Improved power efficiency in blue phosphorescent organic light-emitting diodes using diphenylmethyl linkage based high triplet energy hole transport materials

Article abstract of DOI:10.1016/j.orgel.2012.11.004

Improved power efficiency in blue phosphorescent organic light-emitting diodes (PHOLEDs) was demonstrated by using new high triplet energy hole-transport materials based on the diphenylmethyl linkage. Two high triplet energy hole-transport materials with diphenylamine or ditolyamine moieties linked through a diphenylmethyl linkage, 4,4′-(diphenylmethylene)bis(N,N- diphenylaniline) (TCBPA) and 4,4′-(diphenylmethylene)bis(N,N-di-p- tolylaniline), were synthesized and evaluated as hole-transport materials for blue PHOLEDs. The power efficiency of TCBPA was superior to that of standard 1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl]cyclohexane.

Full text of DOI:10.1016/j.orgel.2012.11.004

Organic Electronics 14 (2013) 370–377
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Improved power efficiency in blue phosphorescent organic
light-emitting diodes using diphenylmethyl linkage based high triplet
energy hole transport materials
⇑
Chil Won Lee, Jun Yeob Lee
Department of Polymer Science and Engineering, Dankook University, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do 448-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 August 2012
Received in revised form 3 November 2012
Accepted 4 November 2012
Available online 22 November 2012
Improved power efficiency in blue phosphorescent organic light-emitting diodes
(PHOLEDs) was demonstrated by using new high triplet energy hole-transport materials
based on the diphenylmethyl linkage. Two high triplet energy hole-transport materials
with diphenylamine or ditolyamine moieties linked through a diphenylmethyl linkage, 4,
40-(diphenylmethylene)bis(N,N-diphenylaniline) (TCBPA) and 4,40-(diphenylmethylene)
bis(N,N-di-p-tolylaniline), were synthesized and evaluated as hole-transport materials
for blue PHOLEDs. The power efficiency of TCBPA was superior to that of standard 1,1-
bis[4-[N,N0-di(p-tolyl)amino]phenyl]cyclohexane.
Keywords:
High triplet energy hole transport material
Diphenylmethyl linkage
Power efficiency
Ó 2012 Elsevier B.V. All rights reserved.
Quantum efficiency
Blue phosphorescent device
1. Introduction
in decreasing the driving voltage of blue PHOLEDs because
of the high electron mobility of pyridine-based electron
High power efficiency is an important aspect of device
performance for organic light-emitting diodes (OLEDs) be-
cause the power consumption of organic light-emitting
diode panels can be reduced by improving their power effi-
ciency. In general, power efficiency can be enhanced by
lowering the driving voltage and increasing the quantum
efficiency of organic light-emitting diodes. Therefore, high
efficiency phosphorescent organic light-emitting diodes
(PHOLEDs) with low driving voltage have been developed
[1–5]. In particular, many studies have attempted to en-
hance the power efficiency of blue PHOLEDs due to the
low power efficiency of current blue fluorescent OLEDs.
Much effort to improve the power efficiency of blue
PHOLEDs was devoted to reducing the driving voltage of
blue PHOLEDs by using host, hole and electron transport
layers with high triplet energy and good charge transport
properties [6–21]. Several electron transport materials
based on the pyridine moiety were reported to be effective
transport materials [6–8]. High power efficiency of 56 lm/
W at 100 cd/m2 was demonstrated using the high mobility
electron transport materials. In addition to the electron
transport materials, mixed host or bipolar host materials
could also improve the driving voltage of blue PHOLEDs
[9–14]. Several mixed host structures made up of strong
hole and electron transport materials were reported and
could lower the driving voltage due to low energy barrier
for hole and electron injection and good charge transport
properties [9–11]. Similarly, bipolar host materials with
strong hole and electron transport groups in the molecular
backbone structure were also effective to achieve the low
driving voltage [12–14]. In the case of hole-transport
materials, 1,1-bis[4-[N,N0-di(p-tolyl)amino]phenyl]cyclo-
hexane (TAPC) is known as
a high triplet energy
hole-transport material capable of decreasing the driving
voltage of blue PHOLEDs because of its high hole mobility
[17]. However, few materials have been developed to
improve the driving voltage of blue PHOLEDs [18–21].
Therefore, it is strongly required to develop high triplet
energy hole-transport materials with good hole-transport
⇑
Corresponding author. Tel./fax: +82 31 8005 3585.
E-mail address: leej17@dankook.ac.kr (J.Y. Lee).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.11.004

C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
371
properties to enhance the power efficiency of blue
PHOLEDs.
bath. Trifluoromethanesulfonic anhydride (24.0 g, 85.1
mmol) in 50 mL of dichloromethane was added dropwisely
to the solution under nitrogen. The reaction was allowed to
proceed for 2 h and quenched by adding methanol and
water followed by dilution with dichloromethane. The
crude solution was washed with dichloromethane and
water. The organic layer was dried over anhydrous magne-
sium sulfate and evaporated in vacuo to give the crude
product, which was purified by column chromatography
using dichloromethane/hexanes eluent. The final white
powdery product was obtained in 16.0 g.
In this work, two high triplet energy hole-transport
materials with good hole-transport properties, 4,40-(diphe-
nylmethylene)bis(N,N-di-p-tolylaniline) (TCBTA) and 4,
40-(diphenylmethylene)bis(N,N-diphenylaniline) (TCBPA),
were synthesized and compared with TAPC as hole-trans-
port materials for blue PHOLEDs. The power efficiency of
blue PHOLEDs was shown to be improved by more than
30% using TCBPA as the hole-transport material of blue
PHOLEDs instead of TAPC.
TC-OTf yield 91%. 1H NMR (500 MHz, CDCl3): d 7.13 (d,
4H, J = 4.0 Hz), 7.18 (d, 4H, J = 4.0 Hz), 7.23–7.31 (m, 10H).
MS (FAB) m/z 616 [M+].
2. Experimental section
2.1. General information
3.2. 4,40-(diphenylmethylene)bis(N,N-di-p-tolylaniline)
(TCBTA)
Tritylaniline, sodium-t-butoxide, tri-t-butyl phosphine,
4,40-dihydroxytetraphenylmethane and p,p’-ditolylamine
(TCI Chem. Ind. Co.) were used without further purifica-
tion. 4-bromotoluene, palladium acetate(II), pyridine,
trifluoromethanesulfonic anhydride, 2-dicyclohexylphos-
The mixture of TC-OTf (3.00 g, 4.87 mmol), p,p0-ditolyl-
amine (2.40 g, 12.2 mmol), sodium t-butoxide (2.34 g,
24.4 mmol), 2-dicyclohexylphosphino-20,60-dimethoxybi-
phenyl (S-Phos, 0.40 g, 0.975 mmol), and tris(dibenzyli-
deneacetone)dipalladium (Pd2(dba)3ꢂ0.23 g, 0.244 mmol)
in 100 mL of dry xylene was refluxed under N2 for 36 h.
Then reaction mixture was cooled down to room tempera-
ture, diluted with 100 mL water and extracted with ethyl
acetate. The organic extract was dried over anhydride
magnesium sulfate, filtered, and evaporated to yield a
brown solid. The crude material was purified by column
chromatography on silica gel using dichloromethane/n-
hexane as eluent. Additional purification by sublimation
(290 °C. at 10ꢁ5 mm Hg) resulted in 1.9 g of pure white
compound.
phino-20,60-dimethoxybiphenyl,
tris(dibenzylideneace-
tone)dipalladium and diphenylamine (Aldrich Chem. Co.)
were used without further purification. All reaction
solvents were distilled over drying agent. The 1H and 13C
nuclear magnetic resonance (NMR) were recorded on
Avance 500 (Bruker). Fourier transform infrared (FT-IR)
spectra were recorded on a Nicolet 380 Fourier transform
spectrometer. The mass spectra were recorded using a
JEOL, JMS-600 W spectrometer in FAB mode. Elemental
analysis of the materials was carried out using EA1110
(CE instrument). The DSC measurements were performed
on a Mettler DSC822e differential scanning calorimeter
under nitrogen at a heating rate of 10 °C/min. Photolumi-
nescence (PL) spectra were recorded on a fluorescence
spectrophotometer (HITACHI, F-7000) and the ultravio-
let–visible (UV–Vis) spectra were obtained by means of a
UV–Vis spectrophotometer (Shimadzu, UV-2501PC). Sam-
ple was dissolved in tetrahydrofuran at a concentration
of 1.0 ꢀ 10ꢁ4 M for UV–Vis and PL measurements. Triplet
energy analysis was carried out using low temperature
PL measurement in liquid nitrogen. The highest occupied
molecular orbital of compounds was measured with a cyc-
lic voltammetry. Cyclic voltammetry measurement of
organic materials was carried out in acetonitrile solution
with tetrabutylammonium perchlorate at 0.1 M concentra-
tion. Ag was used as the reference electrode and Pt was the
counter electrode.
TCBTA yield 55%. mp 306 °C. Tg 100 °C. FT-IR (KBr)
3030.1, 1739.2, 1601.1, 1499.1, 1444.9, 1369.0, 1320.4,
1272.8, 1213.2, 1110.6, 1036.1, 815.6, 759.4, 723.0,
702.1 cmꢁ1 1H NMR (500 MHz, CDCl3): d 2.29 (s, 12H),
.
6.86 (d, 4H, J = 4.3 Hz), 6.98–7.00 (m, 12H), 7.04 (d, 8H,
J = 4.0 Hz), 7.15–7.25 (m, 10H). 13C NMR (125 MHz, CDCl3):
d 20.8, 64.1, 120.9, 124.7, 125.8, 127.3, 129.8, 131.2, 131.7,
132.4, 139.9, 145.3, 145.8, 147.2. MS (FAB) m/z 710 [M+].
Anal. Calcd. for C53H46N2: C(89.54%); H(6.52%); N(3.94%).
Found: C(89.47%); H(6.52%); N(3.94%).
3.3. 4,40-(Diphenylmethylene)bis(N,N-diphenylaniline)
(TCBPA)
The mixture of TC-OTf (3.00 g, 4.87 mmol), diphenyl-
amine (2.06 g, 12.2 mmol), sodium t-butoxide (2.34 g,
24.4 mmol), 2-dicyclohexylphosphino-20,60-dimethoxybi-
phenyl (S-Phos, 0.40 g, 0.975 mmol), tris(dibenzylidene-
acetone)dipalladium (Pd2(dba)3. 0.23 g, 0.244 mmol) in
100 mL of dry xylenes was refluxed under N2 for 36 h. Then
reaction mixture was cooled down to room temperature,
diluted with 100 mL water and extracted with ethyl
acetate. The organic extracts were dried over anhydride
magnesium sulfate, filtered, and evaporated to yield a
brown solid. The crude material was purified by column
chromatography on silica gel using dichloromethane/n-
hexane for eluent. Additional purification by sublimation
3. Synthesis
Synthetic scheme of TCBTA and TCBPA is presented in
Scheme 1.
3.1. Synthesis of 4,40-(diphenylmethylene)bis(4,1-phenylene)
bis(trifluoromethanesulfonate) (TC-OTf)
4,40-Dihydroxytetraphenylmethane (10.0 g, 28.4 mmol)
and pyridine(9.0 g, 113.5 mmol) were added to 100 mL
dichloromethane and the solution was cooled in an ice salt

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C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
Scheme 1. Synthetic scheme of TCBTA and TCBPA.
(270 °C at 10ꢁ5 mm Hg) resulted 2.2 g of pure white
compound.
transport units such as diphenylamine and ditolylamine
should be introduced in the molecular structure for good
hole transport properties and an interconnecting unit
which can isolate each aromatic amine unit should be
included in the backbone structure for high triplet energy.
In our previous work, we demonstrated that the triphenyl-
methyl unit is better than the triphenylsilyl unit in terms
of hole transport and maintains the high triplet energy of
the aromatic amine unit [22]. Therefore, TCBTA and TCBPA
were designed to have a diphenylmethyl linkage and hole-
transport type diphenylamine or ditolylamine moieties.
TCBTA and TCBPA were synthesized by using 4,40-
(diphenylmethylene)bis(1,4-phenylene) bis(trifluorometh-
anesulfonate) (TC-OTf) prepared from 4,40-dihydroxytetra-
phenylmethane. TC-OTf was modified with diphenylamine
or ditolylamine using S-Phos and Pd2(dba)3 catalyst to pro-
duce TCBTA and TCBPA. Synthetic yields of TCBTA and
TCBPA were 55% and 69%, respectively.
TCBPA Yield 69%. mp 230 °C. Tg 92 °C. FT-IR (KBr)
3029.6, 1587.3, 1486.5, 1312.6, 1273.4, 1179.4, 1112.8,
1077.7, 1030.9, 889.3, 825.8, 746.2, 692.0 cmꢁ1 1H NMR
.
(500 MHz, CDCl3): d 6.93 (d, 4H, J = 4.5 Hz), 6.98 (t, 4H,
J = 5.0 Hz), 7.04 (d, 4H, J = 4.3 Hz), 7.08(d, 8H, J = 3.8 Hz),
7.16–7.26 (m, 18H). 13C NMR (125 MHz, CDCl3): d 64.0,
122.1, 122.8, 124.5, 125.9, 127.4, 129.2, 131.1, 131.9,
140.7, 145.5, 147.0, 147.7. MS (FAB) m/z 654 [M+]. Anal.
Calcd. for C49H38N2: C(89.87%); H(5.85%); N(4.28%). Found:
C(89.88%); H(5.82%); N(4.27%).
4. Device fabrication and measurement
Device structure of blue PHOLED fabricated using
TCBTA and TCBPA was indium tin oxide (ITO, 50 nm)/
N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-
biphenyl-4,40-diamine (DNTPD, 60 nm)/N,N0-di(1-naph-
thyl)-N,N0-diphenylbenzidine (NPB, 5 nm)/TCBPA or TCBTA
The photophysical properties of TCBTA and TCBPA were
analyzed using UV–Vis and PL spectrometry. Fig. 1 shows
UV–Vis, solution PL and low temperature PL spectra of
TCBTA and TCBPA. Both compounds exhibited strong
absorption of diphenylamine moiety at 305 nm, which
or
TAPC
(25 nm)/9-(3-(9H-carbazol-9-yl)phenyl)-9H-
carbazol-3-yl)diphenylphosphine oxide (mCPPO1): bis
((3,5-difluorophenyl)pyridine) iridium picolinate (FIrpic)
(25 nm, 3%)/diphenylphosphine oxide-4-(triphenylsilyl)
phenyl (TSPO1)/LiF (1 nm)/Al (200 nm). Hole only devices
with a device structure of ITO (50 nm)/DNTPD (60 nm)/
NPB (5 nm)/TCBPA or TCBTA (25 nm)/Au (100 nm) were
also fabricated to compare hole transport properties of
hole transport materials. Device performances of blue
PHOLEDs were measured using Keithley 2400 source
measurement unit and CS1000 spectroradiometer.
was assigned to p–
pꢃ transition of the diphenylamine unit.
Optical bandgaps of TCBTA and TCBPA, as calculated from
the absorption edge of the UV–Vis spectra, were 3.59 eV
and 3.64 eV, respectively. A wide bandgap was observed
in the two hole-transport materials because of limited con-
jugation by the diphenylmethyl linkage. PL emission of
TCBTA and TCBPA was observed at 380 nm and 372 nm,
respectively. The red shift of the PL emission of TCBTA
was due to the weak electron-donating character of the
methyl substituent in the tolyl unit. Low temperature PL
analysis was also carried out to measure the triplet energy
of the hole-transport materials. Triplet energies calculated
from the first phosphorescent emission peak were 2.75 eV
and 2.86 eV for TCBTA and TCBPA, respectively. Two diphe-
5. Results and discussion
In order to develop a high triplet energy hole-transport
material with good hole-transport properties, strong hole-

C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
373
Fig. 1. UV–Vis absorption, solution PL and low temperature PL spectra of TCBTA and TCBPA.
TCBTA
TCBPA
HOMO
LUMO
Fig. 2. HOMO and LUMO distribution of TCBTA and TCBPA.
nylamine or ditolylamine groups were isolated by the
diphenylmethyl connecting unit, which resulted in high
triplet energy in the hole-transport materials.
difference of the HOMO levels between TCBTA and TCBPA
because HOMO was localized on similar aromatic amine
units. The HOMO levels of the two hole-transport materials
were suitable for hole injection into the emitting layer and
the LUMO level was low enough for electron blocking from
the emitting layer. Therefore, both materials can be effec-
tive as hole-transport materials for blue PHOLEDs.
Molecular simulation of TCBTA and TCBPA was per-
formed to study the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) distributions. Fig. 2 shows the HOMO/LUMO dis-
tributions of TCBTA and TCBPA. A suite of the Gaussian
03 program and the nonlocal density functional of Becke’s
3-parameters employing Lee–Yang–Parr functional
(B3LYP) with 6–31Gꢃ basis sets was used for the calcula-
tion [23]. HOMO of the two hole-transport materials was
dispersed over the peripheral phenyl unit of TCBTA and
TCBPA, while LUMO was localized on the central phenyl
unit of TCBTA and TCBPA due to the strong electron-donat-
ing character of diphenylamine and ditolyamine.
Thermal analysis of TCBTA and TCBPA was also per-
formed due to the importance thermal stability in overall
device stability. Fig. 3 shows DSC thermograms of TCBTA
and TCBPA. The glass transition temperatures (Tgs) of
TCBTA and TCBPA were 100 °C and 92 °C, respectively.
The Tg of TCBTA was slightly higher than that of TCBPA,
due to the additional methyl group in the molecular struc-
ture. There have been several studies about the effect of
methyl substituent on the glass transition temperatures
of hole transport materials. In the case of N,N0-diphenyl-
The HOMO and LUMO levels of TCBTA and TCBPA were
measured using cyclic voltammetry (CV). The HOMO/
LUMO levels of TCBTA and TCBPA were ꢁ5.70/ꢁ2.11 eV
and ꢁ5.68/ꢁ2.04 eV, respectively. There was little
N,N0-bis
(3-methylphenyl)-1,10-diphenyl-4,40-diamine
derivatives [24], the methyl substituent increased the Tg
due to restricted molecular motion, while the methyl

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C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
Fig. 3. DSC thermograms of TCBTA and TCBPA.
Fig. 4. TGA thermograms of TCBTA and TCBPA.
substituent decreased the Tg in the case of 4,40,400-
tris(diphenylamino) triphenylamine [25] and 3,6-di(N-
diphenylamino)-9-phenylcarbazole [26] derivatives due
to favorable amorphous phase formation by the methyl
group. In the case of TCBTA and TCBPA, the methyl group
slightly increased the Tg due to bulkiness of the methyl
group. The Tg of the two hole-transport materials was
much higher than that of TAPC (79 °C) [27,28].
but the Tms disappeared during 2nd scan after cooling
from isotropic liquid to room temperature and only Tgs
were observed. This indicates that both TCBTA and TCBPA
form stable amorphous glassy state due to tetrahedral
diphenylmethyl linkage. There was little difference of Tm
between the two materials.
Thermogravimetric analysis (TGA) was also carried out
to study the thermal decomposition of the two hole-trans-
port materials. Fig. 4 presents the TGA curves of TCBTA and
TCBPA. The relative weight stability of both materials up to
The two materials showed Tms of 306 °C and 302 °C,
respectively during the 1st scan of the sublimed crystal,

C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
375
100
80
60
40
20
0
TAPC
TCBTA
TCBPA
N
N
TAPC
2.00
0
1
2
3
4
5
6
Voltage (V)
2.04
2.1
2.11
Fig. 5. Current density–voltage curves of hole only devices of TCBTA,
TCBPA and TAPC.
2.4
5.5
2.52
2.95
5.60
2.56
6.13
10000
1000
100
10
60
50
40
30
20
10
0
TAPC
TCBTA
TCBPA
5.1
5.50
5.68
1
5.70
0.1
0.01
6.79
0
2
4
6
8
10
Voltage(V)
Fig. 7. Chemical structure of TAPC and energy level diagram of blue
PHOLEDs.
Fig. 6. Current density–voltage–luminance curves of blue PHOLEDs with
TCBTA, TCBPA and TAPC hole transport materials.
25
TAPC
300 °C revealed their thermal stability. The temperatures
for 5% weight loss were 394 °C and 388 °C for TCBTA and
TCBPA, respectively.
TCBTA
20
TCBPA
15
To compare the hole-transport properties of the two
hole-transport materials, hole-only devices were fabri-
cated. Fig. 5 shows the current density–voltage curves of
the hole-only devices. The hole current density was high
in the TCBTA device at the same driving voltage. The differ-
ence between TCBTA and TCBPA was attributed to the
molecular structure of the aromatic amine units: ditolyl-
amine was used as the hole-transport unit in TCBTA,
compared to diphenylamine in TCBPA. Ditolylamine is well
known as a better hole-transport unit than diphenylamine
[29,30], which explains the high hole current density ob-
served in TCBTA. TCBTA was even better than TAPC which
is well known as a hole transport material with high hole
mobility, which may be due to small size of diphenyl-
methyl linking unit of TCBTA compared with cyclohexyl
linking unit of TAPC.
Blue PHOLEDs were fabricated using the TCBTA and
TCBPA hole-transport materials. Blue emitting FIrpic was
used as the blue phosphorescent dopant and mCPPO1 as
the host material. The mCPPO1 host was reported as a
bipolar host material for blue PHOLEDs and showed high
quantum efficiency due to balanced hole and electron
density in the emitting layer [31]. Therefore, mCPPO1 is
10
5
0
0
1
100
10000
Luminance (cd/m2)
Fig. 8. Quantum efficiency–luminance curves of blue PHOLEDs with
TCBTA, TCBPA and TAPC hole transport materials.
suitable as the host material for the FIrpic dopant. The cur-
rent density–voltage–luminance curves of blue1 PHOLEDs
with the TCBTA and TCBPA hole-transport materials are
shown in Fig. 6. TAPC was used as the standard hole-trans-
port material to compare the device performances. The
current density of blue PHOLEDs was high in the TCBTA
device, while the TCBPA device showed a lower current
1
For interpretation of color in Figs. 6, 8, and 9 the reader is referred to
the web version of this article.

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C.W. Lee, J.Y. Lee / Organic Electronics 14 (2013) 370–377
35
30
25
20
15
10
5
electrons in the emitting layer and thus improves the
TAPC
recombination efficiency in the emitting layer. In the case
of TCBTA, the quantum efficiency was rather low, in spite
of the low driving voltage. Although the driving voltage
was low due to the high hole current density in TCBTA
device, this high hole density disrupted the holes and elec-
trons balance in the emitting layer, resulting in a low quan-
tum efficiency. The low quantum efficiency of TAPC device
is due to too low hole current density in the emitting layer
due to high energy barrier for hole injection.
The power efficiency–luminance curves of blue PHOL-
EDs are shown in Fig. 9. The power efficiency of TCBPA blue
PHOLEDs was higher than that of TCBTA because of the
high quantum efficiency of the TCBPA device, although
the driving voltage of TCBPA was not as low as that of
TCBTA. The power efficiency of the TCBPA device was even
higher than that of the TAPC device by 20%. The maximum
power efficiency of the TCBPA device was 29.2 lm/W.
TCBTA
TCBPA
0
0
1
100
10000
Luminance (cd/m2)
Fig. 9. Power efficiency–luminance curves of blue PHOLEDs with TCBTA,
TCBPA and TAPC hole transport materials.
density than TCBTA. As shown in the hole current density
device data, the high hole current density of TCBTA
increased the current density of TCBTA blue PHOLED.
Although the current density of the TCBPA device was
lower than that of TCBTA, it was higher than that of TAPC,
indicating that TCBPA is better than TAPC to increase the
current density in the emitting layer. The high current den-
sity of TCBTA and TCBPA devices implies high hole current
density in the emitting layer because the same electron
transport layer was used in all devices. This is closely
related with the energy barrier for hole injection between
the hole transport layer and emitting layer. Considering
the hole only device data of TAPC, TCBTA and TCBPA, the
TAPC device should show higher current density than the
TCBPA device. However, the high energy barrier of
0.63 eV between TAPC and mCPPO1 as shown in energy le-
vel diagram in Fig. 7 hindered hole injection in the TAPC
device, leading to low current density in the TAPC blue
PHOLED [31]. The energy barriers for hole injection of
TCBTA and TCBPA with the mCPPO1 emitting layer were
0.43 eV and 0.45 eV, respectively. The luminance showed
a similar relationship as the current density.
The quantum efficiency–luminance curves of blue
PHOLEDs are shown in Fig. 8. The quantum efficiency of
blue PHOLEDs was high in the TCBPA device with a maxi-
mum quantum efficiency of 21.8%. This high quantum effi-
ciency was also maintained at high luminance and the
quantum efficiency at 1000 cd/m2 was 20.2%. Compared
with TAPC, which showed a maximum quantum efficiency
of 18.5% and a quantum efficiency at 1000 cd/m2 of 16.4%,
TCBPA showed an approximate 20% improvement of quan-
tum efficiency. The high quantum efficiency of the TCBPA
device was attributed to the charge balance, high triplet
energy and electron-blocking effect of TCBPA. The triplet
energy of TCBPA was 2.86 eV, which was much higher than
that of FIrpic (2.65 eV) [32]. Therefore, the triplet exciton
quenching of FIrpic by TCBPA is suppressed and the triplet
excitons are confined in the emitting layer, leading to high
quantum efficiency. The LUMO level for electron blocking
also contributed to the charge confinement in the emitting
layer. The LUMO level of TCBPA was ꢁ2.04 eV, compared
with ꢁ2.64 eV for the host material. There is a 0.6 eV
energy barrier for electron leakage from the emitting layer
to the TCBPA hole-transport layer, which confines
6. Conclusions
Two high triplet energy hole-transport materials based
on the aromatic amine moiety and the diphenylmethyl
linkage were developed as hole-transport materials for
blue PHOLEDs. TCBTA lowered the driving voltage of blue
PHOLEDs, while TCBPA improved the quantum efficiency
of blue PHOLEDs. The power efficiency of blue PHOLEDs
was improved by more than 20% by using TCBPA instead
of the standard TAPC.
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