White organic light-emitting diodes with evenly separated red, green, and blue colors for efficiency/color-rendition trade-off optimization

Article abstract of DOI:10.1002/adfm.201100895

A novel yellowish-green triplet emitter, bis(5-(trifluoromethyl)-2-p- tolylpyridine) (acetylacetonate)iridium(III) (1), was conveniently synthesized and used in the fabrication of both monochromatic and white organic light-emitting diodes (WOLEDs). At the optimal doping concentration, monochromatic devices based on 1 exhibit a high efficiency of 63 cd A ^-1 (16.3% and 36.6 lm W^-1) at a luminance of 100 cd m ^-2. By combining 1 with a phosphorescent sky-blue emitter, bis(3,5-difluoro-2-(2-pyridyl)phenyl)-(2-carboxypyridyl)iridium(III) (FIrPic), and a red emitter, bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate) iridium(III) (Ir(btp)₂(acac)), the resulting electrophosphorescent WOLEDs show three evenly separated main peaks and give a high efficiency of 34.2 cd A^-1 (13.2% and 18.5 lm W^-1) at a luminance of 100 cd m^-2. When 1 is mixed with a deep-blue fluorescent emitter, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi), and Ir(btp)₂(acac), the resulting hybrid WOLEDs demonstrate a high color-rendering index of 91.2 and CIE coordinates of (0.32, 0.34). The efficient and highly color-pure WOLEDs based on 1 with evenly separated red, green, blue peaks and a high color-rendering index outperform those of the state-of-the-art emitter, fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃), and are ideal candidates for display and lighting applications.

Full text of DOI:10.1002/adfm.201100895

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White Organic Light-Emitting Diodes with Evenly Separated
Red, Green, and Blue Colors for Efficiency/Color-Rendition
Trade-Off Optimization
Shuming Chen, Guiping Tan, Wai-Yeung Wong,* and Hoi-Sing Kwok*
applications in high-resolution, full-color
[
1–15]
A novel yellowish-green triplet emitter, bis(5-(trifluoromethyl)-2-p-tolylpyridine)
acetylacetonate)iridium(III) (1), was conveniently synthesized and used in the
fabrication of both monochromatic and white organic light-emitting diodes
WOLEDs). At the optimal doping concentration, monochromatic devices
displays and solid-state lighting.
Gen-
erally, to construct WOLEDs, two or three
emitters that emit two complementally
(
[
1–4]
[5–10]
colors
or three primary colors
(
should be employed. For display applica-
tions, it is important that the generated
white light can be separated into three pri-
mary colors with equal emission intensity
−1
−1
based on 1 exhibit a high efficiency of 63 cd A (16.3% and 36.6 lm W ) at
−2
a luminance of 100 cd m . By combining 1 with a phosphorescent sky-blue
emitter, bis(3,5-difluoro-2-(2-pyridyl)phenyl)-(2-carboxypyridyl)iridium(III)
[11,12]
after passing through the color filters.
(FIrPic), and a red emitter, bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylaceto-
To this end, the spectra of the WOLEDs
should normally possess three evenly sepa-
rated red (R), green (G) and blue (B) peaks
that correspond to the transmission peaks
of the R, G and B color filters. For lighting
applications, the 1931 Commission Inter-
national de L’Eclairage (CIE) coordinates
of the white spectrum should be close to
the equal-energy-white-point (i.e., (0.33,
nate)iridium(III) (Ir(btp) (acac)), the resulting electrophosphorescent WOLEDs
2
−1
show three evenly separated main peaks and give a high efficiency of 34.2 cd A
−1
−2
(
13.2% and 18.5 lm W ) at a luminance of 100 cd m . When 1 is mixed
with a deep-blue fluorescent emitter, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-
biphenyl (BCzVBi), and Ir(btp) (acac), the resulting hybrid WOLEDs demon-
2
strate a high color-rendering index of 91.2 and CIE coordinates of (0.32, 0.34).
The efficient and highly color-pure WOLEDs based on 1 with evenly separated
red, green, blue peaks and a high color-rendering index outperform those of
0
.33)) and the spectrum should cover the
the state-of-the-art emitter, fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy) ), and
are ideal candidates for display and lighting applications.
whole visible region in order to provide a
high color-rendering capability for objects
viewed under such white light. Thus, it is
preferable to adopt a three-emitter system
3
1
. Introduction
so that the resulting white spectrum can possess R, G and B
components and a high color-rendering index (CRI).
[
5–10]
White organic light-emitting diodes (WOLEDs) have been
of considerable interest in recent years due to their potential
For a three-emitter system, the emitters must be carefully
chosen. The emission spectra of the emitters should compen-
sate each other so that the white spectrum generated can cover
the whole visible region with a uniform emission intensity.
For blue phosphorescent emitters, there are not many choices
but to employ bis(3,5-difluoro-2-(2-pyridyl)phenyl)-(2-carbox-
ypyridyl)iridium(III) (FIrPic), a sky-blue emitter with a central
emission wavelength of 475 nm, due to the scarcity of deep-
blue-phosphorescent emitters in the literature. It is common
to adopt bis(2-benzo[b]thiophen-2-yl-pyridine)(acetylacetonate)
S. Chen, Prof. H.-S. Kwok
Centre for Display Research and Department
of Electronic and Computer Engineering
The Hong Kong University of Science & Technology
Clear Water Bay, Kowloon
Hong Kong, P. R. China
E-mail: eekwok@ust.hk
iridium(III) (Ir(btp) (acac)), with a central emission wave-
G. Tan, Prof. W.-Y. Wong
2
Department of Chemistry and Centre for
Advanced Luminescence Materials
Hong Kong Baptist University
Waterloo Road, Kowloon Tong
Hong Kong, P. R. China
length of 616 nm, as a saturated red emitter. In order to gen-
erate the white spectrum with three evenly separated R, G and
B peaks, the central emission wavelength of the green emitter
should thus be located at (616 + 475)/2 = 545 nm. Commer-
cially available green emitters such as fac-tris(2-phenylpyridine)
E-mail: rwywong@hkbu.edu.hk
S. Chen, G. Tan, Prof. W.-Y. Wong, Prof. H.-S. Kwok
Institute of Molecular Functional Materials
Areas of Excellence Scheme
iridium(III) (Ir(ppy) ) or bis(2-phenylpyridine)(acetylacetonate)
3
iridium(III) (Ir(ppy) (acac)), with a central emission wavelength
2
of ≈512 nm, cannot meet this spectral requirement. WOLEDs
employing the above-mentioned benchmark green emitters
exhibit a low color-rendering capability due to the undesirable
University Grants Committee of HKSAR, P. R. China
DOI: 10.1002/adfm.201100895
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Cl
CF3
(
i) BuLi, THF
ii) B(OMe)3
OH
OH
N
H3C
Br
H3C
B
H3C
CF3
(
Pd(PPh ) , toluene,
3 4
N
o
(iii) HCl
2 M Na2CO3, 110 C, 48 h
HL1
CH3
CH3
CF3
IrCl3 nH O
2
o
O
O
acetylacetone, Na CO
Cl
Cl
N
90 C, overnight
2
3
Ir
2
Ir
2
Ir
2
-ethoxyethanol
o
2-ethoxyethanol / H O
2
N
N
9
0 C, overnight
CF3
CF3
CH3
2
1
Scheme 1. Synthetic sketch leading to the iridium(III) heteroleptic complex 1.
[
5–10]
deep valley present between the R and G peaks.
To address
and 2′-p-tolylpyridine (Ir(tpy) (acac)) are known in the litera-
ture, there have been no reports to date of the iridium(III)
2
[13]
[16]
this problem, one more yellow emitter
or even two more
emitters[ have been employed to achieve a good white-light
spectrum, which inevitably increases the complexity of the
system. It is thus necessary to develop new, efficient, phospho-
rescent, yellowish-green emitters with a central emission wave-
length close to 545 nm, so that a broadband white spectrum
with a uniform emission intensity covering the whole visible
spectrum can be realized in a relatively simple three-emitter
system. Previously, Park et al.[ synthesized a high-energy-level
yellowish-green phosphor for constructing WOLEDs in such a
three-color stratagem. The resulting WOLEDs exhibited a mod-
14]
derivative with both CF and CH groups attached to the ppy
3
3
ligand. The synthesis of 1 is shown in Scheme 1. The key ligand
precursor is the 4-methylphenylboronic acid, synthesized from
4-bromotoluene, n-butyl lithium and trimethyl borate in a dry
ice–acetone bath. HL1 was conveniently synthesized from the
palladium-catalyzed Suzuki cross-coupling of 2-chloro-5-trif-
[17]
luoromethylpyridine with the boronic acid derivative. Then,
15]
IrCl · nH O reacted with an excess of the HL1 to give a chloride-
3
2
bridged iridium(III) dimer. The dimer could be readily cleaved
to the monomeric complex by replacing the bridging chlorides
with acetylacetonate anion (acac) derived from the deprotonation
−1
−1
erate peak efficiency of 11.7%, 23.4 cd A and 13.2 lm W and
showed a high CRI of 87, mainly due to the evenly separated R,
G and B peaks, by adopting the yellowish-green dopant.
In this contribution, we easily synthesized a new yellowish-
green emitter, 1, with a central electroluminescence (EL) wave-
length of 544 nm, for application in WOLEDs. Complex 1 pos-
sesses a high EL efficiency; for example, at an optimal doping
concentration, monochromatic devices based on 1 exhibited a
high luminance efficiency (LE) of 63 cd A at a luminance of
00 cd m , corresponding to an external quantum efficiency
EQE) of 16.3% and a power efficiency (PE) of 36.6 lm W .
Upon combination of 1 with FIrPic and Ir(btp) (acac) in the
active layer, the resulting electrophosphorescent WOLEDs
showed three evenly separated peaks and gave a high efficiency
of 34.2 cd A (13.2% and 18.5 lm W ) at the relevant bright-
ness level of 100 cd m . By combining 1 with the deep-blue
fluorescent emitter 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-
biphenyl (BCzVBi) and Ir(btp) (acac), the resulting hybrid
WOLEDs demonstrated a very-high CRI of 91.2 and CIE coordi-
nates of (0.32, 0.34). The efficient WOLEDs demonstrated here,
with separated R, G, and B peaks and high CRI, are ideal candi-
dates for display and lighting applications.
[18]
of acetylacetone (Hacac) by sodium carbonate.
Purification
of the mixture by preparative thin-layer-chromatography (TLC)
plates furnished 1 as an air-stable orange powder in a high
purity. All of the compounds were thoroughly characterized by
NMR spectroscopy and fast-atom-bombardment mass spectrom-
1
etry (FAB-MS). H-NMR-spectroscopy data suggested that com-
plex 1 was always in its pure stereoisomeric form, with one set
of proton signals.
−1
−2
1
The thermal stability of 1 was evaluated using thermogravi-
metric analysis (TGA) measured under a nitrogen stream
(Table 1). Complex 1 was thermally stable up to 327 °C, sug-
gesting that it could be vacuum-evaporated to form a good-
quality thin film.
−1
(
2
−1
−1
The UV–vis and photoluminescence (PL) spectra of 1 were
measured in CH Cl solution (Table 1, Figure 1). The higher-
−2
2
2
energy absorption peaks below 350 nm were assigned to ligand-
based spin-allowed 1π–π* transitions. The weaker absorption
peaks above 350 nm with lower extinction coefficients pos-
sessed substantial mixing of ligand-based 3π–π* states, spin-
2
1
allowed singlet metal-to-ligand charge-transfer ( MLCT) and
3
spin-forbidden triplet metal-to-ligand charge-transfer ( MLCT)
states. The spin-orbit coupling was enhanced by the presence
of closely spaced π–π* and MLCT states and the presence of
the heavy-atom effect of the iridium(III) center. Complex 1
2
. Results and Discussion
[19]
emitted a strong phosphoresecence at 547 nm at 293 K (543 nm
at 77 K). The slight blue shift was caused by solvent recorgani-
zation from a fluid solution to a rigid matrix when the tempera-
ture changed from 293 K to 77 K. The change of the solution
phase impeded the stabilization of charge-transfer states before
emission occurred; therefore, phosphorescence predominantly
2
.1. Synthetic Strategies, Chemical Characterization and
Thermal, Photophysical and Electrochemical Properties
^
Although heteroleptic iridium(III) complexes Ir(C N) (acac), with
C N = 5-trifluoromethyl-2-phenylpyridine (Ir(CF -ppy) (acac)),
2
^
3
2
3
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T a b le 1 . Photophysical, electrochemical and thermal data for complex 1.
Absorptiona)
Emission
CH2Cl2)
E ox
[V]
HOMO
[eV]
LUMO
[eV]
Eg
[eV]
ΔTdec
[°C]
d)
e)
f)
g)
h)
(
λabs [nm]
λem at 293 K/77 K [nm]b)
τP [μs]c)
2
77 (3.74), 313 (2.30), 361 (0.46), 379 (0.30), 473 (0.09)
547/543 (0.47)
0.92
0.52
–5.32
–2.84
2.48
327
a)
4
−1
cm−1) are shown in parentheses; The phosphorescence quantum yield, ΦP, at 293 K is
b)
In degassed CH2Cl2 at 293 K; molar extinction coefficient ε values (×10
M
c)
d)
+
e)
shown in parentheses against Ir(ppy)3 (ΦP = 0.40), λex = 400 nm; Triplet lifetime at 293 K; 0.1 M [Bu4N]PF6 in tetrahydrofuran (THF), versus Fc/Fc couple; HOMO =
f)
g)
h)
−1
−(Eox + 4.8); LUMO = HOMO + Eg; Estimated from the onset wavelength of the optical absorption; At a heating rate of 5 °C min under N2.
of the 3MLCT nature took place at higher energy.[20] The
PL emission maximum of 1 was between that for (Ir(CF3-
ppy) (acac)) (554 nm) and (Ir(tpy) (acac)) (512 nm). Compared
1: ITO/NPB (60 nm)/2–7% 1:CDBP (20 nm)/TAZ (40 nm)/
LiF (1 nm)/Al (100 nm); and b) type 2: ITO/NPB (60 nm)/4%
1:CBP (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (where
ITO = indium tin oxide; NPB = N,N’-bis(naphthalen-1-yl)-N,N’-
2
2
with the emission spectrum at room temperature, the PL spec-
trum at 77 K showed an apparent vibrational fine structure (with
a shoulder peak at about 572 nm), and this structured emission
bis(phenyl)-benzidine; CDBP
= 4,4´-bis(carbazol-9-yl)-2,2´-
dimethylbiphenyl; TAZ = 3-(4-biphenyl)-4-phenyl-5-tert-butylp
henyl-1,2,4-triazole; CBP = 4,4´-bis(carbazol-9-yl)biphenyl; and
TPBi = 2,2´,2˝-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimida-
zole)). In the type-1 devices, the CDBP host material and the
TAZ exciton-blocking material were chosen to effectively con-
fine the excitons within 1. For comparison, CBP as host mate-
rial and TPBi as exciton-blocking material were chosen in the
type-2 device. To find out the best doping level, the doping con-
centration of 1 was varied from 2 wt% to 7 wt%. Meanwhile,
green OLEDs based on the commercial green emitter Ir(ppy)3
3
3
revealed that the mixing between the MLCT and π–π* levels
was so effective that an almost ligand-centred emission was
observed upon freezing the matrix.[ The phosphorescence
quantum yield (Φ ) of 1 in CH Cl solution excited at 400 nm
21]
P
2
2
was 0.47, which is comparable to the Ir(ppy) standard (ΦP
=
3
0
.40). The short phosphorescence lifetime, τ , of 0.92 μs indi-
P
cates a weaker triplet-triplet annihilation, which reduces the
[
22]
chance of a decay in the efficiency of the devices.
The electrochemical behavior of 1 was investigated by cyclic
voltammetry using ferrocene as the internal standard and the
results are listed in Table 1. A reversible wave was found in
with a type-3 structure: ITO/NPB (60 nm)/7% Ir(ppy) :CDBP
3
(20 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm) were also fab-
ricated and compared. Figure 2a displays the typical current-
density–voltage–luminance (J–V–L) curves for the devices. All
of the devices with CDBP as the host and TAZ as the exciton-
blocking layer exhibited similar J–V–L characteristics. At higher
doping concentrations, the current density decreased slightly,
indicating that more excitons were trapped by the phosphor,
the anodic scan (E
Ir /Ir oxidation.
= 0.52 V), which can be assigned to the
The highest-occupied-molecular-orbital
1
/2,ox
IV
III
[23]
(HOMO) energy level was estimated from the oxidation couple
to be −5.32 eV, with respect to the energy level of the ferrocene
reference (4.8 eV below the vacuum level).[
24]
1
, at higher doping levels. The devices with CBP as the host
2
.2. Characterization and Optimization of 1-Doped
and TPBi as the exciton-blocking layer exhibited a substantially
larger current density at a certain voltage, mainly due to the
lower triplet energy level of the CBP and TPBi compared with
that of CDBP and TAZ, resulting in a lower carrier-injection
barrier and hence a larger current density. Figure 2b shows
the current-efficiency–luminance characteristics of the devices.
It is clear that the efficiency of the devices with CDBP as the
host and TAZ as the exciton-blocking layer increased with
increasing doping concentration up to a certain level and then
decreased when the doping level was increased. For example, at
Monochromatic OLEDs
To optimize the device performance, two kinds of device with
the following structures were fabricated and compared: a) type
[25]
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
a
b
c
−2
−1
a luminance of 1000 cd m , the LE increased from 54 cd A
for a doping concentration of 2 wt% to 61.6 cd A for a doping
−1
−1
concentration of 4 wt% and then decreased to 51.8 cd A for a
doping concentration of 7 wt%. Thus, the optimal doping con-
centration was around 4 wt%. At lower doping concentrations,
the energy transfer from the host to 1 may not be complete,
while at higher doping concentrations, triplet-triplet annihila-
tion may reduce the efficiency. At the optimal doping concen-
tration of 4 wt%, devices with CDBP as the host and TAZ as
the exciton-blocking layer exhibited a substantially higher
200
300
400
500
600
700
Wavelength (nm)
−1
−2
−1
LE of 61.6 cd A at 1000 cd m , compared with 40 cd A at
−2
1
000 cd m for the devices with CBP as the host and TPBi
Figure 1. Optical absorption and photoluminescence spectrum of 1 in
CH Cl (a: UV spectrum at 293 K; b: PL spectrum at 293 K; c: PL spec-
trum at 77 K).
as the exciton blocking layer, which is likely to be due to the
higher triplet energy levels of CDBP (3.0 eV) and TAZ (2.7 eV),
2
2
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5
(a)
10
compared with those of CBP (2.6 eV) and TPBi (2.67 eV),
respectively, resulting in better triplet exciton confinement and
hence higher efficiency. The performance of the devices with
400
350
300
250
200
150
100
1
:CDBP
2
%
[25]
4
10
4%
the commercial green emitter Ir(ppy) is also included for com-
7%
:CBP
3
1
parison. It was noted that the efficiency of the devices with 1
was significantly higher than that of the devices with Ir(ppy)3.
Most likely, the improved device efficiency of 1 can be traced
3
4%
1
1
1
0
Ir(ppy) :CDBP
3
7%
[
26]
2
0
to the steric hindrance of both CF and methyl groups. For
3
−2
example, at a luminance of 1000 cd m , the LE of the devices
−1
with 4 wt% 1 reached 61.6 cd A , which is 1.27 times higher
1
0
50
−1
than the 48.4 cd A for the devices with Ir(ppy) . The key char-
3
0
acteristics of the devices are summarized in Table 2. Figure 2c
0
-
50
10
shows the EL spectra of the devices with 1 and Ir(ppy) . Com-
3
0
2
4
6
8
10
12
14
16
plex 1 emitted a yellowish-green color with a central emission
wavelength of 544 nm, which was red shifted by 32 nm com-
pared with that for Ir(ppy)3 at 512 nm. The yellowish-green
emission color and excellent performance of 1 suggest the good
potential of employing 1 as the emitter for the construction of
efficient WOLEDs showing evenly separated R, G and B colors.
Voltage (V)
(b)
70
60
50
40
30
20
10
0
1
:CDBP
2
.3. Fabrication and Characterization of Efficient WOLEDs with
2%
%
%
:CBP
4
7
Evenly Separated Red, Green and Blue Peaks
1
4
%
The excellent performance of 1 prompted us to explore its
application in WOLEDs. A series of phosphorescent WOLEDs
were fabricated with the structure: ITO/NPB (40 nm)/6%
Ir(ppy) :CDBP
3
7
%
-
10
Ir(btp) (acac):CDBP (10 nm)/4% 1:CDBP (0–4 nm)/12%
2
0
2000
4000
6000
8000
10000
FIrPic:CDBP (5 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm),
−
2
Luminance ( cd m )
where 10 nm-thick CDBP doped with 6 wt% Ir(btp) (acac)
2
(
c)
served as the red-emitting layer and 5 nm-thick CDBP doped
with 12 wt% FIrPic worked as the sky-blue-emitting layer. The
thickness of the yellowish-green-emission layer, 1:CDBP, was
varied from 0 to 4 nm in order to tune the purity of the white
spectrum as well as the device efficiency. Figure 3a shows the
spectra of the devices with a 0 nm thickness of the 1:CDBP
layer. The resulting white spectra exhibited two main peaks
centered at 475 nm and 616 nm, which arose from the emis-
1
.0
.8
.6
.4
.2
.0
0
1
Ir(ppy)
3
0
0
0
0
sion of the FIrPic and the Ir(btp) (acac), respectively. Though
2
the CIE coordinates of (0.35, 0.38) are very close to the equal-
energy-white-point, such white spectra may not find applica-
tion in display or lighting, mainly due to the lack of a green
component. For display applications, the green color is mainly
resolved from the tail of the sky-blue emission after such white
light passes through the color filter, which would result in a
weak green-light intensity as well as poor color saturation.
For lighting applications, the lack of green peaks in the spec-
trum leads to a poor color-rendering capability (i.e., green and
4
00 450 500 550 600 650 700 750
Wavelength (nm)
Figure 2. a) Current-density–voltage–luminance characteristics of the devices
based on 1 and Ir(ppy) . b) Efficiency–luminance characteristics of the devices
based on 1 and Ir(ppy) . c) EL spectra of the devices based on 1 and Ir(ppy) .
3
3
3
T a b le 2 . Key performance characteristics of devices based on 1 and Ir(ppy)3.
Lmax @ 15 V [cd m−2 ]
62921
ηext [%]
ηL [cd A−1
]
ηP [lm W−1
22.7, 24.9
36.6, 29.3
39.9, 23.9
19.3, 20.9
34.6, 26.2
]
λmax [nm]
544
Structure
Phosphor dopant
2%
Von [V]
4.2
a)
b)
b)
b)
1
:CDBP/TAZ
10.5 , 14.0
40.5, 54.0
63.0, 61.6
51.4, 51.8
30.8, 40.0
50.7, 48.4
a)
4
7
%
%
4.2
63059
16.3 , 15.9
544
a)
4.2
71685
13.3 , 13.4
544
a)
b)
1
:CBP/TPBi
4%
7%
3.8
89339
8.2 , 10.6
544
a)
b)
Ir(ppy)3:CDBP/TAZ
3.4
61214
14.5 , 13.9
512
a)Values collected at 100 cd m−2 ; b)Values collected at 1000 cd m−2
.
3
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(
a)
(a)
1.0
.8
0.6
1.0
0.8
0.6
0.4
0.2
0.0
Ir(ppy) :CDBP 2 nm
475
3
1
:CDBP 0 nm
0
512
616
6 V
6 V
8
1
V
0 V
8
V
0.4
0.2
0.0
CIE = (0.35,0.38)
37
104
400
450
500
550
600
650
700
750
400
450
500
550
600
650
700
750
(b)
Wavelength (nm)
1
.0
Ir(ppy) :CDBP 4 nm
3
(
b)
1.0
0.8
0.6
0.4
0.2
0.0
4
75
1:CDBP 2 nm
6 V
8 V
616
0.8
0.6
0.4
0.2
0.0
6
V
544
8 V
0 V
1
69
72
400
450
500
550
600
650
700
750
400
450
500
550
600
650
700
750
Wavelength (nm)
(
c)
Figure 4. a,b) EL spectra of WOLEDs with Ir(ppy) :CDBP green-emission
layers with thicknesses of 2 nm (a) and 4 nm (b).
1.0
0.8
0.6
0.4
0.2
0.0
3
1
:CDBP 4 nm
6 V
8 V
10 V
could still be observed, the green and blue peaks were too close
to each other with a wavelength separation of only 37 nm, while
the green and red peaks were too far apart, with a separation of
400
450
500
550
600
650
700
750
Wavelength (nm)
1
04 nm, resulting in a deep valley between the green and red
emissions. Thus, it is difficult to achieve a good color-rendering
capability in such a three-emitter system due to the uneven sep-
aration of the emission peaks. To address this problem, at least
one more yellow emitter has to be employed to compensate
Figure 3. a–c) EL spectra of WOLEDs with 1:CDBP yellowish-green-emis-
sion layers with thicknesses of 0 nm (a), 2 nm (b) and 4 nm (c).
[13–15]
yellow objects cannot be correctly rendered under the illumi-
nation of such white light). Thus, it is necessary to remedy
this deficiency in such white spectra with a yellowish-green
color element. By employing a 2 nm thickness of 1:CDBP as
the yellowish-green emission layer, a new peak, centered at
the spectra for the white-color deficiency,
which inevitably
increases the complexity of the system. In addition, in contrast
to the WOLEDs using 1 as the emitter, in which the spectra
were less sensitive to the thickness of the green-emission
layer, the spectra of the WOLEDs with Ir(ppy) were found to
3
5
44 nm, originating from 1 was clearly observed. With the
be more sensitive. Due to the vicinity of the blue and green
peaks, the green spectrum was actually the superimposition of
compensation of the yellowish-green emission, the resulting
white spectra exhibited three evenly separated peaks, as shown
in Figure 3b. The spectra also demonstrated moderate color
stability. With the increase of the driving voltage, the blue and
red peaks decreased slightly in intensity with the CIE coor-
dinates changing from (0.35, 0.46) at 6 V to (0.34, 0.46) at
both emission peaks of Ir(ppy) and the tail emission of FIrPic.
3
Thus, its intensity increased dramatically with an increase of
the thickness of the green-emission layer, resulting in an unbal-
anced white emission, as shown in Figure 4b. For example, the
CIE coordinates shifted from the white region of (0.32, 0.44) for
8
V, which signifies that more excitons recombined in the yel-
the devices with the 2 nm thickness of Ir(ppy) :CDBP layer to
3
lowish-green region with an increased driving voltage. It was
also noted that the red emission intensity decreased slightly
when the thickness of the yellowish-green emission layer was
increased, as depicted in Figure 3c, resulting in the CIE coor-
dinates changing from (0.35, 0.46) at 6 V for the devices with a
the green region of (0.28, 0.53) for the devices with the 4 nm
thickness of the Ir(ppy) :CDBP layer. Thus, to obtain a balanced
3
white emission in such a three-emitter system, the thickness of
the green emission layer has to be controlled carefully, which
further increases the complexity of the fabrication process for
practical use.
2
nm thickness of 1:CDBP to (0.34, 0.46) at 6 V for the devices
with a 4 nm thickness of 1:CDBP. However, the EL efficiency
was enhanced significantly on increasing the thickness of the
yellowish-green emission layer (vide infra).
Figure 5a shows typical J–V–L characteristics of the devices.
The current density decreased slightly with an increase of the
thicknesses of the yellowish-green- or green-emission layers,
which was expected due to the larger resistance induced
by the thicker layer. All of the devices turned on at 4 V for
For comparison, WOLEDs using the commercial benchmark
green-emitter Ir(ppy) were also fabricated with the device struc-
3
−2
ture: ITO/NPB (40 nm)/6% Ir(btp) (acac):CDBP (10 nm)/7%
1 cd m ; however, at higher voltages, the devices with thicker
yellowish-green layers exhibited a higher luminance. For
example, at a driving voltage of 15 V, the luminance increased
2
Ir(ppy) :CDBP (2–4 nm)/12% FIrPic:CDBP (5 nm)/TAZ
3
(40 nm)/LiF (1 nm)/Al (100 nm). Figure 4a shows the spectra
−2
of the WOLEDs with a 2 nm thickness of Ir(ppy) :CDBP.
from 9829 cd m for the devices without the 1:CDBP layer to
3
−2
−2
Although three main peaks centered at 475, 512 and 616 nm
20 024 cd m and 24 090 cd m for the devices with the 2 nm- and
Adv. Funct. Mater. 2011, 21, 3785–3793
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(
a)
5
4
3
2
1
0
Figure 5b. Obviously, the devices with thicker yellowish-green-
emission layers showed a higher EL efficiency. For example,
3
2
2
1
1
00
50
00
50
00
10
1
:CDBP x nm
nm
Ir(ppy) :CDBP x nm
0
2nm
4 nm
−2
at a luminance of 100 cd m , the devices with the 4 nm-thick
3
10
−1
1
:CDBP layer showed a high efficiency of 34.2 cd A (13.2%
2
nm
4 nm
−1
−1
and 18.5 lm W ), which is higher than the 31.8 cd A (12.8%
−1
−1
−1
10
and 17.2 lm W ) and 17.9 cd A (10.9% and 9.7 lm W ) for
the devices with the 2 nm- and 0 nm-thick 1:CDBP layers,
−1
respectively. In this case, the peak LE was 39.9 cd A , corre-
10
−1
sponding to a peak PE of 33.0 lm W and an EQE of 15.5%.
The efficiency roll-off occurred quickly at high luminance; for
5
0
10
−2
example, at a high luminance of 1000 cd m , the efficiency was
−1
−1
0
27.4 cd A (10.7% and 11.3 lm W ), which is about 69% of
its peak value, and is mainly due to the triplet-triplet annihi-
lation and/or exciton leakage at high current density, a typical
phenomenon in phosphorescent OLEDs. To address this issue,
emitters with short exciton lifetimes may be adopted, device
structures with a wide exciton recombination zone to reduce
10
2
4
6
8
10
12
14
16
Voltage (V)
(
b)
1
:CDBP x nm
4
0
0
0
0
0
0
nm
2 nm
4 nm
Ir(ppy) :CDBP x nm
[27]
3
2
1
3
the exciton density can be designed, and/or exciton-blocking
2
nm
4 nm
layers with high triplet energy level can be employed to prevent
[28]
exciton leakage at high current density.
It should also be noted that the efficiencies of the devices
doped with 1 were substantially higher than those with the
0
2000
4000
6000
8000
2 nm
10000
−2
Ir(ppy) emitter; for example, at a luminance of 100 cd m , the
3
3
2
1
0
0
0
0
1:CDBP x nm
efficiency of the devices with the 2 nm-thick 1:CDBP layer exhib-
0
nm
4 nm
−
1
−1
Ir(ppy) :CDBP x nm
ited a high efficiency of 31.8 cd A (12.8% and 17.2 lm W ),
3
−1
−1
2
nm
4 nm
remarkably higher than the 19.1 cd A (8.9% and 10.4 lm W )
for the devices with the 2 nm thickness of Ir(ppy) :CDBP, which
3
was expected due to the higher efficiency of 1 than that of
Ir(ppy) . The key characteristics of the devices are listed in
3
0
2000
4000
6000 −
8000
10000
2
Table 3. When these WOLEDs are used for solid-state lighting,
all of the photons should be taken into account for illumination
since the originally trapped photons can be redirected to the
forward-viewing direction by applying light out-coupling tech-
Luminance ( cd m )
Figure 5. a) Current-density–luminance–voltage characteristics of the
electrophosphorescent WOLEDs with different thickness of the yellowish
green/green emission layer. b) Efficiency–luminance characteristics of the
electrophosphorescent WOLEDs with different thickness of the yellowish
green/green emission layer.
[
2d]
niques such as by engineering the lighting fixtures, roughing
[29]
[30]
the substrate or patterning a micro-lens array. It has been
experimentally shown that the efficiency can be improved by
[
31]
a factor of 1.2–2.3
are employed. Under these conditions, a maximal total LE of
≈71.8 cd A and a maximal total PE of ≈59.4 lm W can be
if certain light out-coupling techniques
4
nm-thick 1:CDBP layers, respectively, mainly owing to the
−1
−1
high efficiency of the green emission. To verify this asser-
tion, the efficiency–luminance characteristics are plotted in
obtained if a factor of 1.8 is adopted.
T a b le 3 . Key performance characteristics of the electrophosphorescent WOLEDs with different yellowish-green/green-emission-layer thicknesses.
ηL [cd A−1 ]
ηP
[lm W−1 ]
Structure
Thickness
Von
[V]
Lmax @ 15 V
ηext
[%]
CIE co-
ordinates [x, y]
−2
[
nm]
[cd m
]
With 1
0
4
4
9829
10.9a)
17.9
12.7
31.8
25.2
34.2
27.4
19.1
14.5
32.2
25.3
9.7
5.0
(0.35, 0.38)c)
(0.33, 0.38)d)
(0.35, 0.46)c)
(0.34, 0.46)d)
(0.34, 0.46)c)
(0.33, 0.46)d)
(0.32, 0.44)c)
(0.30, 0.43)d)
(0.28, 0.53)c)
(0.28, 0.51)d)
7
.7b)
12.8a)
0.2b)
13.2a)
0.7b)
8.9a)
.8b)
12.0a)
.4b)
2
4
2
4
20 024
24 090
13 021
19 384
17.2
9.9
1
4
18.5
11.3
10.4
5.8
1
With Ir(ppy)3
4.2
4.2
6
16.3
9.9
9
a)Values collected at 100 cd m−2 ; b)Values collected at 1000 cd m−2 ; c)Values collected at 6 V; d)Values collected at 8 V.
3
790 wileyonlinelibrary.com
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.MaterialsViews.com
Although high-efficiency WOLEDs with
three evenly separated peaks have been dem-
onstrated in the three-emitter system by
employing 1 as a yellowish-green emitter,
one may note that the CIE coordinates of
(a)
1.0
0.8
0.6
CIE = (0.32,0.34)
CRI = 91.2
CCT = 6042 K
(0.34, 0.46) of such WOLEDs are still far
0.4
away from the equal-energy-white-point,
which has to be improved for high-quality-
illumination applications. In addition, the
sky-blue emission stemming from FIrPic
is far from saturation, which would signifi-
cantly reduce the color gamut of a display.
To alleviate these problems, a fluorescent
emitter, BCzVBi, was adopted to replace
the FIrPic as the deep-blue emitter. Hybrid
WOLEDs were fabricated with a structure of
6
V
8 V
10 V
0
.2
.0
0
4
00
450
500
550
600
650
700
750
(b)
1
.0
CIE = (0.31,0.32)
CRI = 63.6
CCT = 6984 K
0.8
0
0
.6
.4
ITO/NPB (40 nm)/10% Ir(btp) (acac):CDBP
0.2
0.0
2
6
V
8 V
10 V
(
(
(
10 nm)/4% 1:CDBP (4 nm)/CDBP
3 nm)/10% BCzVBi:CDBP (5 nm)/TAZ
40 nm)/LiF (1 nm)/Al (100 nm), where
4
00
450
500
550
600
650
700
750
a 3 nm thickness of CDBP was inserted
between the BCzVBi, the fluorescent emitter,
and 1, the phosphorescent emitter, to con-
fine the singlet exciton recombining in the
Wavelength (nm)
Figure 6. a,b) EL spectra of the hybrid WOLEDs with 1 (a) and Ir(ppy) (b) as the emitters.
3
BCZVBi:CDBP region. With such an arrangement, both sin-
glet and triplet excitons were expected to be harvested.
For comparison, WOLEDs were also fabricated with the com-
yellowish-green emitter, 1, with a high synthetic accessibility
demonstrated a remarkably higher efficiency together with
an excellent color-rendering capability, as compared with the
[
32,33]
mercial green emitter Ir(ppy) , with a structure of ITO/NPB
commercial benchmark green emitter Ir(ppy) . This is mainly
3
3
(
(
(
40 nm)/10% Ir(btp) (acac):CDBP (10 nm)/7% Ir(ppy) :CDBP
attributable to the higher efficiency and red-shifted spectrum of
2
3
4
nm)/CDBP (3 nm)/10% BCzVBi:CDBP (5 nm)/TAZ
1 over that of Ir(ppy) . Our WOLEDs outperform most white-
3
40 nm)/LiF (1 nm)/Al (100 nm). Figure 6 shows the EL spectra
emitting RGB devices that have been reported in the literature,
with a simultaneous enhancement of both WOLED efficiency
and color rendition. The efficient WOLEDs, here with sepa-
rated R, G and B peaks and a high color-rendering index of up
to 91, would be ideal candidates to bring OLEDs into the next
generation of full-color displays and the solid-state lighting
market.
of these devices with 1 and Ir(ppy) . By adopting the deep-blue
3
emitter, BCzVBi, both types of WOLED emitted a very-pure
white color with CIE coordinates of (0.32, 0.34) and a color-
correlated temperature (CCT) of 6042 K for the devices with
1
and (0.31, 0.32) and 6984 K, repectively, for the devices with
Ir(ppy) . Although the CIE coordinates of both kinds of WOLED
3
are close, the WOLEDs with 1 exhibited a significantly higher
color-rendering capability as compared with those with Ir(ppy)3.
For example, the WOLEDs with 1 showed a respectably high
CRI of 91.2, remarkably higher than the 63.6 for WOLEDs with
4
. Experimental Section
General Information: All of the reactions were performed under
Ir(ppy) . The high CRI was mainly caused by the three evenly
3
nitrogen. The solvents were carefully dried and distilled from appropriate
drying agents prior to use. Commercially available reagents were used
without further purification unless otherwise stated. All of the reactions
were monitored by TLC with Merck precoated glass plates. Flash
column chromatography and preparative TLC were carried out on silica
gel from Merck (0.063–0.200 mm). FAB-MS was carried out using a
Finnigan MAT SSQ710 system. The NMR spectra were measured in
separated emission peaks, which desirably covered the whole
visible spectra. It is impressive that our best devices are supe-
rior to most currently known RGB-designed WOLEDs, showing
a concomitant set of high-efficiency and CRI values (see Sup-
porting Information). The poor CRI reported in the literature
is attributable to the deep valley present between the green
and red peaks: it is therefore important to select appropriate
emitters with each emission peak separated evenly so that
a high CRI can be attained, which is appropriate to indoor-
lighting applications.
CDCl using a Bruker Ultrashield 400 MHz FT-NMR spectrometer; the
3
1
13
H and C chemical shifts are quoted relative to the internal standard,
tetramethylsilane.
Physical Measurements: UV–vis spectra were obtained using an
HP-8453 spectrophotometer. The photoluminescent properties and
lifetimes of the compounds were probed using a Photon Technology
International (PTI) Fluorescence Master Series QM1 system. The
phosphorescence quantum yields were determined in CH Cl solution
2
2
[34]
3
. Conclusions
at 293 K against Ir(ppy) as a reference (ΦP = 0.40). Electrochemical
3
measurements were made using a CHI model 600C electrochemistry
station. A conventional three-electrode configuration, consisting of
a platinum working electrode, a Pt-wire counter electrode and a Ag/
AgCl reference electrode, was used. The supporting electrolyte was
In conclusion, efficient WOLEDs with three evenly sepa-
rated peaks have been demonstrated in a new three-emitter
platform. In such a system, WOLEDs based on a tailor-made
Adv. Funct. Mater. 2011, 21, 3785–3793
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.MaterialsViews.com
0
.1 M [Bu N]PF in MeCN. All of the potentials reported are quoted with
4 6
Supporting Information
+
reference to the ferrocene-ferrocenium (Fc/Fc ) couple at a scan rate
of 100 mV s . The oxidation potential (E ) was used to determine the
−1
Supporting Information is available from the Wiley Online Library or
from the author.
ox
HOMO energy level using the equation: HOMO =−(Eox + 4.8) eV, which
was calculated using the reference standard ferrocene value of −4.8 eV
with respect to vacuum.[ Thermal analyses were performed using a
Perkin–Elmer TGA6 thermal analyzer.
Preparation of Ligand HL1: Pd(PPh3)4 (160 mg) was added to a
mixture of 4-methylphenylboronic acid (1.07 g, 7.88 mmol), 2-chloro-5-
24]
Acknowledgements
S.C. and G.T. contributed equally to this work. The work was supported
by the Hong Kong Research Grants Council (HKBU202709 and HKUST2/
CRF/10), the University Grants Committee of HKSAR, China (AoE/P-
03/08) and Hong Kong Baptist University (FRG2/10-11/101). We thank
Prof. W.-H. Leung and Mr. V. H.-Y. Ng at the Hong Kong University of
Science & Technology for assistance in the electrochemical studies of 1.
(trifluoromethyl)pyridine (0.48 mg, 2.62 mmol) in toluene (50 mL) and
a 2 M aqueous solution of Na CO (8 mL) under an inert atmosphere
2
3
of nitrogen. The reaction mixture was heated at 110 °C for 48 h. After
cooling to room temperature, water was added and the solution mixture
was extracted with ethyl acetate. The combined organic layer was dried
over Na SO , filtered, and concentrated under reduced pressure. The
2
4
residue was purified on a silica column by gradient elution with a mixture
of hexane and CH Cl (2:1, v/v) to afford a white solid (93%, 574 mg).
Received: April 21, 2011
Published online: August 3, 2011
2
2
1
H NMR (400 MHz, CDCl , δ): 8.92 (s, 1H, Ar), 7.96–7.92 (m, 3H,
3
Ar), 7.82–7.80 (d, J = 8.6 Hz, 1H, Ar), 7.32–7.30 (d, J = 8.1 Hz, 2H,
1
3
Ar), 2.42 (s, 3H, CH3); C NMR (100 MHz, CDCl , δ): 160.65, 146.51,
3
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1
40.31, 135.17, 133.80, 129.70, 127.14, 119.58 (Ar), 124.95, 124.62,
+
24.29, 123.96 (CF ), 21.33 (CH ); FAB-MS (m/z): 238.2 [M] .
3
3
Preparation of 1: IrCl ·nH O (137 mg, 0.46 mmol) was allowed to react
3
2
with the cyclometalating ligand HL1 (275 mg, 1.16 mmol) in a mixture of
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3
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6
1
1
40.87, 140.33, 125.26, 125.02, 123.67, 122.73, 117.71 (Ar), 124.99,
24.28, 123.64, 123.30 (CF ), 100.85 (CH), 28.61 (CH ), 21.83 (CH );
[
3
3
3
+
FAB-MS (m/z): 764.2 [M] ; Anal. calcd (%) for C H N F O Ir: C 48.75,
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6
2
H 3.30, N 3.67; found: C 48.96, H 3.58, N 3.85.
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.2 Torr for 10 s in the pretreatment chamber. Then, the samples were
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−7
transferred to the organic chamber with a base pressure of 5 × 10 Torr
without breaking the vacuum for depositing the organic layer. The samples
were then transferred to a metal chamber for cathode deposition, which
was composed of 1 nm LiF capped with 100 nm Al. The light-emitting
2
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using an HP4145B semiconductor parameter analyzer. The forward-
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