High-efficiency undoped blue organic light-emitting device

Article abstract of DOI:10.1016/j.dyepig.2010.01.009

A high-efficiency, undoped, blue, organic light-emitting device was devised that employed a novel electroluminescent material 4-(7,10-diphenylfluoranthen-8-yl)-N,N-diphenylbenzenamine as electrofluorescence emitter. The device with a simple structure of ITO/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/4-(7,10-diphenylfluoranthen-8-yl)-N,N-diphenylbenzenamine/4,7-diphenyl-1,10-phenanthroline/LiF/Al exhibited stable blue light emission of CIE chromaticity coordinates (x=0.19±0.01 and y=0.45±0.02), and a current efficiency of 6.0cd A^-1 and a power efficiency of 4.6lm W^-1.

Full text of DOI:10.1016/j.dyepig.2010.01.009

Dyes and Pigments 86 (2010) 233e237
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
High-efficiency undoped blue organic light-emitting device
,
b
,
c
b
,
,
Qing-Xiao Tong a , Mei-Yee Chan , Shiu-Lun Lai d, Tsz-Wai Ng b d, Peng-Fei Wang e,
*
Chun-Sing Lee b d, Shuit-Tong Lee b
a Department of Chemistry, Shantou University, Guangdong 515063, China
b Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong Kong, Hong Kong SAR, China
c Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
d Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, China
e Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China
,
,d,
**
a r t i c l e i n f o
a b s t r a c t
Article history:
A high-efficiency, undoped, blue, organic light-emitting device was devised that employed a novel electrolu-
minescent material 4-(7,10-diphenylfluoranthen-8-yl)-N,N-diphenylbenzenamine as electrofluorescence
emitter. The device with a simple structure of ITO/N,N0-bis(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-
diamine/4-(7,10-diphenylfluoranthen-8-yl)-N,N-diphenylbenzenamine/4,7-diphenyl-1,10-phenanthroline/LiF
/Al exhibited stable blue light emission of CIE chromaticity coordinates (x ¼ 0.19 ꢀ 0.01 and y ¼ 0.45 ꢀ 0.02),
Received 10 December 2009
Received in revised form
20 January 2010
Accepted 21 January 2010
Available online 28 January 2010
and a current efficiency of 6.0 cd Aꢁ1 and a power efficiency of 4.6 lm Wꢁ1
.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
High-efficiency
Non-doped
Blue
Organic light-emitting diode
Electroluminescence
Small-molecule
1. Introduction
To improve electroluminescence (EL) quantum efficiency,
doping of fluorescent or phosphorescent dyes in the emissive layer
(EML) is the most effective approach adopted nowadays. However,
the success of this doping method relies on the precise control of
the small dopant amounts used, which are of the order w0.1% [2,4].
This imposes pronounced manufacturing process control which
increases production costs. In addition, phase separation in a guest-
host system is also a potential problem [5e7]; recent investigation
has proven that phase separation upon heating is an important
cause of performance degradation in some guest-host system-
based devices. Although host and guest molecules are initially
homogeneously mixed, guest molecules commonly aggregate
leading to phase separation during operation and/or upon heating
[5e7]. Phase separation increases the host-guest distance beyond
the capture radius R (1e10 nm) of a guest molecule for efficient
host-guest energy transfer, thus rendering energy transfer inef-
fective [8]. This phenomenon can further reduce device lifetime,
especially under high-temperature operation. In view of these
shortcomings, considerable efforts have recently been devoted to
the synthesis of high-efficiency non-doped blue-emitting materials
[9e23].
Since Tang and VanSlyke [1] first introduced efficient organic
light-emitting devices (OLEDs), organic electroluminescent devices
have attracted much scientific and commercial interest owing to
their promising application in full-colour displays as well as large-
area, flexible, lightweight light sources [2,3]. Full-colour displays
require blue, green, and red emission elements with high device
stability, high luminous efficiency and colour purity. Although
significant improvement in OLED performance has been achieved
over the past decades, further improvement is still required,
especially in terms of the performance of blue OLEDs in the context
of current efficiency and external quantum efficiency, which are
relatively poor compared with red and green OLEDs. Blue-emitting
materials are of interest not only as a major constituent of
redegreeneblue full-colour displays, but also as the key emitting
element for generating white light in combination with the
complementary yellow colour.
* Corresponding author. Tel.: þ86 754 8290 2772; fax: þ86 754 8290 2767.
** Corresponding author. Tel.: þ86 852 2788 9606; fax: þ86 852 2784 4696.
E-mail addresses: qxtong@gmail.com (Q.-X. Tong), apannale@cityu.edu.hk (S.-T.
Lee).
Indeed, the molecular design of the structure of blue-emitting
materials can influence their performance and thermal stability.
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.01.009

234
Q.-X. Tong et al. / Dyes and Pigments 86 (2010) 233e237
Scheme 1. Synthetic route of TPADPF.
Spiro-fluororene has received much attention as blue emitters
because of its outstanding luminescent properties [24e26],
although the multiple-step synthesis, coupled with low yields, are
not commercially attractive. Chiechi et al. [9] recently developed
a robust, fluoranthene derivative, fluorophore 7,8,10-triphenyl-
fluoranthene (TPF), as a blue-emitting material in OLEDs. This
compound was simply obtained in two synthetic steps from
commercial starting materials and exhibited high quantum yield
(0.86) in the solid-state. In addition, TPF can be employed as
a single layer for light emission, in which the undoped device dis-
played maximum current and power efficiencies of 3.02 cd Aꢁ1 and
1.1 lm Wꢁ1, respectively.
were dissolved in o-xylene (40 mL) under an argon atmosphere and
the resultant mixture was heated for 24 h at 170 ꢂC (oil bath
temperature). After the mixture was cooled to room temperature,
ethanol (200 mL) was added. The precipitate was filtered, washed
with ethanol (300 mL) and dried in a vacuum, followed by column
chromatography (dichloromethane/petroleum ether, 1:6) on silica
gel, TPADPF was obtained as a slightly yellow and strongly fluo-
rescent solid. Yield: 85%. 1H NMR (CD2Cl2, 400 MHz):
d 7.78e7.68
(m, 4H), 7.61e7.52 (m, 3H), 7.48e7.18 (m, 12H), 7.12e6.95 (m, 9H),
7.14 (d, J ¼ 7.2, 2H), 6.74 (d, J ¼ 7.2, 1H),. MS: m/z 597. Anal. Calcd for
C46H31N: C, 92.43; H, 5.23; N, 2.34. Found: C, 94.55; H, 5.28; N, 2.28.
This paper concerns the synthesis and characterization of the
fluoranthene derivative, 4-(7,10-diphenylfluoranthen-8-yl)-N,N-
diphenylbenzenamine (TPADPF) which is obtained by a four step
synthesis in high yield according to the DielseAlder reaction.
Comparing with unsubstituted TPF [9], diphenylamine was intro-
duced at carbon 4 position of the benzene ring 8 because of its rigid
structure and hole transporting properties that are important for
producing an undoped emitting material.
2.3. Device fabrication and characterization
Fig.
1
shows the structures of N,N0-bis(1-naphthyl)-N,N0-
diphenyl-1,10-biphenyl-4,40-diamine (NPB) and 4,7-diphenyl-1,10-
phenanthroline (BPhen). Devices were constructed on a glass
substrate which had been pre-coated with a patterned layer of
indium-tin oxide (ITO) having a sheet resistance of 30
U .
squareꢁ1
Substrates were cleaned with isopropyl alcohol, Decon 90, and
deionized water and then dried in an oven, before being finally
treated in a UV-ozone chamber. All organic and metal layers were
grown in succession without breaking vacuum (at a base pressure
of 10ꢁ6 mbar). Deposition rates were monitored with a quartz
oscillation crystal and controlled at 0.1e0.2 nm/s for both organic
and metal layers. A shadow mask was used to define the cathode to
make four 0.1 cm2 devices on each substrate. EL spectra and current
density-voltage-luminance (J-V-L) characteristics of OLEDs were
measured with a programmable Keithley model 237 power source
and a Spectrascan PR650 photometer under ambient condition. 1H
NMR spectra were recorded with a Bruker DPX-400 spectrometer
(400 MHz). Mass spectrometry (MS) was performed on a PE SCIEX
LC/MS spectrometer. Elemental analyses (EA) were performed on
a Vario EL Elementar by the Flash EA 1112 method. Absorption and
photoluminescence (PL) spectra of TPADPF were measured with
2. Experimental
2.1. Material synthesis
The fluoranthene derivative TPADPF was synthesized according
to the DielseAlder reaction [9,27,28] (Scheme 1). 4-ethynyl-N,N-
diphenylbenzene amine [28] and 7,9-diphenyl-8H-cyclopenta[l]
acenaphthylen-8-one [29] were synthesized according to literature
procedures. All solvents were purified by routine procedures.
2.2. Synthesis of TPADPF
4-ethynyl-N,N-diphenylbenzene amine (807 mg, 3.0 mmol) and
7,9-diphenyl-8H-cyclopenta[l]acenaphthylen-8-one (1.07 g, 3.0 mmol)
Fig. 1. Chemical structures of NPB and BPhen.

Q.-X. Tong et al. / Dyes and Pigments 86 (2010) 233e237
235
Fig. 4. Energy level diagram for the TPADPF-based device.
The Eg for TPADPF is 2.7 eV, calculated from the onset of the thin
film absorption spectrum.
Fig. 2. Absorption and PL spectra of thermally evaporated thin film of TPADPF on
a quartz substrate.
3.2. Electronic structure of TPADPF
UPS is the useful tool for obtaining the electronic band struc-
tures of organics. Some important values, for instance, IP and the
highest occupied states (HOS) can be obtained from UPS. Fig. 3
shows a He I (hv ¼ 21.22 eV) UPS spectrum of the TPADPF film
under ultra-high vacuum conditions. The inelastic electron cutoff
and the HOS of the highest occupied molecular orbital (HOMO) are
also enlarged. With reference to these values, IP could be calculated
by IP ¼ 21.22 ꢁ (25.32 ꢁ 9.70) ¼ 5.6 eV. Subtracting the Eg of 2.7 eV
from the IP, the EA of TPADPF was estimated to be 2.9 eV. Fig. 4
illustrates the energy level diagram for the TPADPF-based device.
a Perkin Elmer Lambda 2S US/VIS spectrometer and a Perkin Elmer
LS 50B luminescence spectrometer, respectively. The ionization
potential (IP) of TPADPF was measured with ultraviolet photoelec-
tron spectroscopy (UPS) in a VG ESCALAB 220i-XL surface analysis
system, whereas the electron affinity (EA) was estimated by sub-
tracting from the optical bandgap (Eg) determined by the absorp-
tion spectrum of its solid-state film.
3. Results and discussion
3.1. Photo-physical characteristics of TPADPF
3.3. Device characteristics of TPADPF
TPADPF was synthesized in-house according to the synthesis
route in Scheme 1, and its molecular structure was confirmed with
mass spectrometry, 1H nuclear magnetic resonance, and elemental
analyses. Fig. 2 shows the absorption and PL spectra of vacuum-
deposited thin film of TPADPF on a quartz substrate. It can be seen
that the PL spectrum has a strong blue emission peak at 486 nm
under direct excitation at 386 nm, while the absorption spectrum
has its characteristic peaks at 385, 314, and 238 nm, respectively.
In order to investigate EL properties of organic TPADPF, OLEDs
with architectures of: (A) ITO/TPADPF (100 nm)/BPhen (20 nm)/LiF
(0.5 nm)/Al (100 nm); and (B) ITO/NPB (70 nm)/TPADPF (30 nm)/
BPhen (20 nm)/LiF (0.5 nm)/Al (100 nm) were fabricated by
thermal vapor deposition, whereas NPB and BPhen served as
a
hole transporting and an electron transporting materials,
respectively, and TPADPF functioned as both hole transporting and
light-emitting material. Fig. 5 shows the EL spectra for devices A
Fig. 3. UPS spectrum of a TPADPF film obtained with He I excitation.

236
Q.-X. Tong et al. / Dyes and Pigments 86 (2010) 233e237
Fig. 7. Current efficiency-current density-power efficiency characteristics of the
TPADPF-based devices.
Fig. 5. EL spectra of device A and device B viewed in the normal direction at the
luminescence of 10, 100, 1000, and 10,000 cd/m2, respectively.
3477 cd/m2, respectively; whilst those for device B are only
77.4 mA/cm2 and 4597 cd/m2. Surprisingly, the driving voltage for
device A is much lower than that for device B at the same current
density. It is commonly realized that NPB can act as a stepping-
stone to enhance hole injection from ITO to EML, resulting in
a reduction of driving voltage. However, it is not the case for the
present TPADPF-based devices. The reasons behind this phenom-
enon may be attributed to the elimination of current/luminescence
quenching sites at the contact of ITO/TPADPF; particularly NPB with
relatively small EA (w2.3 eV) can function as an electron-blocking
layer at the ITO/TPADPF interface. Another factor may be ascribed to
different carrier mobilities in organic layers [30], in which the hole
mobility in TPADPF is comparatively higher than that in NPB, thus
resulting in a lower driving voltage.
and B viewed in the normal direction at different luminance (L)
levels. As seen in Fig. 5, both devices A and B exhibited blue light.
The EL spectral peaks centered at 492 nm with the full spectral
widths at half maxima (FWHM) of 73.5, 73.5, 74.3, and 77.9 nm for
device A while the peaks located at 496 nm with FWHM of 76.3,
76.5, 76.8, and 77.0 nm for device B at the L ¼ 10, 100, 1000, and
10,000 cd/m2, respectively. The corresponding CIE 1931 chroma-
ticity coordinates of (x ¼ 0.19 ꢀ 0.01, y ¼ 0.43 ꢀ 0.02) for device A
and (x ¼ 0.19 ꢀ 0.01, y ¼ 0.45 ꢀ 0.02) for device B, respectively. It
should be highlighted that our recent study on the use of another
new blue emitter 4,40,400-trispyrenylphenylamine (TPyPA) showed
exciplex formation at the TPyPA/BPhen contact as a consequence
of an additional yellow emission peak at 570 nm [17]. On the
contrary, exciplex formation does not occur in the present devices,
which may be attributed to a weaker electron donating property
of TPADPF as compared to TPyPA.
Apart from the J-V-L characteristics, the incorporation of NPB
can improve device efficiencies. As shown in Fig. 7, the maximum
current and power efficiencies of device A are 3.0 cd/A (hEQE
¼
1.20%) and 3.1 lm/W, respectively, whereas those of device B are
6.0 cd/A (hEQE ¼ 2.37%) and 4.6 lm/W, respectively (hEQE is the
external quantum efficiency). The performance improvement for
device B can be assigned to be a better confinement of hole and
electron currents within the TPADPF layer, which may be due to the
lower hole mobility in NPB than that in TPADPF as well as the
elimination of the quenching sites by incorporation of low EA of
NPB, consistent with the J-V-L characteristics. It is worth pointing
out that the problematic roll-off characteristic of current efficiency
at high operational brightness is not observed in the present
devices. There is only a slight change in current efficiencies of
2.9 and 5.8 cd/A for devices A and B (only w3% drop from
the maximum values) even at the current density as high as
250 mA/cm2. Although its colour purity has room for improvement,
these excellent device performance does show that TPADPF is the
suitable candidate for commercial lighting/display applications.
The electrical characteristics of devices A and B are depicted in
Fig. 6. Under the same operating voltage, device B demonstrates
a lower current density and a higher luminance than device A, even
though the turn-on voltage (defined as voltage required to obtain
a luminance of 1 cd/m2) of both devices are the same (Vturn-
¼ 2.6 V). For instance, at the operating voltage of 5 V, the current
on
density and the luminance for device A are 114.6 mA/cm2 and
4. Conclusions
In summary, we have fabricated a highly efficient non-doped
blue OLED by using a novel EL material TPADPF. The device with
a simple structure of ITO/NPB/TPADPF/BPhen/LiF/Al demonstrated
a high current efficiency of 6.0 cd/A, and a high power efficiency of
4.6 lm/W. It shows stable blue emission with the CIE coordinates of
(x ¼ 0.19 ꢀ 0.01, y ¼ 0.45 ꢀ 0.02), with the CIE coordinates and the
FWHM remaining unchanged over a wide luminance range.
Fig. 6. J-V-L characteristics of the TPADPF-based devices.

Q.-X. Tong et al. / Dyes and Pigments 86 (2010) 233e237
237
[14] Lo SC, Namdas EB, Burn PL, Samuel IDW. Synthesis and properties of highly
efficient electroluminescent green phosphorescent iridium cored dendrimers.
Macromolecules 2003;36:9721.
The present results demonstrate that TPADPF is a promising
candidate as a blue-emitting material in OLEDs.
[15] Wong WY, He Z, So SK, Tong KL, Lin Z. A multifunctional platinum-based
triplet emitter for OLED applications. Organometallics 2005;24:4079.
[16] Li Y, Rizzo A, Salerno M, Mazzeo M, Huo C, Wang Y, et al. Multifunctional
platinum porphyrin dendrimers as emitters in undoped phosphorescent
based light emitting devices. Appl Phys Lett 2006;89:061125.
[17] Tong QX, Lai SL, Chan MY, Tang JX, Kwong HL, Lee CS, et al. High-efficiency
nondoped white organic light-emitting devices. Appl Phys Lett 2007;91:
023503.
Acknowledgements
This work was supported by the Key Laboratory of Photo-
chemical Conversion and Optoelectronic Materials,TIPC,CAS and Li
Ka Shing Foundation.
[18] Tong QX, Lai SL, Chan MY, Lai KH, Tang JX, Kwong HL, et al. Efficient green
organic light-Emitting devices with a nondoped dual-functional electrolumi-
nescent material. Appl Phys Lett 2007;91:153504.
References
[19] Zhao L, Zou JH, Huang J, Li C, Zhang Y, Sun C, et al. Asymmetrically 9,10-
disubstituted anthracenes as soluble and stable blue electroluminescent
molecular glasses 2008;9:649.
[20] Huang J, Qiao X, Xia Y, Zhu X, Ma D, Cao Y, et al. A dithienylbenzothiadiazole
pure red molecular emitter with electron transport and exciton self-confine-
ment for nondoped organic red-light-emitting diodes. Adv Mater
2008;20:4172.
[21] Tong QX, Lai SL, Chan MY, Zhou YC, Kwong HL, Lee CS, et al. High-efficiency
nondoped green organic light-emitting devices. Chem Phys Lett 2008;455:79.
[22] Tong QX, Lai SL, Chan MY, Zhou YC, Kwong HL, Lee CS, et al. Highly efficient
blue organic light-Emitting device based on a nondoped electroluminescent
material. Chem Mater 2008;20:6310.
[1] Tang CW, Van Slyke SA. Organic electroluminescent diodes. Appl Phys Lett
1987;51:913.
[2] Tang CW, Van Slyke SA, Chen CH. Electroluminescence of doped organic thin
films. J Appl Phys 1989;65:3610.
[3] Chen CH, Shi J, Tang CW. Recent developments in molecular organic electro-
luminescent materials. Macromol Symp 1997;125:1.
[4] Lee MT, Chen HH, Liao CH, Tsai CH, Chen CH. Stable styrylamine-doped blue
organic electroluminescent device based on 2-methyl-9,10-di(2-naphthyl)
anthracene. Appl Phys Lett 2004;85:3301.
[5] Zhong GY, Xu Z, Zhang ST, Zhan YQ, Wang XJ, Xiong ZH, et al. Aggregation and
permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-
4H-pyran molecules in Alq. Appl Phys Lett 2002;81:1122.
[6] Chang SC, He G, Chen FC, Guo TF, Yang Y. Degradation mechanism of phospho-
rescent-dye-doped polymerlight-emitting diodes. Appl PhysLett2001;79:2088.
[7] Gong JR, Wan LJ, Lei SB, Bai CL, Zhang XH, Lee ST. Direct evidence of molecular
aggregation and degradation mechanism of organic light-emitting diodes
under joule heating: an STM and photoluminescence study. J Phys Chem B
2005;109:1675.
[23] Tong QX, Lai SL, Chan MY, Zhou YC, Kwong HL, Lee CS, et al.
A high
performance nondoped blue organic light-Emitting device based on a diphe-
nylfluoranthene-substituted fluorene derivative. J Phys Chem C 2009;113:
6227.
[24] Katsis D, Geng YH, Ou JJ, Culligan SW, Trajkovska A, Chen SH, et al. Spiro-
linked ter-, penta-, and heptafluorenes as novel amorphous materials for blue
light emission. Chem Mater 2002;14:1332.
[25] Wong KT, Chien YY, Chen RT, Wang CF, Lin YT, Chiang HH, et al. Ter(9,9-dia-
rylfluorene)s: highly efficient blue emitter with promising electrochemical
and thermal stability. J Am Chem Soc 2002;124:11576.
[26] Wu CC, Liu TL, Hung WY, Lin YT, Wong KT, Chen RT, et al. Unusual nondis-
persive ambipolar carrier transport and high electron mobility in amorphous
ter(9,9-diarylfluorene)s. J Am Chem Soc 2003;125:3710.
[27] Park YI, Lee SE, Park JW, Oh SY. New multi-phenylated carbazole derivatives
for OLED through Diels-Alder reaction. Mol Cryst Liq Cryst 2007;470:223.
[28] Tong QX, Lai SL, Chan MY, Lai KH, Tang JX, Kwong HL, et al. High Tg triphe-
nylamine-based starburst hole-transporting material for organic light-emit-
ting devices. Chem Mater 2007;19:5851.
[8] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151.
[9] Chiechi RC, Tseng RJ, Marchioni F, Yang Y, Wudl F. Efficient blue-light-emitting
electroluminescent devices with
a robust fluorophore: 7,8,10-triphenyl-
fluoranthene. Adv Mater 2006;18:325.
[10] Robinson MR, Bazan GC, O'Regan MB. Synthesis, morphology and optoelec-
tronic properties of tris[(N-ethylcarbazolyl)(3,5-hexyloxybenzoyl)methane]
(phenanthroline)europium. Chem Commun 2000:1645.
[11] Murata H, Kafafi ZH, Uchida M. Efficient organic light-emitting diodes with
undoped active layers based on silole derivatives. Appl Phys Lett 2002;80:189.
[12] Lee CL, Kang NG, Cho YS, Lee JS, Kim JJ. Polymer electrophosphorescent device:
comparison of phosphorescent dye doped and coordinated systems. Opt
Mater 2003;21:119.
[29] Wehmeier M, Wagner M, Müllen K. Novel perylene chromophores obtained
by a facile oxidative cyclodehydrogenation route. Chem Eur J 2001;7:2197.
[30] Xia ZY, Xiao X, Su JH, Chang CS, Chen CH, Li DL, et al. Low driving voltage and
efficient orange-red phosphorescent organic light-emitting devices based on
a benzotriazole iridium complex. Synthetic Metals 2009;159:1782.
[13] Xin H, Li FY, Guan M, Huang CH, Sun M, Wang KZ, et al. Carbazole-function-
alized europium complex and its high-efficiency organic electroluminescent
properties. J Appl Phys 2003;94:4729.