Efficient bipolar host materials based on carbazole and 2-methyl pyridine for use in green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c5nj00929d

Two host materials based on the carbazole/pyridine end-capping group were designed and synthesized to produce green phosphorescent organic light-emitting diodes. One of the host materials, 9-[9,9-diethyl-2-(6-methylpyridin-2-yl)-9H-fluoren-7-yl]-9H-carbaz

Full text of DOI:10.1039/c5nj00929d

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Efficient bipolar host materials based on carbazole and 2-methyl
pyridine for green phosphorescent organic light-emitting diodes
.
a
a
b
b
b
a
Jwajin Kim , Se Hyun Kim , Ho Won Lee , Song Eun Lee , Young Kwan Kim * and Seung Soo Yoon *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two host materials based on carbazole/pyridine endꢀcapping group were designed and synthesized for
green phosphorescent organic lightꢀemitting diodes (PHOLEDs). One of those, 9ꢀ[9,9ꢀdiethylꢀ2ꢀ(6ꢀ
methylpyridinꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ7ꢀyl]ꢀ9Hꢀcarbazole (2) exhibited excellent properties for efficient green
PHOLEDs as host material. A device using compound 2 as host material with the green phosphorescence
1
0
dopant bis[2ꢀ(1,1′,2′,1″ꢀterphenꢀ3ꢀyl)pyridinatoꢀC,N]iridium(III)(acetylacetonate), (tphpy) Ir(acac),
2
showed the external quantum efficiency (EQE) of 10.7 %, a power efficiency (PE) of 17.27 lm/W and
2
luminous efficiency (LE) of 39.72 cd/A at 20 mA/cm , respectively, with the Commission International
de L’Eclairage (CIE) chromaticity coordinates of (0.34, 0.62) at 8.0 V.
1
6,17
and high triplet energy.
1
5
Introduction
In the display industry, PHOLEDs have attracted much attention
due to their theoretically higher internal quantum efficiency than
fluorescent OLEDs. It is because PHOLEDs can use not only
Results and Discussion
Synthesis of bipolar materials
singlet excitons but also triplet excitons to generate emission by 50 Bipolar host materials were synthesized with a yield of 93% and
2
2
3
3
4
4
0
5
0
5
0
5
originated from spinꢀorbital coupling caused by heavy transition
metal such as Ir and Pt. However, it has problems that must be
solved which derived from numerically increased triplet excitons
such as tripletꢀtriplet annihilation and tripletꢀpolaron annihilation
78%. As described in Scheme 1, product 1 and 2 were
synthesized by Suzuki coupling from the corresponding boronic
esters and 2ꢀbromoꢀ6ꢀmethylpyridine in the presence of
1
palladium catalyst Pd(PPh ) and base K CO . These products
3 4 2 3
1
13
in neat film state. To prevent those efficiency decrement 55 were characterized by H and C NMR, mass spectrometry, and
2,3
problems, PHOLEDs usually adopt a hostꢀdopant system. To
obtain high efficiency from hostꢀdopant system PHOLEDs, the
choice of host materials has great effects for efficiencies of
device and host materials should satisfy the following conditions.
element analysis.
O
N
N
B
N
O
(
i) the triplet energy (E ) of the host material must be higher than
1
T
a)
N
Br
that of the dopant to prevent of flow back the energy from dopant
4,5
O
to host, (ii) the host material needs to wellꢀmatched highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels with the neighboring
layers such as hole transporting layer and electron transporting
layer to get hole and electron easily, and (iii) good and balanced
carrier mobility to make the broad electronꢀhole recombination
zone within the emitting layer (EML) and reduce the efficiency
rollꢀoff. In these aspects, bipolar materials having high triplet
energy is suitable for host materials with their good dual mobility
N
B
N
O
N
2
o
Scheme 1. a) Pd(PPh
3
)
4
, 2M K
reflux for 2 hours.
2
CO
3
, aliquat 336, toluene, ethanol, 90 C
6
0
Physical and Photo-physical Properties of compounds
Thermogravimetric analysis (TGA) were performed to determine
the thermal stability of compounds 1 and 2, respectively. The
6
ꢀ12
decomposition temperature (T ), corresponding to 5% weight loss,
d
of hole and electron to lead to the high efficiency of device.
o
o
were detected at 266 C and 305 C, respectively. Increased T
d
In this paper, we report two bipolar host materials 9ꢀ[4ꢀ(6ꢀ
methylpyridinꢀ2ꢀyl)phenyl]ꢀ9Hꢀcarbazole (1) and 9ꢀ[9,9ꢀdiethylꢀ
o
65
value about 40 C implies that the introduction of the fluorene
moiety bring thermal stability of compound due to its stable
structure.
2
ꢀ(6ꢀmethylpyridinꢀ2ꢀyl)ꢀ9Hꢀfluorenꢀ7ꢀyl]ꢀ9Hꢀcarbazole
(2)
bearing carbazole, which have sufficiently large triplet energy
1
3ꢀ15
and good hole transporting ability
and 2ꢀmethylpyridine
having good electron accepting ability with high electron affinity
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Table 1. Physical and photoꢀphysical properties of host materials 1 and 2.
Performance of electroluminescence devices
abs, a
em, a
b
c
d
e
λ
max
λ
max
E
g
HOMO /LUMO
E
T
35 Green electroluminescence (EL) devices A-C were fabricated by
Compounds
(
nm)
283
344
(nm) (eV)
(eV)
(eV)
using (tphpy) Ir(acac) as the dopant in emitting layer (EML) and
2
1
2
394
402
3.55
3.37
5.77/2.22
5.89/2.52
2.71
3.05
compounds 1, 2 and CBP as the host materials. The energy levels
of the materials used in each layer of the devices are displayed in
Fig 2.
a
abs
em
ꢀ5
b
λ
2 2
max and λmax measured in CH Cl solution (ca.1×10 M). The energy
band gaps were determined from the intersection of the absorption and
photoluminescence spectra. The HOMO energy levels of the compounds
c
N
O
O
Ir
N
5
were measured using a lowꢀenergy photoꢀelectron spectrometer (Rikenꢀ
d
Keiki, ACꢀ2). The LUMO energy levels were estimated by subtracting
e
2
N
the energy band gap from the HOMO energy levels. E
T
determined by
(
tphpy)
2
Ir(acac)
N
N
the absorption energy from the absorption spectra of the materials in 2ꢀ
MeTHF solution at 77K.
TcTa
N
N
(
a)1.0
1
2
1.0
0.8
0.6
0.4
0.2
0.0
NPB
Li
N
O
0
0
0
0
0
.8
.6
.4
.2
.0
BPhen
Liq
N
N
40
Fig. 2. Energy-level diagram of devices A-C.
Devices A and B consist of the following layer structure : Indium
tin oxide (ITO)/ N,N'ꢀbis(naphthalenꢀ1ꢀyl)ꢀN,N'ꢀbis(phenyl)ꢀ
benzidine (NPB)(50nm)/
tris(4ꢀcarbazoylꢀ9ꢀylphenyl)amine
3
00
350
400
450
500
45 (TcTa)(10nm)/ (tphpy)
hroline (Bphen)(30nm)/ 8ꢀhydroxyquinoline lithium (Liq)(2nm)/
Al(100nm). NPB and TcTa are selected as hole transport layers
2
Ir(acac)(8%):Host(30nm)/ bathophenantꢀ
Wavelength [nm]
1
0
(
b) 1.0
1.0
0.8
0.6
0.4
0.2
0.0
1
2
(
HTL). Compounds 1 and 2 are used for host materials. Bphen is
0.8
0.6
0.4
0.2
0.0
CBP
tphpy) Ir(acac)
used for the electron transport layer (ETL), and Al is the cathode.
50 The reference device C consists of the same structure except
using CBP instead of compound 1 and 2 as host. EL spectra of
(
2
(
tphpy) Ir(acac) doped devices A-C are plotted in Fig. 3 and EL
2
data of devices A-C are listed in Table 2. In all devices, quite
similar green emission peaks were exhibited at 526 nm, 527 nm
and 529 nm, respectively. It means that the emissions from device
A-C are not originated from host material but triplet excitons of
55
3
50
400
450
500
550
dopant, (tphpy) Ir(acac) and the energy transfer from hosts to
2
Wavelength [nm]
dopant occured perfectly.
Fig.1. (a) UV-Vis absorption and PL emission spectra host materials 1, 2
and (b) PL emission spectra of host materials 1, 2, and CBP and UV/Vis
A
B
C
1.0
0.8
0.6
0.4
0.2
2
absorption spectra of dopant materials, (tphph) Ir(acac).
15
20
25
30
Photoꢀphysical properties of compounds 1 and 2 were analyzed
by using UVꢀvis and photoluminescence (PL) spectrometers in
Table 1. Fig. 1 shows UVꢀVis spectra compounds 1 and 2, and
PL spectra of compounds 1, 2, and CBP as well as UV/Vis
absorption spectra of dopant materials, (tphph) Ir(acac). In Fig.
2
1(a), compound 2 is exhibiting redꢀshifted absorption and
emission spectra due to longer πꢀconjugation length than that of
compound 1 by replacing phenyl by fluorene moiety. Also,
compound 2 shows smaller Stokes shift than compound 1 about
0.0
450
500
550
600
650
700
750
Wavelength [nm]
60
Fig. 3. Normalized EL emission spectra of devices A-C.
5
3 nm. It implies that the replacement phenyl by fluorene brings
Fig. 4 shows (a) current densityꢀ and luminanceꢀvoltage, (b)
luminous efficiencyꢀ and power efficiencyꢀcurrent density, and (c)
the external quantum efficienciesꢀcurrent density plots of devices
A-C. In particular, device B shows higher external quantum
efficiencies than those of device A and C. Device B exhibited an
external quantum efficiency of 10.7 %, a power efficiency of
structural rigidity for molecule. Triplet energy of compounds 1
and 2, which were determined from the first phosphorescent
emission peaks in 77 K, were 2.71eV and 3.05eV, respectively.
These values are higher than the 2.43 eV triplet energy of
typhpy) Ir(acac) dopant and 4,4
(CBP, E = 2.56 eV), which commonly used material as host
material for green and red PHOLEDs.
triplet energy of compound 1 and 2 than dopant material meets
the condition for good host materials for PHOLEDs.
65
(
′
ꢀbis(
Nꢀcarbazolyl)ꢀ1,1′ꢀbiphenyl
2
T
1
7.27 lm/W, and a luminous efficiency of 39.72 cd/A at 20
1
8,19
Sufficiently higher
2
mA/cm , respectively, with the Commission International de
L’Eclairage chromaticity coordinates of (0.34, 0.62) at 8.0 V.
2
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Table 2. EL performance characteristics of devices AꢀC.
Conclusions
a/b
a/b
a/b
c
EL, c
Devices Luminance
LE
PE
EQE
CIE
λ
max
In summary, we have synthesized new bipolar green host
materials based on carbazole and 2ꢀmethylpyridine. Green
phosphorescent devices using these compounds as hosts were
fabricated and their electroluminescence (EL) properties were
investigated. Especially, a device using 9ꢀ[9,9ꢀdiethylꢀ7ꢀ(6ꢀ
2
(host)
(cd/m )
(cd/A) (lm/W)
(%)
7.87/
5.19
(x, y)
(0.34,
0.62)
(nm)
A
7207
(10V)
18680
(9.5V)
40520
(11V)
30.9/
19.11
57.25/
39.72
45.6/
36.2
18.69/
6.67
45.05/ 15.42/ (0.34,
17.27 10.7 0.62)
31.84/ 12.44/ (0.35,
13.10 9.74 0.60)
5
5
5
26
27
29
(1)
55
B
(2)
C
CBP)
methylpyridinꢀ2ꢀyl)ꢀ9
Hꢀfluorenꢀ2ꢀyl]ꢀ9
Hꢀcarbazole (2) as the
(
host and bis[2ꢀ(1,1 ,2
′
′
,1
″
ꢀterphenꢀ3ꢀyl)pyridinatoC,N
]iridium(III)
a
b
2 c
Maximum values. At 20 mA/cm . At 8V.
(
acetylacetonate), (tphpy) Ir(acac), as the dopant exhibited the
2
The significant improvement of whole efficiencies of device B in
comparison with device A and C could be explained based on (i)
the efficient energy transfer from host to dopant through the
Förster energy transfer pathway and (ii) the appropriate HOMO
and LUMO energy levels with adjacent layers such as HTL, ETL,
and (iii) the wellꢀbalanced carrier mobility derived from bipolar
structure.
6
0
improved EL efficiencies (an external quantum efficiency of 10.7
, a power efficiency of 17.27 lm/W and luminous efficiency of
%
5
2
3
9.72 cd/A at 20 mA/cm , respectively), in comparison with a
device using CBP as host (an external quantum efficiency of 9.74
, a power efficiency of 13.10 lm/W and luminous efficiency of
%
2
65
36.2 cd/A at 20 mA/cm , respectively). The effective energy
transfer, charge injection and the wellꢀmatched electronꢀ
accepting/ electronꢀdonating capabilities of compound 2 would
play important roles in improving the EL efficiencies of OLED
device using compound 2 as host.
1
1
2
2
3
3
4
4
5
0
5
0
5
0
5
0
5
0
In Fig. 1(b), PL emission peaks of compound 1 and 2 are
observed at 394 nm, 402 nm and that of CBP are exhibited at 350
1
nm. Also, a singlet metalꢀtoꢀligand transfer ( MLCT) absorption
band of dopant material is located at range of 400ꢀ450 nm,
broadly. It implies that the Förster energy transfer from
compound 1 and 2 to dopant would be more efficiently occured
than that of CBP. The resulting singlet excitons of dopants are
transformed to triplet excitons by strong spinꢀorbital coupling
originated from Ir. It would be an important part for improving
EL efficiencies of device A and B equally, compared to device
a)16
(
A
B
C
14
12
10
8
6
20,21
C.
However, the much smaller energy gap between the
4
LUMOs energy levels of compound 2 and ETL than those of
compound 1 and ETL makes electron transfer easily from ETL to
EML in device B. This effective electron injection of device B
would contribute the improved EL efficiencies of device B in
comparison with device A. In an aspect of energy gaps between
EML and neighboring layers, CBP in device C is more
effectively positioned to get higher efficiencies than compound 2
in device B. Nevertheless, device B shows more high efficiencies
than device C. The pyridine moiety in compound 2 of device B
2
0
20
40
60
80
100
2
Current density [mA/cm ]
70
(b) 60
LE A 60
LE B
LE C
PE A
PE B
PE C
40
40
20
0
20
provides
a good electronꢀaccepting ability (or electronꢀ
withdrawing ability) to compound 2, and it improves the electron
mobility of compound 2. CBP is endꢀcapped with two carbazole
unit and thus has hole transfer unit only. This difference brings a
wellꢀbalanced charge carrier mobility to compound 2, compared
to CBP. The resulting wellꢀbalanced carrier population in the
emitting layer of device B using compound 2 as host would
contribute the improve EL efficiencies of device B in comparison
with device C. However, in high currentꢀdensity region, the
larger energy gap between LUMOs of ETL and EML in device B
than those in device C brings higher efficiency rollꢀoff of device
B than that of device C. This larger energy gap of device B than
device C would make the electron injection in device B more
difficult than device C, and thus induce the narrower exciton
recombination zone near the ETL in the EML of device B in
comparison with device C. Particularly, in high current density,
the populated tripletꢀexcitons in narrow recombination zone of
device B would become to interact with each other. The resulting
tripletꢀtriplet annihilation in the EML of device B would
contribute to the reduced EL efficiency, and thus high efficiency
0
0
20
40
60
80
100
2
Current density [mA/cm ]
(c) 2
00
80
60
A
B
C
1
0000
1
1
1000
140
1
00
1
20
00
1
10
1
8
0
0
6
40
0.1
20
0
0.01
0
2
4
6
8
10
Voltage [V]
Fig. 4. (a) The external quantum efficiencies-current density, (b)
luminance efficiency- and power efficiency-current density, and (c)
current density- and luminance-voltage plots of devices A-C.
75
Experimental Details
General information
22
rollꢀoff of device B in the high currentꢀdensity region.
1
13
The Hꢀ and Cꢀ nuclear magnetic resonance (NMR) spectra
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were recorded on a Varian Unity Inova 300Nb spectrometer. 60 (tphpy) Ir(acac) (30nm, 8%)/ 4,7ꢀdiphenylꢀ1,10ꢀ phenanthroline
2
Elemental analysis (EA) was measured using an EA 1108
spectrometer. The low resolution mass spectra were measured
using a Jeol JMSꢀAX505WA spectrometer in EI and APCI mode
and a JMSꢀT100TD (AccuTOFꢀTLC) in positive ion mode. The
(Bphen)(30nm)/ lithium quinolate (Liq)(2nm)/ Al (100nm). The
current density ( ), luminance( ), LE, PE, and CIE chromaticity
coordinates of the OLEDs were measured with a Keithly 2400,
Chroma meter CSꢀ1000A. EL was measured using a Roper
J
L
5
0
5
0
UVꢀVis absorption and photoluminescence spectra of the newly 65 Scientific Pro 300i.
ꢀ
5
designed host materials were measured in a CH Cl solution (10
2
2
M) using a Shimadzu UVꢀ1650PC and AMINCOꢀBowman
Series 2 Luminescence Spectrometer. The ionization potentials
(or HOMO energy levels) of the compounds were measured
using a lowꢀenergy photoꢀelectron spectrometer (RikenꢀKeiki,
ACꢀ2). The energy band gaps were determined from the
intersection of the absorption and photoluminescence spectra.
The LUMO energy levels were estimated by subtracting the
energy band gap from the HOMO energy levels. The thermal
properties were measured by thermogravimetric analysis (TGA)
Acknowledgements
This research was supported by the Basic Science Research
Program through the NRF funded by the Ministry of Education,
Science and Technology (NRFꢀ2013R1A1A2A10008105).
1
1
2
7
0
Notes and references
a
Department of Chemistry, Sungkyunkwan University, Suwon, 440-746,
Korea. Tel: 82-031-290-7071; Fax: 82-031-290-7075;
E-mail: ssyoon@skku.edu
(
(
1
DTAꢀTGA, TAꢀ4000) and differential scanning calorimetry
b
Department of Information Display, Hongik University, Seoul, 121-791,
DSC) (Mettler Toledo; DSC 822) under N at a heating rate of
2
75
Korea. Tel: 82-2-320-1646; Fax: 82-2-3141-8928.;
E-mail: kimyk@hongik.ac.kr
o
0
C/min. A heatꢀcoldꢀheat method was used with an initial
o
heating rate of 10 C/min, rapidly quenchedꢀcooled in liquid
nitrogen, and finally heated at rate of 10 C/min.
o
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ꢀBromoꢀ6ꢀmethylpyridine (1.0 mol) and the corresponding
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aqueous 2.0 M K CO (10.0 mol), Aliquat 336 (0.1 mol), and
2
5
2
3
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3
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691.
recrystallized from CH Cl /EtOH.
2
2
8
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9
9
2
-[4-(6-Methylpyridin-2-yl)phenyl]-9
H
-carbazole (1) : Yield =
3%. H NMR (300 MHz, CDCl3): δ ppm 8.19 (d, = 8.5 Hz,
H), 8.14 (d, = 7.8 Hz, 2H), 7.67ꢀ7.62 (m, 3H), 7.56 (d, = 7.6
= 7.8 Hz, 2H), 7.28 (t,
1
J
J
J
9
5
35
40
45
50
Hz, 1H), 7.47ꢀ7.45 (m, 2H), 7.40 (t,
.8 Hz, 2H), 7.11 (d, = 7.3 Hz, 1H), 2.65 (s, 3H); C NMR (75
MHz, CDCl ): δ ppm 158.9, 156.3, 141.0, 139.1, 138.4, 137.3,
J
J =
1
3
7
J
1
0. K. S. Yook and J. Y. Lee, J. Lumin. 2015, 161, 271.
3
11. H. Hong, W. Yixin, Z. Shaoqing, Y. Xiao, W. Lei, and Y. Chuluo, J.
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1
2
28.8, 127.4, 126.3, 123.7, 122.2, 120.6, 120.3, 117.9, 110.1,
5.1; MS (EI+, m/z): 334 [M+]; Anal. calcd for C H N : C
2
4
18
2
100 12. G. H. Kim, R. Lampande, J. H. Kong, J. M. Lee, J. H. Kwon, J. K.
Lee, and J. H. Park, RSC. Adv. 2015, 5, 31282.
86.20, H 5.43, N 8.38. Found C 85.83, H 5.37, N 8.31.
-[9,9-Diethyl-7-(6-methylpyridin-2-yl)-9 -fluoren-2-yl]-9
carbazole (2) : Yield = 78%. H NMR (300 MHz, CDCl3): δ
ppm 8.19 (d, = 7.8 Hz, 2H), 8.09ꢀ8.06 (dd, = 1.5, 6.6 Hz, 1H),
.02 (s, 1H), 7.96 (d, = 3.6 Hz, 1H), 7.87 (d,
= 7.8 Hz, 1H), 105
7.69ꢀ7.60 (m, 2H), 7.56 (d, = 8.4 Hz, 2H), 7.44 (d, = 8.1 Hz,
H), 7.35ꢀ7.29 (m, 2H), 7.12 (d, = 6.9 Hz, 1H), 2.67(s, 3H),
.24ꢀ2.03(m, 4H), 0.49(t, = 7.2 Hz, 6H); C NMR (75 MHz,
9
H
H-
13. M.ꢀH. Tsai, H.ꢀW. Lin, H.ꢀC. Su, T.ꢀH. Ke, C. ꢀC. Wu, F.ꢀC. Fang,
Y.ꢀL. Liao, K.ꢀT. Wong, and C.ꢀI. Wu, Adv. Mater. 2006, 18, 1216.
14. Q. Gao, M. Luo, X. H. Sun, H. L. Tam, M. S. Wong, B. X. Mi, P. F.
Xia, K. W. Cheah, and C. H. Chen, Adv. Mater. 2009, 21, 688.
15. J. W. Yang and J. Y. Lee, Org Electron. 2015, 22, 74.
1
1
1
1
J
J
8
J
J
J
J
6. S. ꢀJ. Su, H. Sasabe, T. Takeda, and J. Kido, Chem. Mater. 2008, 20,
691–1693
7. H. Ye, D. Chen, M. Liu, S. ꢀJ Su, Y. ꢀF. Wang, C. ꢀC. Lo, A. Lien,
and J. Kido, Adv. Funct. Mater. 2014, 24, 3268.
8. S. Lamansky, P. Djurovich, D. Murphy, F. AbdelꢀRazzaq , H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest, and M. E. Thompson, J. Am.
Chem. Soc. 2001, 123, 4304.
9. W. ꢀY. Wong, C. ꢀL. Ho, Z. ꢀQ. Gao, B. ꢀX. Mi, C. ꢀH. Chen, K. ꢀW.
Cheah, and Z. Lin, Angew. Chem. Int. Ed. 2006, 45, 7800.
0. W. Jiang, L. Duan, J. Qiao, D. Zhang, G. Dong, L. Wang, and Y.
Qiu, J. Mater. Chem., 2010, 20, 6131.
4
2
J
1
3
1
J
CDCl ): δ ppm 158.8, 157.4, 152.7, 151.0, 141.7, 141.4, 140.8,
3
1
1
1
10
1
39.4, 137.2, 136.9, 126.7, 126.3, 123.7, 122.2, 121.8, 121.7,
121.4, 120.7, 120.4, 120.2, 118.0, 110.1, 56.9, 33.1, 25.2, 9.1;
MS (APCI+, m/z): 479 [M+]; Anal. calcd for C H N : C 87.83,
H 6.32, N 5.85. Found C 87.22, H 6.25, N 5.81.
3
5
30
2
1
2
2
15
Device fabrication and characterization
OLEDs using greenꢀlightꢀemitting molecules were fabricated by
ꢀ
7
1. H. Liu, W. Gao, K. Yang, B. Chen, S. Liu, and Y. Bai, Chem. Phys.
Lett., 2002, 352, 353.
55
vacuum (5×10 torr) thermal evaporation onto precleaned ITO
coated glass substrates. The Structure was as follows: ITO/ N,N’
diphenylꢀN,N’ꢀ(1ꢀnapthyl)ꢀ(1,1’ꢀphenyl)ꢀ4,4’ꢀdiamine (NPB)(50
ꢀ
20 22. J. W. Kang, S. H. Lee, H. D. Park, W. I. Jeong, K. M. Yoo,Y. S. Park,
and J. J. Kim, Appl. Phys. Lett., 2007, 90, 223508.
nm)/ 4,4’,4’’ꢀtris(
Nꢀcarbazole)triphenylamine (TcTa)(10nm)/ Phꢀ
osphorescent host materials: Green dopant material
4
| Journal Name, [year], [vol], 00–00
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New Journal of Chemistry
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DOI: 10.1039/C5NJ00929D
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Device A (host:1)
Device B (host:2)
Device C (host:CBP)
10000
1
1
N
1000
100
N
1
1
2
1
1
0
8
0
0
0
0
0
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4
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N
0.1
0.01
N
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Voltage [V]