Novel efficient blue fluorophors with small singlet-triplet splitting: Hosts for highly efficient fluorescence and phosphorescence hybrid WOLEDs with simplified structure

Article abstract of DOI:10.1002/adma.201204724

The exact hosts for F-P hybrid WOLEDs have been first demonstrated following a new design strategy for blue fluorophors with small singlet-triplet splitting. Two novel compounds DPMC and DAPSF exhibit efficient blue fluorescence, high triplet energies and good conductivities. These merits allow us to use new simplified device designs to achieve high efficiency, slow efficiency roll-off and stable emission color. Copyright

Full text of DOI:10.1002/adma.201204724

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Novel Efficient Blue Fluorophors with Small Singlet-Triplet
Splitting: Hosts for Highly Efficient Fluorescence and
Phosphorescence Hybrid WOLEDs with Simplified
Structure
Cai-Jun Zheng, Jing Wang, Jun Ye, Ming-Fai Lo, Xiao-Ke Liu, Man-Keung Fung,
Xiao-Hong Zhang,* and Chun-Sing Lee*
To solve the stability problem of blue phosphor in phosphores-
cent white organic light-emitting device (WOLED),[1] Forrest
considered as the current key point for further enhancing the
performance of F-P hybrid WOLEDs.[4]
et al proposed
a
fluorescence-phosphoresce (F-P) hybrid
Singlet energy level of typical blue fluorophors and triplet
energy level of common green phosphors are about 2.9 and
2.4 eV, respectively. This implies that to fulfill the new require-
ment of the ideal F-P hybrid WOLEDs host, EST of the blue
fluorophor has to be smaller than 0.5 eV. Up till now, most
high performance blue fluorescent materials are based on π−π∗
transition and their EST are usually larger than 0.6 eV.[5] This
explains why such fluorophor has not yet been found.
Further considering the n–π∗ transition based compounds
normally have very weak fluorescence, the most probable way
to achieve the efficient fluorophor with small singlet-triplet
splitting is via compounds based on intramolecular charge
transfer (ICT) transition.[5] In an ICT based compound, the
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) would be located at the
electron-donating group (D) and the electron-accepting group
(A), respectively. Thus, the spatial overlap between HOMO
WOLED. In their devices, a blue fluorescent emitter is used for
better stability, while green and red phosphors are used for high
efficiency triplet emissions[2]. The use of blue fluorescent emit-
ters implies that only roughly 25% (i.e. only the singlet) of the
excitons produced in them are used for photon generation. It is
usually difficult to make use of the remaining triplet excitons
due to their typically low-lying triplet states. Recently, Leo et al
have reported a blue fluorescent material (4P-NPD) with a high
triplet energy, such that its triplet excitons can sensitive triplet
emission of an orange phosphor.[3] This represents the first F-P
hybrid WOLED with close to 100% exciton utilization.
While this work of Leo et al represents an important break
through, there is still room for improvement in two aspects.
Firstly, as there is only one exciton generation zone in devices
of this concept, carrier utilization would decrease evidently
under high current density. Indeed, obvious efficiency roll-off
has been observed for this device. Besides, as the triplet energy
level of 4P-NPD is not high enough to sensitize the green phos-
phor, the red and the green phosphors have to be doped in dif-
ferent hosts. Thus, color of the WOLED varies considerably at
different driving voltages. Therefore, finding an efficient blue
and LUMO would be minimal, which is required for the small
[4]
EST
.
However, too small overlap will reduce the fluorescence
efficiency,[4] therefore, to achieve efficient ICT-based fluorophor,
the electron-donating group and the electron-accepting group
should be bridged into a suitable π-conjugation system (π) as
shown in Figure 1. In such a D-π-A structure, the π segment
should be of an appropriate length to maintain a small HOMO-
LUMO overlap and thus a small EST, while not suppressing
the fluorescence of the compound. Following such strategy,
in this work, we report the design and synthesis of two blue
fluorophor with sufficiently small singlet-triplet splitting (EST
to allow the sensitization of other green phosphors has been
)
Dr. C.-J. Zheng, Dr. J. Wang, Dr. J. Ye,
X.-K. Liu, Prof. X.-H. Zhang
Nano-organic Photoelectronic Laboratory and
Key Laboratory of Photochemical Conversion and
Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190, P.R. China
E-mail: xhzhang@mail.ipc.ac.cn
Dr. C.-J. Zheng, Dr. M.-F. Lo, Dr. M.-K. Fung, Prof. C.-S. Lee
Center of Super-Diamond and Advanced Films (COSDAF) &
Department of Physics and Materials Sciences
City University of Hong Kong
Hong Kong SAR, P.R. China
E-mail: apcslee@cityu.edu.hk
DOI: 10.1002/adma.201204724
Figure 1. Design strategy for new ICT transition based blue fluorophors.
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T a b le 1 . Summary of physical properties of the two fluorophors.
ox c)
red c)
d)
e)
a)
a)
b)
b)
Φfa)
[%]
λmax,f
λmax,p
compound
T / Tm/ Td
[°C]
E1/2
[V]
HOMOc)
[eV]
E1/2
[V]
LUMOc)
[eV]
Eg
Eg
λmax,abs
[nm]
λmax,f
g
[eV]
3.10
2.87
[eV]
3.13
2.95
[nm]
464
475
[nm]
410
424
[nm]
470
507
DPMC
DAPSF
360
366
79
74
50/157/269
112/255/426
1.04
1.01
−5.78
−5.75
−2.06
−1.86
−2.68
−2.98
c)
a)Measured in CH2Cl2 solution at room temperature; b)Measured in 2-MeTHF solution at 77K;
E
1/2
ox: half-wave oxidation potentials vs. SCE; E1/2red: half-wave reduc-
tion potentials vs. SCE. HOMO = − E1/2 (vs SCE) − ESCE and LUMO = E1/2 (vs SCE) − ESCE, the energy level of SCE (ESCE) is 4.74eV vs. vacuum level.[18]; d)The energy
ox
red
bandgap, calculated from the CV. [e] The energy band gap, calculated from the absorption spectra.
fluorophors, 7-(di-phenylamino)-4-methoxycoumarin (DPMC)
and di[4-(4-diphenylaminophenyl) phenyl] sulfone (DAPSF).
As shown in Figure 1, in DPMC molecule, the di-phenylamino
group is the electron-donor moiety, the formate group is the
electron-acceptor moiety and the two moieties are linked
by the styryl group. Similarly, in a DAPSF molecule, the di-
phenylamino groups are also electron-donor moiety, the sulfonyl
group is the electron-acceptor moiety and the biphenyl group
is the conjugation bridge linkage group. Based on such appro-
priate styryl and biphenyl bridge linkage groups, as expected,
the ICT transition does endow the two compounds with small
singlet-triplet splittings (0.39 and 0.48 eV) and high fluorescent
quantum yields (79% and 74%). Furthermore, containing both
hole- and electron-transporting moieties, the two compounds
were found to have bipolar transporting properties. These
enable the design of a new simplified device strategy in which
the blue fluorophors are used (1) to generate blue singlet emis-
sion and (2) as hosts and sensitizers for both the red and green
phosphors. In addition to the theoretical feasibility of utilizing
all generated excitons, the present device design also allows a
wide spread emission zone within a single blue-emitting host.
These should lead to better carrier utilization at high cur-
rent densities and a more stable color with respect to voltage
changes. Indeed, our optimized WOLEDs based on DPMC
and DAPSF respectively show low turn-on voltages of 3.4 and
2.4 V, high maximum total external quantum efficiency (EQE)
of 21.0% and 20.2%, slow efficiency roll-offs (EQEs are 20.2%
and 16.1% even at 1000 cd m−2 ) and stable white emissions,
which present the best results in reported F-P hybrid WOELDs,
and are even close to the best fully phosphorescent WOLEDs.
This study could provide a useful strategy for designing blue
fluorophors and device configuration for F-P hybrid WOLEDs.
The synthetic routes of the two new compounds DPMC and
DAPSF are shown in Scheme S1. The aryl group was introduced to
the amino group via Ullmann reaction[6] starting with 3-methoxy-
aniline, followed by BBr3 treatment to give the corresponding
phenol, which was further condensed with malonic acid using
phosphorus oxychloride, and reacted with dimethyl sulfate to yield
the target DPMC. To get DAPSF, 4,4′-dibromodiphenylsulfide
was readily prepared by electrophilic aromatic substitution of Br2
onto diphenyl sulfide, and then followed by oxidation reaction to
yield di(4-bromophenyl)sulfone. The target compound was finally
obtained via Suzuki cross-coupling reaction[7] of the resulting
bromized aromatic sulfone with the corresponding boronic acid.
DPMC and DAPSF were characterized by high-resolution mass
spectrometry, 1H NMR and 13C NMR spectroscopy.
absorption peaks of DPMC and DAPSF in CH2Cl2 are at 360
and 366 nm, respectively, which could be assigned to ICT transi-
tions of the compounds. Both compounds show blue emissions
(464 nm for DPMC, and 475 nm for DAPSF) in CH2Cl2 at room
temperature. As shown in Figure S2, the PL spectra of two com-
pounds are red-shift with the increase of the solvent’s polarity,
which could once again prove that both DPMC and DAPSF are
based on ICT transitions. Fluorescent quantum yields (Φf) of
DPMC and DAPSF were respectively determined to be 79% and
74% in CH2Cl2, by respectively using coumarin 1 (Φf = 99% in
ethyl acetate)[8] and quinine sulfate (Φf = 56% in 1 N H2SO4)[9] as
calibration standards. As listed in Table 1, the shortest-wavelength
phosphorescence peaks of DPMC and DAPSF in 2-MeTHF at
77 K are at 470 and 507 nm. Triplet energy levels of DPMC and
DTMC were thus respectively determined to be 2.64 and 2.45 eV,
which are higher than the triplet energy of common green phos-
phorescence dopants such as fac-tris(2-phenylpyridine) iridium
(Ir(ppy)3) (2.4 eV). From the fluorescence and phosphorescence
emissions of two compounds at 77 K,[10] the ESTs of DPMC and
DAPSF were determined to be 0.39 and 0.48 eV, respectively.
The high fluorescent quantum yields, suitable triplet energy
levels and small EST values of DPMC and DAPSF fully fulfill the
requirements of the exact hosts for F-P hybrid WOLEDs.[4]
The electrochemical properties of two blue fluorophors were
investigated by cyclic voltammograms. DPMC and DAPSF
both exhibit the quasi-reversible waves in their cyclic voltam-
mograms, which hints the possible bipolar transporting proper-
ties of the two compounds.[11,12] The HOMO and LUMO energy
levels of DPMC and DAPSF were determined from the half-
wave potentials of the oxidation and reduction curves (relative
to vacuum level),[10] and listed in Table 1.
Thermal properties of DPMC and DAPSF were characterized
using differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) in a nitrogen atmosphere at a scanning
rate of 10 K min−1 . Measured glass transition temperatures (Tg),
melting temperatures (Tm) and decomposition temperatures
(Td) of two blue fluorophors have been listed in Table 1. It can
be see that DPMC has a Tg of only 50 °C which is obviously too
low to enable a good thermal stability in its solid film. Com-
paring to the D-π-A structure of DPMC, the D-π-A-π-D struc-
ture of DAPSF has a much more bulky structure which leads to
a higher Tg (112 °C) and better thermal stability.
To check whether DPMC and DAPSF can be used as blue
host emitters, devices with a configuration of ITO/NPB (30 nm)/
TCTA (10 nm)/DPMC or DAPSF (30 nm)/TPBI (30 nm)/
LiF (1.5 nm)/Al were fabricated. In these devices, ITO (indium
tin oxide) and LiF/Al are the anode and the cathode, respec-
tively; 4,4′-bis[N-(1-naphthyl)-N-phenyl amino]biphenyl (NPB)
The absorption and photoluminescence (PL) spectra of the two
blue fluorophors are shown in Figure S1. The longest-wavelength
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1.0
(a)
DAPSF
(b)
DPMC
1.0
0.8
0.6
0.4
0.2
0.0
10 cd/m2
10 cd/m2
100 cd/m2
1000 cd/m2
0.8
0.6
0.4
0.2
0.0
100 cd/m2
1000 cd/m2
350
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength(nm)
Wavelength (nm)
10
10 cd/m2
(d)
1.0
0.5
0.0
(c)
DPMC
DAPSF
100 cd/m2
1000 cd/m2
10000 cd/m2
8
6
4
2
0
400
500
600
700
800
0.1
1
10
100
Current density (mA/cm2)
Wavelength(nm)
(e)
1.0
0.8
0.6
0.4
0.2
0.0
10 cd/m2
100 cd/m2
1000 cd/m2
400
500
600
700
800
Wavelength(nm)
Figure 2. (a) EL spectra of the DPMC based blue fluorescent OLED. (b) EL spectra of the DAPSF based blue fluorescent OLED. (c) EQE-current density
plots of the DPMC and DAPSF based blue fluorescent OLEDs. (d) EL spectra of the DPMC based green phosphorescent OLED. (e) EL spectra of the
DAPSF based green phosphorescent OLED.
As shown in Figures 2(a) and 2(b), both devices exhibit stable
blue electroluminescence (EL) spectra at different luminances.
CIE coordinates of the DPMC and DAPSF based devices are
respectively (0.154, 0.085) and (0.138, 0.152). Figure 2(c)
shows EQE-current density characteristics of the two devices.
is the hole-transporting layer (HTL); 4,4′,4″-tris(N-carbazolyl)
triphenylamine (TCTA) is the electron-blocking layer (EBL),
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) is the
electron-transporting layer (ETL) and hole-blocking layer (HBL);
and DPMC and DAPSF are used as the emitting layer (EML).
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The maximum EQEs of the DPMC- and
the DAPSF- based devices are 4.93 and
7.64%, respectively. The lower efficiency
of the DPMC-based device might be due to
the stronger exciton annihilation between
small-steric-bulk DPMC compounds. Con-
sidering the 5% limit for EQE of fluorescent
devices,[13] both blue fluorescent devices
exhibit extremely high EQE. The EQE value
of 7.64% is even higher than the theoretical
limit, which probably because the small EST
enables delayed fluorescence arising from
triplet-triplet annihilation (TTA).[14] To prove
the existence of delayed fluorescence, tran-
sient PL decay characteristics of DAPSF were
measured in oxygen-free methanol solution
at 300K. As shown in Figure S5, two lifetimes
of 4 ns and 40 ms have been observed, which
could be assigned to ordinary fluorescence
and delayed fluorescence, respectively.
To check the bipolar transporting prop-
erties of the blue fluorophors, two types
of devices with configurations of ITO/
NPB(30nm)/TCTA(10nm)/DPMC or DAPSF
(60nm)/LiF (1.5nm)/Al and ITO/DPMC or
DAPSF
(70nm)/TPBI(30nm)/LiF(1.5nm)/
Al were fabricated. In addition to their role
as blue emitter, the two blue fluorophors
as function as the ETL and HTL respec-
tively in the first and second configurations.
As shown in Figure S6, all the four devices
successfully exhibited blue emissions from
DPMC or DAPSF, which indicates that both
compounds can transport hole and electron.
Furthermore, from Figure S7, DAPSF shows
better and more balanced carriers trans-
porting properties than DPMC.
Considering that the triplet energy level
in film at room temperature may be red-
shifted comparing with that measured at
77 K in solution, Ir(ppy)3-doped devices
were fabricated to check whether two blue
fluorophors could indeed be used as hosts
to sensitize green phosphorescent materials.
The devices have a simple configuration of
ITO/NPB (30 nm)/TCTA (10 nm)/DPMC or
Figure 3. Energy level diagrams in DPMC and DAPSF based F/P-WOLEDs. Solid lines corre-
DAPSF: 2% Ir(ppy)3 (30 nm)/TPBI (30 nm)/ spond to HOMO and LUMO energy levels; circles and triangles refer to the exciton (S0, S1, and
T1) energies; the gray filled box depicts the main exciton generation zones.
LiF (1.5 nm)/Al. Figures 2(d) and 2(e) show
EL spectra of two devices at different lumi-
nances. It can be seen that the EL spectra mainly show the
green Ir(ppy)3 emission. These confirm that two compounds
can sensitize triplet emission of the green phosphor.
DPMC- and DAPSF- based F-P hybrid WOLEDs with a con-
figuration of ITO/NPB (30 nm)/TCTA (10 nm)/EML (30 nm)/
TPBI (30 nm)/LiF (1.5 nm)/Al were then fabricated. Details
of the DPMC and DAPSF-based EML have been shown in
Figure 3.[2,3,15,16] The EML has a composite structure of undoped
host (6 nm)/Ir(ppy)3 doped host (8 nm)/Ir(2-phq)3 doped host
(10 nm)/undoped host (6 nm). This unique simplified device
configuration for WOLED is enabled by the two hosts with
(i) blue fluorescence, (ii) high enough triplet energy to sensitive
both green and red phosphors, and (iii) abilities to transport
both electron and hole.
As TCTA and TPBI mostly conduct holes and electrons
respectively, the main exciton generation zones would be
located at both the TCTA/EML and EML/TPBI interfaces.[2]
Considering that singlet excitons only have very short diffusion
lengths of about 3 nm, we place 6–10 nm of undoped DMPC
or DAPSF spacers at the two sides of the EML to capture the
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(b) DAPSF
(a) DPMC
200
150
100
50
10000
1000
100
300
200
100
0
10000
1000
100
10
10
0
1
1
0
2
4
6
8
10
0
2
4
6
8
10
Voltage (V)
Voltage (V)
100
100
(c)
(d) DPMC
1.0
0.5
0.0
DPMC
DAPSF
10 cd/m2
100 cd/m2
1000 cd/m2
10000 cd/m2
10
10
1
300
400
500
600
700
800
1
10
100
1000
10000
Luminance (cd/m2)
Wavelength(nm)
10 cd/m2
1.0
0.8
0.6
0.4
0.2
0.0
(e) DAPSF
100 cd/m2
1000 cd/m2
10000 cd/m2
400
500
600
700
800
Wavelength (nm)
Figure 4. (a) Current density-voltage and luminance-voltage characteristics of the DPMC based F-P hybrid WOLED. (b) Current density-voltage and
luminance-voltage characteristics of the DAPSF based F-P hybrid WOLED. (c) EQE-luminance and PE-luminance plots of the DPMC and DAPSF based
F-P hybrid WOLED. (d) The EL spectra of the DPMC based F-P hybrid WOLED. (e) The EL spectra of the DAPSF based F-P hybrid WOLED.
singlet excitons for blue emission. The triplet excitons can then
transfer to the green or the red phosphors doped in the middle
regions of the EML for their long diffusion lengths of 100 nm.
For the large gaps and high triplet energy levels of TCTA and
TPBI would effectively confine all excitons such that they can
be used in the EML with minimal wastage.
Figures 4(a) and 4(b) show current density-luminance-voltage
characteristics of the two devices. The DPMC- and DAPSF-
based white devices have low turn-on voltages (voltage to get
1 cd/m2) of 3.4 and 2.4 V, respectively, which is remarkable for
WOLEDs without using the p-i-n structures.[17–21] The turn-
on voltage of 2.4 V is in fact comparable with those of devices
using the p-i-n structures.[2,3] Such superior performance could
be attributed to the bipolar conductivity of the fluorophors. As
illumination sources are generally characterized by their total
emitting power,[2,17] the maximum total EQEs of DPMC and
DAPSF based white devices reach 21.0% and 20.2%, respec-
tively, which is even slightly higher than the 20% limit of EQE
of conventional OLEDs.[4] With two exciton generation zones in
devices and excellent carrier blocking effects of TCTA and TPBI,
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both devices exhibit relatively mild efficiency roll-off. Even at a
high luminance of 1000 cd m−2 , the DPMC and DAPSF based
white devices still gives a high total EQE levels of 20.2% and
16.1%. Due to the extremely low turn-on voltages, the DAPSF
based white device has a maximum total power efficiency (PE)
of 48.2 lm W−1 , and keep a high total PE of 26.8 lm W−1 at a
high luminance of 1000 cd m−2 . As listed in Table S3, such high
and stable efficiency performances of these structure-simplified
hybrid white devices have already presented the best results
in reported F-P hybrid WOELDs, even are closed to the best
WOLEDs based on p-i-n structures or using a fully phosphores-
cent strategy.[2–4,15,17–36]
EL spectra of the two F-P hybrid WOLEDs at different
luminances are shown in Figures 4(d) and 4(e), and the corre-
sponding CIE values are listed in Table S1. With increasing cur-
rent density, relative intensities of both blue and green emis-
sions increase. The reason for green emission increase might
be that the higher concentration of triplet excitons can let more
triplet excitons stay on the green phosphor, which has a higher
triplet energy level for emission. The increase in blue emission
is probably due to TTA at high exciton concentrations. At dif-
ferent luminances, both devices exhibits warm white emission.
At a practical luminance of 1000 cd m−2 , DPMC and DAPSF
based white devices exhibit good color rendering index (CRI) of
75 and 72, respectively.
More structurally-simplified single emission layer white
device based on DAPSF has also been fabricated with a con-
figuration of ITO/NPB (30 nm)/TCTA (10 nm)/DAPSF: 0.5%
Ir(ppy)3: 0.2% Ir(2-phq)3 (30 nm)/TPBI (30 nm)/LiF (1.5 nm)/
Al. The low dopant concentrations in the EML could increase
the average distance between dopant and host molecules, thus,
part of the singlet excitons could remain on the DAPSF host
to give blue fluorescence due to the different diffusion lengths
for singlet and triplet excitons. As shown in Figure S9, the
single-EML white device also exhibit outstanding performance
with a low turn-on voltage of 2.4 V, maximum total EQE and
PE of 21.8% and 57.3 lm W−1 , and high EQE and PE of 14.6%
and 24.4 lm W−1 at a high luminance of 1000 cd m−2 with a
CRI of 60. Compared with the multi-EML white device, the
single-EML device shows more obvious efficiency roll-off and
spectral changes at high current densities, which is probably
caused by incomplete triplet utilization due to the low dopant
concentration.
devices.[2–4,15,17–36] This research provides a useful strategy in
the design of new blue fluorophors and device configuration for
F-P hybrid WOLEDs.
Experimental Section
DPMC: 1H NMR (400 MHz, Acetonitrile-d3) δ = 7.56(d, J = 8.8 Hz,
1H), 7.35-7.30(m, 4H), 7.17–7.13(m, 6H), 6.87(d, J = 8.8 Hz, 1H), 6.80(s,
1H), 5.52(s, 1H), 3.95(s, 3H); 13C NMR (400 MHz, CDCl3) δ = 166.9,
163.6, 154.9, 152.0, 146.3, 129.8, 126.2, 125.1, 123.6, 116.4, 108.6, 106.9,
87.5, 56.3; HRMS calcd for C22H17NO3: 343.1208; found, 343.1212. Anal.
calcd. for C22H17NO3: C, 76.95; H, 4.99; N, 4.08. Found: C, 76.92; H,
5.02; N, 4.05.
DAPSF: 1H NMR (400 MHz, CDCl3, δ): 7.99 (d, J = 8.5 Hz, 4H), 7.67
(d, J = 8.5 Hz, 4H), 7.43 (d, J = 8.7 Hz, 4H), 7.28 (t, J = 8.2 Hz, 8H),
7.14–7.04 (m, 16H). 13C NMR (100 MHz, CDCl3, δ): 148.6, 147.4, 145.6,
139.7, 132.3, 129.5, 128.3, 128.1, 127.3, 125.0, 123.6, 123.1. HRMS
(m/z): calcd for C48H36N2O2S: 704.2497; found, 704.2052. Anal. calcd.
for C48H36N2O2S: C, 81.79; H, 5.15; N, 3.97. Found: C, 81.73; H, 5.17;
N, 4.02.
OLED Fabrication and measurement: ITO coated glass with a sheet
resistance of 30 Ω square−1 was used as the substrate. Before device
fabrication, the ITO glass substrates were cleaned with isopropyl
alcohol and deionized water, dried in an oven at 120 °C, treated with
UV-ozone, and finally transferred to a vacuum deposition system with a
base pressure better than 1 × 10−6 torr for organic and metal deposition.
The devices were fabricated by evaporating organic layers with an
evaporation rate of 1-2 Å s−1 . The cathode was completed through
thermal deposition of LiF at a deposition rate of 0.1 Å s−1 , and then
capped with Al metal through thermal evaporation at a rate of 10 Å s−1
.
EL luminescence, spectra and CIE color coordinates were measured with
a Spectrascan PR650 photometer and the current-voltage characteristics
were measured with a computer-controlled Keithley 2400 SourceMeter
under ambient atmosphere.
Acknowledgements
The authors thank Dr B. H. Zhang for CRI calculation. This work was
supported by the National Natural Science Foundation of China (Grant
51103169, 50825304, 51033007, 51128301), Beijing Natural Science
Foundation (Grant No. 2111002), the National High-tech R&D Program
of China (863 Program) (Grant No. 2011AA03A110) and the Instrument
Developing Project of the Chinese Academy of Sciences (Grant No.
YE201133), P. R. China. Prof. C. S. Lee would like to acknowledge
support from the Research Grants Council of the Hong Kong (Project
No. T23-713/11).
In conclusion, a new design strategy for ICT transition
based blue fluorophors has been proposed to address a cur-
rent key issue in F-P hybrid WOLED research. Following such
strategy, two novel blue fluorophors DPMC and DAPSF have
been designed and synthesized with efficient blue fluorescence,
high triplet energies and good conductivities. These enable us
to design a new simplified F-P hybrid WOLED device using a
single host material to give blue fluorescence and to sensitize
both green and red phosphorescent dopants. The white devices
based on DPMC and DAPSF show warm white emissions at
different luminances, and exhibit outstanding performance
respectively with the low turn-on voltages of 3.4 and 2.4 V, max-
imum total EQEs of 21.0% and 20.2%, and high total EQEs of
20.2% and 16.1% at a high luminance of 1000 cd m−2 , which
present the best results in reported F-P hybrid WOELDs and
are closed to those of high performance fully phosphorescent
Received: November 15, 2012
Revised: December 25, 2012
Published online: February 18, 2013
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Adv. Mater. 2013, 25, 2205–2211
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