Molecular orientation of a new anthracene derivate for highly-efficient blue fluorescence OLEDs

Article abstract of DOI:10.1016/j.orgel.2015.05.050

A new anthracene derivative of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenylanthracene (BDNPA) was designed and synthesized as a non-doped blue emitter in organic light emitting diodes (OLEDs). BDNPA has highly rigid structure and thermal

Full text of DOI:10.1016/j.orgel.2015.05.050

Organic Electronics 24 (2015) 234–240
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Molecular orientation of a new anthracene derivative for highly-efficient
blue fluorescence OLEDs
Sunyoung Sohn a, ,1, Myeong-Jong Kim b,1, Sungjune Jung c, Tae Joo Shin d, Han-Koo Lee d, Yun-Hi Kim b,
⇑
⇑
a Future IT Innovation Laboratory, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
b Department of Chemistry and RIGET, Gyeonsang National University, Jinju 660-701, Republic of Korea
c Dept. of Creative IT Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
d Pohang Accelerator Laboratory, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A new anthracene derivative of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenylanthra
cene (BDNPA) was designed and synthesized as a non-doped blue emitter in organic light emitting diodes
(OLEDs). BDNPA has highly rigid structure and thermal stability with decomposition temperature (corre-
sponding to 5% weight loss) of 490 °C, because p-naphthyl xylene groups in 9,10-positions were highly
twisted to anthracene core due to steric hindrance of xylene groups, and because the 2,3-diphenyl groups
were also twisted about 45–49° to anthracene. OLEDs with BDNPA non-doped emitter showed high effi-
Received 23 February 2015
Received in revised form 26 May 2015
Accepted 31 May 2015
Available online 3 June 2015
Keywords:
ciency of 5.21 cd/A due to the carrier mobility with well-aligned
out-of-plane by the face-on orientation by grazing incidence X-ray diffraction.
Ó 2015 Elsevier B.V. All rights reserved.
p-stacking structure toward
Anthracene derivative
Molecular orientation
GIXD analysis
Non-doped emitter
Rigid structure
1. Introduction
organic materials are required because high efficient charge injec-
tion and transport into OLEDs are the limiting factors in determin-
ing operating voltage and device efficiency. However, the device
characteristics with synthesized emitters for small molecule and
polymers are mostly analyzed by highest occupied molecular orbi-
tal (HOMO) and lowest unoccupied molecular orbital (LUMO)
Organic light emitting diodes (OLEDs) have potential applica-
tions in next-generation electronics due to advantages such as
low driving voltage, wide view-angle, self-emitting property, and
applicability to flexible large-area display and lighting [1–3]. Blue
OLEDs have low efficiency, low color purity, and short lifetime
compared to green and red OLEDs, and are therefore not well sui-
ted to commercial applications. One way to increase the efficiency
of blue OLEDs is to increase the efficiency of exciton generation,
which is controlled by charge injection and transport or hole block-
ing [4,5]. As common requirement, development of blue-emitting
organic materials with wide intrinsic bandgap will be a great chal-
lenge to develop an efficient and thermal stable properties for pure
blue-color materials [6].
energy level alignment, carrier mobility (
ogy [11–13].
l), and surface morphol-
Molecular orientation in organic photovoltaics (OPVs) and
organic field-effect transistors (OFETs) can be determined using
grazing incidence X-ray diffraction (GIXD) analysis. For example,
side chain composition (linear alkyl or branched alkyl chains)
on backbone orientation in OPVs, or dependence of
polymer regioregularity, -stacking of quasiplanar molecules in
hole-transporting materials, and (along the inter-chain,
l on the
p
l
p–p
Therefore, many researchers are evaluating the blue fluores-
cence of novel emitters based on small molecules (e.g., anthracene,
fluorene, truxene derivatives, hybrid anthracene and fluorene) that
are suitable to give high efficiency and stable properties, or are
developing novel dual-core and triple-core organic emitters
[6–10]. To improve device efficiency, syntheses with functional
side-chain, intra-chain direction) in OFETs were investigated [14–
17]. In contrast, research on molecular orientation in OLEDs have
mainly used variable angle spectroscopic ellipsometry to consider
the outcoupling enhancement [8,18,19].
Because the molecular orientation of the emitting material in
OLEDs is still not unclear, in this study we have focused on deter-
mining the molecular orientation change according to substituent
position of anthracene core in a new anthracene derivative that is
intended for use as a non-doped emitter. To accomplish this quan-
tization we used two-dimensional GIXD (2D-GIXD) analysis. To the
⇑
Corresponding authors.
E-mail addresses: sysohn@postech.ac.kr (S. Sohn), ykim@gsnu.ac.kr (Y.-H. Kim).
1
These two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.orgel.2015.05.050
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

S. Sohn et al. / Organic Electronics 24 (2015) 234–240
235
best of our knowledge, this is the first report of molecular orienta-
tion using 2D-GIXD analysis in the synthesized small-molecule
emitters, and it can give new insights for the design and synthesis
of high-performance fluorescence emitters for use in OLEDs.
magnesium sulfate. The solvent was removed under reduced pressure.
The resulting compound was a yellow power. Yield (5.5 g, 42%).
2.2.5. Synthesis of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-
2,3-diphenylanthracene (BDNPA)
BDNPA was synthesized as follows (Scheme 1). 9,10-bis(2,5-di
methyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenyl-9,10-dihydroan
thracene-9,10-diol (3.0 g, 3.63 mmol), NaH2PO4ꢁH2O (5.0 g,
36.3 mmol) and KI (1.81 g, 10.89 mmol) were dissolved in 50 ml
of acetic acid and then the reaction mixture was refluxed for 4 h.
After the reaction finished, the mixture was worked up using
water. The crude solid product was filtered several times by water.
Yield: (2.02 g, 70%). 1H NMR (500 MHz, CDCl3, ppm) d 8.0–7.95 (m,
8H), 7.83 (s, 2H), 7.76–7.73 (m, 2H), 7.70–7.69 (m, 2H), 7.58–7.54
(m, 4H), 7.45–7.44 (m, 4H), 7.34–7.33 (m, 2H), 7.25–7.24 (m, 6H),
7.19–7.17 (m, 4H), 2.41–2.39 (d, 6H), 2.09–2.08 (d, 6H) 13C NMR
(500 MHz, CDCl3, ppm) 141.68, 141.25, 139.61, 138.70, 137.27,
136.38, 135.17, 133.43, 133.35, 133.29, 132.84, 132.33, 131.74,
130.37, 130.18, 129.21, 128.17, 128.07, 127.99, 127.76, 127.73,
127.51, 127.05, 126.45, 126.15, 125.82, 125.31. 20.22, 19.55
EI-MS: m/z 790.
2. Experiment
2.1. Materials
All starting materials were purchased from Aldrich, and
Pd(pph3)4 catalyst was purchased from Umicore. 9,10-bis(2,5-dime
thyl-4-(naphthalen-2-yl)phenyl)anthracene (BDNA), 9,10-bis(2,5-
dimethylbiphenyl-4-yl)anthracene (BDPA) and 2,3-dibromoan-
thraquinone were synthesized as described in the literature.
2.2. Synthesis
2.2.1. Synthesis of 2,3-dibromoanthraquinone
2,3-Dibromoanthraquinone was synthesized according to liter-
ature procedure (Dyes Pigm 2012; 95:384–391). Mp 278–279 °C.
IR (KBr): 1681 cmꢀ1 (C@O), 3075 cmꢀ1 (sp2 C–H), 1656 cmꢀ1
(C@C). 1H NMR (300 MHz CDCl3 ppm): d 8.49 (2), 8.38–8.40 (2),
7.86–7.89 (2).
2.3. Analysis
1H NMR spectra were recorded using a Bruker Avance-300 MHz
FT-NMR spectrometer, and chemical shifts were reported in parts
per million with tetramethylsilane as internal standard.
Thermogravimetric analysis (TGA) was performed under nitrogen
2.2.2. Synthesis of 2,3-diphenylanthracene-9,10-dione
2,3-Dibromoanthraquinone (5 g, 13.66 mmol), phenyl boronic
acid (7.5 g, 61.51 mmol), and 2 M K2CO3 (20 mL) were dissolved in
100 mL of toluene. Pd(pph3)4 (0.5 g, 0.43 mmol) was added to the
mixture, then it was stirred at 90 °C for 48 h. After the reaction fin-
ished, the mixture was extracted with dichloromethane. The organic
solution was dried with MgSO4. After the solvent was evaporated,
the crude product was purified by column chromatography using
n-hexane/dichloromethane (1:1, v/v) as eluent. Yield (3.0 g, 61%).
1H NMR (300 MHz, DMSO-d6, ppm) d 8.25–8.20 (m, 2H), 8.13 (s,
2H), 7.97–7.94 (m, 2H), 7.34–7.30 (m, 6H), 7.23–7.20 (m, 4H). 13C
NMR (300 MHz, DMSO-d6, ppm) 183.04, 146.48, 139.64, 134.15,
133.74, 132.27, 129.66, 128.24, 127.72, 127.29. EI-MS: m/z 360.
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 at a rate of 10 °C/min. UV–visible (UV–Vis) spectra were
measured using a Shimadsu UV-1065PC UV–visible spectropho-
tometer. Photoluminescence (PL) spectra were measured using a
Perkin Elmer LS50B fluorescence spectrophotometer, respectively.
The electrochemical properties of the materials were measured
by cyclic voltammetry (CV) using an Epsilon C3 in a 0.1 M solution
of tetrabutyl ammonium perchlorate in acetonitrile. The topogra-
phy and phase images of BDNPA and BDNA films were analyzed
2.2.3. Synthesis of 2-(4-bromo-2,5-dimethylphenyl)naphthalene
Naphthalen-2-ylboronic acid (10 g, 58.14 mmol), and 2 M
K2CO3 (87 mL) were dissolved in 200 mL of Toluene. Pd(pph3)4
(0.9 g, 0.77 mmol) was added to the mixture, then it was stirred
at 90 °C for 48 h. After the reaction finished, the mixture was
extracted with dichloromethane. The organic solution was dried
with MgSO4. The solvent was evaporated, then the crude
product was purified by column chromatography using
n-hexane/dichloromethane (1:1, v/v) as eluent. Yield (10.5 g,
58%). 1H NMR (300 MHz, DMSO-d6, ppm) 7.97–7.95 (m, 3H), 7.85
(s, 1H), 7.58 (m, 4H), 7.29 (s, 1H), 2.35 (s, 3H). 2.22 (s, 3H)
with scan size of 1 lm using an atomic force microscope (AFM,
VEECO Dimension 3100 + Nanoscope V) system.
Near edge X-ray absorption fine structure (NEXAFS) measure-
ments were performed at room temperature at the 4D beamline
of the Pohang Light Source (PLS-II) of Pohang Accelerator
Laboratory (PAL). We used the partial-electron-yield detection
mode for NEXAFS spectra by recording the sample current normal-
ized to a signal current measured simultaneously using a gold
mesh in ultrahigh vacuum, and used a polarized (p-polarized) syn-
chrotron photon beam (ꢂ85%) with an energy in the range of 279–
325 eV with a spectral energy resolution of
DE = 150 meV and
probing depth of ꢂ50 Å for surface-sensitive measurements.
2D-GIXD measurement was conducted at the PLS-II 9A U-SAXS
beamline of PAL. The X-rays coming from the in-vacuum undulator
were monochromated (Ek = 11.06 keV, wavelength k = 1.121 Å) using
a double crystal monochromator and focused both horizontally and
2.2.4. Synthesis of 9,10-bis(2,5-dimethyl-4-(naphthalen-2-yl)phenyl)-
2,3-diphenyl-9,10-dihydroanthracene-9,10-diol
2-(4-Bromo-2,5-dimethylphenyl)naphthalene (9.99 g, 32.13 mmol)
and anhydrous tetrahydrofuran (250 mL) were added to a 3-neck
flask. N-butyllithium was added dropwise over 10 min at ꢀ78 °C,
and the reaction solution was stirred at that temperature for
30 min. Separately, 2,3-diphenylanthracene-9,10-dione (5.79 g,
16.06 mmol) was diluted in anhydrous tetrahydrofuran (250 mL);
this solution was then slowly added to the reaction mixture. The
temperature was increased to 0 °C, and the reaction was stirred
at that temperature for 6 h. After the reaction was finished, the
mixture was extracted with diethyl ether and washed with ammo-
nium chloride. The organic layer was dried over anhydrous
vertically (FWHM 300
lm (H) ꢃ 30 (V)
lm at sample position)
using K-B type mirrors. Vacuum GIXD system was equipped
with a 7-axis motorized sample stage for the fine alignment of
thin film. The incidence angle of X-ray beam was set to 0.125°,
which is close to the critical angle of BDNPA and BDNA thin films.
GIXD patterns were recorded with a 2D CCD detector (Rayonix
SX165, USA) and X-ray irradiation time was 30 s. Diffraction angles
were calibrated by a pre-calibrated sucrose (Monoclinic, P21,
a and the sample-to-detector distance was about 226.5 mm [20].

236
S. Sohn et al. / Organic Electronics 24 (2015) 234–240
OH
B
OH
O
O
O
O
Pd(pph3)4
2M K2CO3
Toluene
Br
Br
+
OH
B
Br
Pd(pph3)4
Br
Br
OH
2M K2CO3
Toluene
OH
O
O
Br
NaH2PO4/KI
CH3COOH
n-BuLi
THF
HO
+
Scheme 1. Synthesis of anthracene derivative BDNPA.
LUMO : -1.702
HOMO : -5.049
(a)
Fig. 1. Density functional theory (DFT) calculation. Calculated HOMO and LUMO
electron density map of BDNPA.
2.4. OLED device fabrication
We fabricated OLED devices with a structure of ITO/TAPC/
BDNPA/Bphen/LiF/Al or ITO/TAPC/BDNA/Bphen/LiF/Al. All processes
were performed in a class-1000 cleanroom. To form anodes,
ITO-coated glasses with a resistance of 15
X/sq were sequentially
cleaned with deionized (DI) water, acetone, and isopropyl alcohol
for 15 min in an ultrasonic bath, then treated with O2 plasma for
60 s before use. Blue-emitting OLEDs with different non-doped
emitters of BDNPA and BDNA (40 nm thick) were fabricated on
ITO-coated glasses. To fabricate the OLEDs, di-[4-(N,N-ditoyl-amin
o)-phenyl]cyclohexane (TAPC, 50 nm) was used as a as a hole
transport layer (HTL) and 4,7-diphenyl-1,10-phenanthroline
(Bphen, 10 nm) was used as a as an electron transport layer (ETL).
(b)
Fig. 2. Normalized UV-absorption and photoluminescence (PL) spectra of (a)
BDNPA and (b) BDNA fabricated on quartz plates.

S. Sohn et al. / Organic Electronics 24 (2015) 234–240
237
TAPC or Bphen were deposited onto the ITO substrate using a ther-
mal evaporation system. Then an LiF interlayer (1 nm) and an Al
cathode (120 nm) layer were deposited. Current density–voltage
(J–V), luminance–voltage (L–V) characteristics, current efficiency
(cd/A), and power efficiency (lm/W) were analyzed using keithley
236 and CS-1000 (Konica Minolta Co.) system.
cene-9,10-diol, which was obtained by nucleophilic addition of
lithiated (2,5-dimethyl-4-naphthyl)phenyl to 2,3-diphenylanthra-
quinone. The structure of the compound was confirmed by
1H-NMR, 13C-NMR, and mass spectroscopies. To fully optimize the
molecular structure, density function theory (DFT) calculation
using B3LYP/6-31G⁄ methods in the Gaussian 03 program were
conducted to characterize the 3-dimensional structures and elec-
tron densities of the HOMO and LUMO states (Fig. 1) The calculated
geometries of p-naphthyl xylene groups in the 9,10-positions are
highly twisted toward the anthracene core due to steric hindrance
of xylene groups, whereas the 2,3-diphenyl groups are twisted
about 45–49° to anthracene. As a consequence, the electron densi-
ties of HOMO and LUMO were distributed in anthracene units and
slightly distributed in 2,3-substituted phenyl groups.
In order to compare their thermal stability, thermal stability of
BDNPA was investigated by TGA and DSC under a nitrogen atmo-
sphere (see Supporting information Fig. S1). Decomposition tem-
perature (Td: corresponding to 5% weight loss) of BDNPA was
490 °C, which was higher than that of the BDNA with 390 °C as
shown in Ref. [6]. And also, BDNPA did not show any transition even
when heated to 250 °C, which means that the highly-twisted and
rigid structure of BDNPA has high thermal stability.
3. Results and discussion
3.1. Materials
The new limb-structured anthracene derivative, 9,10-bis(2,5-di
methyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenylanthracene
(BDNPA) was synthesized successfully (scheme 1). For comparison,
the photophysical information of 9,10-bis(2,5-dimethyl-4-(naph
thalen-2-yl)phenyl)anthracene (BDNA) are summarized in previ-
ous literature [4].
3.2. General physical properties
2,3-Diphenylanthraquinone was obtained by a Suzuki coupling
reaction between 2,3-dibromoanthraquinon and phenyl boronic
acid. BDNPA was synthesized by reduction of 9,10-bis(2,5-dime
thyl-4-(naphthalen-2-yl)phenyl)-2,3-diphenyl-9,10-dihydroanthra
The photophysical properties of (a) BDNPA and (b) BDNA in a
dilute solution and in the film state were investigated using
Fig. 3. Topography (left) and phase (right) images of (a) BDNPA and (b) BDNA thin films at 1 lm scan size.

238
S. Sohn et al. / Organic Electronics 24 (2015) 234–240
UV–Vis and PL spectroscopy (Fig. 2). BDNPA solution showed UV–
Vis absorption maxima at 370, 388, and 410 nm, whereas the film
showed maxima at 373, 391, and 412 nm; these which are charac-
teristic anthracene vibrational patterns, as shown in Fig. 2(a).
BDNPA solution showed PL maxima at 424 and 446 nm, whereas
the film showed maxima at 435 and 451 nm. BDNPA with xylene
unit at the 9,10-position also showed weak mirror image similarity
between absorption and emission spectra, which is related to
inhibited vibronic coupling. For comparison, the UV–Vis absorp-
tion and PL spectra of BDNA solution and film were shown in
Fig. 2(b). BDNA solution showed PL maxima at 408, 431, 453, and
485 nm, whereas the film showed maxima at 431, 453, and
485 nm (see the physical properties of BDNA in Ref. [6]). The elec-
trochemical properties of BDNPA were investigated using cyclic
voltammetry (CV). From the oxidation potential and the band
gap of UV onset (430 nm), the HOMO was calculated to be
ꢀ5.92 eV, and the LUMO was calculated to be ꢀ3.03 eV.
morphology with root-mean-square (rms) roughness of 0.36 nm
and 0.4 nm for the scan size of 1 m. These structural
properties are caused by the non-planar molecular structure with
lm ꢃ 1
l
steric hindrance which enables pinhole-free surfaces [4].
The features in the NEXAFS spectra (see Supporting information
Fig. S2) acquired from the BDNPA and BDNA films are assigned as
the p⁄ (C@C) orbital at 285.4 eV, the r⁄ (C–H) mixed with Rydberg
orbitals around 287–291 eV, and the several r⁄ orbitals in the
energy range 292–307 eV. We attempted to determine the tilt
angle
a between the C@C double bond in the conjugated planes
and the substrate surface with the photon beam size of approxi-
mately 0.1 ꢃ 0.3 mm. We believe that the BDNPA and BDNA mate-
rials as non-doped emitters form amorphous organic films because
the peak intensity of the p⁄ resonance of the films does not change
with an increase in h, and because the change in crystallinity of
BDNA after C@C/C–H change is insignificant. These amorphous
molecular structures of BDNPA and BDNA resulted in high unifor-
mity of nanometer-scale surface smoothness as shown in previous
AFM results; such smoothness of these surfaces is a basic require-
ment for their use as hosts in OLEDs [6,19].
For structural analysis, the topography and phase images of
BDNPA and BDNA films (40 nm thick) were obtained using AFM
(Fig. 3). Although the BDNPA film showed partial nanocrystals of
ꢂ30 nm width and 3 nm height and BDNA film showed some
pores, both BDNPA and BDNA films had uniform surface
To clarify the molecular orientations as a function of substituent
positions, we analyzed the 2D-GIXD patterns of BDNPA and BDNA
2.0
1.5
1.0
0.5
0.0
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
qxy (Å-1)
(a)
2.0
1.5
1.0
0.5
0.0
-1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
qxy (Å-1)
(b)
Fig. 4. Two-dimensinal grazing incidence X-ray diffraction (2D-GIXD) images of scattered X-ray intensity from vacuum deposited, 40 nm thick (a) BDNPA and (b) BDNA on
SiO2/Si substrates. The vertical and horizontal axes correspond to scattering in-plane and out-of-plane to the plane of the films. In the films, BDNPA assumed the face-on
orientation; BDNA assumed the edge-on orientation.

S. Sohn et al. / Organic Electronics 24 (2015) 234–240
239
films (Fig. 4). BDNPA shows the preferred face-on orientation of
nano-crystals with apparent crystallite size of ꢂ1.5 nm, calculated
from FWHM of diffraction peak in the qz profile (Fig. 4a). In this fig-
ure, face-on means conjugated backbone planes are paralleled to
transporting property of the anthracene derivative. Charge trans-
port in the BDNPA is improved due to the shortened stacking
distance and face-on orientation compared to the BDNA of the
edge-on orientation (Fig. 4). Electron mobilities ( ) are calculated
p–p
l
the substrate, thus
p
-staking is also paralleled to the substrate.
using the space-charge-limited-current model and the BDNPA
has higher electron mobility of ꢂ5.04 ꢃ 10ꢀ5 cm2 Vꢀ1 sꢀ1 than that
of BDNA of ꢂ8.65 ꢃ 10ꢀ5 cm2 Vꢀ1 sꢀ1, which means higher mobil-
ity in face-on molecules than in the edge-on molecule, as shown in
Ref. [15]. Particularly, the BDNPA–OLEDs show excellent current
efficiency of 5.21 cd/A at CIE coordinate (0.16, 0.19) compared to
those of BDNA–OLEDs (1.05 cd/A). We believe that this difference
occurs because the electron mobility as well as the outcoupling
efficiency is affected due to molecular orientation. In the reference
[9,10,19], the emitted light from vertically and horizontally ori-
ented molecules are investigated, especially, the horizontally ori-
ented molecule (face-on) is possible to enhance the light
outcoupling efficiency of small-molecule OLEDs using emitting
materials. The external quantum efficiency (EQE) of uex due to
the molecular orientation of fluorescent emitter is improved as
follows:
In contrast, the BDNA film shows mainly edge-on orientation
(Fig. 4b) with moderate distribution abroad 15° calculated from
diffraction peak. These results suggest that these new anthracene
derivatives can form different types of molecular orientation by
introduction of phenyl substituents in the 2,3-positions of the
anthracene core; this introduction could be a useful method to
change molecular orientation.
3.3. Device characteristics
The J–V and L–V, cd/A, and lm/W curves of the OLEDs with
BDNPA (BDNPA–OLEDs) and BDNA (BDNA–OLEDs) as
a
non-doped emitter were measured (Fig. 5). The maximum lumi-
nance of BDNPA–OLEDs was three times higher than that of the
BDNA–OLEDs at the same voltage. Because the turn-on voltages
of BDNPA–OLEDs and BDNA–OLEDs affected by hole injection effi-
ciency are almost the same, we believe that high current flow that
is a consequence of electron injection efficiency leads to high lumi-
nance in BDNPA–OLEDs because the phenyl substituents in the
2,3-positions of anthracene core improved the ambipolar
uex
¼
c
ꢁ
ur
ꢁ
uPL
ꢁ
uOC
;
ð1Þ
where
c
is the carrier balance factor,
u
r is the singlet exciton forma-
tion ratio, uPL is the PL quantum efficiency of the emitter, uOC is
outcoupling efficiency [18].
4. Conclusion
500
Brightness of BDNPA
1400
Brightness of BDNA
We have studied the molecular orientation of new anthracene
derivatives and its effect on OLED current density, brightness,
and current efficiency. BDNPA with substituent 2,3-diphenyl
groups to anthracene core assumed a highly-twisted, rigid struc-
ture, and high thermal stability with no transition up to 250 °C.
According to the structural investigation combined with device
analysis, we concluded that the BDNPA with face-on orientation
Current density of BDNPA
Current density of BDNA
1200
1000
800
600
400
200
0
400
300
200
100
0
as a non-doped emitter could lead to small p–p stacking distance
and efficient molecular charge transfer. These advantages occur
because the face-on orientation structure of non-doped emitter
sandwiched between the electrodes allows efficient out-of-plane
transport both along the backbone and the direction of p–p stack-
ing as compared to the edge-on orientation from 2D-GIXD mea-
surement, thereby improving current efficiency by more than a
factor of five. Therefore, molecular orientation affected by either
substituent position of anthracene core or twisted structure could
be a key to determine the efficiency of blue OLEDs.
2
3
4
5
6
7
8
9
Voltage (V)
(a)
6
6
5
4
3
2
1
0
Acknowledgments
cd/A of BDNPA
cd/A of BDNA
lm/W of BDNPA
lm/W of BDNA
5
4
3
2
1
0
This research was supported by the MSIP (Ministry of Science,
ICT and Future Planning), South Korea, under the ‘‘IT Consilience
Creative Program’’ (NIPA-2014-H0201-14-1001) supervised by
the NIPA (National IT Industry Promotion Agency),
a Grant
(CASE-2014M3A6A5060952) from the Center for Advanced Soft
Electronics under the Global Frontier Research Program of the
MSIP, and Nano-Material technology Development Program
through the National Research Foundation of South Korea
(NRF) funded by the South Korean Government (MISP)
(2012M3A7B4049647). Experiments at PLS-II 4D and 9A beamli-
nes were supported in part by MEST and POSTECH.
2
3
4
5
6
7
8
9
Voltage (V)
Appendix A. Supplementary data
(b)
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.orgel.2015.05.
050.
Fig. 5. (a) J–V and L–V characteristics of OLEDs with BDNPA and BDNA as a non-
doped emitter. (b) Current efficiency (cd/A) and power efficiency (lm/W) of the
same devices.

240
S. Sohn et al. / Organic Electronics 24 (2015) 234–240
[10] B. Kim, Y. Park, J. Lee, D. Yokoyama, H.H. Lee, J. Kido, J. Park, J. Mater. Chem. C 1
References
(2013) 432.
[11] I.G. Hill, A. Kahn, J. Appl. Phys. 84 (1998) 5583.
[12] L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 23 (2011)
926.
[13] T. Wakimoto et al., Handbook of Luminescence, Display Materials, and Devices,
American Scientific Publishers, California, 2003.
[14] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.M.
Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwig, D.M.D.
Leeuw, Nature 401 (1999) 685.
[15] I. Osaka, M. Saito, T. Koganezawa, K. Takimiya, Adv. Mater. 26 (2014) 331.
[16] A. Wakamiya, H. Nishimura, T. Fukushima, F. Suzuki, A. Saeki, S. Seki, I. Osaka, T.
Sasamori, M. Murata, T. Murata, H. Kaji, Angew. Chem. Int. Ed. 53 (2014) 5800.
[17] I. Botiz, N. Stingelin, Materials 7 (2014) 2273.
[18] H. Kuma, C. Hosokawa, Sci. Technol. Adv. Mater. 15 (2014) 034201.
[19] D. Yokoyama, J. Mater. Chem. 21 (2011) 19187.
[1] M. Janka, E. Saukko, P. Raumonen, D. Lupo, Org. Electron. 15 (2014) 3431.
[2] Y. Ji, F. Ran, H. Xu, W. Shen, J. Zhang, Org. Electron. 15 (2014) 3137.
[3] M.A. McCarthy, B. Liu, E.P. Donoghue, I. Kravchenko, D.Y. Kim, F. So, A.G.
Rinzler, Science 332 (2011) 570.
[4] Y. Kim, W.B. Im, Phys. Stats. Sol. 201 (2004) 2148.
[5] C.-C. Chi, C.-L. Chiang, S.-W. Liu, H. Yueh, C.-T. Chen, C.-T. Chen, J. Mater. Chem.
19 (2009) 5561.
[6] H. Park, J. Lee, I. Kang, H.Y. Chu, J.-I. Lee, S.-K. Kwon, Y.-H. Kim, J. Mater. Chem.
22 (2011) 2695.
[7] C.-F. Wang, W.-Y. Hung, M.-H. Cheng, J.-S. Hwang, M.-K. Leung, K.-T. Wong,
Org. Electron. 14 (2013) 1958.
[8] J. Lee, B. Kim, J.E. Kwon, J. Kim, D. Yokoyama, K. Suzuki, H. Nishimura, A.
Wakamiya, S.Y. Park, J. Park Chem. Commun. 50 (2014) 14145.
[9] D. Yokoyama, Y. Park, B. Kim, S. Kim, Y.-J. Pu, J. Kido, J. Park, Appl. Phys. Lett. 99
(2011) 123303.
[20] R.C. Hynes, Y.L. Page, J. Appl. Cryst. 24 (1991) 352.