Effect of magnesium oxide buffer layer on performance of inverted top-emitting organic
light-emitting diodes
Ho Won Choi, Soo Young Kim, Woong-Kwon Kim, Kihyon Hong, and Jong-Lam Lee
Citation: Journal of Applied Physics 100, 064106 (2006); doi: 10.1063/1.2349552
View online: http://dx.doi.org/10.1063/1.2349552
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/100/6?ver=pdfcov
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JOURNAL OF APPLIED PHYSICS 100, 064106 共2006兲
Effect of magnesium oxide buffer layer on performance of inverted
top-emitting organic light-emitting diodes
Ho Won Choi, Soo Young Kim, Woong-Kwon Kim, Kihyon Hong, and Jong-Lam Leea兲
Department of Materials Science and Engineering, Pohang University of Science and Technology
(POSTECH), Pohang, Kyungbuk 790-784, Korea
共Received 10 May 2006; accepted 7 July 2006; published online 26 September 2006兲
The effect of magnesium oxide 共MgO兲 buffer layer between cathode and emitting materials on
performance of inverted top-emitting organic light-emitting diodes 共ITOLEDs兲 was investigated.
The operation voltage at the current density of 100 mA/ cm2 decreased from 14.9 to 9.7 V for
ITOLEDs with 1 nm thick MgO buffer layers. The maximum luminance value increased about 78%
in ITOLEDs using MgO buffer layer, which is 1000 cd/ m2 at the current density of 191 mA/ cm2.
Synchrotron radiation photoelectron spectroscopy results revealed that the atomic concentration of
Al–O bond increased after deposition of MgO on Al, indicating the oxidation of Al surface.
Secondary electron emission spectra showed that the work function increased about 0.8 eV by
inserting the insulating MgO buffer layer. Therefore, the enhancement of device performance results
from the decrease of the energy barrier for electron injection based on the tunneling model. © 2006
American Institute of Physics. 关DOI: 10.1063/1.2349552兴
I. INTRODUCTION
Top-emitting organic light-emitting diodes 共OLEDs兲
have been introduced for use in high-contrast, full-color, flatpanel, and head-up displays. Such a top-emitting structure
has attracted considerable interest for an active matrix OLED
共AMOLED兲 display fabricated on opaque substrates. In particular, inverted top-emitting OLEDs 共ITOLEDs兲 that have a
reflective cathode at the bottom and a 共semi兲transparent anode on top are more preferable due to the use of generally
superior n-type transistors rather than p-type transistors in
AMOLEDs.1 In order to improve the performance of
ITOLEDs, it is necessary to prepare a high reflective bottom
cathode and enhance the injection efficiencies of electrons.
Aluminum 共Al兲, being much more stable and resistant to
oxidation than low work function metals such as Mg, Ca, or
Li,2 is a highly desired ITOLED cathode due to its high
reflectivity 共⬃93% 兲3 at the wavelength of 520 nm. However, it makes a poor OLED cathode due to its comparative
high work function 共⬃4.3 eV兲.4 Thus, in order to enhance
the electron injection and achieve improved device performance a proper electron injection layer 共EIL兲 must be inserted at the interface of organic layers with Al electrodes.
Two models for enhancing electron injection have been
discussed.5 Both models commonly demonstrated a method
to control electron injection by inserting a thin insulating
layer between the cathode and the emitting materials. The
first one is the tunneling probability enhancement model5–9
which states that the use of an ultrathin insulating layer can
generate interfacial dipoles to drop voltage across this layer.
This resulted in alignment of the Fermi level of the cathode
and the lowest-unoccupied molecular orbital 共LUMO兲 energy level of the emitting layer, enhancing electron injection.
The other is the chemical reaction model5,8 which states that
alkali or alkaline earth atoms in the compounds formed by
a兲
Electronic mail: [email protected]
0021-8979/2006/100共6兲/064106/6/$23.00
doping the near interface region of emitting materials
through coevaporation with Cs,10 or by depositing a thin
layer of LiF,11 or other alkaline metal insulators12 can improve electron injection due to their low work functions. It
was suggested that the enhancement of electron injection by
two models results from the decrease of the energy barrier
for electron injection. It was reported that CsF,5 NaSt,5 poly共methyl methacrylate兲,7 or SiO29 were inserted between the
cathode and emitting layer as an EIL to reduce turn-on voltage. However, no works were conducted on magnesium oxide 共MgO兲 as an EIL. It is well known that MgO with a wide
band gap 共6.0– 7.8 eV兲13 and high dielectric constant of 9.96
has good insulating properties.14 Thus, it is expected that
electron injection could be improved by inserting a thin MgO
buffer layer, lowering the electron tunneling barrier.
In this work, we report the effect of MgO buffer layer on
enhancement of electron injection in ITOLEDs. The change
in the work function with the insertion of MgO layer was
examined using synchrotron radiation photoelectron spectroscopy 共SRPES兲. From this, the model for improvement of
device performance by insertion of the thin MgO layer is
discussed.
II. EXPERIMENT
Schematic structure of ITOLEDs is shown in Fig. 1. The
devices were built on glass substrates precoated with Ti/ Al
which was used as the cathode directly 关Fig. 1共a兲兴 or covered
with an ultrathin insulating layer on the Ti/ Al cathodes 关Fig.
1共b兲兴. The glass was used as the starting substrate. The glass
surface was first immersed sequentially in ultrasonic baths of
acetone and isopropyl alcohol, followed by cleaning in deionized water. Six kinds of samples were prepared. Titanium
共Ti兲 was used as a glue layer for Al film in order to enhance
the adhesion of Al film on glass.15 The cleaned glass substrates were loaded into a thermal evaporator, followed by
the deposition of Ti 共20 nm兲 and Al 共150 nm兲 layers. The
100, 064106-1
© 2006 American Institute of Physics
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FIG. 1. Schematic device structures of inverted top-emitting devices: 共a兲
without and 共b兲 with the insulating buffer layer.
substrates were then transferred into a plasma treatment
chamber and exposed to N2 plasma for 30 s at 35 W under
100 mTorr. Then, the insulating layer of LiF and MgO was
deposited on Al or N2 plasma treated Al cathode by a thermal
evaporator and rf magnetron sputtering, respectively. The
MgO sputtering deposition was carried out at 5 mTorr with a
mixed gas of argon and oxygen at room temperature and a
base pressure of 4 ⫻ 10−6 Torr. The rf sputtering power was
100 W, resulting in a deposition rate of 0.1 Å / s. The six
types of samples were loaded to the thermal evaporator, on
which tris 共8-hydroxyquinoline兲 aluminum 共Alq3, 22 nm兲,
共␣-NPD,
4 , 4⬘-关N-共1-naphthyl兲-N-phenyl-amino兴biphenyl
35 nm兲, copper phthalocyanine 共CuPc, 18 nm兲 were deposited in sequence. Finally, a Au 共30 nm兲 layer, which acted as
an anode, was deposited. The deposition rates of organic and
Au layers were 0.5– 1.0 Å / s. Film thickness was determined
in situ using a crystal monitor. The active area of the device
was 3 ⫻ 3 mm2. The current density-voltage 共J-V兲 characteristics and luminance of the samples were measured by a
Hewlett-Packard 4156A and a Yokogawa 3298F, respectively. All the measurements were carried out in N2-ambient
and at room temperature.
In order to investigate the core level spectra, six kinds of
samples were loaded into a vacuum chamber, equipped with
an electron analyzer at the 2B1 beam line of the Pohang
Accelerator Laboratory. An incident photon energy of
600 eV was used to obtain Al 2p, O 1s, Mg 2p, Li 1s, and F
1s core level spectra. The onset of photoemission, corresponding to the vacuum level at the surface of Al, was measured with a negative bias 共−20 V兲 on the sample to avoid
the work function of the detector. The incident photon energy
was calibrated with the core level spectrum of Au 4f.
FIG. 2. J-V characteristics of ITOLEDs with LiF thickness.
age of 9 V to generate a current density of 50 mA/ cm2,
which is lower by 6 V than that for devices without LiF
layer.
J-V characteristics of ITOLEDs with MgO thickness are
shown in Fig. 3. ITOLED with a 1 nm thick MgO layer
required a drive voltage of 8 V to generate a current density
of 50 mA/ cm2, which is lower by 7 V than that for devices
without MgO layer. In the case of ITOLED with 0.5 nm
thick MgO layer, the drive voltage is higher than that with
1 nm thick MgO. In the case of ITOLED with 2 nm thick
MgO layer, the insulating nature of the MgO layer acts as an
electron injection barrier, showing also higher drive voltage.
Note that the drive voltage at 50 mA/ cm2 can be reduced
about 1.0 V by replacing the interfacial layer of LiF with
MgO.
The J-V characteristics are shown in Fig. 4 as a function
of ITOLEDs with different cathode structures. A variety of
cathode structures and the performance of ITOLEDs are
summarized in Table I. The operation voltage at the current
density of 100 mA/ cm2 decreased from 14.9 to 11.7 V
when the Al cathode was treated by N2 plasma. In addition, it
is clearly seen in the figure that insertion of a thin insulating
buffer layer on Al cathode displaces the characteristic J-V
curves toward low operation voltage. Furthermore, samples
III. RESULTS AND DISCUSSION
Figure 2 shows the J-V curves as a function of different
LiF thicknesses. Electrical performance in ITOLEDs with
LiF layers was enhanced in comparison to those without LiF
layer. It is noteworthy that the ITOLED performance depends upon the thickness of LiF layer. This result coincides
with previous reports that the electron injection barrier between the cathode and the emitting materials depends on the
thickness of the insulating layer.16 Of particular note,
ITOLED with a 0.5 nm thick LiF layer required a drive volt-
FIG. 3. J-V characteristics of ITOLEDs with MgO thickness.
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FIG. 4. J-V characteristics of ITOLEDs with various cathode structures: 共a兲
Al 共150 nm兲, 共b兲 Al 共150 nm兲 / N2 plasma, 共c兲 Al 共150 nm兲 / LiF 共0.5 nm兲,
共d兲 Al 共150 nm兲 / MgO 共1 nm兲, 共e兲 Al 共150 nm兲 / N2 plasma/LiF 共0.5 nm兲,
and 共f兲 Al 共150 nm兲 / N2 plasma/MgO 共1 nm兲.
FIG. 5. L-V characteristics of ITOLEDs with various cathode structures: 共a兲
Al 共150 nm兲, 共b兲 Al 共150 nm兲 / N2 plasma, 共c兲 Al 共150 nm兲 / LiF 共0.5 nm兲,
共d兲 Al 共150 nm兲 / MgO 共1 nm兲, 共e兲 Al 共150 nm兲 / N2 plasma/LiF 共0.5 nm兲,
and 共f兲 Al 共150 nm兲 / N2 plasma/MgO 共1 nm兲.
with the insulating buffer layer deposited on N2 plasma
treated Al cathode, i.e., samples E and F, have lower operation voltage than those of samples with insulating buffer layers on Al cathode. Although the operation voltage of
ITOLEDs is effectively reduced by the insertion of LiF
buffer layer between the N2-plasma-treated Al cathode and
the emitting layer, it is most significantly reduced by the
insertion of MgO. Therefore, it is suggested that the N2
plasma treatment on Al surface and the insertion of the insulating buffer layer between the cathode and emitting materials are effective in reducing the operation voltage of
ITOLEDs.
The dependence of the emission intensity on the injected
current 共L-V兲 is shown in Fig. 5. The emission intensity increases with the injection current for all the samples. It was
found that the luminance increased with the N2 plasma and
LiF buffer layer. The luminance was the highest in sample F,
in good agreement with the result of Fig. 4. The emission
intensity of sample F was enhanced by about two times compared to the bufferless sample A. At 191 mA/ cm2, the emission intensity of sample F reaches 1000 cd/ m2. It is thought
that the improvement of the emission intensity by the MgO
buffer layer might be attributed to the lowering of the energy
barrier, which enhances the injection of electron into the organic layer.
Figure 6共a兲 shows the Al 2p SRPES spectra for six
samples. In order to separate the chemical bonding states
including those in the spectra, the spectral line shape was
simulated using a suitable combination of Gaussian and
Lorentzian functions. The resolution of binding energy in
measurements was 0.1 eV. The spectra were curve fitted and
the relative contributions of metallic Al, Al–O, and Al–N
bonds to the total Al photoelectron signal are summarized in
Table II. The Al 2p peak was separated into two or three
components. The lower binding energy peak “P1” centered at
73.0 eV in sample A corresponds to metallic Al.17 The higher
binding energy peak “P2” centered at 75.6 eV was assigned
to the Al–O bond due to the existence of native oxide on Al
surface.17 In sample B, each peak shift was negligible and a
new peak “P3” centered at 75.1 eV appeared. The peak P3
corresponds to the Al–N bond obtained by the N2 plasma
treatment.17 Table II shows that samples C and E have a
smaller relative contribution of metallic Al to the total Al
photoelectron signal than sample A. Furthermore, samples D
and F show a much lower concentration of metallic Al with
a major contribution of Al–O bond to the total Al photoelectron signal. The O 1s SRPES spectra are shown in Fig. 6共b兲.
For the O 1s spectra, the samples with a MgO layer showed
pronounced low binding energy asymmetry. This means that
the intensity of a superimposed peak on the side of the low
binding energy increased with MgO deposition, in good
agreement with previous reports.18 Anyway, this experiment
result reveals that the Al surface was oxidized by the deposition of a thin MgO layer.
The composition of LiF with an Al undercoating layer
TABLE I. Various cathode structures and the ITOLED performance.
Samples
Cathode structures
Voltage 共V兲
at 100 mA/ cm2
Luminance 共cd/ m2兲
at 191 mA/ cm2
Sample A
Sample B
Sample C
Sample D
Sample E
Sample F
Al 共150 nm兲
Al 共150 nm兲 / N2 plasma
Al 共150 nm兲 / LiF 共0.5 nm兲
Al 共150 nm兲 / MgO 共1 nm兲
Al 共150 nm兲 / N2 plasma/LiF 共0.5 nm兲
Al 共150 nm兲 / N2 plasma/MgO 共1 nm兲
14.9
11.7
12.2
11.8
11.3
9.7
560
630
750
790
850
1000
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FIG. 6. SRPES spectra of 共a兲 Al 2p and 共b兲 O 1s core levels with various
cathode structures.
was examined by SRPES. The Li 1s and F 1s core level
spectra are shown in Fig. 7. Peaks of Li 1s and F 1s can be
fitted at 57.0 and 686.3 eV in sample C, respectively, compared to 55.6 and 684.9 eV reported for bulk LiF.19 No shift
of binding energy was demonstrated in sample E.
Figure 8共a兲 shows Mg 2p SRPES spectra for samples D
and F. The Mg 2p peak could be separated into two components using the same method. Both peaks “P4” and “P5”
centered at 50.8 and 51.1 eV in sample D correspond to the
coexistence of MgO and Mg hydroxide 共Mg共OH兲2兲,18,20 respectively. In addition, two component peaks are evident in
the O 1s core level spectra of the same samples in Fig. 8共b兲.
The lower binding energy peak “P6” centered at 531.6 eV
was attributed to the MgO oxygen atom 共O2−兲 in sample
D.18,20 On the other hand, the higher binding energy peak
“P7” centered at 532.9 eV corresponds to chemisorbed OH−
at MgO.18,20 The binding energy of Mg 2p and O 1s peaks in
sample D remains virtually unchanged in sample F. The
quantitative atomic ratio between Mg 2p and O 1s determined from the integral peak intensities of Mg 2p, O 1s, and
their sensitivity factors are summarized in Table III. The
atomic ratio between Mg 2p at 50.8 eV and the O 1s component at 531.6 eV in sample D is about 0.98, which is fairly
consistent with the theoretical ratio 1.00 for MgO. However,
the atomic ratio is slightly decreased in sample F.
The relative change of work function was measured using secondary electron emission spectra, as shown in Fig. 9.
The onset of secondary electron was determined by extrapolating two solid lines from background and straight onset in
the spectra.21 As shown in Fig. 7, the onset of secondary
electron for the samples with insulating buffer layer shifted
FIG. 7. SRPES spectra of 共a兲 Li 1s and 共b兲 F 1s core levels.
to the higher kinetic energy with respect to the onset for
sample A. This result means that the work function of the
samples with insulating buffer layer is higher than that of
bufferless sample A. Furthermore, the work function of
samples D and F with MgO buffer layer was much higher
than that of samples C and E with LiF buffer layer. Therefore, this result supports the fact that the enhancement of the
electron injection, lowering the potential barrier by insertion
of the insulating buffer layer, does not explain the chemical
reaction model, but does explain the tunneling probability
enhancement model.
Based on the experimental observation in Figs. 4 and 5,
the dependence of operation voltage and luminance on the
contact angles is shown in Fig. 10. The contact angles on the
thin films of Al, N2 plasma treated Al, LiF, and MgO were
measured by the sessile-drop technique.22,23 Each contact
angle quoted is the mean of at least three measurements to
get reliable contact angle data. The typical error is ±3° in all
our experiments. The voltage and luminance shown in Fig.
10 were measured at the current density of 100 and
191 mA/ cm2, respectively. The sample A exhibited a very
hydrophobic surface with a water contact angle of 74°. However, the contact angle dropped from 74° to 56° after N2
plasma treatment on Al. In comparison with the N2 plasma
treated Al film, the LiF film on the N2 plasma treated Al film
TABLE II. Relative contributions of metallic Al, Al–O, and Al–N bonds to
the total Al photoelectron signal.
Samples
Metallic Al 共%兲
Al–O 共%兲
Al–N 共%兲
Sample A
Sample B
Sample C
Sample D
Sample E
Sample F
28
15.7
18.9
1.8
15.5
2.5
72.0
54.5
81.1
98.2
73.5
93.7
¯
29.8
¯
¯
11.0
5.1
FIG. 8. Deconvolution of 共a兲 Mg 2p and 共b兲 O 1s core level spectra.
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TABLE III. Peak intensity ratios of deconvoluted peaks in Mg 2p and O 1s
core level spectra.
Mg 2p / O 1s
Samples
MgO
Mg共OH兲2
Sample D
Sample F
0.98
0.83
0.99
1.06
displayed a much smaller contact angle of 44°. Furthermore,
the result measured for MgO film on N2 plasma treated Al
film exhibited the smallest contact angle of 31°, showing a
hydrophilic surface. The lower value of the contact angle and
correspondingly higher surface energy represented a strong
polar interaction, which can improve adhesion.22 The operation voltage of samples decreased and higher luminance was
obtained as the contact angle was small. Therefore, N2
plasma treatment and buffer layer deposition on Al could
enhance adhesion between cathode and organic materials,
improving device performance.
The enhancement of electron injection by inserting the
insulating buffer layer can be understood on the basis of the
energy band diagram shown in Fig. 11. The electron injection barrier 共⌽b兲 from Al to Alq3 corresponds to the energy
difference between the work function of Al 共4.30 eV兲 and
the LUMO energy level 共2.55 eV兲 of Alq3.9 If no buffer
layer is included, the electron injection barrier ⌽b is a rather
large 1.75 eV. Therefore, the electron tunneling from Al to
Alq3 is very weak due to the large electron injection barrier
at low bias. However, as shown in Fig. 11共b兲, when the ultrathin insulating buffer layer is inserted, the voltage drop
across it lifts the cathode EF and hence reduces the tunneling
barrier. If the buffer layer thickness is optimized as shown in
Fig. 11共c兲, considerable voltage can drop across this layer,
which can align the Fermi level of Al with the LUMO energy
FIG. 10. Dependence of operation voltage and luminance on contact angles.
level of Alq3 and enable direct tunneling of electrons through
the thin insulating layer. Al oxide and MgO buffer layer
could be considered as an insulating layer in this system. The
electrical and optical properties of sample B is superior to
that of sample A, although the relative ratio of Al–O bond in
sample B is lower than that in sample A. This result indicated
that aluminum oxide is not a main cause of device performance improvement. After deposition of MgO on sample B,
i.e., in the case of sample F, the operation voltage reduced
and luminance value increased. Therefore, it is considered
that the improvement of device performance mainly originated from MgO layer. Electron injection can be enhanced
with the introduction of MgO layer, increasing the balanced
recombination with holes. Consequently, the emission intensity increased due to the balanced recombination and operation voltage of ITOLEDs was reduced. The above discussion
is based on the assumptions that the major current flowing
through the samples is the hole current and the emission
intensity is proportional to the electron current injected into
the samples.
IV. CONCLUSION
We have demonstrated the effect of MgO buffer layer
between the cathode and the emitting materials on electrical
and optical properties of ITOLEDs. The operation voltage of
ITOLEDs was effectively reduced by the insertion of LiF
FIG. 9. Change of secondary electron emission spectra with various cathode
structures: 共a兲 Al 共150 nm兲, 共b兲 Al 共150 nm兲 / N2 plasma, 共c兲 Al
共150 nm兲 / LiF 共0.5 nm兲, 共d兲 Al 共150 nm兲 / MgO 共1 nm兲, 共e兲 Al 共150 nm兲 / N2
plasma/LiF 共0.5 nm兲, and 共f兲 Al 共150 nm兲 / N2 plasma/MgO 共1 nm兲.
FIG. 11. Schematic band diagram of tunneling model: 共a兲 without buffer
layer, 共b兲 with ultrathin buffer layer, and 共c兲 with optimal thickness buffer
layer.
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Choi et al.
buffer layer between the N2-plasma-treated Al cathode and
the emitting layer, Alq3, but it is most significantly reduced
by using a thin MgO layer as the interfacial layer. The operation voltage of ITOLEDs at the current density of
100 mA/ cm2 decreased from 14.9 to 9.7 V when the 1 nm
thick MgO layer was present between Al and Alq3. The
maximum
luminance
value
increased
from
560 to 1000 cd/ m2 in ITOLEDs using MgO buffer layer.
SRPES spectra revealed that the surface of Al cathode
changed to Al oxide with the deposition of MgO. And the
work function of MgO-coated Al cathode increased about
0.8 eV in comparison with the Al cathode. Thus, it is concluded that the energy barrier for injection of electrons at the
interface of MgO with Alq3 decreased, leading to the enhancement of ITOLED device performance.
ACKNOWLEDGMENTS
This research was supported in part by the Program for
the Training of Graduate Students in Regional Innovation,
conducted by the Ministry of Commerce, Industry and Energy of the Korean Government, in part by the Korea Science and Engineering Foundation through the Quantumfunctional Semiconductor Research Center at Dongguk
University 共2006兲, and in part by a Korean Research Foundation Grant funded by the Korean Government 共MOEHRD兲
共KRF-2005-005-J13102兲.
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