Vol.2, No.6, 631-634 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.26078
Optical properties for N,N’-bis (lnaphyhly)-
N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine and tris
(8-hydroxyquinolinato) aluminum in organic light
emitting devices
Mei Yee Lim1*, Wan Mahmood Mat Yunus1, Zainal Abidin Talib1, Anuar Kassim2
1Department of Physics, Faculty of Science, University Putra Malaysia, Selangor, Malaysia; *Corresponding Author:
2Department of Chemistry, Faculty of Science, University Putra Malaysia, Selangor, Malaysia
Received 12 January 2010; revised 9 March 2010; accepted 2 April 2010.
ABSTRACT
The optical properties of N,N’-bis (Inaphthyl)-
N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB)
and tris (8-hydroxyquinolinato) aluminum (Alq3)
organic materials used as hole transport and
electron transport layers in organic light-emit-
ting devices (OLED) have been investigated.
The NPB and Alq3 layers were prepared using
thermal evaporation method. The results show
that the energy band gap of Alq3 is thickness
independence while the energy band gap of
NPB decreases with the increasing of sample
thickness. For the case of photoluminescence
the Alq3 with thickness of 84 nm shows the
highest relative intensity peak at 510 nm.
Keywords: Energy Band Gap; N,N’-Bis (Inaphthyl)
-N,N’-Diphenyl-1,1’-Biphenyl-4,4’-Diamine Tris (8
Hydroxyquinolinato) Aluminum; Organic Light
Emitting Diode; Photoluminescence
1. INTRODUCTION
Various organic light emitting devices displays have be-
en investigated in recent years. Many techniques like
anode [1] and cathode [2] modifications, inserting hole
blocking layer [3], control the thermal evaporation rate
[4] and doping system have been recently reported to
further improve display efficiency due to their excellent
performances. Currently, there is great interest in the
study of OLEDs containing small molecules as emitting
layer. Such molecules have been found to be useful in
OLED applications such as optical devices, lumines-
cence probes in biomedical assays, luminescence sensors
for chemical species and fluorescent lighting [5,6]. Ad-
vantages of organic LEDs over inorganic ones are their
high emission efficiency in the visible spectrum, easy to
process, robustness and almost infinite possibility for
modification. They also have low operating voltage, low
power consumption and wide viewing angle. However,
there are still several problems which must be solved
especially for transferring research results to commercial
application of OLED devices in display technology.
Short OLED lifetime, low carrier mobility and high in-
terface barriers must be improved by optimizing the ma-
terial parameters and fabrication steps.
As present, the basic structure of an OLED is shown
in Figure 1 and typically consists of three organic semi-
conductor layers (hole injection layer, organic emitter
and electron transport layer) sandwiched between two
electrodes. The electron-injecting electrode consists of a
Metal Cathode
Electron Transport Layer
Organic Emitter
Hole Injection Layer
Indium tin Oxide Anode
Light Output
Figure 1. The basic structure for or-
ganic light emitting device.

M. Y. Lim et al. / Natural Science 2 (2010) 631-634
632
low work function metal alloy, typically Mg-Ag or Li-Al,
deposited by vacuum evaporation. The bottom, hole-
injecting, electrode is typically a thin-film of the trans-
parent semiconductor indium tin oxide (ITO) [7]. Upon
recombination, energy is released as light, which is emi-
tted from the light-transmissive anode and substrate.
N’-bis (Inaphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-di-
amine (NPB) was chosen as hole injection layer because
it can be easily manufactured and is abundantly available
in powder form. However, tris (8-hydroxyquinolinato)
aluminum (Alq3) is used as an ETL because Alq3 is ther-
mally and morphologically stable therefore can be easily
evaporated into thin films form. Easily synthesized and
purified, molecularly shaped to avoid exciplex formation
(e.g. with N,N’-bis (Inaphthyl)-N,N’-diphenyl-1,1’-bi-
phenyl-4,4’-diamine at the interface), and produce green
fluorescent light become another reason to be a good
host emitter [8].
2. EXPERIMENTAL
For the preparation of Alq3 and NPB organic layers, the
indium tin oxide (ITO) glasses substrate was cut into
square plates (2 cm × 2 cm). The ITO glasses were im-
mersed in ultrasonic baths with acetone for 10 minutes.
Then, the ITO glasses were rinsing in deionised water
for 10 min and then blow dried with nitrogen gas. This
procedure was applied to remove organic contamination
and particles from the ITO surface. Thermal evaporation
technique using resistively heated tantalum boats in
vacuum, at a base pressure of 1.0 × 10-5 Torr was used to
prepare thin film sample. Different thickness of organic
layers was deposited on the ITO at a rate of 2.5 A/s at
room temperature. The thicknesses of the layers were
measured by Tencor P-12 Disk Profile. The PL spectra
of the devices were measured by a EL spectra USB 2000
FLG Spectrofluorometer. The spectra of optical absorp-
tion measurements were made over the wavelength
range of 360 nm-800 nm to obtain the energy band gap.
3. RESULTS AND DISCUSSION
3.1. Energy Band Gap
The spectra of optical absorption measurements were
made over the wavelength range of 360 nm-800 nm. The
variation in the absorbance with wavelength is shown in
Figures 2 and 3 for Alq3 and NPB with varying thick-
ness.
The optical absorption spectra of both samples, Alq3
and NPB are similar thus the optical band gap was cal-
culated using a well known equation as

g
Ehch  
2
)( (1)
400 500 600 700 800
-0.1
0.0
0.1
0.2
0.3
0.4
Absorbance, Abs
Wavelength, nm
ITO
16 nm
33 nm
50 nm
67 nm
84 nm
101 nm
118 nm
134 nm
Figure 2. The optical absorption of Alq3 at various thickness.
400 500 600700 800
0.0
0.5
1.0
1.5
2.0
2.5
Absorbance, Abs
Wavelength, nm
28 nm
55 nm
83 nm
110 nm
138 nm
165 nm
193 nm
Figure 3. The optical absorption of NPB measured at different
thicknesses.
where c is a constant, Eg is the optical band gap, hv is
photon energy. The energy band gap was obtained by
plotting (αhv) 2 as a function of photon energy,

h.
The result shows that the energy band gap for the Alq3,
is tend to be independent on the layer thickness as shown
in Figure 4. However for NPB layers the energy band
gap decreases with the increasing NPB thickness as dis-
played in Figure 5. This result can be easily correlated
with the efficient hole transport in hole injection layer as
discussed by Zhang Zhi-Feng et al. This phenomenon is
due to a good balance between the injected electrons and
holes in the OLED structure [9].
3.2. Photoluminescence
The photoluminescence spectra of Alq3 were success-
fully measured using Ocean Optics spectrofluorometer
operated at 390 nm. Figure 6 shows the normalized pho-
toluminescence intensity for samples at various thick-

M. Y. Lim et al. / Natural Science 2 (2010) 631-634
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633
020 40 60 80100120140
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Energy band Gap, eV
Thickness, nm
Figure 4. Energy band gap for different thickness of Alq3.
20 40 60 80100120140160180200
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Energy Band Gap, eV
Thickness, nm
Figure 5. Energy band gap for different thickness of NPB.
450 500 550 600 650
0
25
50
75
100
Normalized Intensity, a.u
Wavelength, nm
16nm
33nm
50nm
65nm
84nm
101nm
118nm
134nm
Figure 6. The PL performance with the different thickness of
Alq3.
400 500 600 700 800 90010001100
0
100
200
300
400
Normalized Intensity, a.u
Wavelength, nm
28 nm
55 nm
83 nm
110 nm
138 nm
165 nm
193 nm
Figure 7. The PL performance with the different thickness of
NPB.
nesses. It can be observed that Alq3 with 84 nm thickn-
ess gives the highest relative intensity at emission wave-
length of 510 nm. In the case of NPB layer there is no
any photoluminescence phenomenon occurred as dis-
played by Figure 7. The spectra in Figure 7 are the op-
tical reflectance of the excitation source. However NPB
layer with a thickness 110 nm gives the highest relative
reflectance intensity at the peak of excitation source, i.e.
390 nm and 750 nm respectively.
4. CONCLUSIONS
The energy band gap of Alq3 and NPB layers for OLED
structure have successfully been measured. The NPB
layer shows a thickness dependence of energy band gap
while the energy band gap for Alq3 is tend to be inde-
pendence on the sample thickness ranging from 16 nm to
134 nm. As electron transport layer in OLED structure,
the photoluminescence spectrum of Alq3 was observed at
510 nm with the optimum thickness of 84 nm.
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