Using an organic molecule with low triplet energy as a host in a highly efficient blue electrophosphorescent device

Article abstract of DOI:10.1002/anie.201308046

To achieve high efficiencies in blue phosphorescent organic light-emitting diodes (PhOLEDs), the triplet energies (T₁) of host materials are generally supposed to be higher than the blue phosphors. A small organic molecule with low singlet energy (S₁) of 2.80eV and triplet energy of 2.71eV can be used as the host material for the blue phosphor, [bis(4,6-difluorophenylpyridinato-N,C^2′)iridium(III)] tetrakis(1-pyrazolyl)borate (FIr6; T₁=2.73eV). In both the photo- and electro-excited processes, the energy transfer from the host material to FIr6 was found to be efficient. In a three organic-layer device, the maximum current efficiency of 37cd A^-1 and power efficiency of 40Lm W^-1 were achieved for the FIr6-based blue PhOLEDs. Copyright

Full text of DOI:10.1002/anie.201308046

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DOI: 10.1002/anie.201308046
Blue Phosphorescent Organic LEDs
Using an Organic Molecule with Low Triplet Energy as a Host in
a Highly Efficient Blue Electrophosphorescent Device**
Cong Fan, Liping Zhu, Tengxiao Liu, Bei Jiang, Dongge Ma,* Jingui Qin, and Chuluo Yang*
Abstract: To achieve high efficiencies in blue phosphorescent
organic light-emitting diodes (PhOLEDs), the triplet energies
(T1) of host materials are generally supposed to be higher than
the blue phosphors. A small organic molecule with low singlet
energy (S1) of 2.80 eV and triplet energy of 2.71 eV can be used
as the host material for the blue phosphor, [bis(4,6-difluor-
ophenylpyridinato-N,C2’)iridium(III)] tetrakis(1-pyrazolyl)-
borate (FIr6; T1 = 2.73 eV). In both the photo- and electro-
excited processes, the energy transfer from the host material to
FIr6 was found to be efficient. In a three organic-layer device,
the maximum current efficiency of 37 cdAÀ1 and power
efficiency of 40 LmWÀ1 were achieved for the FIr6-based
blue PhOLEDs.
guarantee high triplet energies, wide band-gap bipolar host
materials have been developed, such as bis[4-(N-carbazolyl)-
phenyl]phenylphosphine oxide (BCPO), 2,7-bis[diphenyl-
phosphoryl]-9-[4-(N,N-diphenylamino)phenyl]-9-phenyl-
fluorene (POAPF), and 9-[3-(9H-carbazole-9-yl)phenyl]-3-
(diphenylphosphoryl)-9H-carbazole (mCPPO1), which usu-
ally emit fluorescence in violet or ultraviolet region.[5]
However, owing to their intrinsic low HOMO level and
high LUMO level, simultaneously injecting holes and elec-
trons into these wide band-gap bipolar host materials would
be difficult, which could consequently bring about high drive
voltages and low power efficiencies in the blue PhOLEDs.[6]
Therefore, it is preferable to achieve a trade-off between the
triplet energy and HOMO/LUMO level for host materials.
Recently, Padmaperuma et al. reported a high-efficiency
sky-blue PhOLED hosted by a bipolar material with lower
triplet energy than that of the sky-blue phosphor, [bis{(4,6-
O
wing to spin–orbit coupling, phosphorescent heavy-metal
complexes can harvest both electro-generated singlet and
triplet excitons in the emitting layer (EML) of PhOLEDs,
achieving 100% internal quantum efficiency.[1] To avoid
competitive de-excitation pathways, such as triplet–triplet
annihilation and/or concentration quenching, the emissive
phosphors as guests are generally dispersed into organic
small-molecule or polymer host matrix, which play an
indispensible role for energy transfer and charge transport.[2]
To provide more balanced electron and hole fluxes in EML,
bipolar host materials are currently drawn considerable
attention.[3]
difluorophenyl)pyridinato-N,C2’}iridium(III)]
picolinate
(FIrpic), providing that the adjacent hole- and electron-
transporting materials possessed sufficiently high triplet
energies to confine the triplet excitons in the emitting
layer.[7] It would be of significance to see if this strategy
could be applicable to the blue phosphor of FIr6.[8] Herein, we
designed and synthesized a new host material, namely
POBPmDPA, by integrating diarylamine and diphenylphos-
phine oxide into the biphenyl skeleton. The new host
possesses close triplet energy (2.71 eV) to the blue phosphor
of FIr6 (2.73 eV). Significantly, efficient energy transfer from
the low triplet-state host material to the blue phosphor of FIr6
can be realized both in photo- and electro-excited processes.
By employing FIr6 as the blue phosphor and the new
compound as the host material, the blue PhOLEDs achieved
a maximum current efficiency of 34 cdAÀ1, a maximum power
efficiency of 34 LmWÀ1, and a maximum external quantum
efficiency (EQE) of 18.1% with a good Commission Interna-
tional de IꢀEclairage coordinate (CIE) of (0.15, 0.25).
It is a challenge to design bipolar host materials for blue
PhOLEDs. Apart from possessing hole- and electron-trans-
porting mobility, it is universally believed that bipolar host
materials for blue PhOLEDs should have higher triplet
energies than the blue phosphors. Otherwise the energy of
excitons located on the blue phosphors could exothermically
transfer to the lower triplet states of host materials.[4] To
[*] C. Fan,[+] T. Liu, B. Jiang, Prof. J. Qin, Prof. C. Yang
Hubei Collaborative Innovation Center for Advanced Organic
Chemical Materials, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Department of Chemistry
Wuhan University, Wuhan, 430072 (P.R. China)
E-mail: clyang@whu.edu.cn
Scheme 1 depicts the synthesis of the new compound,
POBPmDPA. The intermediate of 2’-bromo-N,N-di(p-tolyl)-
biphenyl-2-amine (BrBPmDPA) was obtained through the
Suzuki coupling reaction of 2-(di-p-tolylamino)phenylboronic
acid with 1,2-dibromobenzene; BrBPmDPA then underwent
lithiation, coupling with chlorodiphenylphosphine, and oxi-
dation with hydrogen peroxide to afford the final product. All
L. Zhu,[+] Prof. D. Ma
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun, 130022 (P.R. China)
E-mail: mdg1014@ciac.jl.cn
1
of the compounds were fully characterized by H NMR and
13C NMR spectrometry, mass spectrometry, and elemental
analysis (see the Supporting Information). The molecular
structure of POBPmDPAwas further confirmed by the single-
crystal X-ray diffraction analysis (Supporting Information,
Figure S1). The compound showed good thermal stability as
indicated by the decomposition temperature (Td, correspond-
[+] These authors contributed equally to this work.
[**] We are grateful to the National Basic Research Program of China
(973 Program 2013CB834805) and the National Science Fund for
Distinguished Young Scholars of China (No. 51125013).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308046.
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Figure 2. Phosphorescence spectra of POBPmDPA and FIr6 in a 2-
MeTHF matrix (10À5 m) at 77 K.
Scheme 1. Synthesis of POBPmDPA.
ing to 5% weight loss) of 2968C (Supporting Information,
Figure S2) and the glass transition temperature (Tg) of 888C
in the differential scanning calorimetry (DSC) thermogram
(Supporting Information, Figure S2 inset). The electrochem-
ical properties of POBPmDPA were probed by cyclic
voltammetry (CV). It exhibited a reversible oxidation process
in CH2Cl2 solution with a half-wave oxidation potential (Eox
vs. ferrocene/ferricenium) of 0.36 V (Supporting Information,
Figure S3), and accordingly, its HOMO energy was estimated
to be À5.2 eV, which was close to most triphenylamine
derivatives.
the dopant FIr6, we investigated the photoluminescence (PL)
spectrum and lifetime of FIr6 doped in POBPmDPA matrix
at 10% ratio by weight. For comparison, the PL spectrum and
lifetime of FIr6 doped in polymethylmethacrylate (PMMA)
matrix were also measured under the identical conditions. As
Figure 3 showed, the emission of FIr6-POBPmDPA compo-
site film predominantly originated from FIr6 molecule at both
room temperature and 77 K, which is almost the same with
the FIr6-PMMA composite film (Supporting Information,
Figure S4). This is indicative of the efficient energy transfer
from POBPmDPA to FIr6 in the photo-excited process. The
Figure 1 shows the absorption spectrum of POBPmDPA
in CH2Cl2 solution and fluorescence (FL) spectra in toluene
and neat film. The optical energy band gap (Eg) of
POBPmDPA was calculated to be 3.2 eV from the onset of
absorption (388 nm). The fluorescence emission peaks of
POBPmDPA were 444 nm in toluene solution and 440 nm in
neat film; accordingly, its singlet state (S1) was estimated to be
2.80 eV. The phosphorescence spectrum of POBPmDPA in
the 2-methyltetrahydrofuran (2-MeTHF) at 77 K (Figure 2)
exhibited the highest-energy vibronic emission at 458 nm,
from which the triplet energy (T1) of the compound was
determined to be 2.71 eV. Under the same conditions, the
triplet energy of FIr6 was estimated to be 2.73 eV.[8]
The new compound POBPmDPA possessed a nearly
equal triplet energy to that of FIr6. To explore the nature of
the photo-excited process between the host POBPmDPA and
Figure 3. Photoluminescence spectra of FIr6-POBPmDPA composite
film (10 wt%) at room temperature and 77 K (excitation at 330 nm).
PL intensity of FIr6 emission in FIr6-POBPmDPA composite
film displayed almost monoexponential decay (7 ns (6%) and
2.2 ms (94%); Supporting Information, Figure S5). The life-
time was nearly equal to that of FIr6 in FIr6-PMMA
composite film at the same conditions. The lifetime domain
of the FIr6-POBPmDPA composite film was assigned to FIr6
emission, indicating that the photo-excited excitons of FIr6
could not be quenched by the host POBPmDPA.
Though the compound POBPmDPA showed almost equal
triplet energy to FIr6, the efficient energy transfer from
POBPmDPA to FIr6 in the photo-excited process encouraged
us to fabricate the FIr6-based PhOLEDs. We initially
fabricated the blue device Awith the following configuration:
Figure 1. UV/Vis absorption spectrum in CH2Cl2 solution (10À5 m) and
fluorescence spectra in toluene (&, 10À5 m) and a neat film ( ) at
ITO/MoO3
(10 nm)/TAPC
(60 nm)/POBPmDPA:FIr6
*
room temperature.
(20 nm)/TmPyPB (35 nm)/LiF/Al. In device A, MoO3 and
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coordinates of (0.15, 0.25) during operational voltages of 5–
8 V (inset of Figure 4).
To study the influence of adjacent hole- and electron-
transporting materials on the FIr6-based PhOLEDs, we
further fabricated devices B and C. Compared to device A,
NPB (1,4-bis(1-naphthylphenylamino)-biphenyl) with a low
triplet energy of 2.41 eV was used to replace TAPC as the
hole-transporting material in device B, whereas BCP (2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline) with a low triplet
energy of 2.50 eV was used to replace TmPyPB as the
electron-transporting material in device C.[11] The current
efficiency, power efficiency versus current density along with
EL spectra of devices B and C are shown in the Supporting
Information, Figure S7, and all the data are collected in
Table 2. Usually, lower triplet-energy hole-/electron-trans-
porting materials than that of phosphor could not block the
diffusion of triplet excitons from EML. This may result in the
change of EL spectrum or the decreased device efficiency.[12]
Devices B and C, which use low triplet-energy hole- and
electron-transporting materials, still displayed the typical EL
spectra originated from FIr6 emission, with CIE coordinates
of (0.16, 0.30) and (0.15, 0.23), respectively. This implied that
the FIr6 excitons were well formed and manipulated inside
the EML in devices B and C.
Furthermore, we also noted that device B exhibited high
efficiencies with a maximum current efficiency of 37 cdAÀ1,
a maximum power efficiency of 40 LmWÀ1, and a maximum
EQE of 17.9%, which were comparable to those of device A.
To the best of our knowledge, the device represented the
highest power efficiency (PE) for FIr6-based blue PhOLEDs,
and even higher than those of FIr6-based PhOLEDs with n-
doped ETL (PEmax of 39.2 LmWÀ1)[10a] or p-i-n structure
(PEmax of 36 LmWÀ1).[10d] In contrast, device C showed fair
efficiencies with a maximum current efficiency of 9 cdAÀ1,
a maximum power efficiency of 7 LmWÀ1, and a maximum
EQE of 5.3%. This suggested that the formed excitons in
EML could be partially quenched by the low triplet energy of
BCP in device C, whereas they could be hardly quenched by
the low triplet energy of NPB in device B. We presumed that
the exciton-recombination zone be near the interface
between the EML and the ETL in devices. Why has this
situation arisen? One factor is that the holes in device C could
be accumulated in the interface between the EML and ETL
owing to the poor electron mobility of BCP in comparison
with TmPyPB in device A (electron mobility: BCP
ꢀ 10À6 cm2 VÀ1 sÀ1 vs. TmPyPB ꢀ 10À3 cm2 VÀ1 sÀ1).[13] Another
factor was that the host material POBPmDPA may have
significantly higher hole mobility than its electron mobility.
To confirm the hypothesis, we investigated the electron-/
hole-transporting characters of POBPmDPA by fabricating
Figure 4. Current efficiency and power efficiency versus current density
of device A (Inset: EL spectra at 5–8 V).
LiF served as the hole- and electron-injecting materials; FIr6
doped in POBPmDPA with optimized doping level of 10%
was used as the emitting layer. TAPC [1,1-bis{(di-4-tolylami-
no)phenyl}cyclohexane] and TmPyPB [1,3,5-tri(m-pyrid-3-yl-
phenyl)benzene)] acted as the hole- and electron-transporting
materials, respectively. It should be mentioned that TAPC
and TmPyPB possess higher triplet energies (TAPC, T1 =
2.87 eV; TmPyPB, T1 = 2.78 eV) than that of FIr6 to confine
the excitons in the EML.[9] The current efficiency and power
efficiency versus current density of device A were shown in
Figure 4. Remarkably, device A displayed rather high effi-
ciencies with a maximum current efficiency of 34 cdAÀ1,
a maximum power efficiency of 34 LmWÀ1, and a maximum
EQE of 18.1%. At a practical brightness of 100 cdmÀ2
(0.35 mAcmÀ2, 3.9 V), device A still had a high current
efficiency of 33 cdAÀ1, power efficiency of 26 LmWÀ1, and
EQE of 17%. These efficiencies were comparable to the best
results of FIr6-based devices summarized in Table 1. More-
over, these high efficiencies were acquired in a three organic-
layer device structure, in contrast to those high-efficiency blue
PhOLEDs required at least four organic layers.[5a,9a,10] It is
also noteworthy that the electroluminance (EL) of device A
showed typical and stable FIr6 emission with good CIE
Table 1: Summary of some host materials for FIr6-based devices.
Host material
S1
T1
CEmax
PEmax
EQEmax
[%][e]
[eV][a] [eV][b] [cdAÀ1 [c]
]
[lmWÀ1 [d]
]
BCPO[5a]
Ad-Pd[9b]
POBPCz[9c]
3.2
–
3.1
3.01 36.8
2.97
3.01 40
2.71 34
33.1
33
36
19.8
19
19.5
18.1
–
POBPmDPA (this work) 2.8
34
[a] Singlet state. [b] Triplet state. [c] Maximum current efficiency.
[d] Maximum power efficiency. [e] Maximum external quantum efficiency.
Table 2: Summary of devices A–C with the following device configuration: ITO/MoO3 (10 nm)/HTL/POBPmDPA:FIr6 (20 nm)/ETL/LiF/Al.
CIE (x, y)[e]
[a]
[b]
[c]
[d]
Device
HTL
ETL
Von
CEmax
PEmax
EQEmax
A
B
C
TAPC (T1 =2.87 eV) (60 nm)
NPB (T1 =2.41 eV) (70 nm)
TAPC (T1 =2.87 eV) (60 nm)
TmPyPB (T1 =2.78 eV) (35 nm)
TmPyPB (T1 =2.78 eV) (35 nm)
BCP (T1 =2.5 eV) (35 nm)
2.9 V
2.9 V
3.3 V
34 cdAÀ1
37 cdAÀ1
9 cdAÀ1
34 lmWÀ1
40 lmWÀ1
7 lmWÀ1
18.1%
17.9%
5.3%
(0.15, 0.25)
(0.16, 0.30)
(0.15, 0.23)
[a] Turn-on voltages at 1 cdmÀ2. [b] Maximum current efficiency. [c] Maximum power efficiency. [d] Maximum external quantum efficiency. [e] CIE
coordinates at 6 V.
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deposition rates of both host and guest were controlled with their
correspondingly independent quartz crystal oscillators. The evapo-
ration rates were monitored by a frequency counter and calibrated by
a Dektak 6m profiler (Veeco). The current density–voltage–lumi-
nance of the device was measured with a Keithey 2400 Source meter
and a Keithey 2000 Source multimeter equipped with a calibrated
silicon photodiode. The EL spectra were measured by a JY SPEX
CCD3000 spectrometer. All of the measurements were carried out at
room temperature under ambient conditions.
Received: September 13, 2013
Revised: October 29, 2013
Published online: January 21, 2014
Keywords: blue phosphorescence ·
organic light-emitting diodes · triplet state
.
Figure 5. Current density versus voltage of the hole- and electron-only
devices.
the hole-only device with the structure: ITO/MoO3 (10 nm)/
TAPC (10 nm)/POBPmDPA (30 nm)/TAPC (10 nm)/MoO3
(10 nm)/Al, and the electron-only device with the structure:
ITO/LiF (1 nm)/TmPyPB (10 nm)/POBPmDPA (30 nm)/
TmPyPB (10 nm)/LiF (1 nm)/Al. In the two devices, MoO3
and LiF served as the hole- and electron-injecting materials;
TAPC and TmPyPB were used to prevent electron and hole
injecting from the cathode and anode, respectively. As
depicted in the current density versus voltage curve
(Figure 5), the hole-only device exhibited substantially
higher current density than the electron-only device under
the same voltage, manifesting the dominating hole-trans-
porting ability of POBPmDPA.
In conclusion, our research has revealed that efficient
energy transfer from a low singlet-/triplet-state host material
to the blue phosphor of FIr6 can be realized both in photo-
and electro-excited processes. Providing that hole- and
electron-transporting materials with high triplet energies
and good carrier mobilities were used, highly efficient blue
PhOLEDs with the maximum current efficiency of 34 cdAÀ1
and power efficiency of 34 LmWÀ1 were achieved, even
though the triplet energy of host material was not higher than
the blue phosphor. The encouraging results would bring down
the threshold for designing host materials and open a new way
for the development of blue PhOLEDs. The energy transfer
process between the new host and guest in these blue
PhOLEDs will be further investigated.
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