Highly efficient triazine/carbazole-based host material for green phosphorescent organic light-emitting diodes with low efficiency roll-off

Article abstract of DOI:10.1039/c6ra27770e

Two novel host materials, 3-(dibenzo[b,d]furan-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (BFTC) and 3-(dibenzo[b,d]thiophen-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (BTTC) were designed and synthesized. These two compounds exhibited excellent physical properties with high thermal stabilities and reasonable HOMO-LUMO energy levels. Both of them were applied to fabricate green phosphorescent organic light emitting devices (PhOLEDs) as host materials, and the BTTC based device demonstrated outstanding electroluminescence performance with a maximum current efficiency, maximum power efficiency and external quantum efficiency of 69.3 cd A^-1, 54.2 lm W^-1 and 21.9%, respectively, suggesting that BTTC is a promising host for green PhOLEDs.

Full text of DOI:10.1039/c6ra27770e

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Highly efficient triazine/carbazole-based host
material for green phosphorescent organic
light-emitting diodes with low efficiency roll-off†
Cite this: RSC Adv., 2017, 7, 7287
a
a
a
a
c
b
*
Mingming Hu, Yang Liu, Yi Chen, Wenxuan Song, Lei Gao, Haichuan Mu,
d
a
*
Jinhai Huang and Jianhua Su
Two novel host materials, 3-(dibenzo[b,d]furan-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole
(BFTC) and 3-(dibenzo[b,d]thiophen-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (BTTC) were
designed and synthesized. These two compounds exhibited excellent physical properties with high
thermal stabilities and reasonable HOMO–LUMO energy levels. Both of them were applied to fabricate
green phosphorescent organic light emitting devices (PhOLEDs) as host materials, and the BTTC based
device demonstrated outstanding electroluminescence performance with a maximum current efficiency,
maximum power efficiency and external quantum efficiency of 69.3 cd Aꢀ1, 54.2 lm Wꢀ1 and 21.9%,
respectively, suggesting that BTTC is a promising host for green PhOLEDs.
Received 5th December 2016
Accepted 9th January 2017
DOI: 10.1039/c6ra27770e
www.rsc.org/advances
In recent years, a great progress of green PhOLEDs based on
small molecule materials have been made.13–19 Carbazole is
Introduction
a commonly used molecular fragment because its small conju-
gated system, high triplet energy level and the lone pair electrons
in the N atoms can effectively reduce the splitting of the singlet
and triplet energy level, which make carbazole-containing
compounds as promising hole-transport materials in OLEDs.20,21
Besides, triazine derivatives are widely used as electron-
transporting materials due to its high electron affinity.22–25 Su
and his coworkers reported a bipolar host material, 2,4,6-tris(3-
(9H-carbazol-9-yl) phenyl)-1,3,5-triazine, which had low power
voltage and high efficiency originated from its small singlet–
triplet splitting and low LUMO energy level.26 Chang and his
coworkers developed a dicarbazole-triazine baesd bipolar host
material, exhibiting a high glass transition temperature of 134 ꢁC
and triplet energy (ET ¼ 2.67 eV), giving a green PhOLEDs doped
with (TPm)2Ir(acac) with maximum efficiencies (EQE) of 20.1%
(76.3 cd Aꢀ1 and 72.7 lm Wꢀ1).27
Taking the advantage of the carbazole and triazine in the
host, two novel host materials, 3-(dibenzo[b,d]furan-4-yl)-9-(4,6-
diphenyl-1,3,5-triazin-2-yl)-9H-carbazole (BFTC) and 3-(dibenzo-
[b,d]thiophen-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-9H-carbazole
(BTTC) were designed and synthesized. Besides, the introduction
of dibenzo[b,d]furan and dibenzo[b,d]thiophen were expected
to reduce the oxidation potential, enhance the thermal stability
and provide more delocalized electrons for enlarging the
HOMO orbital energy level and beneting the injection and
transmission of holes.28 The green PHOLEDs based on above
mentioned two host materials achieved superior EL perfor-
mance with the maximum current, power and external
quantum efficiency of 69.3 cd Aꢀ1, 54.2 lm Wꢀ1 and 21.9%,
respectively.
Organic light-emitting devices (OLEDs) are considered as the
next generation displays and illumination sources because of
their active and large area emissions, high luminous efficiency
and low power consumption.1–7 It is important to develop effi-
cient host materials for phosphorescent OLEDs with low effi-
ciency roll-off induced by concentration quenching and triplet–
triplet annihilation.8 In general, an appropriate host material is
required to satisfy several fundamental properties. Firstly, the
triplet energy level (ET) of the host should be higher than the
dopant to prevent reverse energy transfer from the guest to the
host. Secondly, the highest occupied molecular orbitals
(HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs) of the host are required to match to adjacent material
layers to benet the injection of the hole and electron, so as to
reduce the turn-on voltage. In addition, the host is supposed to
have good carrier transporting characteristics for effective
exciton recombination. Finally, good thermal stability is also
expected to form a uniform thin lm morphology.9–12
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science & Technology, Shanghai 200237, PR China. E-mail: bbsjh@
ecust.edu.cn
bDepartment of Physics, School of Science, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, PR China. E-mail: hcmu@ecust.
edu.cn
cSchool of Chemistry and Chemical Engineering, Queen's University, Belfast, Northern
Ireland, UK
dShanghai Taoe Chemical Technology Co., Ltd, Shanghai, PR China
† Electronic supplementary information (ESI) available: 1H, HRMS and EL spectra
under different driving voltages of two materials. See DOI: 10.1039/c6ra27770e
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temperature with high yield of 93.5% and 89.7%, respectively.
Results and discussion
Synthesis and characterization
The molecular structures of two intermediates were conrmed
1
by H NMR. Additionally, the molecular structure of the nal
compounds can be conrmed by elemental analyses and high-
resolution mass spectrometry (HRMS).
The synthetic routes for the two compounds are shown in
Scheme 1. The key intermediate, 3-(dibenzo[b,d]furan-4-yl)-9H-
carbazole (3) and 3-(dibenzo[b,d]thiophen-4-yl)-9H-carbazole
(5), were synthesized by Suzuki coupling reaction of 3-bromo-
9H-carbazole (1) and corresponding phenylboronic acid
precursors with high yields of 95.3% and 97.2%, respectively,
while the target compounds of BFTC and BTTC were obtained
aer treating 2-chloro-4,6-diphenyl-1,3,5-triazine and corre-
sponding intermediate in the presence of NaH in DMF at room
Photophysical properties
Fig. 1(a) displays the UV-vis absorption and uorescence spectra
of the two compounds in dichloromethane (DCM) solution, and
the detailed data are presented in Table 1. BFTC and BTTC show
almost identical absorption peaks with only a 5 nm difference
in peak wavelength. The strong absorption peak around 260 nm
can be assigned to the p–p* transition of the compounds, and
the additional absorption peak around 330 nm can be attrib-
uted to the n–p* transition of the carbazole moiety.29 The
optical energy bandgaps (Eg) of the two compounds estimated
from the onset of the absorption spectra are determined to be
3.44 eV and 3.53 eV, respectively. The maximum PL emission
wavelengths of the two compounds are both observed at
511 nm. Furthermore, the triplet energies (ET) estimated from
the phosphorescence spectrum in 2-methyl-tetrahydrofuran at
77 K are 2.58 eV for BFTC, and 2.56 for BTTC. Such high
dibenzofuran substituent has a higher ET than the dibenzo-
thiophene substituent. Therefore, the two compounds with
high bandgaps and triplet energy levels make them promising
host materials for green PhOLEDs.
Scheme 1 Synthetic routes of the compounds.
Thermal properties
The thermal properties of the two compounds were investigated
by thermogravimetric analysis (TGA) and differential scanning
calorimetry analysis (DSC) under nitrogen atmosphere. As
shown in Fig. 2 and Table 1, the TGA measurement reveals their
high thermal-decomposition temperatures (Td, corresponding
to 5% weight loss) of 398 ꢁC for BFTC and 452 ꢁC for BTTC,
respectively. We observed a well-dened melting peak of 278 ꢁC
for BFTC and 280 ꢁC for BTTC, while the endothermic glass
transition temperature (Tg) of both two compounds are not
observed even in the second scan by the DSC. From the data
above, new host materials exhibit high thermal ability because
of their high molecule weight especially for BTTC, which is
favorable to form stable lms surface morphology.
Electrochemical properties
The electrochemical behaviors of the two compounds were
determined by cyclic voltammetry (CV) using a Fc/Fc+ couple as
the internal reference. As shown in Fig. 3, all the CV curves
exhibit irreversible oxidation process in DCM solution, and the
onset irreversible oxidation process in DCM solution, and the
onset potentials are 0.86, 0.89 V (vs. Fc/Fc+) for BFTC and BTTC,
respectively. Compared to the BTTC, the electron-delocalization
of BFTC is larger by replacing the dibenzothiophene ring with
dibenzofuran ring, rendering easier oxidation. Their HOMO
levels, which are ꢀ5.26 eV for BFTC and ꢀ5.29 eV for BTTC,
respectively, are determined from the onset potentials while
ferrocene/ferrocenium (Fc/Fc+) was applied as the internal
standard. Subsequently, based on the optical energy gaps and
Fig. 1 (a) Normalized UV-vis absorption and fluorescence emission
spectra of the compounds in dichloromethane solution at room
temperature; (b) phosphorescence spectra in a frozen 2-methylte-
trahydrofuran at 77 K.
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Table 1 Physical properties of BFTC and BTTC
Absorption
Emission
HOMOd [eV]
LUMOe [eV]
f
Tg [ꢁC]
g
Tm [ꢁC]
h
Td [ꢁC]
Compounds
lmaxa [nm]
lmaxa [nm]
Egb [eV]
ETc [eV]
BFTC
BTTC
325
330
511
511
3.44
3.53
2.58
2.56
ꢀ5.26
ꢀ5.29
ꢀ1.82
ꢀ1.76
NA
NA
278
280
398
452
a
b
Measured in dichloromethane at a concentration of 1.0 ꢂ 10ꢀ5 and excited by 335 nm. Estimated from onset of the absorption spectra (Eg ¼
c
d
1241/lonset). Estimated from the phosphorescence spectrum in 2-methyl-tetrahydrofuran at 77 K. The HOMO energy level was determined
e
f
from cyclic voltammetry (EHOMO ¼ ꢀ4.4 ꢀ Eox). The LUMO energy level was calculated by the equation: ELUMO ¼ EHOMO + Eg. Measured by
DSC, NA: no available. g Tm: melting temperature. h Td: decomposition temperature at 5% weight loss.
Fig. 4 The molecular orbital surface of the HOMO and LUMO levels
Fig. 2 TGA traces of the two compounds.
for the two compounds.
electronegativity of triazine core. The highest occupied molecular
orbital (HOMO) levels of BFTC are distributed among the electron-
donating 3-(dibenzo[b,d]furan-4-yl)-9H-carbazole moiety, while
BTTC are mainly located at the 3-(dibenzo[b,d]thiophen-4-yl)-9-
carbazole moiety, which reveals a desirable separation of
HOMOs and LUMOs beneting for the transporting balance of
holes and electrons. Unconjugated uorene moiety contributes to
the effective separation of the electron density of the HOMO and
LUMO, which induces a high ET.30
Electroluminescence of PHOLEDs
To evaluate the properties of the two compounds as host
materials, the device conguration of [ITO/PEDOT:PSS (6 nm)/
TAPC (35 nm)/TCTA (5 nm)/host: Ir(ppy)3 (20 nm, 8 wt%)/TPBi
(35 nm)/LiF (1 nm)/Al (80 nm)] were fabricated. Here, the
emitting layer (EMT) was composed of 8 wt% Ir(ppy)3 doped
into the host materials. ITO and LiF/Al were used as the
anode and composite cathode, respectively. Poly(3,4-ethylene
Fig. 3 Oxidation behaviors of the two compounds in CH2Cl2 solution
containing 0.1 M TBAPF6 electrolytes, scanning rate: 100 mV sꢀ1
.
HOMO levels, their LUMO levels were deduced to be ꢀ1.82 eV
for BFTC and ꢀ1.76 eV for BTTC.
dioxythiophene)/poly(styrenesulfonate)
(PEDOT:PSS)
was
Theoretical calculation
employed as the hole injection layer, 4,40-(cyclohexane-1,1-diyl)
To further understand the electronic structural properties cor- bis(N,N-di-p-tolylaniline) (TAPC) served as hole-transporting
responding to the molecular level of the two compounds, the layer (HTL), tris(4-(9H-carbazol-9-yl)phenyl)amine (TCTA) was
frontier molecular orbital energy levels were calculated by used as electron-blocking layer (EBL), and 1,3,5-tris(1-phenyl-
density functional theory (DFT) at the B3LYP/6-31G level. Fig. 4 1H-benzo[d]imidazol-2-yl)benzene (TPBi) was employed as hole-
shows that the lowest unoccupied molecular orbital (LUMO) blocking layer (HBL) and electron-transporting layer (ETL). The
energy levels of both two compounds are mainly located at device and the related HOMO and LUMO energy levels of these
the 2,4-diphenyl-1,3,5-triazine moiety due to the stronger materials in devices are shown in Fig. 5. In addition, the current
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Fig. 5 The energy level diagram of the materials in devices.
density–voltage–luminance (J–V–L) characteristics, efficiencies
and EL spectra for devices are shown in Fig. 6, and their key
performances results are summarized in Table 2.
As revealed in Fig. 6 and Table 2, the turn-on voltages are
3.3 eV for BFTC and 3.4 eV for BTTC, respectively, at a lumi-
nance of 1 cd mꢀ2. The low turn-on voltages of two devices
should result from the matched HOMO and LUMO energy levels
of two hosts with their adjacent layers, TCTA and TPBi, which is
favorable for the injection of the hole and electron, so as to
prot the recombination of hole and electron in emitting layer.
The maximum luminance of BFTC is 17 241 cd mꢀ2, and 24 535
cd mꢀ2 for BTTC, respectively. It can be found that both the
luminance and current density of BTTC are higher than BFTC,
suggesting BTTC as a more promising host for green PhOLEDs.
The current/power efficiency-luminance characteristics are
shown in Fig. 6(b). The BTTC based device exhibits more effi-
cient electroluminescence performance with the demonstrated
respective current and power efficiency of 69.3 cd Aꢀ1 and 54.2
lm Wꢀ1 compared to those of 63.1 cd Aꢀ1 and 45.2 lm Wꢀ1 for
BFTC. The high efficient performance is caused by its approxi-
mate LUMO energy levels with the TPBi, which is benecial for
electron transportation. As showed in Fig. 6(c), the maximum
external quantum efficiency of 21.9% is achieved by BTTC,
while 19.7% for BFTC. Both two compounds show high elec-
troluminescence efficiency even at a high luminance, and only
slight efficiency roll-off over a wide range of luminance is
observed. Importantly, a high efficiency of 50.5 cd Aꢀ1, 22.3 lm
Wꢀ1 (15.7% EQE) even at 10 000 cd mꢀ2 was also obtained for
BTTC. From the EL spectra showed in Fig. 6(d), green light
emissions with the respective emission peaks of 507 and
508 nm (the corresponding CIE coordinates of (0.28, 0.61) and
(0.30, 0.60) at 6 V) for two compounds based device are proved.
We believe that the electroluminescence performance will be
further improved via the optimizing device congurations.
To further verify the difference in the charge transport of two
compounds, hole and electron only devices were fabricated with
the structure of [ITO/PEDOT:PSS (5 nm)/TAPC (5 nm)/host (60
nm)/TAPC (5 nm)/LiF (1 nm)/Al (80 nm)] for the for the hole, and
[ITO/TmPyPB (5 nm)/host (60 nm)/TmPyPB (5 nm)/LiF (1 nm)/Al
Fig. 6 (a) Current density–voltage–luminance (I–V–B) characteris-
tics; (b) current and power efficiency; (c) external quantum efficiency
(EQE) versus current density; (d) EL spectra at 6 V of green PhOLEDs.
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Table 2 Characteristics of OLEDs with BFTC and BTTC host materials
Lmaxb [cd mꢀ2
]
hc [cd Aꢀ1
c
]
hp [lm Wꢀ1
]
c
hext [%]
CIE(x,y)
a
c
d
Device
Host
Von [V]
A
B
BFTC
BTTC
3.3
3.4
17 241
24 535
63.1, 61.9, 50.1
69.3, 69.2, 50.5
45.2, 32.4, 18.5
54.2, 41.0, 22.3
19.7, 19.3, 15.5
21.9, 21.4, 15.7
(0.28, 0.61)
(0.30, 0.60)
a
b
Turn-on voltage to give a luminance of 1 cd mꢀ2
.
Lmax: maximum luminance. hc: current efficiency. hp: power efficiency. hext: external quantum
efficiency. Order of measured values: maximum, then at 1000 cd mꢀ2 and 10 000 cd mꢀ2
.
Measured form the EL spectra at 6 V by inverting
c
d
chromaticity coordinates on the CIE 1931 diagram.
UV-vis absorption spectra were recorded on a Varin Cary 500
recording spectrophotometer with baseline correction. Photo-
luminescence (PL) spectra were recorded on a Varian-Cary uo-
rescence spectrophotometer. Cyclic voltammetric studies of the
compounds in oxidation processes were carried out in nitrogen-
purged dichloromethane (DCM) solution at room temperature by
a Versastar II electrochemical workstation. The conventional
three-electrode conguration consists of a glassy carbon working
electrode, a platinum wire counter electrode, and a regular
calomel reference electrode in saturated KCl solution. Bu4NPF6
(0.1 M) was employed as
a supporting electrolyte, and
ferrocenium/ferrocene (Fc/Fc+) was served as an internal stan-
dard. The highest occupied molecular orbital (HOMO) energy
levels (eV) of these compounds were calculated according to the
+
formula: ꢀ[4.8 eV + (Eonset,oxidation ꢀ E1/2(Fc/Fc ))].
Fig. 7 Current density–voltage curves of hole and electron only
devices.
Material synthesis
The synthetic routes for the two compounds are shown in
Scheme 1.
(80 nm)] for the electron. The J–V characteristics of the hole-only
and electron-only devices are shown in Fig. 7.
3-(Dibenzo[b,d]furan-4-yl)-9H-carbazole (3). A mixture of 3-
bromo-9H-carbazole (1.0 g, 4.06 mmol), dibenzo[b,d]furan-4-
ylboronic acid (0.95 g, 4.47 mmol), tetrahydrofuran (20 mL),
potassium carbonate (2 M, 20 mL), and Pd(C2H3O2)2 (44.8 mg,
0.20 mmol) and 2-di-t-butylphosphino-3,4,5,6-tetramethyl-20,40,60-
tri-propylbi-phenyl (Xphos, 0.14 g, 0.40 mmol) were thoroughly
mixed in a 100 mL ask under nitrogen protection and then
heated in the oil bath at 70 ꢁC for 4 h with vigorous stirring. The
mixture was then extracted with CH2Cl2 (3 ꢂ 25 mL). The
combined organic layer was dried with anhydrous Na2SO4. Aer
removal of the solvent by rotary evaporation, the residue was
puried by ethanol to give the title compound as a white solid
Clearly from Fig. 7, the currents for the hole only device of
BTTC are the highest, and the appropriate hole and electron
current densities in both the hole and electron only devices
demonstrates good and balanced charge transport, which provides
the evidence to support its superior EL performance compared to
that of the BFTC. On the other hand, BFTC shows a slightly higher
hole current density compared to its poor electron current density,
which leads to the unbalanced bipolar transporting ability and
then the unsatisfactory EL performance obtained.
Experimental section
General information
1
(1.29 g, yield: 95.3%). H NMR (400 MHz, CDCl3) d 8.60 (s, 1H),
8.18 (d, J ¼ 7.6 Hz, 2H), 8.01 (dd, J ¼ 11.8, 5.0 Hz, 2H), 7.95 (d, J ¼
All the reagents and solvents used for the synthesis or 7.6 Hz, 1H), 7.72 (d, J ¼ 6.6 Hz, 1H), 7.62 (dd, J ¼ 12.8, 8.4 Hz, 2H),
measurements were commercially available without further 7.50–7.43 (m, 4H), 7.37 (d, J ¼ 7.8 Hz, 1H), 7.31–7.27 (m, 1H).
1
purication. The H NMR spectra was recorded on a Brucker
3-(Dibenzo[b,d]furan-4-yl)-9-(4,6-diphenyl-1,3,5-triazin-2-yl)-
AM 400 spectrometer with tetramethylsilane as an internal 9H-carbazole (BFTC). A mixture of 3-(dibenzo[b,d]furan-4-yl)-
reference. Molecular masses were determined by a Waters LCT 9H-carbazole (1.0 g, 3.0 mmol), dimethyl formamide (20 mL)
premier XE spectrometer. Elemental analyses were performed was thoroughly mixed in a 100 mL ask under nitrogen
on a Vario EL-III microanalyzer. Thermogravimetric analysis protection and then sodium hydride was added (3.6 mg, 0.15
(TGA) was performed using a PerkinElmer Pyris Diamond mmol). The mixture was stirred for 15 min. Aer that, 2-chloro-
instrument, and the thermal stability of the samples under 4,6-diphenyl-1,3,5-triazine (0.80 g, 3.0 mmol) was added at
a nitrogen atmosphere was determinedꢀb1y measuring their room temperature with vigorous stirring. The mixture was
weight loss, at a heating rate of 10 ꢁC min from 40 to 700 ꢁC. poured into water and the precipitate was collected by ltra-
The differential scanning calorimetry analysis (DSC) was ob- tion. The residue was recrystallized from ethanol and the
tained by a DSC Q2000 instrument under a nitrogen atmosphere. title compound was obtained as a light yellow solid (1.58 g,
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yield: 93.5%). 1H NMR spectra of the target compound was not
obtained for its poor solubility in common solvent. HRMS (ESI,
m/z) calculated for C39H24N4O: 565.2028, found [M]+: 565.2050.
Anal. calcd for C39H24N4O: C 82.96, H 4.28, N 9.92. Found: C
82.61, H 3.90, N 9.72.
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3-(Dibenzo[b,d]thiophen-4-yl)-9H-carbazole (5). The title
compound was synthesized according to the similar procedure
to 3, with the addition of dibenzo[b,d]thiophen-4-ylboronic acid
and the title compound was obtained as a white solid (yield:
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97.2%). H NMR (400 MHz, CDCl3) d 8.45 (d, J ¼ 1.6 Hz, 1H),
8.24–8.11 (m, 4H), 7.85–7.80 (m, 2H), 7.59 (dd, J ¼ 10.4, 6.4 Hz, 10 P. I. Shih, C. H. Chien, C. Y. Chuang, C. F. Shu, C. H. Yang,
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[M]+: 581.1796. Anal. calcd for C39H24N4S: C 80.67, H 4.17, N
9.65. Found: C 80.54, H 3.66, N 9.51.
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All devices were encapsulated in a N2 purged glove box con-
nected to the evaporator aer the fabrication for device elec-
troluminescence characterization. The ITO/glass substrate was
cleaned sequentially by detergent, de-ionized water and
ethanol. Then the ITO/glass was treated by oxygen (O2) and
polymerized uorocarbon (CFx) plasma before loading into
a 10-source evaporator, with a base pressure of 5.0 ꢂ 10ꢀ4 Pa,
for device fabrication. Organic layers were deposited on the
indium-tin-oxide (ITO)/glass substrate by thermal evaporation.
Conclusion
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13, 1937–1947.
27 C. H. Chang, M. C. Kuo, W. C. Lin, Y. T. Chen, K. T. Wong,
S. H. Chou, E. Mondal, R. C. Kwong, S. Xia, T. Nakagawa
and C. Adachi, J. Mater. Chem., 2012, 22, 3832.
28 J. S. Kang, T. R. Hong, H. J. Kim, Y. H. Son, R. Lampande,
B. Y. Kang, C. Lee, J. K. Bin, B. S. Lee, J. H. Yang,
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In summary, two novel bipolar host materials, BFTC and BTTC
are synthesized and characterized, and electroluminescence
performance of green PhOLED based on those two host mate-
rials are investigated. Eventually, BTTC demonstrates its
promising potential as a desirable host material for green
PHOLED with a high Td of 452 ꢁC. The green PhOLEDs based on
those host materials with the structure of ITO/PEDOT:PSS/
TAPC/TCTA/host: Ir(ppy)3 (8 wt%)/TPBi/LiF/Al show superior
electroluminescence performance, with the turn-on voltage of
3.4 V and maximum current, power and external quantum
efficiency of 69.3 cd Aꢀ1, 54.2 lm Wꢀ1 and 21.9%, respectively.
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7292 | RSC Adv., 2017, 7, 7287–7292
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