Systematic study of TCTA -based star-shaped host materials by optimizing ratio of carbazole/ diphenylphosphine oxide: Achieving both low efficiency roll-off and turn-on voltage for blue PHOLEDs

Article abstract of DOI:10.1039/c4tc00951g

A series of star-shaped host materials DCzPOTA, CzDPOTA and TCTA containing a central triphenylamine (TPA) core, a peripheral modification with different ratios of p-type carbazole units and n-type diphenylphosphine oxide (DPPO) units were purposefully de

Full text of DOI:10.1039/c4tc00951g

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Materials Chemistry C
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Systematic study of TCTA-based star-shaped host
materials by optimizing ratio of carbazole/
diphenylphosphine oxide: achieving both low
efficiency roll-off and turn-on voltage for blue
PHOLEDs
Cite this: J. Mater. Chem. C, 2014, 2,
7428
Biao Pan,a Hong Huang,ab Xiao Yang,a Jiangjiang Jin,a Shaoqing Zhuang,a
a
a
*
Guangyuan Mu and Lei Wang
A series of star-shaped host materials DCzPOTA, CzDPOTA and TCTA containing a central triphenylamine
(TPA) core, a peripheral modification with different ratios of p-type carbazole units and n-type
diphenylphosphine oxide (DPPO) units were purposefully designed and synthesized. Their electronic
nature, electrochemical, thermal and morphological properties were investigated systematically. In
suitable proportions of carbazole and phosphine oxide, blue PhOLEDs based on DCzPOTA exhibits a
maximum power efficiency of 36.9 lm Wꢀ1 and an ideal turn-on voltage (2.7 V at 1 cd mꢀ2). These
results indicate that the electronic nature of the host material can be finely adjusted by gradually
changing the donor–acceptor ratio.
Received 8th May 2014
Accepted 16th July 2014
DOI: 10.1039/c4tc00951g
www.rsc.org/MaterialsC
transporting properties are required to increase the balance of
the carriers and to broaden the excitation recombination zone,
1 Introduction
which is of paramount importance to maintain the charge
balance in the emissive layer so as to improve the device effi-
ciency accompanied by a low roll-off.17–21 To fulll these two
basic requirements, we came up with an appealing strategy,
including the following two aspects: Firstly, choosing a high
triplet energy moiety as the core and appropriate electron donor
(D) and accepter (A) moieties as peripheral groups; Secondly,
constructing new materials by altering the proportion of p-type
and n-type moieties so as to nd the ideal molecular structures.
As is well known, TCTA has been widely used as a host
material for blue, orange, and white OLEDs because of its high
morphological stability (Tg ¼ 152 ꢁC) and reasonable ET level
(ET ¼ 2.85 eV).22–24 However, the performances of blue PhOLEDs
were relatively poor and suffered from a high efficiency roll-off
at high current density, which was presumably due to the fact
that TCTA only behaved as a hole-transporting type host mate-
rial with a low capability to transport electrons.25 To further
optimize the compound TCTA as an appropriate host for the
blue PhOLEDs, J. H. Kwon et al. have proposed a series of
modied-TCTA materials with electron-decient N-a-carboline
moieties and showed a very high external quantum efficiency of
25.7%, low onset and driving voltages of 2.47 V.26 In 2012, our
group has reported a new bipolar host material TCTAPO by
introducing an electron decient DPPO group into the TCTA
Since the organic light emitting diodes (OLEDs) were rst
commercially successful in global markets, phosphorescent
organic light-emitting diodes (PHOLEDs) have shown great
application potential in digital terminal displays and planar
solid-state lighting.1–6 However, even as OLEDs sales currently
exceed $1 B per year and grow exponentially, there are chal-
lenges in the science and technology of OLEDs that need to be
addressed, notably in improving the efficiency of the devices
and their stability at high brightness (>1000 cd mꢀ2).7,8 In order
to achieve this goal, one of the most critical contributions is the
materials development, along with innovations and optimiza-
tions in the device structure and processing. Therefore, there
have been continual studies to improve the blue PhOLEDs
device performances by synthesizing host and dopant
materials.9–15
To be an effective host for the blue phosphor suitable for
vacuum deposition technology, the triplet energy (ET) and
balanced carrier injecting/transporting property must be
considered together. The ET of the host is required to be higher
than 2.70 eV in order to prevent the back energy transfer from
the phosphor to the host.16 On the other hand, bipolar charge-
aWuhan National Laboratory for Optoelectronics, School of Optical and Electronic
Information, Huazhong University of Science and Technology, Wuhan, 430074, P.R.
China. E-mail: wanglei@mail.hust.edu.cn; Tel: +86-27-87793032
bTCL Corporate Research, Nanshan District, Shenzhen, Guangdong, 518052, P.R.
China
molecule and the maximum power efficiency of the blue device
based on TCTAPO was achieved at 40.7 lm Wꢀ1
.
The results
27
show that the DPPO moiety can reduce the lowest unoccupied
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molecular orbital (LUMO) level and increase the electron- intersection of two tangents drawn at the rising and back-
injection property without changing the ET of TCTA. Moreover, ground current of the cyclic voltammogram. Experimental
it also gives us the enlightenment that we can readily control the conditions and the equipment used have been described in our
carrier injecting/transporting ability of TCTA through changing previous works.32
the ratio of carbazole and DPPO. In fact, many research groups
are concentrating on controllable tuning of the ratio between p- 2.2 Computational details
type and n-type units in host materials because a minute change
of mixing ratio of D–A moieties in host materials could change
the resultant device characteristics.28–31 Nevertheless, up to now,
The geometrical and electronic properties were analyzed with
the Amsterdam Density Functional (ADF) 2009.01 program
package. The calculation was optimized by means of the B3LYP
a systematic study elucidating the structure–function relation-
(Becke three parameters hybrid functional with Lee–Yang–Per-
ship of the number and ratio of DPPO and carbazole moieties in
dew correlation functionals)33,34 with the 6-31G(d) atomic
TCTA has not been carried out.
basis set. Then, the electronic structures were calculated at
s-HCTHhyb/6-311++G(d, p) level.35 Molecular orbitals were
visualized using ADF view.
In this article, star-shaped TPA-cored and peripheral DPPO/
carbazole bipolar hosts, (4-(bis(4-(9H-carbazol-9-yl)phenyl)-
amino)phenyl) diphenylphosphine oxide DCzPOTA and (((4-(9H-
carbazol-9-yl)phenyl)azanediyl)bis(1,4-phenylene))bis(diphenyl-
phosphine oxide) CzDPOTA were conveniently synthesized, and
the effect of the different ratio between carbazole and DPPO on
their thermal, photo physical, electro-chemical, bipolar trans-
porting and device properties were investigated in detail. The
two compounds possessed the same kinds of substituted units
and showed a topological structure. As expected, the disrupted
conjugation via phosphine oxide linkage can be well preserved
2.3 Device fabrication and measurement
The EL devices were fabricated by vacuum deposition of the
materials at a base pressure of 5 ꢂ 10ꢀ6 Torr onto glass pre-
coated with a layer of indium tin oxide (ITO) with a sheet
resistance of 20 U per square and a transmissivity of 80% in
blue light. Before the deposition of an organic layer, the clear
ITO substrates were treated with oxygen plasma for 5 min. Tꢀh1e
˚
deposition rate of the organic compounds was 0.9–1.1 A s
.
to ensure their high triplet energies, and the charge balance in
the emitting layer could be tuned by adjusting the ratio between
carbazole and DPPO units.
Finally, a cathode composed of LiF (1 nm) and aluminum
(100 nm) was sequentially deposited onto the substrate under
vacuum (10ꢀ5 Torr). The L–V–J of the devices was measured with
a Keithley 2400 Source meter and PR655. All measurements
were carried out at room temperature under ambient condi-
tions. Experimental conditions and the equipment used have
been described in our previous works.36
2 Experimental section
2.1 General information
All solvents and materials were used as received from
commercial suppliers without further purication. 1H NMR and
13C NMR spectra were measured on a Bruker-AF301 AT 400 MHz
spectrometer. Elemental analyses of carbon, hydrogen, and
nitrogen were performed on an Elementar (Vario Micro cube) amine (2.5 g, 10.0 mmol) in DMF (25 ml) at 0 C, NBS (5.2 g,
analyzer. Mass spectra were carried out on an Agilent (1100 LC/ 32.0 mmol) in DMF (10 ml) was added drop-wise. The mixture
MSD Trap) instrument using APCI ionization. UV-Vis absorp- was stirred for 4 h at room temperature. Then, water was added
tion spectra were recorded on a Shimadzu UV-VIS-NIR Spec- to the mixture to produce a white precipitate. Aer ltration and
trophotometer (UV-3600). PL spectra were recorded on drying, the obtained white solid was recrystallized from petro-
Edinburgh instruments FLSP920 spectrometer. Differential leum to afford tris(4-bromophenyl) amine (1) with a yield of
scanning calorimetry (DSC) was performed on a PE Instruments 90% (4.3 g). 1H-NMR (CDCl3, 400 MHz): d (ppm) 7.36–7.34
DSC 2920 unit at a heating rate of 10 ꢁC minꢀ1 from 30 to 250 ꢁC (m, 6H), 6.93–6.91 (m, 6H). MS (APCI): calcd for C18H12Br3N:
under nitrogen. The glass transition temperature (Tg) was 481.9, found, 482.9 (M + 1)+.
2.4 Synthesis
Tris(4-bromophenyl)amine (1). To a solution of triphenyl-
ꢁ
determined from the second heating scan. Thermogravimetric
N,N-Bis(4-(9H-carbazol-9-yl)phenyl)-4-bromoaniline (2). A
analysis (TGA) was undertaken with a PerkinElmer Instruments mixture of tris(4-bromophenyl)amine (1) (0.48 g, 1.0 mmol),
(Pyris1 TGA) analyzer. The thermal stability of the samples, carbazole (0.33 g, 2.0 mmol), CuI (60.0 mg, 0.3 mmol), 18-
under a nitrogen atmosphere, was determined by measuring crown-6 (80.0 mg, 0.3 mmol), K2CO3 (1.38 g, 10 mmol), and
their weight loss while heating at a rate of 10 ꢁC minꢀ1 from 30 DMPU (3.0 ml) was reuxed under nitrogen for 48 h. Aer
to 700 ꢁC. Cyclic voltammetry measurements were carried out in cooling, the mixture was extracted with CH2Cl2 and washed
a conventional three electrode cell using a Pt button working with a dilute HCl solution, and then the organic layer was dried
electrode of 2 mm in diameter, a platinum wire counter elec- over anhydrous Na2SO4. Aer removal of the solvent, the
trode, and an Ag/AgNO3 (0.1 M) reference electrode on a residue was puried by column chromatography on silica gel
computer-controlled EG&G Potentiostat/Galvanostat model 283 using CH2Cl2 as the eluent to obtain a white powder. Yield: 38%
at room temperature. Reduction CVs of all compounds were (0.25 g). 1H-NMR (CDCl3, 400 MHz): d (ppm) 8.17–8.15 (d, J ¼ 8.0
performed in dichloromethane containing 0.1 M tetrabuty- Hz, 4H), 7.52–7.38 (m, 18H), 7.32–7.28 (m, 4H), 7.22–7.19
lammoniumhexauorophosphate (Bu4NPF6) as the supporting (m, 2H). MS (APCI): calcd for C42H28BrN3: 653.2, found, 654.4
electrolyte. The onset potential was determined from the (M + 1)+.
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(CDCl3, 100 MHz): d (ppm) 149.92, 144.89, 140.74, 134.48,
133.71, 133.61, 133.07, 132.15, 132.05, 128.62, 128.50, 128.34,
127.28, 126.00, 123.44, 123.11, 120.38, 120.11, 109.70. 31P-NMR
(CDCl3, 400 MHz): d (ppm) 29.26. MS (APCI): calcd for
C
C
N-(4-(9H-Carbazol-9-yl)phenyl)-4-bromo-N-(4-bromophenyl)-
aniline (3). Compound 3 was prepared according to the same
procedure as compound 2. Yield: 45%. 1H-NMR (CDCl3, 400
MHz): d (ppm) 8.16–8.14 (d, J ¼ 7.6 Hz, 2H), 7.45–7.41 (m, 10H),
7.29–7.27 (m, 3H), 7.26–7.24 (m, 1H), 7.07–7.05 (m, 4H). MS
(APCI): calcd for C30H20Br2N2: 568.3, found, 569.2 (M + 1)+.
(4-(Bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)diphenyl-
phosphine oxide (DCzPOTA). To a mixture of NiCl2$6H2O (0.07
g, 0.3 mmol), zinc (0.4 g, 6.0 mmol), 2,20-bipyridine (bpy) (0.09 g,
0.6 mmol) and N,N-bis(4-(9H-carbazol-9-yl)phenyl)-4-bromoa-
niline (2) (0.65 g, 1.0 mmol) in DMAc (30.0 ml) solution,
biphenylphosphine oxide (0.4 g, 2.0 mmol) was added. The
reaction mixture was stirred with a stir bar at 110 ꢁC for 24
hours. Aer that, the mixture was allowed to cool to room
temperature and CH2Cl2 and water were added. The organic
layer was isolated, and the remaining aqueous phase was
further extracted with CH2Cl2. Then, the organic phases were
combined and dried with anhydrous MgSO4, puried by silica
gel column chromatography using dichloromethane–methanol
as the eluent to afford the corresponding cross-coupled
product. Yield: 54%. 1H-NMR (CDCl3, 400 MHz): d (ppm) 8.16–
8.14 (d, J ¼ 8.4 Hz, 4H), 7.78–7.73 (m, 4H), 7.62–7.54 (m, 8H),
7.52–7.43 (m, 16H), 7.31–7.30 (m, 6H). 13C-NMR (CDCl3, 100
MHz): d (ppm) 150.67, 145.49, 140.85, 133.84, 133.68, 133.57,
133.27, 132.18, 132.09, 131.93, 131.91, 128.59, 128.47, 128.31,
126.56, 125.98, 123.41, 121.64, 121.52, 120.38, 120.05, 109.75.
31P-NMR (CDCl3, 400 MHz): d (ppm) 29.27. MS (APCI): calcd for
54H40N2O2P2: 810.3, found, 811.2 (M + 1)+. Anal. calcd for
54H40N2O2P2 (%): C, 79.99; H, 4.97; N, 3.45; found: C, 80.34; H,
5.15; N, 3.42.
3 Results and discussion
3.1 Synthesis and characterization
The bipolar molecules were synthesized by the Ullmann and
Ni(II)/Zn catalyzed phosphine oxide coupling reaction in two
steps. The synthetic routes and chemical structures of
compounds, DCzPOTA, CzDPOTA and TCTA, are depicted in
Scheme 1. The intermediate tris(4-bromophenyl)amine (1) was
synthesized according to the literature.37 The compounds N,N-
bis(4-(9H-carbazol-9-yl)phenyl)-4-bromoaniline (2) and N-(4-(9H-
carbazol-9-yl)phenyl)-4-bromo-N-(4-bromophenyl)aniline (3) were
synthesized with the typical Ullmann coupling reaction of tris(4-
bromophenyl)amine (1) and carbazole in the presence of CuI, 18-
crown-6 and K2CO3. The nal products DCzPOTA and CzDPOTA
were synthesized by the Ni(II)/Zn catalyst phosphine oxide
coupling reaction at the temperature of 110 ꢁC, in yields of 54%
and 45%, respectively. All the developed host materials were
puried by silica gel chromatography and by repeated thermal
gradient vacuum sublimation before characterization and device
fabrication. The chemical structures of the compounds DCzPOTA
and CzDPOTA were fully characterized by 1H NMR, 31P NMR and
13C NMR spectroscopy, mass spectrometry and elemental anal-
ysis. All of the physical properties data were summarized in Table
1 and the data of TCTA were quoted from our previous work.27
C
C
54H38N3OP: 775.3, found, 776.5 (M + 1)+. Anal. calcd for
54H38N3OP (%): C, 83.59; H, 4.94; N, 5.42; O, 2.06; found: C,
83.46; H, 5.03; N, 5.46.
(((4-(9H-Carbazol-9-yl)phenyl)azanediyl)bis(1,4-phenylene))-
bis(diphenylphosphine oxide) (CzDPOTA). The compound
CzDPOTA was prepared according to the same procedure as the
compound DCzPOTA. Yield: 45%. 1H-NMR (CDCl3, 400 MHz): d
(ppm) 8.15–8.13 (d, J ¼ 7.6 Hz, 2H), 7.74–7.71 (m, 9H), 7.70–7.42
(m, 25H), 7.36–7.34 (m, 3H), 7.29–7.22 (m, 1H). 13C-NMR
3.2 Thermal properties
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out to examine the thermal
Scheme 1 Synthesis routes of the compounds DCzPOTA, CzDPOTA and TCTA.
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Table 1 Physical data of compounds DCzPOTA, CzDPOTA and TCTA
HOMO/LUMOc (eV)
HOMO/LUMOd (eV)
ETe (eV)
Tg/Td (ꢁC)
a
f
Compound
labs,maxa (nm)
lem,max (nm)
Egb (eV)
DCzPOTA
CzDPOTA
TCTA
328/294
333/294
326/293
432
417
386
3.39
3.41
3.46
ꢀ5.44
ꢀ5.53
ꢀ5.09
ꢀ2.05
ꢀ2.12
ꢀ1.63
ꢀ5.21
ꢀ5.33
ꢀ4.99
ꢀ2.12
ꢀ2.24
ꢀ2.33
2.84
2.86
2.85
148/471
122/476
152/489
Measured in CH2Cl2. b Determined from the onset of oxidation potentials and the Eg ¼ HOMO–LUMO. c Measured with CV. d Values from DFT
a
calculation. e Measured in 2-methyltetrahydrofuran at 77 K. f Obtained from DSC measurements and TGA measurements.
properties of these new compounds. DCzPOTA shows a high 294 and 328 or 333 nm could be assigned to n–p*, p–p* tran-
ꢁ
glass transition temperature (Tg) of 148 C in the DSC heating sitions of TPA and carbazole, respectively. The absorption edges
cycle, which is 26 ꢁC higher than that of CzDPOTA (122 ꢁC), of the UV/vis spectra of DCzPOTA and CzDPOTA are about 366
indicating that their Tg is inuenced by the change of the ratio and 364 nm, respectively, which corresponds to the band gaps
between the carbazole and phosphine oxide units. The Tg of at 3.39 and 3.41 eV, respectively. The PL emission peak of
these three compounds increased in the order CzDPOTA < DCzPOTA and CzDPOTA in dichloromethane was observed at
DCzPOTA < TCTA with the presence of more carbazoles in the 432 and 417 nm, respectively. Compared to TCTA (386 nm),
peripheral sides of the TPA core, which can be attributed to the both of them exhibited an obvious redshi, which could be
increased rigidity of carbazole compared to the DPPO unit. High ascribed to the strong electron-withdrawing capability of the
Tg values are prerequisites for their applications in PhOLEDs. DPPO unit. The three compounds have almost the same ET
We also measured the decomposition temperatures (Td) of ranging from 2.84 to 2.86 eV, which was determined from the
DCzPOTA and CzDPOTA, determined from the onset points of highest energy vibronic sub-band of the phosphorescence
5% weight loss, which were found to be 471 ꢁC and 476 ꢁC, spectrum in 2-methyltetrahydrofuran at 77 K, presented in
respectively (see Fig. 1). Compared with TCTA (489 ꢁC), it further Fig. 2(b). The DPPO does not extend the conjugation of the core
conrmed that increasing the number of carbazole groups was because of its tetrahedral structure, allowing it to affect only
benecial to improve the thermodynamic stability. The excel- charge transport properties without degrading the ET of the TPA
lent thermal properties indicate that the morphology of the core; therefore, the incorporation of DPPO has no inuence on
materials is not easily changed at the high temperatures the ET of TCTA. In other words, by breaking the conjugation
generated by the operation of OLED devices, and it is closely between carbazole and DPPO through the sp3-N of the TPA core,
correlated with the lifetime of the device, suggesting that a high we successfully obtain two bipolar hosts for the blue phospho-
stability of the glassy state is clearly demanded in order to rescent that meets the requirement of high triplet energy while
prevent the degradation of devices in applications.38
simultaneously conserving its thermal stability.
3.4 Electrochemical properties
3.3 Photophysical properties
The electrochemical properties of these compounds were
analyzed by cyclic voltammetry (CV) (Fig. 3). The compounds
exhibit reversible oxidation behaviors, which can be assigned to
the oxidation of the carbazole and TPA moieties. The highest
occupied molecular orbital (HOMO) determined from the onsets
of the oxidation potentials by comparing with ferrocene (Fc) is
ꢀ5.44–5.53 eV (HOMO ¼ ꢀ(Eox + 4.8) eV). The lowest unoccupied
molecular orbital (LUMO) energy levels were calculated from the
HOMO values and the energy gaps (Eg), which were in the range
of ꢀ2.05 to 2.12 eV. The HOMO levels of these compounds are
adjacent to those of the widely used hole transport material NPB
(ꢀ5.4 eV),39 which indicates that the hole-injection barriers from
NPB to the compounds is small. The LUMO level of DCzPOTA
(ꢀ2.05 eV) is slightly higher than that of CzDPOTA (ꢀ2.12 eV),
but is signicantly different than the TCTA value (ꢀ1.63 eV),
which could be attributed to the electron-withdrawing nature
and increasing numbers of DPPO units.
The UV absorption and PL spectra of DCzPOTA and CzDPOTA
in CH2Cl2 are shown in Fig. 2(a). The two compounds exhibit a
similar absorption spectra shape, and the absorption peaks at
3.5 Quantum chemical calculations
Fig. 1 (a) TGA thermo-grams of the compounds DCzPOTA and
CzDPOTA recorded at a heating rate of 10 ꢁC minꢀ1; Inset: DSC traces
of the compounds DCzPOTA and CzDPOTA recorded at a heating rate
Theoretical calculations on the electronic nature of these
compounds were carried out at the DFT//B3LYP/6-31G level in
the Amsterdam Density Functional (ADF) 2009.01 program to
of 10 ꢁC minꢀ1
.
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Fig. 2 (a) Absorption and PL-emission spectra of TCTA, DCzPOTA and CzDPOTA in CH2Cl2; (b) The phosphorescence spectra of DCzPOTA and
CzDPOTA in a frozen 2-methyltetrahydrofuran matrix at 77 K.
Fig. 3 CV curves of DCzPOTA and CzDPOTA. Working electrode: Pt
button; reference electrode: Ag/Ag+. The oxidation CV was performed
in dichloromethane containing 0.1 M n-Bu4NPF6 as supporting elec-
trolyte at a scan rate of 100 mV sꢀ1
.
acquire a better understanding of bipolar properties. As shown
in Fig. 4, the HOMO orbitals for CzDPOTA and DCzPOTA are
mainly located on the electron-donating carbazole moiety or
TPA moiety, while the LUMO orbitals are mainly dispersed in
the electron-accepting DPPO unit and the benzenes linked to it
directly. The separated HOMO and LUMO distribution of
CzDPOTA and DCzPOTA are largely different from the over-
lapped distribution for TCTA, indicating that the donating or
withdrawing properties of the moieties have great inuence on
the distribution of the frontier energy levels. These newly
designed compounds have almost complete separation of the
HOMO and LUMO orbitals at the carbazole and DPPO moieties,
respectively, which can be rationalized by the disruption of the
p-conjugation between the electron-donor and electron-
acceptor moieties because of the special conjugated congura-
tion of the DPPO group. The separated frontier molecular
orbitals of CzDPOTA and DCzPOTA indicate their potential
ambipolar characteristics, which is preferable for balanced
hole- and electron-transporting properties.2
Fig. 4 The molecular-orbital surfaces of HOMO and LUMO of the
compounds TCTA, DCzPOTA and CzDPOTA.
only device had the following structure: ITO/NPB (10 nm)/host
(30 nm)/NPB (10 nm)/Al (100 nm) and the electron-only device
had the structure of ITO/BCP (30 nm)/host (30 nm)/BCP
(30 nm)/LiF (1 nm)/Al (100 nm). It can be assumed that only
single carriers are injected and transported in the devices due to
the fact that the LUMO of the NPB is low and the HOMO of the
BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) is high
enough to block the electron and hole injection, respectively.40
As shown in Fig. 5, the remarkable current densities are obvi-
ously proving the bipolar properties of the two compounds. The
current density of the CzDPOTA based electron-only device is
higher than that of the DCzPOTA at the same voltages, which
indicates that the number of DPPO groups and the electron-
3.6 Carrier-transport properties
The charge-carrier properties of the two compounds were injecting/transporting ability are proportional. In stark contrast
investigated in hole-only and electron-only devices. The hole- to the electron-only device, the current density of the DCzPOTA
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based hole-only device is higher than that of the CzDPOTA,
indicating that the number of carbazole groups and the hole-
injecting/transporting ability are proportional. It could be
ascertained that tuning the ratio of the carbazole and DPPO
could authentically modulate carrier-transport properties.
3.7 Electroluminescent devices
To assess which of DCzPOTA and CzDPOTA will give the better
balanced performance as bipolar host materials in blue PhO-
LEDs, FIrpic-doped devices were fabricated with a simple
conguration of ITO/MoO3 (10 nm)/NPB (40 nm)/mCP (5 nm)/
host: 6 wt% FIrpic (20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al
(150 nm), (host: CzDPOTA for device A; DCzPOTA for device B;
TCTA for device C). For comparison, the control device C was
also prepared. NPB is the hole-transporting layer and mCP is
the exciton blocking layer. FIrpic-doped host was used as the
emitting layer, the best electroluminescence (EL) performance
was achieved with 6 wt% FIrpic for all the hosts. TmPyPB served
as both electron-transporting layer and hole-blocking layer.
MoO3 and LiF (lithium uoride) served as the hole- and elec-
tron-injecting layers, respectively. The schematic energy level
diagrams of the three devices are shown in Fig. 6.
Fig. 5 The current density versus voltage curves of the hole-only and
electron-only devices for the compounds DCzPOTA and CzDPOTA.
Fig. 7 shows the current density–voltage–brightness (J–V–L)
characteristics, efficiency versus current density curves for the
devices and all of the EL spectra of the devices. The EL data of
the devices are summarized in Table 2. The highest efficiency
was achieved for device B based on DCzPOTA, giving a
maximum external quantum efficiency (EQE) of 16.9% and a
Fig. 6 Energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this work.
Fig. 7 . (a) Current density–voltage–brightness (J–V–L) characteristics for devices A–C; (b) external quantum efficiency and power efficiency
versus current density curves for devices A–C; (c) the EL spectra of the devices A–C.
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Table 2 Electroluminescence properties of the devices A–C
Lmax [cd mꢀ2
(V at Lmax, V)
]
hcb [cd Aꢀ1
]
hpb [lm Wꢀ1
]
EQEb [%]
EQEc [%]
CIE [x, y]d
a
Device
Host
Von (v)
A
B
C
CzDPOTA
DCzPOTA
TCTA
3.10
2.70
3.20
7287(10.8)
19 910(10.5)
5560(8.0)
27.4
33.0
29.4
24.1
36.9
24.4
14.1
16.9
14.5
11.9
14.8
12.4
(0.14, 0.32)
(0.14, 0.32)
(0.15, 0.31)
a
Abbreviations: Von: turn-on voltage at 1 cd mꢀ2; Lmax: maximum luminance; V: voltage at the maximum brightness; hc: current efficiency; hp: power
efficiency; CIE [x, y]: commission international de I'Eclairage coordinates. b Maximum efficiency. c EQE at brightness 1000 cd mꢀ2
.
Measured at 8 V.
d
maximum power efficiency (hP) of 36.9 lm Wꢀ1. On the other DCzPOTA as the host exhibited a maximum power efficiency of
hand, obvious lower efficiencies of 14.1% and 24.1 lm Wꢀ1 were 36.9 lm Wꢀ1 and an ideal turn-on voltage (2.7 V at 1.91 cd mꢀ2).
obtained at 82.0 cd mꢀ2 for device A based on CzDPOTA. Device Our ndings provided a valuable strategy for readily controlling
C exhibits a hP of 24.0 lm Wꢀ1 and an EQE of 14.5%, and its the carrier injecting/transporting ability of star-shaped mole-
turn-on voltage (Von) is 3.2 V. The hP of the device B was cules through changing the number, ratio, and type of func-
increased with about 53.8% compared to device C, which can be tional groups and molecular congurations.
attributed to the improved charge balance by the bipolar nature
of DCzPOTA. Nevertheless, the efficiency of device A is equal to
device C, and the maximum value was reached at higher
Acknowledgements
brightness, which could be caused by the unbalanced charge
and larger hole injection barrier between CzDPOTA and NPB.
From the perspective of Von, it is obvious that device C has a
higher turn-on voltage compared to devices A and B, which can
be ascribed to the bipolar transporting property and the suit-
able HOMO and LUMO energy levels of the two compounds. It is
worth noting that device B has a low turn-on voltage of 2.7 V at a
brightness of 1.91 cd mꢀ2. Considering that the ET of FIrpic is
about 2.65 eV, the turn-on voltage value of device B has already
reached the limit of the FIrpic-based blue PhOLEDs. With
regard to efficiency roll-off, device A and B also have excellent
performances: the EQE of device B remains 16.9%, and 14.8% at
100 and 1000 cd mꢀ2, respectively. Compared with TCTA, the
EQE efficiency roll-off of the device based on DCzPOTA is lower.
The slightly reduced efficiency for the devices based on
DCzPOTA should also be attributed to the reduced carrier
balance recombined in the EML. Finally, EL spectra of the blue
phosphorescent OLEDs based on CzDPOTA and DCzPOTA peak
at 473 nm with a shoulder at 500 nm, which are identical to
those reported previously, indicating an effective exothermic
energy transfer from host to guest FIrpic. From the blue device
results, we can conclude that a better charge balance shall be
attained in DPPO-substituted TCTA while the ratio of DPPO
with carbazole is 1 : 2.
This research work was supported by the NSFC/China
(21161160442, 51203056), the National Basic Research Program
of China (973 Program 2013CB922104) and the Analytical and
Testing Centre at Huazhong University of Science and
Technology.
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