Highly efficient blue light-emitting diodes using new fluorescent host materials based on naphthalenes

Article abstract of DOI:10.1080/15421406.2010.499317

The naphthalene-based blue materials 4,4′-(Dinaphthalen-2-yl)-1, 1′-binaphthyl (DNBN) and 1,4-(Dinaphthalen-2-yl)-naphthalene (DNN) were designed and synthesized for OLEDs. A device (non-doped) employing DNN as the emitter exhibited a maximum luminance, luminous efficiency, and external quantum efficiency of 1120cd/m², 1.40cd/A, and 3.83%, respectively. Moreover, its CIE coordinates (0.152, 0.069) are very close to the NTSC blue standard of (0.14, 0.08). In order to improve EL efficiencies, these materials were used as the blue host materials for the blue dopants PFVtPh and PCVtPh. A device 1b (PFVtPh-doped) showed high EL efficiencies of 5.24cd/A, 2.75lm/W, and 3.82% at 20mA/cm². Copyright Taylor & Francis Group, LLC.

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Highly Efficient Blue Light-Emitting
Diodes Using New Fluorescent Host
Materials Based on Naphthalenes
Jeong Keun Park a , Kum Hee Lee a , Jung Sun Park b , Ji Hoon Seo b
Young Kwan Kim b & Seung Soo Yoon a
,
a Department of Chemistry , Sungkyunkwan University , Suwon,
Gyeonggi-do, Korea
b Department of Information Display , Hongik University , Seoul,
Korea
Published online: 14 Nov 2010.
To cite this article: Jeong Keun Park , Kum Hee Lee , Jung Sun Park , Ji Hoon Seo , Young Kwan Kim
& Seung Soo Yoon (2010) Highly Efficient Blue Light-Emitting Diodes Using New Fluorescent Host
Materials Based on Naphthalenes, Molecular Crystals and Liquid Crystals, 531:1, 55/[355]-64/[364],
DOI: 10.1080/15421406.2010.499317
To link to this article: http://dx.doi.org/10.1080/15421406.2010.499317
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Mol. Cryst. Liq. Cryst., Vol. 531: pp. 55=[355]–64=[364], 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.499317
Highly Efficient Blue Light-Emitting Diodes
Using New Fluorescent Host Materials Based
on Naphthalenes
JEONG KEUN PARK,1 KUM HEE LEE,1
JUNG SUN PARK,2 JI HOON SEO,2
YOUNG KWAN KIM,2 AND SEUNG SOO YOON1
1Department of Chemistry, Sungkyunkwan University, Suwon,
Gyeonggi-do, Korea
2Department of Information Display, Hongik University, Seoul, Korea
The naphthalene-based blue materials 4,40-(Dinaphthalen-2-yl)-1,10-binaphthyl
(DNBN) and 1,4-(Dinaphthalen-2-yl)-naphthalene (DNN) were designed and
synthesized for OLEDs. A device (non-doped) employing DNN as the emitter
exhibited a maximum luminance, luminous efficiency, and external quantum
efficiency of 1120 cd=m2, 1.40 cd=A, and 3.83%, respectively. Moreover, its CIE
coordinates (0.152, 0.069) are very close to the NTSC blue standard of (0.14,
0.08). In order to improve EL efficiencies, these materials were used as the blue host
materials for the blue dopants PFVtPh and PCVtPh. A device 1b (PFVtPh-doped)
showed high EL efficiencies of 5.24 cd=A, 2.75 lm=W, and 3.82% at 20 mA=cm2.
Keywords Blue OLEDs; dopant; fluorescence; host; naphthalene; suzuki cross-
coupling reaction
Introduction
Organic p-conjugated materials continue to attract considerable interest because of
their potential applications in various optoelectronic devices, especially in organic
light-emitting diodes (OLEDs) [1,2]. Blue emitters exhibit relatively poor perform-
ance compared with red and green emitters, many studies have been recently under-
taken toward making blue emitters with high efficiency, color purity, and long
´
operation. Blue emitters with excellent Commission Internationale d’Enclairage
(CIE) coordinates (0.14, 0.08) for the National Television Standards Committee
(NTSC) standard blue need to have wide energy band-gaps near 3.0 eV [3]. Such blue
emitters have high HOMO (highest occupied molecular orbital) levels and low
LUMO (lowest unoccupied molecular orbital) levels. Accordingly, the performance
of blue OLEDs is usually inferior to that of green and red OLEDs due to poor
efficiency [4,5]. Blue-emitting materials based on small molecules with various
Address correspondence to Prof. Seung Soo Yoon, Department of Chemistry,
Sungkyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do 440-746,
Korea (ROK). Tel.: (þ82)31-290-7071; Fax: (þ82)31-290-5971; E-mail: ssyoon@skku.edu
55=[355]

56=[356]
J. K. Park et al.
chromophore structures such as distrylarylene [6], spiro [7], biaryls [8], fluorine [9],
and silicon [10] have been reported and developed.
In this article, novel blue fluorescent materials based on naphthalenes;
40-(Dinaphthalen-2-yl)-1,10-binaphthyl (DNBN), and 1,4-(Dinaphthalen-2-yl)-
naphthalene (DNN) were synthesized, and their electroluminescent properties were
investigated (Scheme 1). These non-planar molecules based on naphthalenes have a large
dihedral angle and twisted conformation [11] to mitigate unfavorable intermolecular
interactions, and thus can decrease self-quenching between the host materials. Further-
more, it can form a good amorphous pinhole-free thin film in the solid state. Therefore,
high efficiencies can be expected for blue OLEDs using naphthalenes molecules.
Experimental
General Procedure for the Suzuki Cross-Coupling Reaction
2-Naphthalene boronic acid (2.5 mol) and the corresponding aryl halide derivatives
(1.0 mol), Pd(PPh3)4 (0.04 mol), aqueous 2.0 M Na2CO3 (10.0 mol), ethanol, and tolu-
ene were mixed in a flask. The mixture was refluxed for 4 h. After the reaction finished,
the reaction mixture was extracted with toluene and washed with water. The organic
layer was dried with anhydrous MgSO4 and filtered with silica gel. The solution was
then evaporated. The crude product was recrystallized from CH2Cl2=EtOH.
40-(Dinaphthalen-2-yl)-1,10-binaphthyl (DNBN): Yield: 90.6%. 1H-NMR
(300 MHz, CDCl3) [d ppm]: 8.07 (d, J ¼ 9.6 Hz, 4H), 8.01 (d, J ¼ 8.4 Hz, 2H),
7.98–7.93 (m, 4H), 7.76 (dd, J ¼ 1.5, 8.4 Hz, 2H), 7.68–7.55 (m, 10H), 7.40 (td,
J ¼ 1.5, 7.7 Hz, 2H), 7.34 (td, J ¼ 1.2, 7.5 Hz, 2H). 13C-NMR (75 MHz, CDCl3)
[d ppm]: 140.4, 138.6, 138.5, 133.7, 133.5, 132.9, 132.1, 129.2, 128.9, 128.4, 128.1,
128.0, 127.8, 127.4, 127.1, 126.7, 126.6, 126.4, 126.3, 126.2. FT-IR [ATR]:
n ¼ 3050, 1598, 1502, 843, 822, 769, 745 cmꢀ1. MS(EIþ) m=z 506 (Mþ). Anal. calcd
for C40H26: C 94.83, H 5.17; found: C 93.30, H 5.15.
1,4-(Dinaphthalen-2-yl)-naphthalene (DNN): Yield: 81.3%. 1H-NMR
(300 MHz, CDCl3) [d ppm]: 8.04–8.00 (m, 4H), 7.96–7.90 (m, 6H), 7.70 (dd,
J ¼ 1.5, 8.4 Hz, 2H), 7.60 (s, 2H), 7.55 (dd, J ¼ 3.3, 6.2 Hz, 4H), 7.44 (dd, J ¼ 3.3,
6.5 Hz, 2H). 13C-NMR (75 MHz, CDCl3) [d ppm]: 140.0, 138.5, 133.6, 132.8,
132.3, 129.0, 128.7, 128.3, 127.9, 127.8, 127.0, 126.7, 126.5, 126.2, 126.1. FT-IR
[ATR]: n ¼ 3051, 1738, 1598, 1502, 846, 826, 766, 745 cmꢀ1. MS(EIþ) m=z 380
(Mþ). Anal. calcd for C40H26: C 94.70, H 5.30; found: C 93.95, H 5.44.
Materials and Measurements
1-Bromonaphthalene, 1-Naphthalene boronic acid, 2-Naphthalene boronic acid,
and 1,4-Dibromonaphthalene were used as received from Alfa Asear or TCI.
1,10-Binaphthyl (A), 4,40-Dibromo-1,10-binaphthyl (B) [12], 40-[2-(2-Diphenylamino-
9,9-diethyl-9H-fluoren-7-yl)vinyl]-p-terphenyl (PFVtPh) [13], and 3-(N-phenyl-
carbazol)vinyl-p-terphenyl (PCVtPh) [14] were synthesized as previously reported.
The solvents were dried using standard procedures. All reagents were used as
received from commercial sources, unless otherwise stated. All reactions were
performed under N2 atmosphere.
1H and 13C NMR were recorded on a Varian Unity Inova 300Nb spectrometer.
FT-IR spectra were recorded using a Bruker VERTEX70 FT-IR spectrometer.

Highly Efficient Blue Light–Emitting Diodes
57=[357]
Elemental analysis (EA) was measured using a EA 1108 spectrometer. Low
resolution mass spectra were measured using a Jeol JMS-600 spectrometer in EI
mode. The UV-Vis absorption and photoluminescence spectra of these newly
designed host materials were measured in CH2Cl2 (10ꢀ5 M) using a Shimadzu
UV-1650PC and an Amincobrowman series 2 luminescence spectrometer. The fluor-
escent quantum yields were determined in the CH2Cl2 solution at 293 K against the
host (UDPA ¼ 0.90) as a standard. Energy levels were measured with a low-energy
photo-electron spectrometer (Riken-Keiki, AC-2). Thermal property was measured
using thermogravimetric analysis (TGA) (DTA-TGA, TA-4000) under nitrogen at
a heating rate of 10ꢁC=min.
Fabrication of OLEDs
To fabricate OLEDs, indium-tin-oxide (ITO) thin films coated on glass substrates
were used, which had 30 X resistivity per square of sheets with 100 nm of thickness.
The ITO-coated glass was cleaned in an ultrasonic bath by the following sequence:
acetone, methyl alcohol, and distilled water; it was then stored in isopropyl alcohol
for 48 h and dried by a N2 gas gun. The substrates were treated by O2 plasma under
2.0 ꢂ 10ꢀ2 torr at 125 W for 2 min [15]. All organic materials and metals were
deposited under high vacuum (5 ꢂ 10ꢀ7 torr). The OLEDs were fabricated in the
following sequence: ITO=4,40-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB)
(50 nm)=Blue emitting materials (DNBN and DNN) (30 nm)=4,7-diphenyl-1,10-
phenanthroline (Bphen) (30 nm)=lithium quinolate (Liq) (2 nm)=Al (100 nm), with
NPB as the hole-transporting layer, Bphen as the electron-transporting layer, and
Liq: Al as the composite cathode. The current density (J), luminance (L), luminous
efficiency (LE), and CIE chromaticity coordinates of the OLEDs were measured
with a Keithley 2400 Chroma meter CS-1000A. Electroluminance was measured
using a Roper Scientific Pro 300i.
Results and Discussion
The optical properties of the blue materials (DNBN and DNN) are presented in
Table 1. The decomposition temperature (Td; corresponding to 5% weight loss)
Table 1. Optical properties of Compounds
HOMO=
Td UVmax PLmax FWHMb PLmax
LUMOd
[eV]
a
b
b
c
Compound [ꢁC]
[nm]
[nm]
[nm]
[nm]
Eg
Ue
MADN
DNBN
DNN
397f
398
329
377
305
304
425
412
404
54
62
59
447f ꢀ5.50f=ꢀ2.50f 3.00f 0.54
403
383
ꢀ6.01=ꢀ2.58 3.43 0.45
ꢀ6.00=ꢀ2.52 3.48 0.32
aThe decomposition temperature.
bIn CH2Cl2 solution.
cIn film state.
dObtained from AC-2 and UV-vis absorption measurements.
eUsing 9,10-Diphenylanthracene as a standard; kex ¼ 360 nm (U ¼ 0.90 in CH2Cl2).
fRef. 18.

58=[358]
J. K. Park et al.
was measured with by a thermogravimetric analysis (TGA). The Td for DNBN and
DNN were 398 and 329ꢁC, respectively. The HOMO energy levels were measured by
a low-energy photo-electron spectrometer (Riken-Keiki AC-2). The HOMO=LUMO
energy levels of DNBN and DNN were ꢀ6.01=ꢀ2.58, and ꢀ6.00=ꢀ2.52, respect-
ively. Both the HOMO and LUMO energy level of the blue host materials (DNBN
and DNN) reveal the similar values. Their optical energy band gaps (Eg) were 3.43
and 3.48 eV, respectively, as determined from the absorption spectra. These materials
have higher energy band gaps than those of PFVtPh (2.82 eV) and PCVtPh (3.10 eV).
It is possible that efficient Fo¨rster energy transfer occurs from the host to the
dopant.
Figure 1(a) shows the absorption of the dopant materials (PFVtPh and PCVtPh)
and emission of the blue host materials (DNN and DNBN). The PL spectra of DNN
and DNBN were 412 and 404 nm, respectively. There is a slight red-shift in the PL
spectrum of DNBN as compared to DNN, due to the lengthening p-conjugation
Figure 1. (a) the emission spectra of the host materials and absorption spectra of dopant
materials in solution and (b) the emission spectra of host and dopant materials in solution
and film state.

Highly Efficient Blue Light–Emitting Diodes
59=[359]
Figure 2. Energy diagrams of non-doped devices 1a-2c.
length. Figure 1(b) shows the PL spectra of the dopant materials (PFVtPh and
PCVtPh) and host materials (DNN and DNBN) in solid film state. Interestingly,
compared to PL spectra of DNBN and DNN in solution state, the PL spectra of
those in film state are blue-shifted due to the differences in solvation.
Figure 2 shows the energy band diagrams of the non-doped devices. The EL per-
formance characteristics of the non-doped devices are summarized in Table 2. As
shown in Figure 3, devices 1a–2a emitted at 440 nm with (0.153, 0.064), and
442 nm with (0.152, 0.069) at 8.0 V, respectively, all in the deep-blue region. These
devices with naphthalenes generate a y-CIE coordinate value of less 0.07 in compari-
son to a common host MADN (y ¼ 0.08). Interestingly, the EL spectra of devices
1a–2a are red-shifted about 30 nm as compared to PL spectra of DNN and DNBN
in solution. Presumably, these results are originated from the differences in solvation
states between solution and solid-state devices. The luminance-voltage-current den-
sity (L-V-J), luminous efficiencies (LE), and external quantum efficiency (EQE) of
the devices 1a–2a are shown in Figures 3–4. Among non-doped devices with the
similar EL performances within 10%, the luminous efficiency (LE), and external
quantum efficiency (EQE) of device 2a reach 1.40 cd=A, and 2.15% (1.26 cd=A,
and 1.93% at 20 mA=cm2), respectively. Although the efficiencies of two devices were
higher than that of 2-methyl-9,10-di(naphthalen-2-yl)anthracene (MADN), they had
low EL performances. It is conjectured that the hole-injection from HTL (NPB) to
EML is difficult due to a high energy barrier (ca. 0.60 eV). Thus, the two devices
have overall low EL properties.
In order to enhance EL efficiencies, we doped these host materials (DNBN and
DNN) with blue dopant materials (PFVtPh and PCVtPh) with a concentration of
Table 2. EL performance characteristic of the non-doped devices
a
EL
max
Device
Von
k
Lb
Jb
LEc=d
PEc=d
[lm=W]
EQEc=d
[%]
(compound) [V] [nm] [cd=m2] [mA=cm2] [cd=A]
CIEe (x,y)
MADN
1a (DNBN) 3.7 440
2a (DNN) 3.4 442
4.0 442
954
994
1120
154
151
139
1.35=1.08 1.01=0.48 1.45=0.98 (0.153,0.080)
1.30=1.07 1.04=0.54 2.14=1.81 (0.153,0.064)
1.40=1.26 1.12=0.70 2.15=1.93 (0.152,0.069)
aTurn-on voltage at 1 cd=m2.
bMaximum luminance and current density.
cMaximum value.
dAt 20 mA=cm2.
Commission Internationale d’Enclairage (CIE) coordinates at a 8.0 V.
e
´

60=[360]
J. K. Park et al.
Figure 3. EL spectra and the CIE coordinates at 8.0 V of the non-doped devices.
8% in each device structure. The EL performances of the doped-OLEDs are
summarized in Table 3. The normalized EL spectra of the doped devices revealed
an emission peak around 444–457 nm in Figure 5. The luminance-voltage (L-V),
luminous efficiencies (LE), and external quantum efficiency (EQE) of each of the
Figure 4. (a) L-V-J characteristics, (b) the luminous efficiencies, and (c) external quantum
efficiencies as a function of current density for the non-doped devices.

Highly Efficient Blue Light–Emitting Diodes
61=[361]
CIEe (x,y)
Table 3. EL performance characteristic of the doped-devices
Lb
Jb
LEc=d
PEc=d
[lm=W]
EQEc=d
[%]
a
EL
max
Device
(compound)
Von
k
[V] [nm] [cd=m2] [mA=cm2] [cd=A]
1b (DNBN:
PFVtPh)
2b (DNN:
PFVtPh)
1c (DNBN:
PCVtPh)
2c (DNN:
PCVtPh)
PCVtPh
3.5 457 6128
2.9 453 2342
4.0 449 1960
202
254
169
109
397
8.43=5.24 6.89=2.75 5.98=3.82 (0.153,0.167)
5.13=3.88 5.17=2.26 3.61=2.79 (0.159,0.168)
2.37=1.69 1.86=0.77 3.16=2.29 (0.152,0.081)
1.13=1.11 0.68=0.53 1.50=1.47 (0.154,0.084)
1.20=1.16 1.06=0.87 1.15=1.12 (0.163,0.124)
3.8 444
2.5 452
791
798
(non-doped)
aTurn-on voltage at 1 cd=m2.
bMaximum luminance and current density.
cMaximum value.
dAt 20 mA=cm2.
Commission Internationale d’Enclairage (CIE) coordinates at a 8.0 V.
e
´
four doped devices (1b–2c) are shown in Figure 6. Devices 1b and 2b, using doped
PFVtPh, had more effective energy transfer (Fo¨rster-type energy transfer from host
to dopant) [16] because of good overlap between the absorption of PFVtPh and
the emission of DNBN and DNN in Figure 1. As well, the direct charge trapping
in the dopant [17] could effectively occurred in devices 1b and 2b, because both the
HOMO energy difference (0.03 eV) between levels of PFVtPh (ꢀ5.43 eV) and NPB
(ꢀ5.40 eV), and the LUMO energy difference (0.39 eV) between PFVtPh (ꢀ2.61 eV)
and Bphen (ꢀ3.00 eV) is small. Therefore, devices 1b and 2b had lower turn-on
voltage and higher EL efficiencies than that of the devices 1c and 2c, respectively.
The EL efficiencies of the doped devices were opposite to those of the non-doped
devices. Thus, the doped-devices employing DNBN as host have higher efficiencies
Figure 5. EL spectra and the CIE coordinates at 8.0 V of the doped devices.

62=[362]
J. K. Park et al.
Figure 6. (a) L-V-J characteristics, (b) the luminous efficiencies, and (c) external quantum
efficiencies as a function of current density for the doped devices.
Scheme 1. Synthetic route for new host materials and the structures of the dopant materials.
(a) toluene=ethanol, Na2CO3 (2M), Pd(PPh3)4, reflux, 4h. (b) Br2=CH2Cl2, rt, 2 h.

Highly Efficient Blue Light–Emitting Diodes
63=[363]
than those of DNN. This observation implies that the energy transfer of dopant with
DNBN is more effective than that with DNN. Among those doped devices, device 1b
had an excellent EL performances; its a maximum luminance (L), luminous efficiency
(LE), and external quantum efficiency (EQE) reach 6128 cd=m2, 8.43 cd=A, and 5.98%,
respectively. It is conjectured that DNBN would be a suitable host for PFVtPh blue
dopant material. In case of device 2c, employing DNN as a host for PCVtPh dopant,
the EL performances were remarkably lower than the others. As shown in Table 3,
the efficiencies of device 2c are similar to that of the non-doped device as a PCVtPh
emitter. Presumably, this is an indication that the energy transfer from DNN (host)
to PCVtPh (dopant) is difficult.
Conclusion
Two fluorescent blue-emitting materials based on naphthalenes, DNBN and DNN,
have been synthesized via the Suzuki Cross-Coupling reaction. In non-doped device
using DNBN and DNN in the emitting layer, both devices (1a and 2a) showed
efficient blue electroluminescence (1.30 cd=A, 1.04 lm=W and 2.14% for device 1a;
1.40 cd=A, 1.12 lm=W and 2.15% for device 2a) which were higher than that of
MADN. Furthermore, their CIE coordinates (0.153, 0.064) and (0.152, 0.069) were
very close to the NTSC blue standard of (0.14, 0.08). Using DNBN as host material
for PFVtPh blue dopant material, device 1b shows excellent efficiencies of 8.43 cd=A,
6.89 lm=W, and 5.98% (5.24 cd=A, 2.75 lm=W, and 3.82% at 20 mA=cm2). This study
demonstrates that fluorescent blue materials based on naphthalenes have sufficient
potential for display or lighting applications.
Acknowledgements
This research was supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (20090073679).
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