Novel blue fluorophor with high triplet energy level for high performance single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes

Article abstract of DOI:10.1021/cm403318r

On the basis of a D-π-A structural strategy incorporating diphenylamine as an electron-donor and 1,3,5-triazine as an electron-acceptor with a short benzene π-conjugated feature, 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N- diphenylaniline (POTA), a novel bl

Full text of DOI:10.1021/cm403318r

Article
pubs.acs.org/cm
Novel Blue Fluorophor with High Triplet Energy Level for High
Performance Single-Emitting-Layer Fluorescence and
Phosphorescence Hybrid White Organic Light-Emitting Diodes
Xiao-Ke Liu,†,‡ Cai-Jun Zheng,†,§,* Ming-Fai Lo,§ Jing Xiao,∥ Zhan Chen,†,‡ Chuan-Lin Liu,†
Chun-Sing Lee,§,* Man-Keung Fung,§ and Xiao-Hong 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, People’s Republic of China
‡University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
§Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of
Hong Kong, Hong Kong SAR, People’s Republic of China
∥College of Physics, Electronic Engineering, Taishan University, Taian, Shandong 271021, People’s Republic of China
S
* Supporting Information
ABSTRACT: On the basis of a D-π-A structural strategy incorporating diphenylamine as an
electron-donor and 1,3,5-triazine as an electron-acceptor with a short benzene π-conjugated
feature, 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA), a novel blue
fluorophor with high triplet energy level has been designed and synthesized to achieve
high performance single-emitting-layer (single-EML) fluorescence and phosphorescence
hybrid white organic light-emitting diodes (F−P hybrid WOLEDs). POTA exhibits efficient
blue electrofluorescence and excellent host features for green and red phosphorescent
devices. The single-EML RGB F−P hybrid WOLED based on POTA and green/red
complexes shows a maximum total power efficiency of 59.8 1.0 lm W−1 and a maximum
total external quantum efficiency of 24.7 0.7%, representing the highest efficiencies among
single-EML RGB WOLEDs. Moreover, these performances are comparable with multiple-
EML RGB WOLEDs even based on p-i-n structures, providing verification of the high
performance and simple structure of single-EML RGB F−P hybrid WOLEDs.
KEYWORDS: fluorescence and phosphorescence hybrid WOLEDs, blue fluorophors, single-emitting-layer, triazine
Generally, white light is formed in WOLEDs by two
complementary colors or three primary colors (red, green,
and blue).2b As the backlight of full-color displays, white light
would be separated into red (R), green (G) and blue (B) color
by the color filters. For solid-state lighting applications, white
light with an entire visible region spectrum provides better high
color-rending capability for objects.1c Thus, adopting an RGB-
emitter system is clearly preferable to realize white light
emission that has high efficiencies and high light qualities.4 In
the past decade, great progress has been made in high
performance RGB F−P hybrid WOLEDs with multiemitting-
layer (EML).3a,b,5 For example, in 2006, Forrest’s group
constructed an F−P hybrid WOLED by doping a blue
fluorophor and green/red phosphors into CBP in separate
emitting-layers, showing a maximum total external quantum
efficiency (EQE) and power efficiency (PE) of 18.7% and 37.6
lm W−1, respectively.3a In 2007, Leo and his co-workers
reported an F−P hybrid WOLED with higher performance by
INTRODUCTION
■
White organic light-emitting device (WOLED) technique has
attracted much attention because of its great potential in solid-
state lighting and displays.1 Many innovative device architec-
tures have been devised using various material systems to
achieve high performance WOLEDs.2 Among them, the
fluorescence and phosphorescence hybrid WOLED (F−P
hybrid WOLED), which can separately utilize the singlet
excitons with a blue fluorescent emitter and the triplet excitons
with green, orange, and red phosphorescent dopants, is
considered an ideal candidate.3 Compared with traditional
fully phosphorescent WOLEDs, the F−P hybrid WOLEDs
replace the blue phosphorescent dopant with a highly efficient
blue fluorescent emitter. F−P hybrid WOLEDs are not only
able to harvest 100% internal quantum efficiency due to the
complete utilization of singlet and triplet excitons, but also
achieve better performance since blue fluorescent emitters are
typically more stable than blue phosphors.3b,c They also have
the potential for higher efficiency because there is nearly no
exchange energy loss from the singlet excitons to the triplet
excitons.3a Therefore, F−P hybrid WOLEDs are considered to
be the best pathway for realizing high performance WOLEDs.
Received: October 9, 2013
Revised: October 12, 2013
Published: October 12, 2013
© 2013 American Chemical Society
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doping an orange phosphor into a blue fluorophor 4P-NPD
and a green phosphor into 1,3,5-tris(N-phenylbenzimidazol-2-
yl)benzene (TPBI). Their p-i-n structured white device
exhibited a total PE of 57.6 lm W−1 at a brightness of 100 cd
m−2.3b However, these multi-EML F−P hybrid WOLEDs are
complicated, expensive, and difficult to reproduce. Moreover,
the solution process, the next generation process for low-cost
and large-area manufacturing,1b,6 cannot be realized on such
complicated multi-EML F−P hybrid WOLEDs. Therefore,
realizing high performance single-EML RGB F−P hybrid
WOLEDs has become increasingly important.3c,7
multi-EML RGB F−P hybrid WOLEDs even those having p-i-n
structures.
EXPERIMENTAL SECTION
■
General Procedures. NMR spectra were determined on a Bruker
Advance-400 spectrometer with chemical shifts reported as ppm. Mass
spectrum data were obtained with a Finnigan 4021C GC-MS
spectrometer. Absorption and photoluminescence spectra were
recorded with a Hitachi UV−vis spectrophotometer U-3010 and a
Hitachi fluorescence spectrometer F-4500, respectively. Cyclic
voltammetry was performed on a CHI660E electrochemical analyzer.
Synthesis. Commercially available reagents were used without
purification. The synthetic route of POTA is outlined in Scheme 1.
In single-EML F−P hybrid WOLEDs, green/red phosphors
are directly doped into a blue fluorophor, which acts as both the
blue emitter and the host material for the phosphorescent
dopants. With this arrangement, the blue fluorophor must
simultaneously achieve an efficient blue fluorescence emission
and a triplet energy level (T1) higher than 2.4 eV for sensitizing
the green phosphor. However, common blue fluorophors
cannot realize such high T1 due to their large singlet−triplet
splitting (ΔEST),5d and blue fluorophors with T1 higher than
2.4 eV are rarely reported. Considering that the singlet energy
level (S1) should be between 2.6 and 2.9 eV for blue
fluorescence emission and T1 must be higher than 2.4 eV to
fit the energy level of green phosphors, the ΔEST of the blue
fluorophor should be smaller than 0.5 eV. To achieve such a
small ΔEST, the blue fluorophor should have a minimal overlap
between the HOMO/LUMO,8 whereas too small an overlap
will reduce the fluorescence efficiency.9 These contradictory
requirements can be appropriately managed in a molecule by
using a suitable π-conjugation group to bridge an electron-
donating group to an electron-accepting group (D-π-A). In
such a molecule, the HOMO and LUMO would be located
mainly on the electron-donating group and the electron-
accepting group respectively, and the overlap between the
HOMO and LUMO would be nearly equal in the π-bridging
group. Thus, we can optionally adjust the bulk of the overlap by
switching the bridging groups to achieve efficient blue
fluorophors that have high T1s. Compared with the similar
structure D−A bipolar host materials which have been widely
used in phosphorescent OLEDs recently,10 D-π-A compounds
can not only provide balanced hole and electron flows, but also
remarkably improve their poor fluorescent efficiency by
appropriately increasing the overlap between the HOMO/
LUMO.9
Scheme 1. Synthetic Route and Chemical Structure of POTA
2-Chloro-4,6-diphenoxy-1,3,5-triazine. Cyanuric chloride (1.840 g,
10 mmol) was dissolved in acetone (100 mL) and cooled to 0 °C. In a
separate flask, phenol (1.880 g, 20 mmol) was reacted with NaOH
(0.800 g, 20 mmol) in water (100 mL) to form a clear aqueous
solution. Then the aqueous solution was added drop by drop to the
cyanuric chloride solution. After stirring at 0 °C for 8 h, the mixture
was poured into water (100 mL) to form white precipitate. The white
precipitate was filtered and washed with water and then ethanol. The
product was purified by recrystallization with hexane to produce a
1
white solid. Yield: 73%. H NMR (400 MHz, CDCl3, δ): 7.43−7.36
(m, 4H), 7.28 (dd, J = 7.8, 1.4 Hz, 2H), 7.17−7.11 (m, 4H). HRMS
(EI, m/z): [M] + calcd for C15H10ClN3O2, 299.0462; found: 299.0546.
4-(4,6-Diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA).
Toluene (12 mL), ethanol (6 mL), and 2 M aqueous Na2CO3 (9 mL)
were added to a mixture of 2-chloro-4,6-diphenyl-1,3,5-triazine (0.536
g, 2 mmol), 4-(diphenylamino)phenylboronic acid (0.578 g, 2 mmol),
and tetrakis(triphenylphosphine)platinum (0.234 g, 0.2 mmol). The
mixture was refluxed for 8 h under a nitrogen atmosphere. When
cooled to room temperature, the mixture was extracted with
dichloromethane and dried over Na2SO4. After the solvent had been
removed under reduced pressure, the residue was purified by column
chromatography on silica gel using dichloromethane/petroleum ether
1
(4:1) as the eluent to produce a yellow solid. Yield: 66%. H NMR
(400 MHz, DMSO-d6, δ): 7.99 (d, J = 8.8 Hz, 2H), 7.45 (t, J = 4.0 Hz,
4H), 7.39 (t, J = 8.0 Hz, 4H), 7.28 (t, J = 8.4 Hz, 6H), 7.19 (t, J = 7.4
Hz, 2H), 7.15−7.13 (m, 4H), 6.96 (d, J = 8.8 Hz, 2H). 13C NMR (100
MHz, CDCl3, δ): 175.81, 173.50, 153.30, 152.80, 147.35, 131.39,
130.39, 130.20, 127.34, 126.78, 126.58, 125.39, 122.49, 120.81. HRMS
According to such a design strategy, in this work we report
on a novel triazine/amino-based D-π-A blue fluorophor 4-(4,6-
diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA).
The short and direct benzene π-conjugated linkage between
the electron donor diphenylamine and the acceptor 1,3,5-
triazine endows the POTA with efficient blue fluorescent
emission and a high T1 of 2.44 eV. The nondoped blue
fluorescent device based on POTA exhibits a maximum EQE of
4.2 0.3% and the phosphorescent devices containing POTA
as host material show maximum EQEs as high as 17.1 0.2%
+
(EI, m/z): [M] calcd for C33H24N4O2, 508.1899; found, 508.1793.
Quantum Calculation. Theoretical calculation of the compound
was carried out using the Gaussian-03 program. Density functional
theory (DFT) B3LPY/6-31G(d) was used to determine and optimize
the structure.
Device Fabrication and Measurement. ITO-coated glasses with
a sheet resistance of 30 Ω per square were used as substrates. The
substrates were first cleaned with isopropyl alcohol and deionized
water, then dried in an oven at 120 °C, treated with UV-ozone, and
finally transferred to a deposition system with a base pressure of about
1 × 10−6 Torr. Thermally evaporated organic materials were
sequentially deposited at a rate of 1−2 Å s−1 onto the ITO substrates.
The cathode was completed by thermal deposition of LiF at a
deposition rate of 0.1 Å s−1, and then capped with Al metal deposited
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.
for green and 18.8
0.2% for red. These excellent
electroluminescent (EL) results demonstrate POTA as an
excellent blue fluorophor as well as an outstanding host
material for phosphorescent dopants. The single-EML RGB F−
P hybrid WOLED based on POTA realizes a maximum total
EQE of 24.7 0.7%, a maximum total PE of 59.8 1.0 lm
W−1, and a PE of 31.3 0.3 lm W1− at a luminance of 1000 cd
m−2, representing the highest efficiencies found among single-
EML RGB WOLEDs. The efficiencies are also higher than in
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RESULTS AND DISCUSSION
Table 1. Physical Properties of POTA
■
a
Synthesis. The intermediate of 2-chloro-4,6-diphenoxy-
1,3,5-triazine was prepared from the reaction between cyanuric
chloride and phenol with the existence of sodium hydroxide
(Scheme 1).11 Then, the chlorine-substituted 1,3,5-triazine unit
was attached to the 4-position of triphenylamine using a
palladium-catalyzed cross-coupling reaction to give POTA in
good yield.11a,12 The chemical structure of POTA was fully
λabs/λem (nm)
295 378/422 437
0.75
b
Φf
c
λfluo/λphos (nm)
443/508
d
S1/ T1 (eV)
2.80/2.44
0.36
e
ΔEST (eV)
f
E1/2ox/E1/2 (V)
0.67/−2.15
−5.41/−2.59
2.82/2.84
red
g
HOMO/LUMO (eV)
1
characterized by H NMR and 13C NMR spectroscopy and
h
Eg/Egopt (eV)
mass spectrometry.
a
Measured in dilute cyclohexane solution at room temperature.
Measured in dilute cyclohexane solution, using 9,10-diphenylan-
thracence (Φf = 0.90 in cyclohexane solution) as a standard.
Fluorescence and phosphorescence spectrum peaks measured in 2-
b
Optical Properties. The UV−vis absorption and photo-
luminescence (PL) spectra of POTA in dilute cyclohexane
solution were investigated (Figure 1 and Table 1). The
c
d
methyltetrahydrofuran at 77 K. S1, singlet energy level, calculated
from the fluorescence peak at 77 K; T1, triplet energy level, calculated
e
from the phosphorescence peak at 77 K. ΔEST, Singlet−triplet energy
f
difference, ΔEST= S1 − T1. E1/2ox, half-wave oxidation potential vs Fc/
g
Fc+; E1/2red, half-wave reduction potential vs Fc/Fc+. HOMO =
−E1/2ox − ESCE and LUMO = −E1/2red − ESCE, the energy level of SCE
h
(ESCE) is 4.74 eV vs vacuum level.21 Eg, energy band gap, calculated
from the CV; Egopt, calculated from the absorption spectrum.
The fluorescence and phosphorescence spectra of POTA in 2-
methyltetrahydrofuran glass at 77 K were also determined, with
peaks at 443 and 508 nm, respectively (Figure 1 and Table 1).
Accordingly, the S1 and T1 of POTA are calculated to be 2.80
and 2.44 eV, respectively, indicating that POTA has a small
ΔEST of 0.36 eV.10d Additionally, the T1 of POTA is higher
than the triplet energy of common green phosphorescent
dopants such as fac-tris(2-phenylpyridine) iridium (Ir(ppy)3)
(2.41 eV),16 ensuring that POTA can be used as a host material
for green phosphors. POTA’s high fluorescent quantum yield
and suitable S1 and T1 indicate that POTA is a potential host
material for single-EML RGB F−P hybrid WOLEDs.
Theoretical Calculations and Electrochemical Proper-
ties. To better understand POTA’s structure−property
relationship at the molecular level, quantum chemical
calculations were employed at the B3LYP/6-31G(d) theoretical
level (Figure 2). In the ICT transition-based POTA, the
Figure 1. (a) UV−vis absorption and PL spectra of POTA in dilute
cyclohexane solution at room temperature, and the fluorescence and
phosphorescence spectra of POTA in 2-methyltetrahydrofuran at 77
K. (b) PL spectra of POTA in different solvents at room temperature.
absorption peak at 295 nm is associated with the diphenylamine
centered n-π* transition,13 while the longer wavelength peak at
378 nm can be attributed to the intramolecular charge transfer
(ICT) transition from the electron-rich diphenylamine moiety
to the electron-deficient 1,3,5-triazine core. POTA shows a
deep blue emission in a dilute cyclohexane solution with a peak
at 422 nm and a shoulder peak at 437 nm, which are attributed
to the n-π* transition and ICT transition, respectively. To
prove that POTA undergoes an ICT transition, we measured
the PL spectra of POTA in solvents with different polarity
(Figure 1). POTA displays strong solvatochromaticity such that
the emission spectrum exhibits a clear bathochromic shift from
nonpolar cyclohexane to higher polar toluene, dichloromethane
(DCM), acetone, and dimethylformamide (DMF).14 The
fluorescent quantum yield (Φf) of POTA in cyclohexane
solution was determined to be 0.75 using 9,10-diphenylan-
thracence (Φf = 0.90 in cyclohexane solution)15 as a standard.
Figure 2. Calculated spatial distributions of the HOMO/LUMO of
POTA.
HOMO is primarily located on the electron-rich diphenylamine
unit and benzene π bridge, whereas the LUMO mostly sits on
the electron-deficient 1,3,5-triazine group and benzene π
bridge. The overlap in the POTA molecule between the
HOMO and LUMO is limited on the benzene π bridge, which
tallies with our assumption and results in a small ΔEST of 0.36
eV and a high fluorescence efficiency of 0.75. To measure the
experimental values of the HOMO/LUMO energy levels, we
investigate the electrochemical property of POTA using a
three-electrode cell (Figure S1 of the Supporting Information,
SI). During the anodic scan, POTA undergoes a quasi-
reversible oxidation curve, originating from its electron-rich
diphenylamine unit. Upon the cathodic sweep, POTA also
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Table 2. Electroluminescence Properties of the Devices
b
c
PE/CE/EQE [lm W−1/ cd A−1/ %]
CIE(x, y)
a
@1000 cd m−2
Von [V]
maximum
device B
3.1
3.0
2.9
2.8
2.9
5.0 0.3/5.9 0.4/4.2 0.3
52.3 0.1/57.6 0.8/17.1 0.2
28.1 0.2/36.7 0.3/18.8 0.2
59.8 1.0/57.2 0.9/24.7 0.7
60.5 0.9/60.4 0.9/25.1 0.5
2.6 0.3/4.8 0.3/3.4 0.2
38.4 0.4/51.9 0.1/15.2 0.2
26.8 0.4/36.7 0.3/18.8 0.2
31.3 0.3/40.6 0.3/18.3 0.2
(0.15, 0.26)
(0.28, 0.63)
(0.55, 0.44)
(0.41, 0.46)
(0.34, 0.45)
device G
device R
device W1
device W2
32.3 2.0/44.3 0.9/20.0 0.6
a
b
c
Turn-on voltage. Maximum power efficiency, current efficiency and external quantum efficiency. Estimated at 1000 cd m−2.
exhibits a quasi-reversible reduction curve, which can be
attributed to electron-deficient 1,3,5-triazine moiety. The
reversible oxidation and reduction behavior of POTA implies
that it has potential for efficient hole and electron transport.13
The HOMO/LUMO energy levels of POTA are estimated
from the half-wave potentials of the oxidation and reduction
curves (relative to vacuum level),10d which are −5.41 eV and
−2.59 eV, respectively. Calculated from the HOMO/LUMO
energy levels, the energy band gap (Eg) value of 2.82 eV is
obtained for POTA, similar to the result estimated from the
optical absorption edges of solid film on quartz substrate
(Table 1). The rather narrow Eg with its suitable HOMO/
LUMO energy levels is beneficial for lessening the driving
voltage and augmenting the PE of the OLEDs.17
Electroluminescence Performance. A nondoped blue
fluorescent OLED (device B) was first fabricated with a device
structure of ITO/NPB (30 nm)/TCTA (10 nm)/POTA (30
nm)/TPBI (30 nm)/LiF (1.5 nm)/Al. In this device, ITO
(indium tin oxide) and LiF/Al are the anode and the cathode,
respectively; NPB (4,4′-bis[N-(1-naphthyl)-N-phenyl amino]-
biphenyl) is the hole-transporting layer (HTL); TCTA
(4,4′,4″-tris(N-carbazolyl)triphenylamine) is the exciton-block-
ing layer; and TPBI serves as the electron-transporting layer
(ETL), the hole-blocking layer and the exciton-blocking layer.
The nondoped blue device based on POTA exhibits stable blue
fluorescent spectra at different luminance with peaks at 472 nm
and CIE coordinates of (0.15, 0.26) (SI Figure S2 and Table 2).
The maximum PE, current efficiency (CE) and EQE of device
B are 5.0 0.3 lm W−1, 5.9 0.4 cd A−1 and 4.2 0.3%,
respectively (Figure 3 and Table 2). Considering the 5%
theoretical limit in fluorescent devices,18 POTA is deemed to
be an outstanding blue fluorophor.
Green and red phosphorescent devices based on POTA were
then constructed to testify the host properties of POTA for
green and red phosphors. They have a configuration of ITO/
NPB (30 nm)/TCTA (10 nm)/POTA: 2 wt % Ir(ppy)3 or 6
wt % Ir(2-phq)3 (30 nm)/TPBI (30 nm)/LiF (1.5 nm)/Al
(device G and R). Herein, Ir(2-phq)3 is tris(2-phenylquino-
line)iridium. Device G exhibits a maximum EQE of 17.1
0.2%, a maximum PE of 52.3 0.1 lm W−1 and a maximum CE
of 57.6 0.8 cd A−1 (Figure 3 and Table 2). Similarly, device R
also gives a high performance with a maximum PE of 28.1 0.2
lm W−1, a maximum CE of 36.7 0.3 cd A−1 and a maximum
EQE of 18.8 0.2%. Considering the 20% theoretically limit
for EQE in phosphorescent devices,18,19 both devices exhibit
extremely high EQEs, indicating that POTA can not only
sensitize a triplet emission of green phosphor, but also achieve
excellent host performance for phosphors.
Figure 3. PE-EQE-luminance plots of (a) device B, (b) device G, and
(c) device R.
concentrations of Ir(ppy)3 and Ir(2-phq)3 have been optimized
to 0.2 and 0.3 wt %, respectively. At such low doping
concentrations, it is likely that some blue fluorophor molecules
will find no phosphorescent molecules within their 3 nm
vicinity (the diffusion length of a singletexciton); while
phosphorescent dopant molecules will exist within 100 nm
(the diffusion length of a triplet exciton).3a,c Therefore, singlet
excitons will be reserved on the blue fluorophors and utilized
POTA was finally applied in a single-EML RGB F−P hybrid
WOLED (device W1) with a configuration of ITO/NPB (30
nm)/TCTA (10 nm)/POTA: Ir(ppy)3: Ir(2-phq)3 (30 nm)/
TPBI (30 nm)/LiF (1.5 nm)/Al. In the EML, the
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for blue fluorescence; whereas the triplet excitons will transfer
to the dopant molecules for green and red phosphorescence,
resulting in white emission. Owing to the suitable HOMO/
LUMO and singlet/triplet energy levels (SI Figure S4), small
injection barriers exist between the HTL/ETL and EML, and
excitons are effectively limited in the EML. Thus, the device
W1 has a low turn-on voltage of 2.8 V (Figure 4a and Table 2).
POTA-based single-EML RGB F−P hybrid WOLED repre-
sents the highest efficiencies among single-EML RGB
WOLEDs with hybrid emitters or all phosphors. Furthermore,
such high performance is also comparable with the multiple-
EML RGB F−P hybrid WOLEDs even based on p-i-n
structures (Table 3).
Table 3. Summary of Recently Reported High Performance
RGB WOLEDs
maximum
@1000 cd m−2
PE/CE/EQE
PE/CE/EQE
[lm W−1/ cd A−1/ %]
[lm W−1/ cd A−1/ %]
a
a
d
ref 5a
−/−/−
46.8/−/16.5
57.3/−/21.8
52.3/−/−
57.6/−/20.3
14.8/25.0/10.7
ref 5b
ref 21
ref 4b
−/−/−
24.4/−/−
35.2/−/−
37.5/−/16.1
a
b
c
e
e
ref 3b
ref 3a.
c
d
37.6/−/18.7
32.0/30.2/13.4
23.8/−/18.4
c
e
ref 5c
ref 21
27.1/30.8/13.7
f
28.6/−/21.0
22.1/−/−
device W1
59.8 1.0/57.2 0.9/
24.7 0.7
31.3 0.3/40.6 0.3/
18.3 0.2
device W2
60.5 0.9/60.4 0.9/
25.1 0.5
32.3 2.0/44.3 0.9/
20.0 0.6
a
b
Single-EML F−P hybrid WOLEDs. Single-EML fully phosphor-
c
escent WOLEDs. Multiple-EML F−P hybrid WOLEDs with p-i-n
d
e
f
structures. At 500 cd m−2. At 100 cd m−2. Multiple-EML F−P
hybrid WOLEDs.
To further prove that the performance of this single-EML
device structure could keep up with the multi-EML for F−P
hybrid WOLEDs, we fabricated a POTA-based multiple-EML
RGB F−P hybrid WOLED (device W2) with a structure of
ITO/NPB (30 nm)/TCTA (10 nm)/POTA (8 nm)/POTA: 2
wt % Ir(ppy)3 (6 nm)/POTA: 2 wt % Ir(2-phq)3 (8 nm)/
POTA (8 nm)/TPBI (30 nm)/LiF (1.5 nm)/Al, which is a
typical device structure in multi-EML F−P hybrid WOLED-
s.3a,21 Device W2 also shows warm white light emission (SI
Table S1) with a maximum total CE of 60.4 0.9 cd A−1, a
maximum total PE of 60.5 0.9 lm W1− and a maximum total
EQE of 25.1
0.5% (SI Figure S5 and Table 2). The
efficiencies of device W1 are equally matched with device W2.
The single-EML F−P hybrid WOLED based on POTA
performs as highly as the multiple-EML, while the former has
a simpler structure and better economy.
CONCLUSIONS
■
In summary, a novel blue fluorophor POTA has been designed
and synthesized by using a D-π-A structural strategy
incorporating diphenylamine as an electron-donor and 1,3,5-
triazine as an electron-acceptor with a short benzene π-
conjugated feature. POTA simultaneously exhibits an efficient
blue fluorescence emission, a high T1 of about 2.44 eV, and
appropriate HOMO/LUMO energy levels. The excellent EL
performances demonstrate that POTA is an excellent blue
fluorophor as well as an outstanding host material for
phosphorescent dopants. The single-EML RGB F−P hybrid
WOLED based on POTA shows a maximum total PE of 59.8
Figure 4. (a) Current density-luminance-voltage characteristics, (b)
the EL spectra, and (c) PE-EQE-luminance plots of device W1.
As shown in Figure 4b and SI Table S1, warm light emission
was realized in this device, and the CIE coordinates show
moderate blue-shift from (0.44, 0.48) to (0.36, 0.44) when the
luminance increases from 100 to 10 000 cd m−2, which may due
to the triplet−triplet annihilation at high current densities. As
illumination sources are generally characterized by their total
emitted power,3a,20 device W1 realizes a maximum total PE of
59.8 1.0 lm W−1, a maximum total CE of 57.2 0.9 cd A−1
1.0 lm W−1 and a maximum total EQE of 24.7
0.7%, the
and a maximum total EQE of 24.7
efficiencies maintain at 31.3 0.3 lm W1−, 40.6 0.3 cd A−1
and 18.3
0.2% even at brightness of 1000 cd m−2. Our
0.7%. The total
highest efficiencies among the single-EML RGB WOLEDs.
These performances are also comparable with multiple-EML
RGB WOLEDs, even those with p-i-n structures. Moreover,
4458
dx.doi.org/10.1021/cm403318r | Chem. Mater. 2013, 25, 4454−4459

Chemistry of Materials
Article
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this work provides verification of the high performance and
simple structure of single-EML RGB F−P hybrid WOLEDs.
ASSOCIATED CONTENT
* Supporting Information
■
S
Cyclic voltammograms, EL spectra, energy level diagrams,
current density-luminance-voltage characteristics, PE-EQE-
luminance plots and CIE coordinates. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: xhzhang@mail.ipc.ac.cn (X.-H.Z.); zhengcaijun@mail.
ipc.ac.cn (C.-J.Z.); apcslee@cityu.edu.hk (C.-S.L.).
■
(13) Tao, Y. T.; Wang, Q. A.; Yang, C. L.; Zhong, C.; Qin, J. G.; Ma,
D. G. Adv. Funct. Mater. 2010, 20, 2923.
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14, 292.
Notes
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B. X.; Xia, P. F.; Cheah, K. W.; Chen, C. H. Adv. Mater. 2009, 21, 688.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 51373190, 51033007, 51103169,
and 51128301), the Beijing Natural Science Foundation (No.
2111002), the National High-tech R&D Program of China
(863 Program) (Grant No. 2011AA03A110) and the Instru-
ment Developing Project of the Chinese Academy of Sciences
(Grant No. YE201133), P. R. China.
(21) Wan, J.; Zheng, C. J.; Fung, M. K.; Liu, X. K.; Lee, C. S.; Zhang,
X. H. J. Mater. Chem. 2012, 22, 4502.
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