Phenothiazine dioxide based high triplet energy host materials for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c5ra19722h

Donor-acceptor type high triplet energy host materials were derived from a phenothiazine dioxide based electron transport moiety and a carbazole based hole transport moiety. Three phenothiazine dioxide type materials, 10-(3,5-di(9H-carbazol-9-yl)phenyl)-10H-phenothiazine 5,5-dioxide (DCzPO), 10-(3′,5′-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-10H-phenothiazine 5,5-dioxide (DCzmPPO), and 10-(3′,5′-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)-10H-phenothiazine 5,5-dioxide (DCzpPPO), could transport both holes and electrons effectively and showed high triplet energy for use in blue phosphorescent organic light-emitting diodes. Device optimization of the DCzPO, DCzmPPO, and DCzpPPO host based blue devices could realize high EQE of 24.3%.

Full text of DOI:10.1039/c5ra19722h

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Phenothiazine dioxide based high triplet energy
host materials for blue phosphorescent organic
light-emitting diodes
Cite this: RSC Adv., 2015, 5, 97903
In Ho Lee and Jun Yeob Lee*b
a
Donor–acceptor type high triplet energy host materials were derived from a phenothiazine dioxide based
electron transport moiety and a carbazole based hole transport moiety. Three phenothiazine dioxide type
0
0
materials, 10-(3,5-di(9H-carbazol-9-yl)phenyl)-10H-phenothiazine 5,5-dioxide (DCzPO), 10-(3 ,5 -di(9H-
0
0
0
carbazol-9-yl)-[1,1 -biphenyl]-3-yl)-10H-phenothiazine 5,5-dioxide (DCzmPPO), and 10-(3 ,5 -di(9H-
Received 24th September 2015
Accepted 4th November 2015
0
carbazol-9-yl)-[1,1 -biphenyl]-4-yl)-10H-phenothiazine 5,5-dioxide (DCzpPPO), could transport both
holes and electrons effectively and showed high triplet energy for use in blue phosphorescent organic
light-emitting diodes. Device optimization of the DCzPO, DCzmPPO, and DCzpPPO host based blue
devices could realize high EQE of 24.3%.
DOI: 10.1039/c5ra19722h
www.rsc.org/advances
In this work, three host materials with a novel phenothiazine
Introduction
dioxide electron transport unit, 10-(3,5-di(9H-carbazol-9-yl)
0
0
In general, host materials for blue phosphorescent organic phenyl)-10H-phenothiazine 5,5-dioxide (DCzPO), 10-(3 ,5 -
0
light-emitting diodes (PHOLEDs) have been developed to satisfy di(9H-carbazol-9-yl)-[1,1 -biphenyl]-3-yl)-10H-phenothiazine 5,5-
0
0
0
the key requirements of high triplet energy and bipolar charge dioxide (DCzmPPO), and 10-(3 ,5 -di(9H-carbazol-9-yl)-[1,1 -
carrying properties because the quantum efficiency (QE) and biphenyl]-4-yl)-10H-phenothiazine 5,5-dioxide (DCzpPPO), were
lifetime of blue PHOLEDs are dominantly determined by these prepared as the host materials for blue PHOLEDs. The role of
two requirements. In particular, the two requirements are crit- the phenothiazine dioxide, as the triplet energy keeping elec-
1
–5
ical to the QE of blue PHOLEDs.
The high triplet energy and bipolar character requirements transition energy, charge carrying performance, and device
tron transport unit, was elucidated by examining the electronic
17
of the host materials can be fullled by including both hole and characteristics of the three host materials. Experimental
electron transport units in the design of the host materials. results veried that phenothiazine dioxide performed well as
6–16
Typically, carbazole is the most popular hole transport unit
the triplet energy keeping electron transport unit in blue
and various electron transport units are merged with a carba- PHOLEDs to provide a high EQE of 24.3%.
zole unit to prepare host materials suitable for blue triplet
emitters. The electron transport units were selected from elec-
tron decient units which have electron withdrawing functional Experimental
groups with little inuence on the triplet energy of the host
0
0
9
,9 -(5-Bromo-1,3-phenylene)bis(9H-carbazole) and 9,9 -(5-(4,4,5,5-
materials. Representative electron transport units are diphe-
8
–13
14
15,16
tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)bis(9H-carbazole)
were prepared according to reported methods.
analysis of the synthesized materials followed the procedure
reported in the literature.
nylphosphine oxide, pyridine and triazine.
Although the
18,19
Instrumental
pyridine and triazine derivatives were used as high triplet
energy host materials, the heterocyclic aromatic units reduced
the triplet energy of the host materials and were rather limited
in their application as the electron transport moiety of the hosts
for blue PHOLEDs. Among the several electron transport
moieties, diphenylphosphine oxide had no effect on the triplet
energy of the backbone structure.
18
Synthesis
10H-Phenothiazine 5,5-dioxide (1). 10H-Phenothiazine (5.00
g, 25.1 mmol) was dissolved in dichloromethane with stirring
under a nitrogen atmosphere, and then glacial acetic acid and
3
0% H O were added to the reactant. The mixture was heated
a
2 2
Department of of Polymer Science and Engineering, Dankook University, 152,
under reux for 12 h and then cooled to room temperature. The
Jukjeon-ro, Suji-gu, Yongin, Gyeonggi, 448-701, Korea
b
reaction mixture was extracted with dichloromethane and the
School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-
gu, Suwon, Gyeonggi, 440-746, Korea. E-mail: leej17@skku.edu
4
organic solution was dried over anhydrous MgSO followed by
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1
solvent removal. The crude product was washed with hexane,
Yield: 2.10 g (68%), H NMR (400 MHz, DMSO): d 8.23 (d,
4H, J ¼ 4 Hz), 8.06 (s, 2H), 7.84 (m, 3H), 7.65 (d, 2H, J ¼ 4.2 Hz),
and ltration afforded the corresponding pure white crystal.
1
Yield: 3.60 g (72%). H NMR (400 MHz, DMSO): d 10.92 (S, 7.6 (d, 4H, J ¼ 4 Hz), 7.44 (t, 4H, J ¼ 7.8 Hz), 7.28 (t, 4H, J ¼
H), 7.92 (d, 2H, J ¼ 4.8 Hz), 7.65 (t, 2H, J ¼ 8.4 Hz), 7.35 (d, 2H, 7.8 Hz).
1
+
0 0 0
10-(3 ,5 -Di(9H-carbazol-9-yl)-[1,1 -biphenyl]-4-yl)-10H-pheno-
J ¼ 4.4 Hz), 7.24 (t, 2H, J ¼ 8.2 Hz). MS (FAB) m/z: 232 [(M + H) ].
1
0-(3,5-Di(9H-carbazol-9-yl)phenyl)-10H-phenothiazine-5,5- thiazine 5,5-dioxide (DCzpPPO). The product was prepared
dioxide (DCzPO). 9,9 -(5-Bromo-1,3-phenylene)bis(9H-carba- by the same synthetic method as DCzPO except that 3 was used.
0
1
zole) (1.00 g, 2.05 mmol), 10H-phenothiazine 5,5-dioxide (0.52
Yield: 1.07 g (84%), H NMR (400 MHz, DMSO): d 8.31 (d, 2H,
g, 2.26 mmol), potassium tert-butoxide (0.34 g, 3.08 mmol), and J ¼ 4.4 Hz), 8.25 (d, 6H, J ¼ 4.2 Hz), 8.05 (d, 2H, J ¼ 4.8 Hz), 7.9
palladium acetate (0.14 g, 0.62 mmol) were dissolved in toluene (s, 1H), 7.65 (d, 4H, J ¼ 4.2 Hz), 7.61 (d, 2H, J ¼ 4.2 Hz), 7.6 (t,
(
50 mL) and tri-tert-butylphosphine (3.08 mL, 3.08 mmol) was 2H, J ¼ 9.0 Hz), 7.48 (t, 4H, J ¼ 8.2 Hz), 7.34–7.29 (m, 6H, J ¼
1
3
added to the solution. The mixture was heated for 12 h and then 11.2 Hz), 6.72 (d, 2H, J ¼ 4.4 Hz). C NMR (400 MHz, CDCl ):
3
cooled to room temperature. The reaction mixture was extracted d 143.2, 140.8, 140.7, 140.3, 139.1, 132.9, 131.5, 130.2, 126.4,
with dichloromethane and the organic solution was dried over 124.9, 124.6, 123.9, 123.6, 123, 122.3, 120.7, 117.2, 109.7. MS
+
anhydrous MgSO
4
,
followed by evaporation of dichloro- (FAB) m/z: 714 [(M + H) ]. Anal. calcd for C48
H
31
N
3
O
2
S: C, 80.76;
methane. The crude product was puried by silica gel chro- H, 4.38; N, 5.89. Found: C, 80.44; H, 4.4; N, 5.85.
matography (ethyl acetate/n-hexane eluent) affording a white
solid.
Device fabrication
1
Yield: 1.00 g (77%). H NMR (400 MHz, DMSO): d 8.22 (d, 4H,
The device design adopted in the device evaluation was indium tin
oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS, 60 nm)/4,4 -cyclohexylidenebis[N,N-bis(4-methyl-
J ¼ 4.0 Hz), 8.17 (s, 1H), 8.07 (d, 2H, J ¼ 4.8 Hz), 8.0 (s, 2H), 7.75–
7
7
.68 (m, 6H, J ¼ 15.0 Hz), 7.46 (t, 4H, J ¼ 8.2 Hz), 7.38 (t, 2H, J ¼
0
13
.6 Hz), 7.29 (t, 4H, J ¼ 7.2 Hz), 7.18 (d, 2H, J ¼ 4.4 Hz). C NMR
): d 142.4, 141.7, 140.5, 140, 133.4, 126.7, 126.6,
25.5, 124.2, 124.1, 123.2, 122.8, 121.3, 120.9, 116.7, 109.5. MS
phenyl)aniline] (TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene
(
1
(
400 MHz, CDCl
3
2
(
mCP, 10 nm)/host:bis[2-(4,6-diuorophenyl)pyridinato-C ,N](pi-
+
colinato) iridium(III) (FIrpic, 25 nm, 10% doping)/diphenyl-
phosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 5 nm)/1,3,5-
tris(N-phenylbenzimidazole-2-yl)benzene (TPBI, 20 nm)/LiF (1
nm)/Al (200 nm). Hole only devices of ITO/PEDOT:PSS (60 nm)/
TAPC (20 nm)/mCP (10 nm)/host (25 nm)/TAPC (5 nm)/Al (200
nm) and electron only devices of ITO/PEDOT:PSS (60 nm)/
TSPO1 (10 nm)/host (25 nm)/TSPO1 (5 nm)/TPBi (30 nm)/LiF
FAB) m/z: 637 [(M + H) ]. Anal. calcd for C42
27 3 2
H N O S: C, 79.10;
H, 4.27; N, 6.59. Found: C, 78.96; H, 4.25; N, 6.55.
0 0 0
,9 -(3 -Bromo-[1,1 -biphenyl]-3,5-diyl)bis(9H-carbazole) (2).
,9 -(5-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenyl-
9
0
9
ene)bis(9H-carbazole) (4.30 g, 8.05 mmol), 1,3-dibromobenzene
9.50 g, 40.2 mmol), and tetrakis(triphenylphosphine)palla-
(
dium(0) (0.28 g, 0.24 mmol) were dissolved in anhydrous
tetrahydrofuran (90 mL) with stirring under a nitrogen atmo-
(1 nm)/Al (200 nm) were used as single carrier devices. The
detailed device preparation and evaluation method is the same
2 3
sphere, and then 2 M K CO solution (30 mL) was added to the
10
as that reported in the literature.
solution. The solution was heated under reux for 6 h and
0
0
cooled to room temperature. The work-up procedure for 9,9 -(3 -
0
bromo-[1,1 -biphenyl]-3,5-diyl)bis(9H-carbazole) was the same
Results and discussion
as that for DCzPO.
1
Yield: 2.30 g (53%), H NMR (400 MHz, DMSO): d 8.26 (d, The phenothiazine dioxide moiety was introduced as a triplet
H, J ¼ 3.6 Hz), 8.13 (s, 3H), 7.92 (d, 1H, J ¼ 3.8 Hz), 7.85 (s, energy keeping electron transport unit in the host materials
H), 7.63 (d, 5H, J ¼ 4.2 Hz), 7.5 (t, 5H, J ¼ 7.4 Hz), 7.32 (t, 4H, as the tetrahedral geometry of the sulfone moiety of pheno-
4
1
J ¼ 7.6 Hz).
thiazine dioxide may preserve the original high triplet energy
0
0
0
1
0-(3 ,5 -Di(9H-carbazol-9-yl)-[1,1 -biphenyl]-3-yl)-10H-phenothi- of the building unit of the host materials. Three compounds,
azine 5,5-dioxide (DCzmPPO). The product was prepared by the DCzPO, DCzmPPO, and DCzpPPO, were built based on
same synthetic method as DCzPO except that 2 was used.
a platform of a dicarbazolylphenyl hole transport unit and
1
Yield: 0.85 g (67%), H NMR (400 MHz, DMSO): d 8.29–8.23 a phenothiazine dioxide electron transport unit. Direct
m, 8H, J ¼ 12.4 Hz), 8.05 (d, 2H, J ¼ 4.8 Hz), 7.9 (t, 1H, J ¼ 8.0 coupling of the dicarbazolylphenyl unit and phenothiazine
(
Hz), 7.8 (s, 1H), 7.62–7.53 (m, 7H, J ¼ 18.8 Hz), 7.47 (t, 4H, J ¼ dioxide unit produced DCzPO, and coupling of the two units
8
.4 Hz), 7.35–7.29 (m, 6H, J ¼ 12.2 Hz), 6.76 (d, 2H, J ¼ 4.2 Hz). via a phenyl linker provided DCzmPPO and DCzpPPO. The
1
3
3
C NMR (400 MHz, CDCl ): d 143, 140.8, 140.7, 140.4, 140, 133, two units were linked through the meta-position of the phenyl
1
1
32.3, 130.6, 129.1, 128.6, 126.4, 125.3, 124.6, 123.8, 123.7, 123, (DCzmPPO) or the para-position of the phenyl (DCzpPPO).
22.3, 120.7, 117.2, 109.6. MS (FAB) m/z: 714 [(M + H) ]. Anal. The preparation procedure of the three compounds is simply
+
calcd for C H N O S: C, 80.76; H, 4.38; N, 5.89. Found: C, described in Scheme 1.
4
8
31 3 2
8
0.76; H, 4.39; N, 5.89.
Electronic calculations of the three compounds may reveal
0
0
0
9,9 -(4 -Bromo-[1,1 -biphenyl]-3,5-diyl)bis(9H-carbazole) (3). the unique role of phenothiazine dioxide as the triplet energy
The product was prepared by the same synthetic method as 2 keeping moiety and they are shown in Fig. 1. DCzPO, DCzmPPO
except that 1,4-dibromobenzene (9.49 g, 40.2 mmol) replaced and DCzpPPO showed localized HOMO distribution on the
1,3-dibromobenzene.
electron rich dicarbazolylphenyl moiety. Whereas, the LUMO
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Scheme 1 Synthetic procedure of DCzPO, DCzmPPO, and DCzpPPO.
was localized on phenothiazine dioxide in DCzPO, and the absorption peaks were observed at long wavelengths for
phenyl linker in DCzmPPO and DCzpPPO. The phenyl linker of DCzmPPO and DCzpPPO, arising from the aromatic phenyl
DCzmPPO and DCzpPPO greatly changed the LUMO distribu- linker. From this result, it can be presumed that the pheno-
tion. The LUMO localization in the phenyl linker of DCzmPPO thiazine dioxide moiety has little effect on the triplet energy of
and DCzpPPO might be due to the electron donating nitrogen the host materials because of the tetrahedral geometry of SO in
2
atom of phenothiazine dioxide in spite of the electron with- the phenothiazine dioxide moiety.
drawing nature of the SO
localized on phenothiazine dioxide, which implies that the was conducted to study the HOMO and LUMO. The oxidative
phenothiazine dioxide is also a key contributor to the LUMO. and reductive scans in Fig. 3 show the HOMO/LUMO of DCzPO,
2
unit. However, the second LUMO was
Electrochemical characterization of the three host materials
The triplet energy of the host materials was experimentally DCzmPPO and DCzpPPO as ꢀ6.17/ꢀ2.68, ꢀ6.09/ꢀ2.80, and
measured using low temperature photoluminescence (PL) ꢀ6.11/ꢀ2.82 eV, respectively. The HOMO–LUMO gap was nar-
analysis, presented in Fig. 2, in addition to ultraviolet-visible rowed in DCzmPPO and DCzpPPO by the phenyl linker. The
(UV-vis) and solution PL results of the host materials. Phos- direct connection of the dicarbazolylphenyl unit and pheno-
phorescent PL emission peak positions from the low tempera- thiazine dioxide unit widened the HOMO–LUMO gap due to
ture PL spectra were 416 (2.98 eV), 416 (2.98 eV) and 455 nm a weakened donor and acceptor character.
(2.72 eV) for DCzPO, DCzmPPO and DCzpPPO, respectively. The
Thermal characterization of DCzPO, DCzmPPO and
meta-substituted phenyl linker did not degrade the high triplet DCzpPPO was also performed to conrm the thermal stability of
energy, but the para-substituted phenyl linker decreased the the host materials because a high glass transition temperature
triplet energy. PL emission peaks and ultraviolet-visible (UV-vis) (T ) and high decomposition temperature (T ) are required for
g d
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Fig. 1 Electronic molecular orbital calculation results for DCzPO, DCzmPPO, and DCzpPPO.
Fig. 2 UV-vis, room temperature PL and low temperature PL of
DCzPO, DCzmPPO, and DCzpPPO.
Fig. 3 CV oxidation and reduction scans of DCzPO, DCzmPPO, and
DCzpPPO.
the host materials. Differential scanning calorimetry (DSC)
thermal scans of the host materials are displayed in Fig. 4. The to the high T values in all of the host materials, and the phenyl
g
inection point of the DSC thermograms suggested T
g
values of linker further strengthened the thermal stability of the host
ꢁ
161, 170, and 186 C for DCzPO, DCzmPPO and DCzpPPO, materials. The T values of the host materials, dened as the
d
g
respectively. High T values were obtained in all host materials, temperature at 5% weight loss during thermogravimetric anal-
ꢁ
substantiating that the three host materials could satisfy the ysis (TGA), were above 400 C in all of the host materials as
thermal requirements of host materials. The rigid and bulky shown in the thermograms in Fig. 5. All materials exhibited
dicarbazolylphenyl unit and the phenothiazine dioxide unit led
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Fig. 6 Current density–voltage–luminance curves of blue PHOLEDs
with DCzPO, DCzmPPO, and DCzpPPO host materials.
Fig. 4 DSC thermograms of DCzPO, DCzmPPO, and DCzpPPO.
results for the blue PHOLEDs are presented in Fig. 6. The
current density and luminance, collected by scanning the
driving voltage of the devices, were high in the DCzPO device at
low driving voltage, while they were high in the DCzmPPO
device at high driving voltage. This behavior of the current
density and luminance dependence on the host materials can
be correlated with the current density of the single carrier
devices of DCzPO, DCzmPPO and DCzpPPO in Fig. 7. The single
carrier current density data revealed that both hole and electron
current densities were high in the DCzmPPO device, while they
were low in the DCzPO device. Especially, the current density
curves of the blue PHOLEDs were well correlated with the
electron current density curves of the host materials. As pre-
sented in the single carrier device data, the electron current
density of the DCzPO device was higher than that of the other
host materials at low driving voltage, but it was lower than that
of the other host materials at high driving voltage. This trend in
the electron current density might be due to the relatively poor
electron carrying capability of DCzPO. In the case of DCzmPPO
and DCzpPPO, the LUMO is widespread over the phenyl linker
and phenothiazine dioxide, which can assist electron hopping
between molecules through the large dispersion of the LUMO.
However, the LUMO of DCzPO is locally found in the pheno-
thiazine dioxide moiety, which reduced the hopping between
molecules, resulting in a relatively poor electron transport
character.
Fig. 5 TGA thermograms of DCzPO, DCzmPPO, and DCzpPPO.
good thermal stability at high temperature, and the phenyl
linker also increased the Td values of the host materials. All
measured material parameters are presented in Table 1.
The DCzPO, DCzmPPO and DCzpPPO compounds were
characterized as host materials in blue PHOLEDs doped with
a common FIrpic emitter. Electrical and optical measurement
The EQE plot of the blue PHOLEDs according to luminance
in Fig. 8 shows that DCzmPPO is better than other host
Table 1 Measured material parameters of host materials
a
b
c
c
d
e
ꢁ
ꢁ
d
UV-vis (nm)
Solid PL (nm)
HOMO (eV)
LUMO (eV)
Bandgap (eV)
Triplet energy (eV)
T
g
( C)
T ( C)
DCzPO
DCzmPPO
DCzpPPO
219, 230, 289, 323
215, 233, 289, 334
216, 233, 289, 334
348
372
370
ꢀ6.17
ꢀ6.09
ꢀ6.11
ꢀ2.68
ꢀ2.80
ꢀ2.82
3.49
3.29
3.29
2.98
2.98
2.72
161
170
186
410
450
457
a
b
Data were measured in THF solution. Solid PL were measured from peak wavelength of the 1 wt% doped lm emission spectra in polystyrene on
c
d
e
quartz. HOMO and LUMO were measured using CV. Bandgaps were calculated from the difference of HOMO and LUMO. Triplet energy was
calculated from the rst phosphorescent emission peak of the low temperature PL spectra.
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Fig.
9
PL intensity of non-doped and FIrpic doped DCzPO,
DCzmPPO, and DCzpPPO films.
DCzmPPO device can be explained by the high triplet energy of
DCzmPPO and the balanced carrier density. Comparing
DCzmPPO and DCzpPPO, the relatively low triplet energy of
DCzpPPO led to incomplete energy transfer from DCzpPPO to
FIrpic as substantiated by the decreased PL intensity of the
DCzpPPO:FIrpic lm in Fig. 9. The comparison of DCzmPPO
and DCzPO substantiated that the poor electron carrying ability
of DCzPO reduced the electron density in the emitting layer and
disrupted the carrier balance in the device. Therefore, both the
high triplet energy and balanced charge carrying properties of
DCzmPPO were key factors for the high EQE of the DCzmPPO
device.
Fig. 7 Current density–voltage curves of hole only and electron only
devices of DCzPO, DCzmPPO, and DCzpPPO.
Conclusions
In conclusion, phenothiazine dioxide derived compounds,
DCzPO, DCzmPPO and DCzpPPO, were developed as high
triplet energy host materials for blue PHOLEDs and demon-
strated a high triplet energy and high EQE in the blue devices.
DCzmPPO worked better than the other compounds as the host
material of FIrpic and showed a high EQE of 24.3%. Therefore,
phenothiazine dioxide derivatives are promising as host mate-
rials for blue PHOLEDs with better EQE.
Acknowledgements
This work was supported by development of high performance
and long term stable deep blue phosphorescence OLED mate-
rials funded by the MOTIE.
Fig. 8 External quantum efficiency–luminance curves of blue PHO-
LEDs with DCzPO, DCzmPPO, and DCzpPPO.
References
1
2
Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943.
A. Chaskar, H. Chen and K. Wong, Adv. Mater., 2011, 23,
materials at a reaching high EQE. The maximum EQE of the
DCzmPPO device was 24.3%, which was much higher than the
3876.
16.9% for DCzPO and 9.4% for DCzpPPO. The high EQE of the
3
K. S. Yook and J. Y. Lee, Adv. Mater., 2012, 24, 3169.
97908 | RSC Adv., 2015, 5, 97903–97909
This journal is © The Royal Society of Chemistry 2015

View Article Online
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4
5
6
L. Duan, J. Qiao, Y. Sun and Y. Qiu, Adv. Mater., 2011, 23, 12 J. Ding, Q. Wang, L. Zhao, D. Ma, L. Wang, X. Jing and
137. F. Wang, J. Mater. Chem., 2010, 20, 8126.
L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido, 13 L. S. Sapochak, A. B. Padmaperuma, X. Cai, J. L. Male and
Adv. Mater., 2011, 23, 926. P. E. Burrows, J. Phys. Chem. C, 2008, 112, 7989.
R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, 14 E. L. Williams, K. Haavisto, J. Li and G. E. Jabbour, Adv.
1
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
003, 82, 2422.
S. Tokito, T. Iijima, Y. Suzuri, H. Kita and T. Tsuzuki, Appl.
Phys. Lett., 2003, 83, 569.
Mater., 2007, 19, 197.
2
15 H. Inomata, K. Goushi, T. Masuko, T. Konno, T. Imai,
H. Sasabe, J. J. Brown and C. Adachi, Chem. Mater., 2004,
16, 1285.
7
8
9
S. O. Jeon and J. Y. Lee, J. Mater. Chem., 2012, 22, 4233.
H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
16 M. Kim, S. K. Jeon, S.-H. Hwang and J. Y. Lee, Phys. Chem.
Chem. Phys., 2015, 17, 13553.
10 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Funct. 17 Y. Li, X. Li, X. Cai, D. Chen, X. Liu, G. Xie, Z. Wang, Y. Wu,
Mater., 2009, 19, 3644.
1 K. S. Yook, S. E. Jang, S. O. Jeon and J. Y. Lee, Adv. Mater.,
C. Lo, A. Lien, J. Peng, Y. Cao and S. Su, J. Mater. Chem. C,
2015, 3, 6986.
1
2010, 22, 4479.
18 Y. J. Cho and J. Y. Lee, Adv. Mater., 2011, 23, 4568.
19 Y. J. Cho and J. Y. Lee, Adv. Mater., 2014, 26, 4050.
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