshold of charge injection, and the other is the
improvement of interfacial contact between the electrode and
organics. In our case, the HOMO values of TPA and of TPD
are not significantly different to lower the turn-on voltage
can endure more than two times higher current, 390 mA cm22
.
This is why the SAM-modified device shows higher brightness
than the control device. In addition to this durability effect,
Fig. 5(b) shows an enhancement of the external quantum
efficiency of the SAM-modified device compared with the
control device. A maximum external quantum efficiency of
1.1% was measured in the SAM-modified device, compared
to 0.92% in the control device. As mentioned earlier, in the
discussion of the surface characterization, the SAM-modified
ITO has a surface work function of 5.30 eV. Fig. 4(b) shows the
energy band diagram of the device. An energy level difference
between the ITO anode and the TPD of 0.7 eV is quite a high
barrier for hole injection, so the presence of a hole-transporting
SAM reduces this barrier to facilitate hole injection and
consequently enhances the quantum efficiency of the device.
In order to assess the durability of the device, the effect of the
SAM on the interfacial morphology30 between the ITO anode
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Acknowledgements
This work was supported by the Center for Advanced
Functional Polymers (CAFPoly) through KOSEF and the
Ministry of Information and Communications (MIC),
Republic of Korea.
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Conclusion
An ethoxysilane functionalized hole-transporting triphenyl-
amine (TPA–CONH–silane) was synthesized and this was self-
assembled to form a monolayer on the ITO anode. Various
aspects of SAM formation were characterized and an OLED
device based on a SAM-modified ITO was fabricated. The
SAM-modified device could endure higher current and showed
much higher luminance than the bare ITO device. External
quantum efficiency as well as maximum luminance was shown
to improve as a result of SAM formation. A study of the TPD
film morphology revealed an enhanced thermal stability of the
device modified with the SAM. The variation of the terminal
group of the SAM and possible further optimization of the
SAM-modified OLEDs is under investigation.
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C.-H. Kim, J. H. Hahn and J. W. Park, Langmuir, 1997, 13, 4305;
(b) S.-Y. Oh, Y.-J. Yun, D.-Y. Kim and S.-H. Han, Langmuir,
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using an MM2 energy minimization method. Refer to Electronic
Supplementary Information{.
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electrode cell in a solution of Bu4NBF4 (0.10 M) in acetonitrile at
a scan rate of 50 mV s21. The HOMO level of the compound
was calculated regarding the oxidation potential of ferrocene as
24.80 eV.
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3498
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