High efficient and high color pure blue light emitting materials: New asymmetrically highly twisted host and guest based on anthracene

Article abstract of DOI:10.1016/j.dyepig.2011.06.008

New asymmetrically highly twisted anthracene derivatives serve as a matched host and guest material in high efficiency blue OLEDs. 2-(2- Methylnaphtathalene-1-yl)-9,10-di(naphthalene-2-yl)anthracene and N-(4-(10-naphthalene-2-yl)anthracene-9-yl)phenyl-N-p

Full text of DOI:10.1016/j.dyepig.2011.06.008

Dyes and Pigments 92 (2011) 588e595
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
High efficient and high color pure blue light emitting materials: New
asymmetrically highly twisted host and guest based on anthracene
,
a,
Il Kang a, Jang-Yeol Back c, Ran Kim b, Yun-Hi Kim b , Soon-Ki Kwon
*
*
a School of Materials Science and Engineering & Engineering Research Institute (ERI), Gyeongsang National University, Jinju-660 701, South Korea
b Department of Chemistry and Research Institute of Natural Sciences (RINS), Gyeongsang National University, Jinju-660 701, South Korea
c EL Materials Development Division Duksan Hi-Metal Co., Bundang 463-420, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New asymmetrically highly twisted anthracene derivatives serve as a matched host and guest material in
high efficiency blue OLEDs. 2-(2-Methylnaphtathalene-1-yl)-9,10-di(naphthalene-2-yl)anthracene and
N-(4-(10-naphthalene-2-yl)anthracene-9-yl)phenyl-N-phenylnaphthalene-2-amine were prepared as
host material and as guest material, respectively. Multilayer organic electroluminecent devices con-
structed using these foregoing twisted anthracene derivatives as the emitting layer gave quantum effi-
ciencies of 5% and exhibited a pure blue emission with CIE chromaticity coordinates x ¼ 0.15, y ¼ 0.14e0.18.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 15 March 2011
Received in revised form
6 June 2011
Accepted 7 June 2011
Available online 21 June 2011
Keywords:
Organic light emitting diodes
Asymmetrically highly twisted
anthracene derivative
Blue emission
Color purity
High efficiency
Matched host-guest
1. Introduction
materials is a subject of current interest [12e14]. This is espe-
cially true for a deep-blue color, which is usually defined as blue
Organic light emitting devices (OLEDs) have been attracting
considerable attention for potential application in flat-panel
displays [1,2]. Efficient blue-light OLEDs are of particular interest,
because they are desired for use as blue light sources in full color
display applications [3e8]. Significant progress in materials
synthesis and device construction has led to the realization of full
color as well as white OLEDs with improved efficiencies and life-
times [9]. Extensive efforts have been made to develop high
performance materials with desirable properties, and devices with
optimized architectures to develop marketable OLEDs [10]. Fluo-
rophores with attractive emission characteristics such as a good
range of colors, and high efficiency are desired as emitters of OLEDs.
Furthermore, the realization of a thermally and morphologically
stable amorphous film is another important factor that can dras-
tically improve the physical performance of OLEDs [11]. Given this
background, the development of high performance blue-emitting
electroluminescent emission having Commission Internationale de
l’Enclairage (CIE) coordinates (y-coordinate value < 0.15) [15].
Guest/host systems have been demonstrated as a approach to
achieve high efficiency and color purity, and much work has been
undertaken in developing the guest and host materials and their
combinations. Generally, a good host material should possess
a wide energy gap, good carrier transporting abilities and film
forming abilities. The most widely used host materials involve
di(styryl)arylene derivatives and anthracene derivatives [16].
The guest materials should possess high luminescent efficiency as
well as the ability to accept energy from host materials via the
Foster energy transfer and/or direct recombination via charge
trapping [17].
Many efficient blue emitting materials for hosts and guests have
been designed, such as anthracene [18e25], fluorene [26], di(styryl)
arylene [27], tetra(phenyl)pyrene [28] and tetra(phenyl)silyl
derivatives [29]. As part of our ongoing efforts to search for robust
materials for full color OLED display devices, we recently submitted
a
highly twisted asymmetric anthracene derivative, 2-(2-
methylnaphthalene)-9,10-di(naphthalene-2-yl)anthracene
(MNAn), which was an efficient blue color emitter [30].
* Corresponding authors. Tel.: þ82 55 751 5296.
E-mail addresses: ykim@gnu.ac.kr (Y.-H. Kim), skwon@gnu.ac.kr (S.-K. Kwon).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.06.008

I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
589
In this paper, we studied the highly twisted asymmetric 2-(2-
methylnaphthalene-)-9,10-di(naphthalene-2-yl)anthracene as a wide
energy gap host for blue OLEDs. Although the 9,10-di(naphthalene-
2-yl)lanthracene (ADN) and 2-(2-methylnaphthalene)-9,10-di(naph
thalene-2-yl)anthracene (MNAn) have been reported as host mate-
rials, the new host having ortho-methylated naphthalene on the 2-
position of the anthracene has increased charge transport ability
thickness of the film were measured with a quartz oscillator. OLED
performance was studied by measuring the currentevoltagee
luminescence (IeVeL) characteristics, EL, and PL spectra at room
temperature. IeVeL characteristics and CIE color coordinates were
measured with a Keithley SMU238 and Spectrascan PR650. EL
spectra of the devices were measured utilizing a diode array rapid
analyzer system (Professional Scientific Instrument Corp.) Fluores-
cence spectra of the solutions in chloroform were measured using
a spectro fluorimeter (Shimadzu Corp.).
due to increased p-electron density as well as a highly twisted struc-
ture, leading to excellent host behavior for blue OLEDs. We have
designed a new asymmetrical anthracene derivative, N-(4-(10-
naphthalene-2-yl)anthracene-9-yl)phenyl-N-phenylnaphthalene-
2-amine, as a dopant. Generally, arylamine groups increase the hole
transporting ability of the materials as well as reduce their crystal-
linity, improving device stability due to the amorphous structure,
increase efficiency due to charge balance and reduce energy
consumption due to reduced driving voltages [18]. Thus, anthracene
derivatives containing arylamine have been reported, however, the
reported compounds usually have a symmetrical structure because
of theirease ofsynthesis. Thesesymmetrically introduced arylamine
groups usually deteriorate color purity for blue emission [13].
Therefore, we designed an asymmetrical anthracene derivative with
an arylamine on the 9-position and naphthalene on the 10-position
of the anthracene unit, respectively. These well matched host-guest
materials can improve efficiency due to balanced energy levels and
morphological stability due to similar structure based on twisted
anthracene.
2.3. Synthesis of naphthalene-2-yl boronic acid (1)
2.5 M n-butyllithium (80 mL, 31.9 mmol) was slowly added to
2-bromonaphthalene (60 g, 29.0 mmol) in tetrahydrofuran (THF)
(100 mL) at ꢂ78 ꢀC. The mixture was stirred for 30 min at ꢂ40 ꢀC.
Triethyl borate (127 g, 869.0 mmol) was slowly dropped into the
mixture and stirred at room temperature. After 12 h, the reaction
was terminated by the addition of 2 M HCl (200 mL) and extracted
with dichloromethane (300 mL). The crude product was purified by
recrystallization with hexane. Yield: 40 g (80%); mp. 268 ꢀC, 1H
NMR (300 MHz, CDCl3, ppm): 8.37(s, 1H), 8.18(s, 2H), 7.95e7.83 (m,
4H), 7.55e7.48 (m, 2H).
2.4. Synthesis of 9-(naphthalene-2-yl)anthracene (2)
9-Bromoanthracene (15 g, 58 mmol) and naphthalene-2-yl
boronic acid (13 g, 76 mmol) was added in the mixture of
toluene (80 mL), K2CO3 (40 mL), THF (25 mL) and of tetrakis(-
triphenylphosphine) palladium (0) (0.27 g, 2 mol%). The solution
was stirred in N2 for 24 h at 110 ꢀC. After reaction, the mixture was
quenched with 2 NeHCl (400 mL). The crude product was extrac-
ted and dried. The purification was carried out by column chro-
matography with hexane. The product was purified by column
chromatography with hexane as eluent Yield: 11.8 g (67%); m.p.
2. Experimental
2.1. Materials
All starting materials were purchased from Aldrich and Strem.
All reagents purchased commercially were used without further
purification.
2.2. Instrument
154 ꢀC, 1H NMR (300 MHz, CDCl3, ppm):
d
¼ 8.56 (s, 1H). 8.02e8.11
(m, 4H), 7.91e7.96 (d, 2H), 7.70e7.73 (d, 2H), 7.57w7.62 (m, 3H),
7.46e7.51 (m, 2H), 7.32e7.37 (m, 2H).
1H NMR spectra were recorded using a Bruker Avance-300 MHz
FT-NMR spectrometer, and chemical shifts were reported in ppm
with tetramethylsilane as internal standard. FT-IR spectra were
recorded using a Bruker IFS66 spectrometer. Thermogravimetric
analysis (TGA) was performed under nitrogen using a TA instruments
2050 thermogravimetric analyzer. Differential scanning calorimeter
(DSC) was conducted under nitrogen using a TA instrument DSC Q10.
The both samples were heated using a 10 ꢀC/min. UV-visible spectra
and photoluminescence (PL) spectra were measured by Shimadsu
UV-1065PC UV-visible spectrophotometer and Perkin Elmer LS50B
fluorescence spectrophotometer, respectively. The electrochemical
properties of the materials were measured by cyclic voltammetry
(Epsilon C3) in a 0.1 M solution of tetrabutyl ammonium perchlorate
in acetonitrile. The organic EL devices were fabricated using succes-
sive vacuum-deposition of N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-
2.5. Synthesis of 9-bromo-10-(naphthalene-2-yl)anthracene (3)
In the 500 mL flask, of 9-(naphthalene-2-yl)-anthracene (20 g,
66 mmol), N,N-dimethylformamide (DMF) (30 mL) was stirred. N-
bromosuccinimide (20 g, 0.12 mmol) was slowly dropped. After
stirring for 7 h, water (700 mL) was added to the mixture. The crude
product was filtrated and washed with ethanol. Yield: 16.9 g (67%);
m.p. 173 ꢀC, 1H NMR (300 MHz, CDCl3, ppm): 8.64e8.67 (d, 2H),
8.02e8.08 (m, 2H), 7.92 (d, 2H), 7.52e7.69 (m, 7H), 7.34 (m, 2H).
2.6. Synthesis of 4,4,4,5-tetramethyl-2-(10-(naphthalene-2-yl)
anthracene-9-yl)-1,3,2-dioxaborolane (4)
0
0
ꢀ
ꢀ
tolylamino)-phenyl]-biphenyl-4,4 -diamine (DNTPD, 700 A), N,N -
0
0
0
diphenyl-N,N -di(1-napthyl)-1,1 -biphenyl-4,4 -diamine (NPD, 300 A),
9.10-di(naphthalene-2-yl)anthracene (ADN): p-NAPPN (3%): or
MNAn: p-NAPPN (3%), tris(8-hydroxyquinoline)aluminum (Alq3,
2.5 M n-butyllithium (50.9 mmol) was slowlyadded to 9-bromo-
10-(naphthalene-2-yl)anthracene (39.1 mmol) in THF (100 mL)
at ꢂ78 ꢀC. The mixture was stirred for 30 min at ꢂ40 ꢀC. 2-
Isopropoxy-4,4,5,5,-tetramethyl-[1, 2, 3] dioxaborolane (24 mL,
11.7 mmol) was slowly dropped into the mixture and stirred at room
temperature. After 12 h, the reaction was terminated by the addi-
tion of 2 M HCl and extracted with ethyl acetate. The crude product
was purified by column chromatography with hexane:ethyl acetate
ꢀ
ꢀ
400 A), LiF (5 A), and Al electrode on top of the ITO glass substrate.
The ITO glass with a sheet resistance of about 10 was etched for
U
the anode electrode pattern and cleaned in ultrasonic baths of iso-
propyl alcohol and acetone. The overlap area of Al and ITO elec-
trodes is about 4 mm2. A UV zone cleaner (Jeilight Company) was
used for further cleaning before vacuum deposition of the organic
materials. Vacuum deposition of the organic materials was carried
out under a pressure of 2 ꢁ10ꢂ7 torr. The deposition rate for organic
materials was about 0.1 nm/s. The evaporation rate and the
(10:1). Yield: 10 g (60.1%); 1H NMR (300 MHz, CDCl3, ppm)
(d, 2H), 8.07e8.01 (m, 2H), 7.9 (d, 2H), 7.6 (d, 2H), 7.6e7.5 (m, 2H),
7.54e7.49 (m, 3H), 7.33e7.28 (m, 2H), 1.6 (s, 12H).
d
¼ 8.4

590
I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
Br
OH
B
Br
Triethylborate
OH
n-BuLi/ Ether
2 mol% Pd(PPh3)4 / 2 M K2CO3
(1)
C9H19BO3
N-Bromosuccimide
DMF
n-BuLi / THF
B
Br
O
O
(4)
(3)
(2)
Br
NH
Br
dppf / Pd2dba3 / NaO-t-Bu
2 mol% Pd(PPh3)4 / 2 M K2CO3
N
(5)
Br
p-NAPPN
(1) naphthalene-2-yl boronic acid
(2) 9-(naphthalene-2-yl)anthracene
(3) 9-bromo-10-(naphthalene-2-yl)anthracene
(4) 4,4,4,5-tetramethyl-2-(10-(naphthalene-2-
yl)anthracene-9-yl)-1,3,2-dioxaborolane
(5) 9-(4-bromophenyl)-10-(naphthalene-2-yl)
anthracene
MNAn
Scheme 1. Synthesis of N-(4-(10-naphthalene-2-yl)anthracene-9-yl)phenyl-N-phenylnaphthalene-2-amine (p-NAPPN).
Fig. 1. TGA and DSC thermograms of p-NAPPN.

I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
591
2.8. Synthesis of N-(4-(10-naphthalene-2-yl)anthracene-9-yl)
phenyl-N-phenylnaphthalene-2-amine (p-NAPPN)
PL
UV
p-N-NAPPN solution
p-N-NAPPN film
1
1
Tris(dibenzylidenacetone)dipalladium (0) (Pd2dba3) (0.04 g,
0.05 mmol), 1,1-bis(diphenylphosphino)ferrocene (dppf) (0.04 g,
0.07 mmol), sodium tert-butoxide (0.4 g, 5 mmol) were added
to toluene (40 mL). 9-(4-Bromophenyl)-10-(naphthalene-2-yl)
anthracene (1.4 g, 3.0 mmol) was added to the mixture. After stir-
ring 15 min, naphthalene-2-yl-phenyl amine (2.3 g, 8.1 mmol) was
added to the mixture. After the mixture was refluxed for 24 h, H2O
added to the mixture to stop the reaction. The crude product was
purified by recrystallization with toluene and ethanol. Yield: 0.8 g
N
(44%); m.p. 274 ꢀC, 1H NMR (300 MHz, CDCl3, ppm):
d
¼ 8.0 (m, 2H),
7.93e7.82 (m, 3H), 7.77 (q, 1H), 7.65 (d, 2H), 7.54e7.45 (m,3H),
7.35e7.21 (m, 9H), 7.09 (m, 1H), HRMS: calcd. for C46H31N:
597.2457, found: 597.2453. Anal. Calcd for C46H31N: C, 92.43; H,
5.23; Found: C, 92.40; H, 5.24.
0
300
0
400
500
600
700
Wavelength (nm)
Fig. 2. UVevisible and PL spectra of p-NAPPN.
2.9. Synthesis of 2-(2-methylnaphtathalene-1-yl)-9,10-
di(naphthalene-2-yl)anthracene (MNAn)
UV
PL
p-N-NAPPN UV
MNAn PL
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
The synthesis was followed by literature method. The product
was purified by column chromatography with hexane as eluent.
Yield: 1.96 g (29.17%); 1H NMR (300 MHz, CDCl3, ppm)
d
¼ 8.17e8.12 (m, 2H), 8.09e8.05 (m, 1H), 8.02e7.88 (m, 6H),
7.84e7.73 (m, 5H), 7.71e7.70 (m, 1H), 7.67e7.62 (m, 3H), 7.57e7.47
(m, 3H), 7.38e7.31 (m, 4H), 7.30e7.26 (m, 2H), 2.25 (d, 3H), HRMS:
calcd. for C45H30: 570.2347, found: 570.2353.
3. Results and discussion
Scheme 1 displays the synthetic route used to prepare an
asymmetrically arylamine substituted anthracene derivative as
a dopant and the highly twisted ortho-substituted anthracene as
a new host. 9-(4-Bromophenyl)-10-(naphthalene-2-yl)anthracene
was obtained by Suzuki coupling reaction of 9-bromoanthracene
naphthalene-2-yl boronic acid following bromination. N-(4-(10-
naphthalene-2-yl)anthracene-9-yl)phenyl-N-phenylnaphthalene-
2-amine (p-NAPPN) was synthesized by N-arylation with naph-
thalene-2-yl-phenyl-amine and 9-(4-bromophenyl)-10-(naphtha-
lene-2-yl)anthracene. 2-Bromo-9,10-dinaphthyl anthracene was
obtained by nucleophilic addition of 2-bromonaphthalene and 2-
bromoanthraquinone followed by an oxidation reaction. MNAn
was synthesized by a Suzuki coupling reaction of 2-bromo-9,10-
dinaphthylanthracene with 2-methyl-1-naphthalene boronic acid.
All compounds were purified by the silica column chromatographic
method or recrystallization. Further purification was readily
accomplished by sublimation under vacuum (10ꢂ6 Torr). The target
compounds were characterized by NMR and mass spectroscopy.
The thermal properties of p-NAPPN were determined by
differential scanning calorimetry (DSC) and thermogravimetry
(TGA) measurements. (Fig. 1) MNAn and p-NAPPN exhibit high
thermal stabilities with decomposition temperatures (5% weight
300
400
500
600
700
Wavelength (nm)
Fig. 3. PL spectrum of MNAn as an host and UVevis absorption spectrum of p-NAPPN
as a dopant.
2.7. Synthesis of 9-(4-bromophenyl)-10-(naphthalene-2-yl)
anthracene (5)
1,4-Dibromobenzene (2 g, 8.5 mmol) and of 4,4,5,5-tetramethyl-
2-(10-(naphthalene-2-yl)anthracene-9-yl)-1,3,2-dioxaborolane
(2.56 g, 5.9 mmol) were added to solution of toluene (60 mL),
2 M K2CO3 (30 mL), 1 mol % of tetrakis(triphenylphosphine) palla-
dium (0). The solution was stirring for 24 h at 90 ꢀC. After reaction,
the mixture was quenched with HCl (2 N, 400 mL). The crude
product was extracted and dried. The purificationwas carried out by
column chromatography with hexane and ethyl acetate (10:1) as
eluent. Yield: 1.4 g (51%); m.p. 237 ꢀC, 1H NMR (300 MHz, CDCl3,
ppm):
d
¼ 8.1 (m, 2H), 8.0 (m, 2H), 7.9 (m, 2H), 7.7e7.6 (m, 6H),
7.6e7.4 (m, 5H), 7.4e7.2 (m, 4H) EI-MS: m/z 458.
Fig. 4. Contour plots regarding the HOMO and LUMO of p-NAPPN.

592
I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
Table 1
The optical, thermal and electrical properties of MNAn and p-NAPPN.
Solutiona
UV (nm)
Filma
HOMO (eV)
LUMO (eV)
Egb (eV)
Td (ꢀC)
Tm (ꢀC)
Tg (ꢀC)
PL (nm)
UV (nm)
PL (nm)
MNAn
p-NAPPN
361,381,402
361,377,401
431
481
364,385,406
382
454
470
5.58
5.54
2.60
2.66
2.98
2.88
382
388
292
274
e
135
a
In CHCl3.
The optical energy band gap.
b
loss) of 382 ꢀC and 388 ꢀC. The melting transition temperature (Tm)
of MNAn was observed at 292 ꢀC.[30] Tg and Tm of p-NAPPN are
135 and 274, respectively. The high thermal stability of these two
compounds as well as similar structures inhibited self-aggregation.
This suggests that the emitting layer composed with MNAn and
p-NAPPN has stable morphological properties, which is desirable
for OLEDs with high stability and efficiency.
Fig. 2 shows the UVevisible absorption and photoluminescence
(PL) spectra of p-NAPPN in chloroform solution. The UVevisible
absorption spectrum of p-NAPPN shows the characteristic vibra-
tional patterns of an isolated anthracene group (lmax ¼ 361, 377,
401 nm). Upon irradiation at 365 nm, the PL spectrum of p-NAPPN
in solution exhibited an excellent blue emission with the peak
maximum at lmax 481 nm. In the PL spectrum of p-NAPPN in a film,
the peak maximum of the blue emission is found at 470 nm without
a shoulder emission, which implies the blue emission with
inhibited intermolecular interaction. p-NAPPN containing an aryl
amine group showed a blue shifted emission in the thin film, which
is often observed for arylamine derivatives [31]. Fig. 3 represents
the PL spectrum of MNAn as a host and UVevis absorption spec-
trum of p-NAPPN as a dopant. From this result, it can be suggested
that the efficient energy transfer can occur by the close match of
emission spectrum of the host and the absorption spectrum of
guest.
The optical energy band gap of MNAn and p-NAPPN is 2.98 eV
and 2.88 eV, respectively, calculated from the threshold of the
optical absorption (416 nm for MNAn, 430 nm for p-NAPPN). From
these results, it is speculated that efficient Foster energy transfer
takes place, because the band gap of p-NAPPN is smaller than that
of the MNAn host.
A contour plot regarding the HOMO and LUMO of p-NAPPN
determined by theoretical calculation is shown in Fig. 4. While the
electron density of the HOMO of p-NAPPN is slightly spread to the
arylamine, the electron density of the LUMO is largely concentrated
on anthracene unit.
Cyclic voltammetry (CV) was carried out to identify the elec-
trochemical behavior of p-NAPPN. MNAn, containing methyl-
naphthalene, has a slightly higher oxidation potential (Eox) and
a higher energy gap than those of p-NAPPN which contains an
electron donating p- arylamine group. The HOMO levels of MNAn
and p-NAPPN were 5.58 eV and 5.54 eV, respectively. The LUMO
values of MNAn and p-NAPPN were 2.60 and 2.66 eV, respectively,
which were calculated from the optical band gap and HOMO values.
Table 1 summarized the optical, thermal and electrochemical
results of MNAn and p-NAPPN.
To study the electroluminescence performance of MNAn and
p-NAPPN, two different types of blue emitting device have
been fabricated. Multilayer devices with a configuration of indium
tin oxide (ITO)/N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolylamino)
phenyl]-biphenyl-4,40-diamine (DNTPD, 70 nm)/1,4-bis[(1-naph
thylphenyl)amino]biphenyl (a-NPD) (50 nm)/ADN:p-NAPPN (3%)/
tris(8-hydroxyquinoline) aluminum (Alq3, 40 nm)/LiF (0.5 nm)/
Al (device A) and (ITO)/DNTPD (70 nm)/1,4-bis[(1-naphthyl
phenyl)-amino]biphenyl (
tris(8-hydroxyquinoline) aluminum (Alq3, 40 nm)/LiF (0.5 nm)/Al
a-NPD) (50 nm)/MNAn:p-NAPPN (3%)/
Fig. 5. The voltage versus current density characteristics, luminous efficiency (LE),
power efficiency (PE), and external quantum efficiency (EQE) of device A.

I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
593
Table 2
EL performances of devices Device A: (ITO)/DNTPD (70 nm)/(
ADN:p-NAPPN (3%)/(Alq3, 40 nm) /LiF (0.5 nm)/Al. Device B: (ITO)/DNTPD (70 nm)/
-NPD) (50 nm)/MNAn:p-NAPPN (3%)/(Alq3, 40 nm) /LiF (0.5 nm)/Al.
a
-NPD) (50 nm)/
(
a
Device
Dopant
Host
lmax hc
(nm) (cd Aꢂ1
hP
(lm Wꢂ1
hexp CIE
a
b
)
)
(%)c (x, y)
Device A p-NAPPN ADN
456
4.95
6.88
3.01
4.37
4.57 0.15, 0.14
5.31 0.14, 0.18
Device B
MNAn 464
a
Current efficiency (hc).
Power efficiency (hP).
External quantum efficiency (hexp).
b
c
(device B) were fabricated, where ITO serves as the anode and Al as
the cathode. The stack of organic layers consists of DNTPD as a hole
injection layer, a-NPD as a hole-transport layer, ADN doped with
p-NAPPN (3%) as the emitter or MNAn doped with p-NAPPN as the
emitter, Alq3 as an electron-transport layer, and LiF as an electron-
injection layer. In this study, ADN was used as the host for
comparing MNAn in blue-emitting electroluminescence devices.
Fig. 5 shows the voltage versus current density characteristics,
luminous efficiency (LE), power efficiency (PE), and external
quantum efficiency (EQE) of device A. The EL properties of the
devices are summarized in Table 2. The turn on voltage was around
4.1 V for device A. The current efficiency, power efficiency, and
external quantum efficiency of device A at 10 mA was 4.95 cd/A,
3.01 lm/W and 4.57%, respectively. Fig. 6 shows EL spectrum of
device A. The EL maximum and full width at half maximum
(FWHM) of device A were 456 nm and 50 nm, respectively. The CIE
coordinates of device A was (0.15, 0.14). From the results, the newly
developed p-NAPPN has potential to be used as a highly efficient
pure blue color dopant.
Fig. 7 shows the voltage versus current density characteristics,
luminous efficiency (LE), power efficiency (PE), and external
quantum efficiency (EQE) of device B. The current efficiency, power
efficiency, and external quantum efficiency of device B at 10 mA
was 6.88 cd/A, 4.37 lm/W and 5.31%, respectively. The maximum
EQE of device B was greater than 5%, the known theoretical limit.
The electroluminescence in the host-dopant system can occur by
the Foster-type energy transfer from the host to dopant. As
expected based on the band gap, Foster-type energy transfer can
effectively occur when the band gap of the host is higher than that
of the guest. It has been reported that the band gap of ADN is 3.1 eV.
Although the ADN host has a higher band gap than that of MNAn,
the EL efficiency of device B was 1.5 times higher than that of device
A. It can be seen from Fig. 2 that the absorption spectrum of the
p-NAPPN dopant material overlapped better with the emission
spectra of MNAn than that of ADN, demonstrating that p-NAPPN
can effectively accept energy transfer from the MNAn host material
Fig. 7. The voltage versus current density characteristics, luminous efficiency (LE),
power efficiency (PE), and external quantum efficiency (EQE) of device B.
Fig. 6. EL spectrum of device A.
Fig. 8. EL spectrum of device B.

594
I. Kang et al. / Dyes and Pigments 92 (2011) 588e595
Fig. 9. Energy level diagrams of devices.
by a Foster-type energy transfer, thereby leading to improved EL
efficiencies. Furthermore, it is suggested that the energy levels are
well matched in device B. The proposed energy level diagrams of
the devices are shown in Fig. 8. Device A used ADN as a host and
p-NAPPN as a dopant and had a higher hole injection barrier
(0.48 eV) from the HOMO of NPD to that of the ADN host than the
electron injection barrier from the LUMO of Alq3 to that of the ADN
host (0.03 eV). Thus, it is suggested that the electrons are injected
into the LUMO of the ADN host, meanwhile, holes are injected to
p-NAPPN because the energy barrier between the HOMO level of
p-NAPPN and the HOMO level of NPD is only 0.14 eV and the doping
ratio is 3%. Thus, it is suggested that charge trapping emission takes
place. On the other hand, Device B used MNAn as a host and
p-NAPPN as a dopant, and the HOMO and LUMO levels of the
dopant p-NAPPN are included with those of the host MNAn. Foster
energy transfer can thus effectively occur in device B owing to the
well matched energy levels. Fig. 9 shows the EL spectrum of device
B. The EL maximum of device B and FWHM was 464 nm and 56 nm,
respectively. The CIE coordinates of device B were x ¼ 0.14, y ¼ 0.18.
photoluminescence spectrum of MNAn but also the well matched
energy levels of the dopant and host, allowing efficient host-to-
guest Foster energy transfer and consequently greatly improved
luminance efficiency.
Acknowledgement
This research was financially supported by Basic Science
Research Program through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology (2011-0003336).
Il Kang, Seul-ong Kim and Hyuntae Park thank to MKE and KIAT
through the Workforce Development Program in Strategic Tech-
nology Under Ministry of Knowledge Economy of Korea.
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