Journal of Luminescence 132 (2012) 1–5
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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin
A very simple method of constructing efficient inverted top-emitting organic
light-emitting diode based on Ag/Al bilayer reflective cathode
Xiao-Wen Zhang a,b,c,n, Hua-Ping Lin b, Jun Li b, Liang Zhang b, Bin Wei b, Xue-Yin Jiang b, Zhi-Lin Zhang b
a
School of Materials Science and Engineering, Guilin University of Electronic Technology, 1 Jinji Road, Guilin 541004, People’s Republic of China
Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai 200072, People’s Republic of China
c
Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, People’s Republic of China
b
a r t i c l e i n f o
abstract
Article history:
Received 12 February 2011
Received in revised form
3 July 2011
Accepted 7 July 2011
Available online 26 July 2011
A comparative study on top-emitting organic light-emitting diodes (TOLEDs) with normal and inverted
structures is briefly investigated. In comparison with the normal TOLED having Ag reflective anode,
the inverted device with monolayer Al reflective cathode shows low efficiency as a result of
lower reflectance of Al and inferior electron injection. Using Ag/Al bilayer reflective cathode is
demonstrated to be a simple and effective method of enhancing efficiency in inverted TOLED. With
tris(8-hydroquinoline) aluminum (Alq3) as emitter the luminous efficiency reaches 5.9 7 0.6 cd/A,
which is much higher than those of the corresponding normal TOLED ( 5.1 cd/A) and inverted TOLED
with monolayer Al reflective cathode ( 4 cd/A). The improved performance is attributed to the
enhanced reflectance and significant microcavity effect. The electron-injection barrier height of
0.1 eV from Al to Alq3 via an ultrathin LiF is estimated in the tunneling process for both normal
and inverted devices.
& 2011 Elsevier B.V. All rights reserved.
Keywords:
OLED
Top emitting
Inverted structure
Microcavity
Electron injection
1. Introduction
Organic light-emitting diodes (OLEDs) have been heavily
studied in the recent years because of their promising applications to high-resolution full-color displays and solid state lightings. In comparison with conventional bottom-emitting OLED
(BOLED), top-emitting OLED (TOLED) is considered to hold many
advantages for constructing superior active matrix (AM) OLED
displays due to its enhanced light outcoupling, large aperture ratio
and ease of fabrication on thin-film transistor (TFT) backplanes.
Although the availability of both n- and p-type TFTs in commercial
AM OLED products is demonstrated, there is a growing interest in
developing the n-TFT for its superior performance and feasible
fabrication [1–3]. In addition the combination of n-TFT with
inverted TOLED (ITOLED) having bottom cathode is regarded to
be ideal in AM OLED displays as the drain line of n-TFT can be
directly connected with the bottom cathode of ITOLED, which
considerably decreases driving voltage and improves electrical
stability.
A major challenge to ITOLED is the fabrication of bottom
cathode with high reflectance and efficient electron-injection
ability. The most widely used electrodes such as Al, Ag and
n
Corresponding author at: School of Materials Science and Engineering, Guilin
University of Electronic Technology, 1 Jinji Road, Guilin 541004, People’s Republic
of China. Tel.: þ 86 773 2291179; fax: þ 86 773 2290129.
E-mail address: [email protected] (X.-W. Zhang).
0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jlumin.2011.07.010
indium–tin oxide (ITO) possess high work function, which deteriorates electron-injection ability. Methods considered to circumvent such a challenging task are mainly focused on the following
aspects: (i) incorporating low work function metals of Mg [3–5]
or Sm [6]; (ii) using Li or Cs (or Cs-based compounds) doped
electron-transport layer (e.g., BPhen:Li [7–9], BPhen:Cs [10],
BPhen:Cs2O [3,9] and Alq3:CsOH [11]); (iii) inserting metal oxides
such as TiO2 [12], ZnO [2], MgO [13,14] and PbO [15] as electroninjection layer and (iv) Using superior organic electron-injection
and transport materials [1,16]. However the active metals are
easily oxidized and cause diffusion in organic layers, which
results in operation instability [17]. Metal oxides usually possess
high evaporation temperature and thereby complicating fabrication process. Fortunately, it is well demonstrated that Al/LiF
cathode, the most widely used contact in OLEDs with normal
architecture, still works efficiently in ITOLEDs [18–20]. On the
other hand, some common hole-injection materials incorporated
with ultrathin metals such as WO3/Ag [1], MoO3/Ag [15,20],
[F4-TCNQ:m-MTDATA]/Au [8], O2-plasma-treated CuPc/Au [18]
and MoO3/C60/Al [19] are proven to be effective semitransparent
anodes in ITOLEDs.
To investigate the differences and performances improvement
of TOLEDs with normal and inverted structures, in this study, we
first carried out a brief comparative study on normal and inverted
devices using a conventional Ag (or Al) reflective electrode and LiF
electron-injection layer, since LiF/Al is the most widely used
contact in OLEDs and practically applied in mass production.
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X.-W. Zhang et al. / Journal of Luminescence 132 (2012) 1–5
Fig. 1. Schematic structures of TOLEDs with (a) normal and (b) inverted structures
and (c) BOLED.
Second we proposed a very simple method of constructing
efficient ITOLED using Ag/Al bilayer reflective cathode and LiF
electron-injection layer, suggesting a potential application in AM
OLED displays. Lastly, we estimated the electron-injection barrier-heights ( 0.1 eV) from Al to tris(8-hydroquinoline) aluminum (Alq3) via ultrathin LiF in the tunneling process for both
normal and inverted devices.
Lambertian distribution) vs. current density characteristics of
these devices are depicted in Fig. 2. It is clear that the maximum
luminous efficiency ( 3.3 cd/A) of Device B is much lower than
that ( 5.1 cd/A) of Device A. The poor performance of Device B
may be related to the following three factors: (i) the carrier
recombination and emitting zone (i.e., the position of the emitting
layer in the cavity) is located towards the reflective cathode,
which results in weak enhancement of antinode resonance [21];
(ii) inefficient electron-injection ability from reflective Al cathode,
because the deposition sequence of Alq3–LiF–Al plays a significant
role in electron-injection characteristics, i.e, the electron-injection
ability from Al to Alq3 in inverted architecture is inferior to that of
the normal architecture and (iii) low reflectivity of Al cathode
(will be discussed later). It can also be seen that the luminous and
power efficiencies of both TOLEDs (Devices A and B) are higher
than those of the corresponding BOLED at the same current
densities (Fig. 2). This is due to the fact that the microcavity
effect in TOLED promotes light outcoupling [22]. Moreover, the
driving voltage of Device C is much higher than those of TOLEDs
(Fig. 3). This results from the higher sheet resistance of ITO, which
is about one order of magnitude higher than those of Ag or Al
electrode. The higher driving voltage also leads to lower power
efficiency in Device C, as shown in Fig. 2(b).
2. Experimental details
The BOLED and TOLEDs with normal and inverted structures,
as shown in Fig. 1, were fabricated by thermal evaporation under
a vacuum of 3 10–4 Pa. All devices were fabricated on glass
substrates subjected to routine chemical cleaning. Here, the
ultrathin MoOx and LiF layers served as hole-injection layer
and electron-injection layer, respectively. The organic bilayer
structure consists of N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)
benzidine (NPB) as the hole-transport layer and Alq3 as the
emitting and electron-transport layer. MoOx/Ag served as the
semitransparent anode of ITOLEDs. The layer thickness and
deposition rate ( 1 Å/s for organic materials) was monitored
in situ using an oscillating quartz thickness monitor.
The active area of devices was 5 5 mm2. The current density–
voltage–luminance (J–V–L) characteristics were measured using a
computer controlled programmable Keithley 2400 Source Meter
and a Minolta LS-110 Luminance Meter. The electroluminescence
(EL) spectra and 1931 Commission Internationale d’Eclairage (CIE)
color coordinates were measured with a PR-650 Spectra Scan.
3. Results and discussion
To investigate the effect of device structures on performances,
two types of TOLEDs with normal and inverted structures were
constructed as follows.
Device A (normal structure): Ag (100 nm)/MoOx (2 nm)/NPB
(58 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al (2 nm)/Ag (18 nm).
Device B (inverted structure): Al (100 nm)/LiF (0.5 nm)/Alq3
(50 nm)/NPB (58 nm)/MoOx (2 nm)/Ag (20 nm).
The BOLED (Device C) with a structure of ITO/MoOx (2 nm)/
NPB (58 nm)/Alq3 (50 nm)/LiF (0.5 nm)/Al (100 nm) was also
fabricated for comparison. It is well established that efficient
electron injection works only for an Al cathode in OLED with an
electron-injection layer of LiF and an electron-transport layer of
Alq3. Consequently, Al is selected as the reflective cathode in
Device B. The luminous efficiency and power efficiency (assuming
Fig. 2. (a) Luminous efficiency vs. current density characteristics and (b) power
efficiency vs. current density characteristics of Devices A–D.
X.-W. Zhang et al. / Journal of Luminescence 132 (2012) 1–5
Fig. 3. J–V characteristics of Devices A–D.
3
of NPB thickness alteration on hole transport can be neglected. We
optimized the NPB thickness to 69 nm and fabricated Device D
with a structure of Al (100 nm)/LiF (0.5 nm)/Alq3 (50 nm)/NPB
(69 nm)/MoOx (2 nm)/Ag (20 nm). The performances of Device D
are also incorporated in Figs. 2 and 3 for comparison, while the EL
spectra at different viewing angles are shown in Fig. 4(b). It is
obvious that the efficiencies of Device D were enhanced with
increasing NPB thickness as compared with those of Device B
(Fig. 2). This shows the same characteristics as reported in other
literatures, i.e., the TOLED shows higher efficiency when the
resonant emission was adjusted to 540 nm for green emitters
[24]. However, the efficiencies ( 4.1 cd/A and 1.6 lm/W) of
Device D are still lower than those ( 5.1 cd/A and 2.9 lm/W)
of Device A. In TOLED, it is well established that, besides the
reflectance and transmittance of the semitransparent electrode,
the reflectivity of reflective bottom contact also plays a crucial role
in tuning the microcavity effect. We calculated the reflectivity
from organic stacks with a refractive index of 1.75 into Ag (or Al)
by a transfer matrix method [25]. The results (Fig. 5) indicate that
Al shows much lower reflectance spectra compared with Ag
within the range 450–700 nm. The lower reflectance of Al results
in inferior performance of ITOLED as a result of reduced multiplebeam and wide-angle interferences within the cavity [26]. It is also
noted that the EL spectra of all TOLEDs (Devices A, B and D) show
considerable blueshift with increasing viewing angles off the
surface normal (Fig. 4) as a result of microcavity effect [26].
To take advantage of high reflectance of Ag and without
sacrificing the electron-injection ability of Al in inverted architecture, we develop a bilayer reflective cathode of Ag/Al for
ITOLED with an effort to improve device performances. An ITOLED
(Device E) having a structure of Ag (100 nm)/Al (2 nm)/LiF
(0.5 nm)/Alq3 (50 nm)/NPB (58 nm)/MoOx (2 nm)/Ag (20 nm)
was fabricated. The luminous efficiency of Device E compared
with Device A is shown in Fig. 6. The figure shows that the ITOLED
using Ag/Al bilayer reflective cathode shows improved luminous
efficiency of 5.970.6 cd/A, which is much higher than those of
the normal TOLED ( 5.1 cd/A) and the ITOLEDs with monolayer
Al reflective cathode ( 4 cd/A). This indicates that using bilayer
Ag/Al reflective cathode is a promising easy-to-do method for
constructing efficient ITOLED, suggesting a potential application
in AM OLED displays. The improved performance of Device E is
obviously attributed to the enhanced reflectance and appropriate
electron-injection ability.
Fig. 4. EL spectra of (a) Devices A–B and (b) Device D at viewing angles of 01, 301
and 601 off the surface normal.
In addition, the resonant emission peaks of Devices A and B
show considerable differences [Fig. 4(a)]. This can be explained by
the differences in phase changes of reflective Ag and Al, which
alters the total optical length of the cavity [23]. To adjust the EL
peak of ITOLED at 540 nm (the same as the EL peak of Device A),
we altered the NPB thickness. Taking into account that the typical
hole mobility in NPB is about two orders of magnitude higher than
the electron mobility in electron-transport material Alq3, the effect
Fig.5. Calculated reflective spectra of Ag and Al from organic stacks with a
refractive index of 1.75.
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X.-W. Zhang et al. / Journal of Luminescence 132 (2012) 1–5
Fig. 6. Luminous efficiency vs. current density characteristics of Device E
compared with Device A. Inset: J–V characteristics.
To verify the differences in electron-injection ability of devices
with normal and inverted structures, we fabricated a series of
‘Electron-only’ devices having the following structures
Cell E1: Al (100 nm)/BPhen (19.5 nm)/Alq3 (80 nm)/LiF
(0.5 nm)/Al (100 nm).
Cell E2: Al (100 nm)/LiF (0.5 nm)/Alq3 (80 nm)/BPhen
(19.5 nm)/Al (100 nm).
Cell E3: Ag (100 nm)/Al (2 nm)/LiF (0.5 nm)/Alq3 (80 nm)/
BPhen (19.5 nm)/Al (100 nm).
Here, 4,7-diphenyl-1,10-phenanthroline (BPhen) served as a holeblocking layer. The J–V characteristics of these devices are
compared in Fig. 7(a). It is clear that the inverted cell (Cells E2
and E3) shows much lower current density in comparison with
the normal cell (Cell E1) at the same voltage. This indicates
inferior electron-injection ability from Al to Alq3 in inverted
architecture as compared with the normal architecture. This also
verifies the fact that the deposition sequence of Alq3–LiF–Al has
large effect on electron-injection characteristics. The inferior
electron-injection, to a certain extent, explained the fact that
ITOLED (Devices B and D) shows lower efficiency in comparison
with the normal device (Device A). It can also be found that the
J–V characteristics of these ‘only’ devices are controlled dominantly by the Fowler–Nordheim (F–N) tunneling process in high
electric field, which gives the current density as [27,28]
"
#
q3 F 2
4 2m 1=2 F3=2
J¼
exp ð1Þ
3 _2
4_F
qF
Here J is the current density, F is the electric filed, _ is the reduced
Planck constant, ^ is the zero-field barrier height and m is the
mass of the free electron. We replot the data [Fig. 7(a)] of Cells
E1–E3 as log(J/F2) vs. F 1 [Fig. 7(b)]. An explicit linear dependence
of log(J/F2) on F 1 in high electric field is observed in these
devices, suggesting a tunneling process [28]. From the slopes of
the straight lines, the electron-injection barrier heights are
estimated at ^ ¼0.11, 0.10 and 0.097 eV for Cell E1, E2 and E3,
respectively. The calculated barrier height (0.11 eV) for Alq3/LiF/
Al structure is in good agreement with the measured result
(0.1 eV) using ultraviolet photoelectron spectroscopy (UPS) by
Mori et al. [29], suggesting that our calculation is feasible and
Fig. 7. (a) J–V characteristics of ‘Electron-only’ devices. (b) The F–N plots of log(J/F2)
as a function of 1/F for J–V data as shown in (a). The barrier heights of electron
injection are derived from the slopes of the fitted lines delineated in the figure.
valid. On the other hand, the electric field of triggering tunneling
process in Cells E2 and E3 is much higher than in Cell E1. This
clearly elucidates that the electron-injection ability in inverted
structure is inferior to that in the normal structure. As a whole the
electron injection from Al to Alq3 layer via an ultrathin LiF
interlayer is mostly controlled by the tunneling process in high
electric field for both normal and inverted structures, although
the inferior electron-injection ability in inverted structure is
observed.
4. Conclusions
In comparison with the normal TOLED having Ag reflective
anode, the ITOLED with monolayer Al reflective cathode showed
lower efficiency as a result of inferior electron injection and lower
reflectance of Al. The Ag/Al bilayer reflective cathode was proven
to be a simple and efficient method of improving ITOLED
performance. With Alq3 as emitter the luminous efficiency of
ITOLED having Ag/Al bilayer reflective cathode reached
5.970.6 cd/A, which is much higher than those of normal TOLED
( 5.1 cd/A) and ITOLED having monolayer Al reflective cathode
( 4 cd/A). The electron-injection barrier height from Al to Alq3
via LiF interlayer was estimated to be 0.1 eV in the tunneling
process for both normal and inverted devices.
X.-W. Zhang et al. / Journal of Luminescence 132 (2012) 1–5
Acknowledgments
The authors would like to acknowledge the financial support given
by the National Natural Science Foundation of China (60777018,
60776040, 61077013) and 863 Project (2008AA03A336).
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