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Efficiency enhancement of fluorescent blue organic light-emittin...
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The electroluminescent characteristics of blue organic light-emitting diodes (BOLEDs) fabricated with doped charge carrier transport layers are analyzed. The fluorescent blue dopant BCzVBi is doped in an emissive layer, hole transport layer (HTL) and electron transport layer (ETL), respectively, to optimize the probability of exciton generation in the BOLEDs. The luminance and luminous efficiency of BOLEDs made with BCzVBi-doped HTL and ETL increase by 22% and 17% from 11,683 cd/m2 at 8.5 V and
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Efficiency enhancement of fluorescent blue organic
light-emitting diodes using doped hole
transport and emissive layers
Nam Ho Kim
1
, You-Hyun Kim
1
, Ju-An Yoon
1
, Seung Il Yoo
1
, Kok Wai Cheah
3
,
Furong Zhu
3
, and Woo Young Kim
1,2,
*
1
Department of Green Energy & Semiconductor Engineering, Hoseo University, 513-3, 2nd Engineering Building,
20, Hoeo-ro beon-gil , baebang-eup, Asan-si, Chungcheongnam-do 3149 9, South Korea
2
Department of Engineering Physic s, McMaster University, 1280 Main St W, Hamilton L8S 4L8, Canada
3
Department of Physics, Hong Kong Baptist University, Hong Kong, Kowloon Tong, Waterloo Road 224, China
*Corresponding author: wykim@hoseo.edu
Received November 10, 2015; accepted January 25, 2016; posted online March 22, 2016
The electroluminescent characteristics of blue organic light-emitting diodes (BOLEDs) fabricated with doped
charge carrier transport layers are analyzed. The fluorescent blue dopant BCzVBi is doped in an emissive layer,
hole transport layer (HTL) and electron transport layer (ETL), respectively, to optimize the probability of ex-
citon generation in the BOLEDs. The luminance and luminous efficiency of BOLEDs made with BCzVBi-doped
HTL and ETL increase by 22% and 17% from 11; 683 cd∕m
2
at 8.5 V and 6.08 cd/A at 4.0 V to 14; 264 cd∕m
2
at
8.5 V and 7.13 cd/A at 4.0 V while CIE coordinates of (0.15, 0.15) of both types of BOLEDs remained
unchanged. The electron mobility of BCzVBi is estimated to be 1.02 × 10
−5
cm
2
∕Vs by TOF.
OCIS codes: 300.2140, 300.6170, 230.3670.
doi: 10.3788/COL201614.043001.
Small organic molecules used in organic light-emitting
diodes (OLEDs) have attracted attention for potential
application in the next generation active matrix full-color
OLED displays and solid-state lighting
[1–3]
. Full-color
OLED displays require a set of primary red, green, and
blue emitters
[4]
. Among the three primary emitters,
although red and green emitting materials have achieved
satisfactory efficiency and color purity, the development
of efficient blue-emitting materials is still an important
issue due to the wide energy bandgap (E
g
)
[5]
. Different
methods to enhance the luminous efficiency of the blue
OLEDs (BOLEDs) have been pursued, such as the devel-
opment of new electroluminescent materials and the
design of novel device structures for achieving efficient
light emission
[6–8]
. Development of highly efficient materi-
als for OLEDs is a more important factor for the ultimate
improvement of luminous efficiency, but it is sometimes
dependent on the combination of the organic materials
used for each functional layer in the device. Consequently,
it is essential to consider both material choice and opti-
mized device structure to obtain high efficiency for
OLEDs. Much effort has been made to improve the lumi-
nous efficiency of OLEDs resulting in their performance
approaching that of inorganic LEDs
[9]
. A host-dopant-
based emitting layer (EML) system is the most effective
way to generate an energy transition between organic mol-
ecules for the efficient generation of excitons in OLEDs
[10]
.
The same doping method can also be applied for applica-
tion in other functional layers, such as the hole transport
layer (HTL) and the electron transport layer (ETL), that
can be favorable for efficient operation in OLEDs. In this
study, the properties of BOLED s incorporating blue fluo-
rescent dopant 4,4
0
-Bis(9-ethyl-e-carbazo vinylene)-1,1
0
-
biphenyl (BCzVBi)-doped HTL, and ETL are analyzed.
The effect of doped-HTL and ETL on the luminance
and luminous efficiency of BOLEDs were also discussed.
Indium tin oxide (ITO)-coated glass substrates were
cleaned in an ultrasonic bath by the regular cleaning
sequence: in deionized water, isopropyl alcohol, acetone,
deionized water, isopropyl alcohol; thereafter, the pre-
cleaned ITO/glass was treated with O
2
plasma, under
vacuum conditions of 5.0 × 10
−2
Torr, of 50 W for
2 min. BOLEDs were fabricated by thermal evaporation
under high vacuum conditions of 5.0 × 10
−7
Torr. BO-
LEDs composed of N,N
0
-diphenyl -N,N
0
-bis(l-naphthyl-
phenyl)-(l,1
0
-biphenyl)-4, 4
0
-diamine (NPB) as the HTL
material; 9,10-di(naphth-2-yl) anthracene (ADN) as the-
host material, BCzVBi as the blue dopant, 4,7-diphenyl-
1,10-phenanthroline (Bphen) as the ETL material, and
8-hydroxyquinolinolato-lithium (Liq) as the electron in-
jection layer, respectively. Then the aluminum cathode
electrode was deposited by thermal evaporation. The
electro-optical characteristics of the BOLED devices were
measured and analyzed using a Keithley 238 LMS PR-650
spectrophotometer, colorimeter, and the current voltage
luminance (IVL) system. Electron mobility was measured
by the TOF-401 using an excitation nitrogen laser at
337 nm and 25°C.
In this Letter, BCzVBi, used for blue fluorescent dopant
of EML, was doped in HTL and ETL for fabrication in
blue OLED devices. The electrical and optical properties
of BCzVBi-doped HTL and ETL on the luminous
COL 14(4), 043001(2016) CHINESE OPTICS LETTERS April 10, 2016
1671-7694/2016/043001(5) 043001-1 © 2016 Chinese Optics Letters
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