Spiro-annulated triarylamine-based hosts incorporating dibenzothiophene for highly efficient single-emitting layer white phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c3tc31336k

Four novel host materials were designed and synthesized, incorporating fluorene-spiro-annulated triphenylamine-carbazole (ST-SCz) and dibenzothiophene (DBT) blocks. A meta-linking strategy was applied by introducing DBT moieties to ST-SCz skeletons at the C3 position of the fluorene backbones. As expected, high triplet energies (over 2.80 eV) were achieved in all four materials despite the different linking positions on the DBT. All four materials show a high T_g from 149 to 163 °C, which benefits from their spiro-structure. Their thermal, electrochemical and photo-physical properties were fully characterized. Highly efficient blue and white PHOLEDs were fabricated using these four materials as the hosts. Triphenylamine-containing STDBT4 and STDBT2 demonstrate better device performance due to their relatively high-lying HOMO compared to carbazole-containing SCzDBT4 and SCzDBT2. Maximum η_ext of 19.6% and 18.4% (STDBT4 and STDBT2) were achieved for FIrpic-based devices, and 23.7% and 22.2% (STDBT4 and STDBT2) were achieved for two color-based white PHOLEDs with double emitting layers, using PO-01 as a yellow dopant. Finally, a highly efficient single-emitting layer white PHOLED with maximum efficiencies of 24.0%, 77.0 cd A^-1 and 63.2 lm W ^-1 and CIE coordinates of (0.38, 0.48) was realized using the device configuration of ITO/HAT-CN/TAPC/STDBT4: 8 wt% FIrpic: 0.8 wt% PO-01/TmPyPB/Liq/Al. The device shows good color stability and low efficiency roll-off. Even at a high luminance of 10,000 cd m^-2, it still maintains very high efficiencies of 17.9%, 56.4 cd A^-1 and 26 lm W^-1.

Full text of DOI:10.1039/c3tc31336k

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Journal of Materials Chemistry C
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DOI: 10.1039/C3TC31336K
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ARTICLE TYPE
Spiro-Annulated Triarylamine Based Hosts Incorporating
Dibenzothiophene for Highly-Efficient Single Emitting Layer White
Phosphorescent Organic Light-Emitting Diodes
Shou-Cheng Dong, Yuan Liu, Qian Li, Lin-Song Cui, Hua Chen, Zuo-Quan Jiang and Liang-Sheng Liao*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Four novel host materials were designed and synthesized, incorporating fluoreneꢀspiroꢀannulated
triphenylamine/carbazole (ST/SCz) and dibenzothiophene (DBT) blocks. A metaꢀlinking strategy was
applied by introducing DBT moieties to ST/SCz skeletons on C3 position of the fluorene backbones. As
10 expected, high triplet energies (over 2.80 eV) were achieved in all these four materials despite the
different linking positions on DBTs. All four materials show high Tg from 149 to 163 oC, which is
benefitted from their spiroꢀstructure. Their thermal, electrochemical and photoꢀphysical properties were
fully characterized. Highly efficient blue and white PHOLEDs were fabricated using these four materials
as host. Triphenylamineꢀcontaining STDBT4 and STDBT2 demonstrate better device performances due
15 to their relatively highꢀlying HOMO compared to carbazoleꢀcontaining SCzDBT4 and SCzDBT2.
Maximum ηext of 19.6%/18.4% (STDBT4/STDBT2) were achieved for FIrpic based devices, and
23.7%/22.2% (STDBT4/STDBT2) for two color based white PHOLEDs with double emitting layers
using POꢀ01 as yellow dopant. Finally a highly efficient single emitting layer white PHOLED with
maximum efficiencies of (24.0%, 77.0 cd Aꢀ1, 63.2 lm Wꢀ1) and CIE coordinator of (0.38, 0.48) was
20 realized using the device configuration of ITO/HATꢀCN/TAPC/STDBT4: 8 wt% FIrpic: 0.8 wt% POꢀ
01/TmPyPB/Liq/Al. And the device shows good color stability and low efficiency rollꢀoff. Even at a high
luminance of 10000 cd mꢀ2, it still maintain very high efficiencies of (17.9%, 56.4 cd Aꢀ1, 26 lm Wꢀ1).
general lighting, the cost. Constructing the multilayer structure of
OLED is the prime part of the total cost. The more the layers, the
higher the cost. The short cut to decrease the layers without
compromising the efficiency is to simplify the emitting layer.6
50 Thus developing highly efficient white PHOLED with single
emitting layers is a promising strategy for the commercialization
of white PHOLED.
Introduction
After over two decades of research and development, organic
25 light emitting diode (OLED) have proved itself to be a practical
technique in display with outstanding performance.1 OLED
display panels have already been commercialized and monetized
in portable electronic devices. And large size OLED TVs are on
their way to mass production. After the success in flatꢀpanel
30 display, the next big thing for OLED is its application in general
lighting.2 Kido’s early research on white OLED revealed its
potential and advantages in solidꢀstate lighting.3 But the
efficiency of conventional fluorescent white OLED cannot
compete with fluorescent tube or LED lighting device, since it
35 could only utilize singlet excitons. In 1998, Forrest and
Thompson reported highly efficient phosphorescent organic lightꢀ
emitting diodes (PHOLEDs),4 which boosted the theoretical
internal quantum efficiency (IQE) from 25% to 100% by utilizing
both singlet and triplet excitons. The significant progress made it
40 possible to develop white PHOLED with efficiency comparable
to fluorescent tube. This was realized by K. Leo who developed a
highly efficient white PHOLED using elegantly designed triꢀ
emitting layers.5 By applying light outcoupling techniques, the
power efficiency can outran fluorescent tube. But there is another
45 critical issue baffling the industrialization of white PHOLED for
A typical PHOLED adopts a hostꢀguestꢀstrategy by doping
55 emitting layer with heavy metalꢀcontaining complexes (Ir, Pt or
Os etc.) as emitting dopant, homogeneously dispersed into
appropriate host material. In this regard, host materials play a
crucial role in PHOLEDs. Developing host materials with
supreme performance is as important as the design of efficient
60 phosphorescent emitters for realizing highly efficient PHOLEDs
for practical usage.7 Especially in PHOLED with single emitting
layer, the physical properties of host material need to be finely
tuned to fit all the dopants involved. Firstly, the triplet energy
(ET) level of host material must be higher than that of guest in
65 order to prevent reverse energy transfer from the guest back to the
host and to confine the triplet excitons in the emission zone.
Since the triplet energy of the commonly used blue dopant FIrpic
is 2.65 eV, host materials with triplet energies over 2.7 eV are
favorable for blue and white PHOLEDs.8 Besides adequate triplet
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energy
2.48 eV due to the greatly extended conjugation by paraꢀlinkage.
The high ETs qualify them as hosts
O
N
N
N
Br
N
+
S
Br
OH
N
HO
B
STDBT4
SCzDBT4
S
S
i
ii
S
S
STDBT4
SCzDBT4
STDBT2
SCzDBT2
ST2DBT4
N
iii
S
Scheme 1 Structures of host materials.
N
HO
STDBT2
B
level, a good host material still have other criteria to meet, such
as good carrier mobility, matched HOMO and LUMO energy
levels, good thermal stability etc.7 mCP, which is a widely used
host materials, has very high ET of 2.9 eV and good holeꢀ
transporting property. But its low glass transition temperature (60
oC) greatly restricts its application.9 During the past decade,
SCzDBT2
OH
Br
5
STBr3
SCzBr3
S
Scheme 2 Synthetic routes of host materials. i) n-BuLi, THF, -78 oC. ii) HCl
55
(11 M), AcOH, reflux. iii) Pd(PPh3)4, THF/K2CO3 (2M) (3/1, v/v), 70 oC.
10 numerous host materials were developed to replace mCP from the
perspectives of increasing Tg, adjusting energy levels, balancing
carrier transporting, etc.7 Spiroꢀannulated structures normally
have very high Tg due to their bulky and orthogonal
configuration.10 And the central sp3 hybridized spiroꢀcarbon
15 could disconnect the conjugation of the two units on each side of
spiroꢀcarbon, and hence preserve ET. In addition, the properties of
the spiroꢀannulated materials can be easily tuned by introducing
different functional groups or atoms on each side of the spiroꢀ
carbon, forming symmetric or asymmetric structures.11 All these
20 features make them good skeletons for developing high
performance host materials. Various spiroꢀstructures were
explored as host materials for PHOLEDs, incorporating
phosphine oxide,12 triphenylsilane,13 cyano group,14 oxadiazole,15
etc.16 Dibenzothiophene (DBT) is another building block that
25 receives growing attentions. DBT has high triplet energy of 3.04
eV and good holeꢀtransporting ability, which makes it a good
candidate for constructing host materials. Recently a few DBT
based host materials were reported with very good device
for blue and white PHOLEDs. Electronꢀdonating triarylamines
are introduced to raise the HOMO levels of these materials,
aiming to facilitate the hole injection. And the energy levels can
also be tuned by different arylamines. By installing
60 triphenylamine or Nꢀphenylcarbazole unit on the fluorene ring
through a spiroꢀcarbon, the resulting spiroꢀannulated hosts not
only inherit the good hole injection, but also show enhanced
thermal stability. All these materials presented high Tg ranging
from 149 to 163 oC. The device performances of these host
65 materials were evaluated using common device structures.
Triphenylamine containing materials showed better performances
than their Nꢀphenylcarbazole containing counterparts. Maximum
efficiencies of 19.6%/47.0 cd Aꢀ1/38.7 lm Wꢀ1 and 18.4%/44.5 cd
Aꢀ1/37.8 lm Wꢀ1 for FIrpic based blue PHOLEDs were achieved
70 by STDBT4 and STDBT2. Afterwards, highly efficient white
PHOLEDs were fabricated using both double emitting layer and
single emitting layer configuration, using FIrpic and POꢀ01 as
dopants. Devices hosted by STDBT4 show the best performances
with maximum efficiencies of 23.7%/75.0 cd Aꢀ1/65.0 lm Wꢀ1 for
75 double and 24.0%/77.0 cd Aꢀ1/63.2 lm Wꢀ1 for single emitting
layer white PHOLED. The white PHOLEDs also exhibit low
efficiency rollꢀoff and good color stability.
performances.17 For instance, Han reported
a DBT based
30 phosphine oxide DBTSPO with a maximum EQE of 13.9% for
white PHOLED,18 In our previous work, we reported a DBT
based host material incorporating spirobifluorene through an
orthoꢀlinkage.19 The maximum EQE for white PHOLED reached
16.9%. And very recently, Wong reported a fluorene based host
35 material CzFCN linking through the C3 position to form a metaꢀ
linkage.20 The white PHOLED using CzFCN as host showed a
maximum EQE of 14.2%. We also developed a spirobifluorene
based phosphine oxide SF3PO linked though the C3 position of
the fluorene ring.21 The triplet energy of SF3PO is significantly
40 higher than its paraꢀlinking analog SPPO1, which is a widely
used host based on spirobifluorene. To integrate above
advantages about spiroꢀstructures and further elevate the device
performance, in this paper, we designed and synthesized four
novel host materials incorporating spiroꢀannulated triarylamine
45 and DBT units using metaꢀlinking strategy (Scheme 1). DBT unit
is connected through the C3 position of fluorene. Both C2 and C4
position of DBT unit are explored, forming a total metaꢀlinkage
with the fluorene ring. As expected, all four materials have very
high ET of over 2.8 eV. For comparison, a paraꢀlinking analog
50 ST2DBT4 was also synthesized. Its triplet energy plummets to
Results and Discussion
Synthesis and Characterization
80 Scheme 2 illustrates the synthetic routes to target materials. 2ꢀ
Bromoꢀtriphenylamine or 9ꢀ(2ꢀbromophenyl)ꢀ9Hꢀcarbazole was
treated with nꢀBuLi at ꢀ78 oC, followed by reaction with 3ꢀ
bromofluorenone to afford corresponding alcohol intermediates.
Then, the resulting intermediates were used without purification
85 to obtain the bromine substituted spiroꢀannulated triarylamines
(STBr3 and SCzBr3) via a FriedelꢀCrafts intramolecular ringꢀ
closure in the presence of acid. The final products were readily
afforded by classic SuzukiꢀMiyaura reaction between STBr3 or
SCzBr3 and corresponding boronic acid of dibenzothiophene in
90 good yields. ST2DBT4 was synthesized following similar
procedure. The final products were characterized by 1H NMR and
13C NMR spectroscopies, mass spectrometry and elemental
analysis. Before device fabrication, all the materials were further
2
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DOI: 10.1039/C3TC31336K
purified by vacuum sublimation. STDBT4 and SCzDBT4, which
link through the position 4 of DBT unit, could be readily
dissolved in common organic solvent (CH2Cl2, CHCl3, THF etc.)
structure consisting of a λmax at 365 nm and a shoulder peak at
382 nm. The PL and absorption spectra results suggest that the
different substitution positions on DBT moieties have limited
45 effect on electronic structures of the materials. We also tested
1.0
149 oC
STDBT4
STDBT4
STDBT2
SCzDBT4
SCzDBT2
0.8
T1
151 oC
0.6
0.4
0.2
0.0
STDBT2
161 oC
SCzDBT4
163 oC
SCzDBT2
300
400
500
Wavelength (nm)
600
100
150
200
250
Fig. 2 UV-Vis absorption and PL spectra measured in toluene solution at
10-5 M; phosphorescence spectra measured in frozen 2-
methyltetrahydrofuran (2-MeTHF) matrix at 77 K.
Temperature (oC)
5
Fig. 1 DSC traces recorded at a heating rate of 10oC min-1.
50 their PL and absorption spectra in thin films (Fig S2), which are
similar to the solution spectra with only slight red shifts. The
triplet energies were estimated from the highest vibronic band of
their low temperature phosphorescent spectra. STDBT4 and
SCzDBT4, which link through the C4 position on the DBT units,
55 exhibit sharp peaks at ca. 440 nm corresponding to the T1 band.
While in the spectra of their counterparts linking through the C2
position of DBT, the vibronic bands at 440 nm shrink to less
obvious shoulder peaks. Nevertheless, all four compounds
possessed high ET over 2.8 eV, which qualify them to be potential
60 hosts for blue and white PHOLEDs. The ET of ST is about 2.87
eV (Fig S3), which is inherited by these materials with little
compromise. For comparison, ST2DBT4, the analogue of
STDBT4 which adopts a paraꢀlinkage, exhibits a dramatically
lowered ET of 2.48 eV (Fig S3). It is obvious that our metaꢀ
65 linking strategy on fluorene moiety successfully reduces the
effective conjugation length and preserves the high triplet energy
of the backbone. Timeꢀdependence DFT was utilized to simulate
the natural transition orbitals (NTOs)22 using the optimized
structures of the S0 state. Fig 3 illustrates the spatial distribution
70 of the lowest and highest NTOs of the host materials. The NTOs
mainly locate on the fluorene ring and one adjacent benzene ring
of DBT moiety, indicating the triplet energies of these materials
are determined by the linking ways of DBT to the spiroꢀstructure.
This explains well why the materials with the same linking
75 configuration on DBT exhibited similar phosphorescent spectra.
STDBT4 and SCzDBT4 linked through the C4 position on DBT
adopt more twisted structures with dihedral angles of ca. 48o
(between fluorene and DBT), about 10o larger than that of
STDBT2 and SCzDBT4, resulting in sharper T1 bands.
due to their twisted structure. Their good solubility also qualifies
them to be potential solution processible host materials in the
future study. On the contrary, STDBT2 and SCzDBT2 exhibited
very poor solubility and precipitated from solutions during
10 reaction. On the other hand, this greatly simplified their
purification process, and pure samples of STDBT2 and
SCzDBT2 can be obtained by just filtration followed by vacuum
sublimation to remove catalyst residues.
Thermal Analysis
15 The thermal stabilities of the compounds were measured by
thermogravimetric analysis (TGA). All four materials exhibited
high decomposition temperatures (Td, corresponding to 5%
o
o
weight loss, Fig S1) ranging from 363 C to 464 C. Their glass
transition temperatures (Tg, Fig 1) were observed at 149 oC and
20 151 oC for STDBT4 and STDBT2, 161 oC and 163 oC for
SCzDBT4 and SCzDBT2 by differential scanning calorimetry
(DSC), indicating that it is the difference of configurations at
spiroꢀmoieties not the DBT linking ways which determined the
morphological stability. Benefitted from the more rigid Nꢀ
25 phenylcarbazole, SCzDBT4 and SCzDBT2 showed both higher
Td and Tg than their triphenylamine counterparts. The good
thermal stabilities of these materials are beneficial to their device
applications by improving the film morphology and reducing the
possibility of crystallization and phase separation upon heating.
30 Photophysical Properties
The UVꢀVis absorption and photoꢀluminescence spectra in dilute
toluene solutions, phosphorescent spectra in 2ꢀmethy THF at 77
K are presented in Fig 2. Detailed photoꢀphysical data are
summarized in Table 1. In the absorption spectra of SCzDBT4
35 and SCzDBT2, the relatively weak shoulder peaks at ca. 360 nm
are attributed to the nꢀπ∗ transitions of carbazole group. As a
result, the optical band gaps of SCzDBT4 and SCzDBT2 are
smaller than those of STDBT4 and STDBT2 by ca. 0.2 eV.
STDBT4 and STDBT2 exhibited broad structureless PL spectra
40 with λmax ca. 370 nm. On the other hand, the PL spectra of
SCzDBT4 and SCzDBT2 were almost identical with vibronic
80 Electrochemical Properties and FMO Energy Levels
The electrochemical behaviors of these materials were probed by
cyclic voltammetry (CV) using
a
typical triꢀelectrode
configuration with ferrocene as internal standard. Both distinct
oxidation and reduction processes were observed (Fig 4). On the
85 oxidation side, they all exhibited a reversible oxidation peak
followed by a quasiꢀreversible oxidation process, corresponding
to the oxidation of the nitrogen center of ST/SCz and the DBT
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DOI: 10.1039/C3TC31336K
ring respectively. While their reduction processes are less
characteristical compared to oxidation ones. The oxidation onsets
of the STꢀcontaining compounds are 0.2 eV lower than their SCzꢀ
containing counterparts. This implies the HOMO levels of
STDBT4 and STDBT2 are about 0.2 eV higher than SCzDBT4
5
Table 1 Physical Properties
g
g
Abs λmax
solutiona/filmb
[nm]
PL λmax
solutiona)/filmb)
[nm]
Eg
Eg
c
d
d
d
ET
[eV]
Tm
[oC]
Tg
Td
HOMOe
[eV]
LUMOf
[eV]
solution
[eV]
film
[eV]
3.52
3.57
[oC] [oC]
STDBT4
STDBT2
319/322
373/380
2.83 >350
2.82 297
149
151
383
363
5.69
5.74
2.17
2.17
3.57
317/320
371/378
3.59
355, 336, 319/
359, 340, 292
356, 319, 294/
360, 320, 295
365, 382/
373, 387
365, 382/
372, 389
SCzDBT4
SCzDBT2
2.82 >350
2.81 >350
161
163
464
397
5.91
5.94
2.58
2.61
3.39
3.40
3.33
3.33
a Measured in toluene solution at room temperature; b vacuumꢀdeposited thin film (50 nm); c measured in 2ꢀMeTHF glass matrix at 77 K; d Tm: melting
temperatures, Tg: glass transition temperatures, Td: decomposition temperatures; e HOMO levels were calculated from UPS data; f LUMO levels were
calculated from HOMO and Eg in solid films; g Eg: The band gap energies were calculated from the corresponding absorption onset.
10
Fig. 3 NTOs of the materials.
and SCzDBT2. The CV results only reflect the electrochemical
behavior of materials in solution. Given that the devices are in
materials reported even not any electronꢀwithdrawing groups are
introduced.7, 20 This may facilitate the electron injection and help
balancing the carrier recombination to some degree. To better
35 understand the electronic structures of these materials, DFT
calculations were utilized to simulate their FMO spatial
distributions. As Fig 5 illustrates, these materials have divided
HOMO and LUMO, separated by the central spiroꢀcarbon. The
HOMOs of these materials locate at the electronꢀrich nitrogen
15 solid films, it is not appropriate to use the HOMO or LUMO
levels calculated directly from the oxidation or reduction process
in the device energy diagram.23 In order to get more accurate and
reliable frontier molecular orbital (FMO) energy levels,
ultraviolet photoemission spectroscopy (UPS) was employed to
20 determine the HOMO levels (Fig S4 to S7). And LUMO levels
were calculated from energy band gaps in solid films (Fig S2) 40 centers. The HOMOꢀ1s distribute on the fluorene ring and spread
and HOMO levels. All the data are summarized in Table 1. The
HOMO levels of STDBT4/STDBT2 are 5.69/5.74 eV, about 0.2
eV higher than SCzDBT4/SCzDBT2 (5.91/5.94 eV), matching
25 perfectly with the CV results. The relatively high lying HOMO of
heavily on the sulfur center of DBT unit with similar distribution
patterns as the HOMO of DBT (Fig S8). The HOMO and
HOMOꢀ1 match with the CV results, corresponding to the first
and second oxidation process, respectively. STDBT4 and
STDBT4/STDBT2 may facilitate the hole injection to the 45 SCzDBT4 have similar LUMO distributions with LUMO
emitting layer in devices. SCzDBT4 and SCzDBT2 show
significantly lowered LUMO levels, which are a result of their
lowered HOMO levels compounded with narrowed optical band
30 gaps compared to STDBT4 and STDBT2. The LUMO levels of
these new materials are comparable to many bipolar host
spreading on part of the fluorene ring. On the other hand,
4
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40 efficiencies of 37.3/29.3 cd Aꢀ1 for CE, 27.0/21.9 lm Wꢀ1 for PE
and 15.3%/11.7% for EQE were still realized. All four devices
show rather low efficiency rollꢀoff at low current density. At a
brightness of 1000 cd mꢀ2, the EQE of B1~4 maintains over 90%
of their initial values. But with the increase of current density
45 from 5 mA cmꢀ2 to 40 mA cmꢀ2, the efficiencies of B1 and B2
decay more rapidly than that of B3 and B4. At a high brightness
of 10000 cd mꢀ2, device B1~4 finally reach similar performances
with EQE of ca. 12%. Although B4 presented the lowest
efficiencies among these devices, it shows very low rollꢀoff with
50 only 13% drop of EQE even at 10000 cd mꢀ2. All four devices
show relativley greenish blue CIE of ca. (0.16, 0.39) compared to
STDBT4
STDBT2
SCzDBT4
SCzDBT2
ST
typical FIrpic based devices. These redꢀshifts of CIE are common
DBT
24
in literatures.20,
This phenomenon can be attributed to the
microꢀcavity effect caused by different distance between emission
55 zone (EMZ) and cathode.25 The CIE of blue devices can be
further altered by manipulating the thickness of charge
transporting layers. But in the application of solidꢀstate lighting,
the CIE of blue devices is not so critical. The better performance
of B1 and B2 may be attributed to the relatively higher HOMO
60 level of STDBT4/STDBT2. The hole/electron injection barriers
are about 0.2 eV/0.5 eV for STDBT4/STDBT2 and 0.4 eV/0.1 eV
for SCzDBT4/SCzDBT2 (Fig 7). Considering these four
materials are holeꢀtransporting type hosts (Fig S9), holes can be
efficiently injected into EML. Based on the energy diagram, SCz
65 based materials should reduce hole injection and facilitate
electron injection, resulting in more balanced charge injection
and an improvement of the quantum efficiency of the device.
However, it is also important to consider the energy levels of the
dopant when discussing efficiency. FIrpic has low lying LUMO
70 level that is below that of the electron transporting material
TmPyPB. The ST based materials have higher LUMOs compared
to the SCz based materials. As such, when electrons are injected
into the emitting layer, there is a greater likelihood that there will
be direct recombination on the dopant when the ST based
75 materials are used compared to the SCz based materials. In
essence, the dopant acts as stronger electron traps for the ST
based materials and the resulting direct recombination on the
dopant produced a high efficiency. In contrast, the SCz based
materials have LUMO levels that are close to those of the dopant
80 and the ability of the dopant to trap electrons is reduced. When
electrons are injected into the emitting layer containing the SCz
based hosts, there is less direct recombination on the dopant and
the efficiency is lower due to a charge imbalance. As long as the
hole can be efficiently recombined with electrons, then the
85 efficiencies of ST based devices can be higher than the SCz based
devices. But as the voltage increases, in the case of B1 and B2,
holes can on longer be efficiently recombined with electrons
because of limited dopant in EML. The unbalanced charge
injection results in excessive holes at the EML/ETL interface,
90 leading to efficiency rollꢀoff at high current density. While in the
contrast, as a merit of the relatively low LUMO levels of
SCzDBT4/SCzDBT2, B3 and B4 have much smaller electron
injection barrier (0.1 eV), which lead to more balanced charge
injection and low efficiency rollꢀoff at high current density. These
95 four materials proved themselves to be very good host materials
for blue PHOLEDs. STDBT4 and STDBT2 with maximum EQE
of 19.6% and 18.4% are among the bestꢀperformed blue host
-3
-2
-1
0
1
Potential/V (vs Fc+/Fc)
Fig. 4 Cyclic voltammogram of host materials. The measurement of
oxidation and reduction potentials were performed in CH2Cl2 and DMF
respectively with 0.1 M of n-Bu4NPF6 as supporting electrolyte.
5
STDBT2 and SCzDBT4 show much different LUMO
distributions. As a result of sulfur induced polarization, the C2
and C8 positions of DBT don’t have LUMO distribution, which
restrain the LUMO of STDBT2 on the DBT ring. In the case of
SCzDBT2, the energy of unoccupied FMO on ST is higher than
10 that on SCz. The LUMO of SCz merged with the LUMO of DBT
forming a nearꢀdegenerate orbital. Nevertheless, the FMO energy
levels are mainly determined by the amine parts of the materials.
The different linking ways of DBT show limited affect on the
FMO energy levels because the metaꢀlinking strategy greatly
15 restricts the conjugation.
EL performance of PHOLEDs
To evaluate the performance of these four materials as host
materials, FIrpic based blue phosphorescent devices were
fabricated using the structure of ITO/HATꢀCN (10 nm)/TAPC
20 (45 nm)/Host:8 wt% FIrpic (20 nm)/TmPyPB (40 nm)/Liq (2
nm)/Al. STDBT4 (B1), STDBT2 (B2), SCzDBT4 (B3) or
SCzDBT2 (B4) were doped with 8 wt% FIrpic to form emitting
layer (EML). To confine triplet excitons in the EML, TAPC and
TmPyPB which have high ET and carrier mobility were served as
25 holeꢀtransporting/electronꢀblocking (HTL/EBL) and electronꢀ
transporting /holeꢀblocking layer (ETL/HBL) respectively. And
to ensure efficient charge injection, HATꢀCN and Liq were
utilized as hole injection layer (HIL) and electron injection layers
(EIL), respectively.
30 Fig 6a illustrates the efficiencies of blue phosphorescent devices.
The detail performance data were summarized in Table 2. High
efficiencies were achieved by device B1~4. The maximum
efficiency of B1 (STDBT4) reached 47.0 cd Aꢀ1 for current
efficiency (CE), 38.7 lm Wꢀ1 for power efficiency (PE) and
35 19.6% for external quantum efficiency (EQE). B2 (STDBT2)
shows similar performance with slightly lowered maximum
efficiencies of 44.5 cd Aꢀ1 for CE, 37.8 lm Wꢀ1 for PE and 18.4%
for EQE. Although the performance of B3 (SCzDBT4)/B4
(SCzDBT2) are relatively lower than that of B1/B2, maximum
This journal is © The Royal Society of Chemistry [year]
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Journal of Materials Chemistry C
Page 6 of 11
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DOI: 10.1039/C3TC31336K
materials.
5
possible layouts to generate white light in PHOLEDs, three colorꢀ
based (R+G+B) and two colorꢀbased (B+Y or B+O) devices.
Generally, RGB white PHOLED usually can obtain very high
color rendering index (CRI) and its CIE can be tuned in a wide
The excellent performance of blue PHOLEDs qualified them as
potential host materials for white PHOLEDs. There are two
10
Fig. 5 Optimized geometry and HOMO-LUMO spatial distributions.
Table 2 Electroluminescence characteristics of the devices
Devicea
B1
Host
Vb [V]
4.8
4.8
5.1
5.1
4.2
4.3
4.8
4.8
4.7
4.6
4.6
4.6
ηcec [cd Aꢀ1]
ηpec [lm Wꢀ1]
38.7, 27.6, 12.2
37.8, 28.2, 13.6
27.0, 23.0, 12.8
21.9, 17.9, 10.5
65.0, 54.1, 26.5
57.5, 49.9, 27.6
45.8, 37.2, 21.7
33.8, 29.1, 18.8
46.6, 34.0, 15.2
61.5, 48.8, 25.1
63.2, 50.1, 26.0
62.0, 52.1, 28.4
ηextc [%]
CIE [x, y]d
0.15, 0.38
0.16, 0.39
0.16, 0.40
0.17, 0.41
0.39, 0.49
0.36, 0.48
0.35, 0.47
0.33, 0.46
0.25, 0.42
0.37, 0.48
0.38, 0.49
0.41, 0.50
CCT [K]e
ꢀ
STDBT4
STDBT2
SCzDBT4
SCzDBT2
STDBT4
STDBT2
SCzDBT4
SCzDBT2
47.0, 41.9, 28.4
44.5, 42.5, 30.9
37.3, 37.2, 30.6
29.3, 29.2, 25.6
75.0, 72.7, 55.8
67.7, 67.3, 55.8
59.0, 57.1, 47.8
44.4, 44.1, 41.2
56.5, 51.0, 35.2
76.5, 71.7, 55.2
77.0, 72.8, 56.4
76.2, 75.5, 60.9
19.6, 17.6, 11.9
18.4, 17.5, 12.7
15.3, 15.2, 12.5
11.7, 11.7, 10.2
23.7, 22.7, 17.5
22.2, 21.9, 17.8
20.0, 18.8, 15.6
15.1, 15.0, 13.6
20.7, 18.9, 13.2
24.0, 22.6, 17.7
24.0, 22.8, 17.9
23.2, 23.0, 18.8
B2
ꢀ
B3
ꢀ
B4
ꢀ
W1
4337
4817
5109
5656
8518
4651
4536
4086
W2
W3
W4
WS1
WS2
WS3
WS4
STDBT4
a The notation 1~4 in device B1~B4 and W1~W4 indicate the corresponding device fabricated with STDBT4, STDBT2, SCzDBT4 and SCzDBT2 as
host respectively. Device configuration: B1~B4, ITO/HATꢀCN (10 nm)/TAPC (45 nm)/Host:8 wt% FIrpic (20 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al;
15 W1~W4, ITO/HATꢀCN (10 nm)/TAPC (45 nm)/Host:8 wt% FIrpic (19 nm)/Host:6 wt% POꢀ01 (1 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al; WS1~WS4,
ITO/HATꢀCN (10 nm)/TAPC (45 nm)/STDBT4: 8 wt% FIrpic: x wt% POꢀ01 (20 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al, WS1~WS4 x = 0.2, 0.5, 0.8, 1.2.
b Voltages at 1000 cd mꢀ2. c efficiencies in the order of maximum, at 1000 cd mꢀ2 and at 10000 cd mꢀ2. d Commission International de I’Eclairage
coordinates measured at 5 mA cmꢀ2. e Correlated color temperature at 5 mA cmꢀ2.
range.2 But with three dopants involved, the device fabrication
20 process will be complicated and the device efficiency may also be
compromised due to the multiple recombination interfaces. Even
all the dopants been doped into one layer to simplify the device
the spectra can still be tuned from cold white to warm white,
which is more desirable in humanꢀfriendly lighting.26 Considering
all these features, a two colorꢀbased device configuration was
adopted in our experiments. White PHOLEDs with double EMLs
structure, the control of doping concentration of low ET dopants 35 were fabricated using these four materials as host. The device
can still be tricky. By comparison, two colorꢀbased PHOLED has
25 more simplified device structure with fewer variables. With less
interfaces involved, the charge recombination zone can be more
easily adjusted. Furthermore, single EML devices can be realized
structure is similar to previous blue PHOLEDs: ITO/HATꢀCN
(10 nm)/TAPC (45 nm)/Host:8 wt% FIrpic (19 nm)/Host:6 wt%
POꢀ01 (1 nm)/TmPyPB (40 nm)/Liq (2 nm)/Al (W1~4). A thin
yellow EML was inserted between blue EML and ETL using POꢀ
by doping two dopants into one layer. And the CIE can be easily 40 01 as dopant. As expected, device W1~4 exhibited excellent
tuned by the doping ratio of yellow or orange dopant. Although
30 the CIE is hard to reach (0.33, 0.33) by manipulating two colors,
performances (Fig 6b). Similar to the blue PHOLEDs, W1
(STDBT4) and W2 (STDBT2) showed better performance
6
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Journal of Materials Chemistry C
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DOI: 10.1039/C3TC31336K
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www.rsc.org/xxxxxx
ARTICLE TYPE
Fig. 6 Current efficiency, power efficiency and external quantum efficiency as a function of luminance for Device B1~4 (a) and Device W1~4 (b)
2.0
2.2
2.2
2.6
2.7
2.7
2.7
2.9
2.9
2.7
2.9
PO-01
6 wt%
PO-01
6 wt%
PO-01
0.2~1.2
wt%
TAPC
5.5
N
N
Liq/Al
4.3
FIrpic
8 wt%
FIrpic
8 wt%
FIrpic
8 wt%
ITO
4.7
TmPyPB
5.1
5.1
5.1
5.7
5.8
5.8
5.8
TAPC
5.7
5.7
5.9
STDBT4/STDBT2
STDBT4/STDBT2
SCzDBT4/SCzDBT2
N
6.7
Device W1/W2
Device W3/W4
Device WS1~4
HAT-CN
F
NC
C
NN
N
N
O
N
O
O
F
Ir
Ir
N
N
O
N
N
N
NC
CNN
9.5
N
N
S
2
O-
2
N
NC
CNN
H
A
T
-
N
TmPyPB
FIrpic
P
O
0
1
5
Fig. 7 Proposed energy level diagram showing HOMO and LUMO levels and the molecular structures for the materials used in Device W1~4 and WS1~4.
than W3 (SCzDBT4) and W4 (SCzDBT2). The bestꢀperformed
W1 reached maximum efficiencies as high as 75.0 cd Aꢀ1 for CE,
65 lm Wꢀ1 for PE and 23.7% for EQE with a CIE coordinator of
compared to their maximum values. And at high luminance of
10000 cd mꢀ2, their EQEs still stay above 73% of the maximum
values. It is worth to notice that device W1~4 possess the same
(0.39, 0.49). The maximum EQE of W2 and W3 also exceed 20% 20 device configuration, but exhibit different CIE coordinates, which
10 with 67.7 and 59.0 cd Aꢀ1 for CE, 57.5 and 45.8 lm Wꢀ1 for PE.
Even the leastꢀperformed device W4 still has an EQE over 15%.
W1 and W2 show much flatter efficiency decay curves as a
function of brightness than corresponding blue devices. And W3
show a gradually blue shift from (0.39, 0.49) for W1 to (0.33,
0.46) for W4. According to the energy diagram, the main carrier
recombination zone of W1 will locate at the interface of POꢀ01
layer and ETL, since the hole injection barrier (0.2 eV) is much
and W4 still preserve their low efficiency rollꢀoff as B3 and B4. 25 smaller than electron injection barrier (0.5 eV). Triplet excitons
15 At a brightness of 1000 cd mꢀ2, the efficiencies of W1~4 maintain
as high as over 96% for CE, 80% for PE and 95% for EQE
can be transfer to POꢀ01 more easily because the band gap and T1
of POꢀ01 are smaller than those of FIrpic. But the POꢀ01 layer is
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Journal of Materials Chemistry C
Page 8 of 11
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DOI: 10.1039/C3TC31336K
very thin, which means part of the triplet excitons can migrate to
or formed in the adjacent FIrpic layer, resulting a warm white of
(0.39, 0.49) with color temperature of 4337 K. STDBT2,
(Fig S13) when the current density varied from 0.2 mA cmꢀ2 to 40
mA cmꢀ2. To further simplify the device structure, devices with
single EML were fabricated by doping FIrpic and POꢀ01 into
SCzDBT4 and SCzDBT2 have sequentially lowered HOMOs 25 STDBT4. The device structure of W1~4 has proved itself
5
compared to STDBT4, which may cause the recombination zone
gradually shift towards the anode. More and more excitons can
migrate or be formed in the blue EML, resulting in progressive
blue shifts in CIE coordinates of W2, W3 and W4. This also
applicable and efficient. So we merged the double EMLs together
leaving only one variable, the doping concentration of POꢀ01 in
the single EML. When doping two triplet emitters into one layer,
the doping concentration of the dopant with lower ET, in this case
explains the lower efficiency rollꢀoff of W1 and W2 compared to 30 POꢀ01, should be smaller in order to get a white emission since
10 B1 and B2 because that the carrier recombination zone of W1 and
W2 mainly locate in the more efficient yellow EML with smaller
efficiency rollꢀoff. In the case of W3 and W4, the carrier
recombination zones shift towards the less efficient blue EML
the excitons tend to be transferred to the emitters with lower ET.
Four concentrations of POꢀ01 were adopted, 0.2%, 0.5%, 0.8%
and 1.2% for WS1~4 respectively, resulting emissions from cold
white (0.25, 0.42) to warm white (0.41, 0.50). All four devices
due to their lower HOMO levels, resulting in the similar 35 show excellent performances with EQE exceeding 20%. Fig 8
15 efficiency decay trend as B3 and B4.
illustrates their efficiency curves as a function of luminance and
emission characteristics. Since the dopants were integrated into
one EML, WS1~4 show quite stable CIE coordinates (Fig 8a).
Device WS1 with 0.2% POꢀ01 exhibit a cold white emission with
Apparently, these four materials showed excellent performance in
white PHOLEDs. Among them, W1 hosted by STDBT4
exhibited the best performance at 1000 cd mꢀ2, with efficiency of 40 CIE of (0.25, 0.42) and CCT of 8518 K. The EL spectrum of
20 72.7 cd Aꢀ1 for CE, 54.1 lm Wꢀ1 for PE and 22.7% for EQE. And
W1 also had the best color stability with CIE nearly unchanged
WS1 is dominated by blue emission from FIrpic due to the
relatively low doping concentration of POꢀ01. And the
Fig. 8 a) CIE coordinates of EL spectra recorded at different current densities from 0.2 to 40 mA cm-2. b) External quantum efficiency as a function of
45
luminance. c) EL spectra recorded at 5 mA cm-2. d) Current efficiency and power efficiency as a function of luminance.
efficiencies are also compromised since most of the excitons are
captured by less efficient FIrpic. By raising the doping rate of
POꢀ01 to 0.5% and 0.8%, the emissions of WS2 and WS3 were
Wꢀ1 for PE and 24.0% for EQE. WS2 trails closely behind WS3
with similar performances. Both WS2 and WS3 had very low
efficiency rollꢀoff. Compared to the maximum values, their
tuned to more desirable warm white light with CIE of (0.37, 0.48) 55 efficiencies maintain over 93% for CE, 79% for PE and 94% for
50 and (0.38, 0.49); CCT of 4651 K and 4536 K, respectively. The
maximum efficiencies of WS3 reach 77.0 cd Aꢀ1 for CE, 63.2 lm
EQE at 1000 cd mꢀ2. Even at 10000 cd mꢀ2, the efficiencies of
WS3 still preserve 56.4 cd Aꢀ1 for CE, 26.0 lm Wꢀ1 and 17.9% for
8
| Journal Name, [year], [vol], 00–00
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Journal of Materials Chemistry C
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DOI: 10.1039/C3TC31336K
EQE, which makes it very tolerable to voltage variation. Further
increase of the POꢀ01 concentration to 1.2% rendered the CIE
(No.21202114,
No.21161160446,
No.61036009,
and
No.61177016), the National HighꢀTech Research Development
Program (No.2011AA03A110), and the Natural Science
Foundation of Jiangsu Province (No.BK2010003). This is also a
coordinate into the yellow region, resulting in a yellowish white
light emission of WS4 with CIE of (0.41, 0.50) and CCT of 4086
5
K. WS4 also exhibits very high efficiencies of 76.2 cd Aꢀ1 for CE, 60 project funded by the Priority Academic Program Development
62.0 lm Wꢀ1 and 23.2% for EQE with even better efficiency rollꢀ
off, since the spectrum is dominated by yellow emission from
more efficient POꢀ01. Compared with double emitting layer
device W1, WS3 have slightly higher efficiencies and similar CIE
of Jiangsu Higher Education Institutions (PAPD) and by the
Research Supporting Program of Suzhou Industrial Park.
Noes and references
10 coordinates. The CIE coordinates of WS3 show a small blue shift
with increasing current density. Since that all the dopants are
integrated into one layer, electron injection into EML is
facilitated by dopants at low current density. As the current
density increases, the recombination zone gradually move toward
15 cathode due to the hole transporting nature of the host, resulting
blue shift in CIE value caused by the microꢀcavity effect.
Nevertheless, the CIE coordinates of simplified devices are still
quite stable. Again, the supreme performances of single EML
white PHOLEDs have proved STDBT4 an excellent host material
20 for costꢀeffective white PHOLED. Its efficiencies are among the
best performed single EML white PHOLED.27 In the upcoming
research, the efficiencies of our single EML white PHOLEDs
could easily outrun fluorescent tube by applying stateꢀofꢀthe art
outcoupling techniques.
Jiangsu Key Laboratory for CarbonꢀBased Functional Materials &
65 Devices, Institute of Functional Nano & Soft Materials (FUNSOM),
Soochow University, Suzhou, Jiangsu 215123, China. Email:
zqjiang@suda.edu.cn; lsliao@suda.edu.cn; Fax: +86ꢀ512ꢀ 6588ꢀ2846;
Tel: +86ꢀ512ꢀ 6588ꢀ0945
† Electronic Supplementary Information (ESI) available: [The detailed
70 experimental procedures were listed in the supporting information]. See
DOI: 10.1039/b000000x/
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25 Conclusions
Four new host molecules based on spiroꢀannulated triarylamine
incorporating dibenzothiophene were synthesized and fully
characterized. The DBT moieties were introduced through monoꢀ
substituted metaꢀlinking strategy on the C3 position of the
30 fluorene ring to preserve ET. This molecular design is rarely
reported before and endowed the materials with high thermal
stability and high triplet energy. And the FMO energy levels can
be tuned by different arylamines to balance the charge injection.
The PHOLED results have proved this series of materials
35 competent for both blue and white devices. The best performed
STDBT4 exhibits maximum EQE of 19.6% for blue and 23.7%
for double EML white PHOLED using common device
structures. Highly efficient cost effective single EML white
PHOLEDs were also fabricated using STDBT4 as host with very
40 low efficiency rollꢀoffs. Maximum efficiencies of (24.0%, 77.0
cd Aꢀ1, 63.2 lm Wꢀ1) were reached with CIE coordinator of (0.38,
0.48). At 1000 cd mꢀ2 and 10000 cd mꢀ2, the EQE still preserved
22.8% and 17.9% respectively. These performances are among
the highest efficiencies achieved by single EML white
45 PHOLEDs. To the best of our knowledge, STDBT4 is so far the
best performed host materials based on spiroꢀstructures for white
PHOLEDs. Given the high efficiencies achieved, the
performance of our single EML white PHOLEDs have the
potential to exceed fluorescent tube efficiency by applying
50 outcoupling layers in further research.
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Acknowledgements
We are grateful to the assistance of Dr. Cheng Zhong in DFT
calculations. We acknowledge the financial support from the
National Postdoctoral Science Foundation of China (No.
55 2012M511797), the Natural Science Foundation of China
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Page 11 of 11
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C3TC31336K
The alignments of spiro-triarylamine and DBT unit are successfully utilized in the host materials of
phosphorescent OLEDs. Benefitted from optimized linking way, four new host materials with high triplet
energies and matched HOMO/LUMO are obtained. In particular, the host STDBT4 could achieve impressively
high efficiencies not only in blue but also in warm white (double-layer or single-layer) with quite low roll-
offs.
361x135mm (300 x 300 DPI)