Luminous Efficiency Enhancement of Organic LightEmitting Diodes by an External Electron Source
Chi-Shing Li, Shui-Hsiang Su*, Tzu-Min Lin , Hsiang-Yu Chi and Meiso Yokoyama
Abstract-The external electron source is used to enhance the
luminous efficiency of organic light-emitting diodes (OLEDs).
Based on the electrons emission characteristics of carbon
nanotubes (CNTs), an OLED combined with a CNTs template is
proposed. Electrons emitted from the CNTs template impact the
Al cathode layer of an OLED device and transport arriving at the
emission layer. Balanced recombination of electrons and holes
occurs in the OLED and the luminous efficiency can be enhanced
by 50%.
I.
INTRODUCTION
Organic light emitting diodes (OLEDs) have been attracting
attention because of their advantages in emission in a wide
visible region and for application in flat-panel displays driver
at low voltage.
Improvements have been made since the first efficient
organic light-emitting diode was reported in the late 1980s [1].
There have been various efforts to increase the efficiency of
OLED [2]. Using multi-layer structures, low work function
cathodes and proper injection configuration can be greatly
improved the performance of OLED. However, the extraction
efficiency is related to the intrinsic photoluminescence
efficiency of the organic material and to the output coupling
efficiency of the sandwiched indium-tin-oxide (ITO)/organic
layers [3, 4]. In fact, in the multi-layer organic structure, holes
and electrons not balanced was the principal factor to affect the
luminous efficiency.
In order to improve the performance of the OLED, the
external electron source of CNTs cathode was proposed [5-7].
By using the CNTs cathode as electron sources inject electrons
into the organic thin film. One of the authors had been report
the similar structure and named field emission organic light
emitting diode (FEOLED) [8]. However, by using the external
electron to enhance the luminance efficiency of OLED has not
yet been report. In this paper the fabrication and emission
characteristics of the OLED also discussed, in terms of external
electron sources to supplement the electronic components of
the OLED.
II. EXPERIMENT
Fig. 1 shows the chemical structure of the organic materials
investigate and the device configurations. All of the organic
layers were deposited onto an ITO-coated glass substrate (7 Ω/
□ ) by vacuum vapor deposition at a pressure of 5 × 10−6
Torr.
All authors are with the school of Department of Electronic Engineering, IShou University, Kaohsiung County, Taiwan
*Contacting Author: Shui-Hsiang Su is with the School of Department of
Electronic Engineering, I-Shou University, Kaohsiung County, Taiwan (phone:
886-7-657-7711; email: shsu@isu.edu.tw).
CNT-base filed emitter cathodes have been fabricated by
using the spray method. The 1,2-dichloroethane (DCE) solvent
is used to disparate the CNTs by sonication. Adhesion between
CNTs and ITO glass substrate was resolved by introducing
glass frit in the DCE/CNTs solution. After that, the spray
technique is introduced to fabricate CNTs field emitters,
followed by heat treatment.
After the OLED device has been done, the cathode (Al) of
OLED was faced toward the CNTs cathode and setting in a
vacuum chamber (<5 × 10-6). This triode structures,
measurement and control by using standard commercial
electronic components. A keithley model 2400 was provided
the OLED, and a keithley model 237 was used to control the
voltage between OELD anode plate and CNTs cathode, to
generate an additional electrical field to attract the electrons to
the OLED cathode (Al). The EL spectrum and luminance–
current–voltage characteristics were measured and recorded by
use of the TOPCON SR-1 spectrometer.
In this system, the CNTs cathode emission electrons were
generated with the electric field to attract and accelerate, which
were accelerated toward the OLED and had to supplement the
electrons.
III. RESULTS AND DISCUSSION
Fig. 2 plots the I-L-V characteristics of the OLED device
with the switch of Sw1=close and Sw2=open. The luminance
of the OLED is 550 cd/m2 at 10 V. The OLED will burn down
at a driving voltage more than 10 V. Experimental results
reveal that unmatched energy barriers exit at the interfaces of
layers resulting in an imbalance in electrons and holes
concentrations in the emission layer. The most situations are
holes more than the electrons. Therefore, external electrons
emitted from the CNTs template outside of the OLED are
suggested to supplement the electrons to enhance the efficiency
of electron-hole recombination.
A CNTs template is used as a cathode in the study and it can
emit a high electron current density of 10 mA/cm2 at 1.4 V/μm.
It is attributed to the CNTs have perfectly dispersed on the
template and glass frit has held all the CNTs tight after
annealing process. The inset in Fig. 2 shows the mapping of
CNTs and quite good dispersion can be observed. The CNTsbased template is very suitable to be the external electron
source.
In order to promote the efficiency of OLED, the exterior
electron source enters from the outside of OLED to further
supplement electron. The VEL of OLED is fixed at 8 V and the
luminance is 184 cd/m2. At the same time, Va is turn on
(Sw2=close) to induce the field emission electrons from CNTs
cathode. The external electrons have initially impacted the Al
cathode layer of an OLED and transport toward the ITO anode.
It has provided more electrons for the recombination with
holes in the organic light emission layer.
Fig. 4 shows the luminous characteristics of a green-OLED.
At the same driving voltage of 8 V, the luminance has arisen
from 184 cd/m2 to 300 cd/m2. The improvement has been
attributed to the more balanced hole-electron recombination in
the emission layer of an OLED.
ITO
m-MTDATA (40 nm)
NPB (20 nm)
Alq3 (60 nm)
LiF (0.7 nm)
Al (30nm)
Va
e
IV. CONCLUSIONS
This study has successfully used a CNTs template to act as
an external electron source in the OLED. Under an electrical
field driving, electrons emitted from the CNTs templates
compensate for the lack of electrons in an OLED and
completely recombine with the holes in the emission layer.
Balanced recombination of electrons and holes occurs in the
OLED and the luminous efficiency can be enhanced by 50%.
The construction presented here is a very promising technique
for OLED device applications.
Sw2
-
-
e
e
-
VEL
-
e
Sw1
CNT
CNTs cathode
Fig. 1 The layer structure of a field-emission organic light-emitting device
(FEOLED) and the configuration adopted in this study.
ACKNOWLEDGMENT
The authors would like to thank the National Science
Council of the Republic of China, Taiwan, for financially
supporting this research under Contract No. NSC 98-2221-E214-003-MY3. The authors would also like to thank the
MANALAB at ISU, Taiwan.
Fig. 2 I-L-V characteristics of organic light-emitting diodes. (Sw1:close,
Sw2:open)
REFERENCES
[1] C. W. Tang and S.A. Van Slyke, “Organic electroluminescent diodes.”
Appl Phys Lett., vol. 51, pp. 913–915, September 1987.
[2] M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson, and S. R. Forrest, “Highly efficient phosphorescent emission
from organic electroluminescent devices,” Nature, vol. 395, pp. 151-154,
September 1998.
[3] I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer,
“30% external quantum efficiency from surface textured, thin-film lightemitting diodes.” Appl. Phys. Lett., vol. 63, pp.2174, October 1993.
[4] M. H. Lu and J. C. Sturm, “External coupling efficiency in planar organic
light-emitting devices.” Appl. Phys. Lett., vol. 78, pp. 1927, March 1993.
[5] W. A. Deheer, A. Châtelain, D. Ugarte, “A carbon nanotube field-emission
electron source.” Science, vol. 270, pp. 1179 – 1180, November 1995.
[6] W. B. Choi, D. S. Chung, J. H. Kang, H. Y. Kim, Y. W. Jin, I. T. Han, Y.
H. Lee, J. E. Jung, N. S. Lee, G. S. Park, and J. M. Kim, “Fully sealed,
high-brightness carbon-nanotube field-emission display.” Appl. Phys.
Lett., vol. 75, pp. 3129-3131, November 1999.
[7] J. M. Bonard, J. P. Salvetat, T. Stockli, W. A. Deheer, L. Forro, and A.
Chatelain, ”Field emission from single-wall carbon nanotube films.” Appl.
Phys. Lett., vol.73, pp. 918-920, August 1998.
[8] C. S. Li, M. Yokoyama, and S. H. Su, “Efficiency enhancement of fieldemission organic light emitting diodes using a dynode structure.”
Electrochemical and Solid-State Letters, vol.11, pp. 1-3, July 2007.
Fig. 3 The J-E characteristics of CNTs-based emitters. (Inset) CNTs Mapping
results from a CNTs-base template. (Sw1:open, Sw2:close)
Fig. 4 The characteristics of the field-emission organic light-emitting device
(FEOLED). (Sw1:close, Sw2:close)