Efficient red electroluminescent devices with sterically hindered phosphorescent platinum(II) Schiff base complexes and iridium complex codopant

Article abstract of DOI:10.1002/asia.201402618

Sterically hindered platinum(II) Schiff base complexes were prepared. Complex 4, which displays red emission with a quantum yield of 0.29 in a thin film and a self-quenching rate constant of 1×10^-7 dm³ mol^-1 s^-1

Full text of DOI:10.1002/asia.201402618

FULL PAPER
DOI: 10.1002/asia.201402618
Efficient Red Electroluminescent Devices with Sterically Hindered
Phosphorescent Platinum(II) Schiff Base Complexes and Iridium Complex
Codopant
[
c]
[a]
[a, b]
[a, b]
[a, b]
Liang Zhou, Chun-Lam Kwong, Chi-Chung Kwok,
Gang Cheng,
[
c]
Hongjie Zhang, and Chi-Ming Che*
ꢀ
1
Abstract: Sterically hindered platinu-
as a deep electron trapper, and red-
light-emitting devices with the highest
current, power, and external quantum
ciency and EQE of up to 14.69 cdA
and 8.3%, respectively, were achieved
m(II) Schiff base complexes were pre-
pared. Complex 4, which displays red
emission with a quantum yield of 0.29
in a thin film and a self-quenching rate
ꢀ2
at a high brightness of 1000 cdm . The
significant delay of efficiency roll-off is
attributed to the bulky 3D structure of
the norbornene moiety at the periph-
ery of the Schiff base ligand of 4 and to
the new device design strategy. The
fabricated device had a projected life-
time (LT50) of 18000 h.
ꢀ
1
efficiencies
18.33 Lm W , and 11.7%, respectively,
were fabricated. A high current effi-
of
20.43 cdA
ꢀ
1
ꢀ
7
3
ꢀ1 ꢀ1
constant of 1ꢀ10 dm mol s , was
used to fabricate organic light-emitting
diodes with single or double emissive
Keywords: doping · electrolumines-
cence · energy transfer · platinum ·
Schiff bases
layers (EMLs). An iridium ACHUTNGRNEUNG( III) com-
plex with a wide band gap was codoped
into the electron-dominant EML to act
Introduction
Over the past decade, phosphorescent platinum(II) com-
plexes have been extensively studied for their applications
in materials science. Phosphorescent platinum(II) complexes
with excellent thermal stability and high photoluminescence
[1]
[2]
Since reports by Ma et al. and Thompson et al. in 1998
on electroluminescence (EL) originating from a triplet excit-
ed state, transition-metal complexes have been extensively
studied in the area of organic light-emitting devices
[18–21]
quantum yields have been reported.
cyclometalated iridium(III) emitters, platinum(II) complexes
adopt a planar coordination geometry that favors intermo-
Unlike octahedral
AHCTUNGTRENNUNG
(
OLEDs). Transition-metal complexes can be used to har-
[22,23]
vest both singlet and triplet excitons, and therefore, are ad-
vantageous for achieving 100% internal quantum efficiency.
To date, the efficiency and brightness of green OLEDs has
lecular ligand–ligand and/or metal–metal interactions.
The presence of such intermolecular interactions facilitates
the formation of low-lying aggregation states, leading to
quenching of emission(s) in most cases, and hence, poses
a detrimental effect on both efficiency and color purity of
[
3–5]
been good enough for industrial applications,
whereas
the performance of red and blue OLEDs in the context of
color purity, efficiency, and brightness remain to be opti-
[19]
the as-fabricated EL devices.
Consequently, the perfor-
[6–17]
mized for practical applications.
mance of most reported EL devices based on phosphores-
cent platinum(II) complexes (dopant) showed strong de-
pendence on dopant concentration. Devices with low
dopant concentrations display marked host emission as
a result of inefficient carrier trapping and/or incomplete
energy transfer from host to emitter, whereas devices with
high dopant concentration usually exhibit lower efficiency
+
[
a] C.-L. Kwong, Dr. C.-C. Kwok, Dr. G. Cheng, Prof. C.-M. Che
State Key Laboratory of Synthetic Chemistry
and Department of Chemistry
The University of Hong Kong
Pokfulam Road, Hong Kong (S.A.R. China)
E-mail: cmche@hku.hk
[19]
owing to the presence of aggregation states and/or triplet–
[24]
triplet annihilation. Another shortcoming of the reported
phosphorescent platinum(II) complexes is their relatively
[
b] Dr. C.-C. Kwok, Dr. G. Cheng, Prof. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (P.R. China)
long emission lifetimes compared with those of iridium ACHTUNGTRENNUNG( III)
+
[
c] Dr. L. Zhou, Prof. H. Zhang
emitters; this results in intense exciton density within the
State Key Laboratory of Rare Earth Resource Utilization
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
[25]
emissive layer (EML), particularly at high current density.
Therefore, EL efficiency of platinum(II) complexes based
OLED devices generally display rapid roll-off with increas-
ing current density.
Changchun 130022 (P.R. China)
+
[
] These authors contributed equally to this work.
Previously, we reported a series of platinum(II) Schiff
base complexes having emission lifetimes of 1–5 ms and used
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402618.
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Chi-Ming Che et al.
Scheme 1. Synthetic scheme for the preparation of the platinum(II) Schiff base complexes examined herein.
7
3
ꢀ1 ꢀ1
these complexes to fabricate red-light OLEDs. Platinum(II)
Schiff base complexes are good candidate materials for in-
dustrial applications because scaling up for mass production
is relatively simple. For instance, 10 g of platinum(II) N,N’-
bis-salicylidenephenylenediamine (1; Scheme 1) can be pre-
pared in a one-pot reaction. However, its planar coordina-
tion geometry leads to severe aggregate formation in the
solid state. As described previously, the doping concentra-
tion of 1 and the EL efficiency of the as-fabricated OLED
device could only be confined to a low level; the best ach-
of 1ꢀ10 dm mol s ; this value is almost two orders of
magnitude lower than that of 1. The emission quantum yield
of 4-doped N,N’-dicarbazolyl-3,5-benzene (mCP) thin film is
0.29, which is 60% higher than that of 1.
As a result of the reduced self-quenching rate constant,
a high doping concentration of 4 wt% of 4 could be ach-
ieved without deteriorating the device performance. The
roll-off of efficiency is delayed owing to the mitigation of
self-quenching and triplet–triplet annihilation. At a bright-
ness of 1000 cdm , the device fabricated with 4 exhibited
a current efficiency that was 49% higher than that of the
device fabricated with 1 under the same double EML device
ꢀ
2
ꢀ
1 [19]
ieved current efficiency was 10.8 cdA .
In the literature, it was reported that a double EML
device architecture was effective in improving EL efficiency,
broadening the recombination zone, and delaying the roll-
[30]
configuration.
duced by 35% upon replacement of 1 with 4.
To further optimize the performance of the devices fabri-
The efficiency roll-off has also been re-
[26–29]
off of efficiency.
Major charge carriers in the first EML
are holes and in the second EML are electrons. Recently,
we utilized this strategy to improve the EL performance of
red-light-emitting complex 1. By utilizing suitable p- and
cated with 4, an iridium
and relatively low-lying energy level was minutely codoped
into the electron-dominant EML. The iridium(III) codoped
ACHTUNGTRNENUNG( III) complex with a wide band gap
[30]
ACHTUNGTRENNUNG
n-type host materials with stepwise energy levels, high EL
devices showed higher EL efficiency, slower roll-off of EL
efficiency, higher brightness, and longer device lifetimes. By
ꢀ
1
current and power efficiencies of up to 17.36 cdA and
ꢀ
1
1
4.73 LmW , respectively, were obtained. However, the
optimizing the concentration of iridium
ACHTUNGTRENNUNG( III) codopant, high
ꢀ
1
roll-off of EL efficiency in these double EML devices was
significant. This was attributed to the accumulation of carri-
ers at the interface of hole- and electron-dominant EMLs;
therefore, the recombination zone was confined.
EL current and power efficiencies of up to 20.43 cdA and
ꢀ
1
18.33 LmW , respectively, have been realized. A high cur-
ꢀ
1
rent efficiency of 14.69 cdA could be retained even at
ꢀ
2
a high brightness of 1000 cdm . The codoped iridium ACHTUNGTRENNUNG( III)
Herein, we improved the EL performance of platinum(II)
Schiff base complexes by modifying the molecular structure
and by optimizing the double EML device structure. Based
on 1, a series of new red-light-emitting platinum(II) Schiff
base complexes were prepared by incorporation of bulky
groups at the periphery of the ligand scaffold. These new
platinum(II) Schiff base complexes have a narrow band gap.
Among them, complex 4 has an emission quantum yield of
complex acted as deep electron trap and energy-transfer
ladder as a result of its low-lying LUMO and appropriate
triplet energy level. The enhanced EL performance is attrib-
uted to more balanced carrier distribution, dilated recombi-
nation zone, and improved energy transfer from the host to
the emitter.
[19]
0.20 in dichloromethane with a self-quenching rate constant
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Chi-Ming Che et al.
Results and Discussion
data of 2a and 2b, or 3a and 3b, the attachment of the tert-
butyl group to the phenyl ring has no significant effect on
the absorption and emission spectra, which suggests that the
phenyl moiety plays a minor role in the HOMOs of the plat-
inum(II) Schiff base complexes examined. The absorption
and emission spectra of 4 (Figure 1) are redshifted relative
to that of 1, which is beneficial for red OLED applications.
Molecular Design of New Red-Light-Emitting Platinum(II)
Schiff Base Complexes
In principle, the emission self-quenching rate constant (kq)
of a platinum(II) complex could be reduced through the in-
corporation of bulky substituent(s) onto the ligand scaffold.
Our previous work showed that the attachment of only one
tert-butyl group to the phenoxide ring or phenyl ring of
Schiff base ligand did not have a significant effect on the k
value.
q
[19]
Structures of the red-light-emitting platinum(II) Schiff
base complexes studied herein are shown in Scheme 1. Com-
plexes 2a and 2b were designed to have more tert-butyl
groups at the periphery of the Schiff base ligand. For 3a and
3
b, an adamantyl group was used instead of a tert-butyl
group. Subsequently, it has been found that these alkyl
groups are flexible, leading to a decrease in emission quan-
tum yield. Our recent work showed that incorporation of
a norbornene moiety into the ligand scaffold could signifi-
cantly reduce the kq value and led to an increase in the
[31]
emission quantum yield of the platinum(II) complex. Fol-
lowing this work, complex 4 was prepared by using a Schiff
base ligand with a norbornene moiety to reduce the kq
value, while not providing an additional relaxation pathway.
Figure 1. The absorption and emission spectra of 4.
Emission Self-Quenching
Photophysical Properties
The emission self-quenching rate constants (k ) of the com-
q
Table 1 contains the photophysical properties of the com-
plexes studied herein. Similar to 1, all of these platinum(II)
complexes display intense absorption bands with lmax in the
range of 468–560 nm; these bands are assigned to mixed
plexes are shown in Table 1. Complex 1 has a large k of
6.9ꢀ10 dm mol s ; this led to a deterioration in the
q
8
3
ꢀ1 ꢀ1
device performance when the doping concentration of 1 ex-
[19]
ceeded 1.5%. For complete energy transfer from the host
to the light-emitting complex (dopant), a high dopant con-
centration without formation of excimer is desirable.
1
metal-to-ligand charge-transfer MLCT (Pt 5d!p*(Schiff
1
base)) and (lone pair(phenoxide)!p*
ACHTUNGERTNNUNG( imine)) charge-trans-
[19]
fer transitions. All of these complexes show red emission
with lmax in the range of 618–642 nm. The large Stoke shifts
between the emission and absorption maxima of these com-
plexes are indicative of phosphorescence emissions.
Herein, the incorporation of more tert-butyl groups to the
periphery of the Schiff base ligand is effective in minimizing
7
3
ꢀ1 ꢀ1
the k value. The k value of 2a is 2.0ꢀ10 dm mol s ; this
q
q
is lower than that of 1 by more than one order of magni-
There is a distinctive redshift in both the low energy ab-
sorption and emission maxima upon attachment of alkyl
groups to the phenoxide ring, as revealed by the spectral
data for 1 and 3a. Further redshift is observed upon the in-
corporation of more alkyl groups to the phenoxide ring (2a
versus 3a). These findings are in agreement with our previ-
ous work, which showed that the phenoxide moiety contrib-
uted significantly to the HOMOs of platinum(II) Schiff base
tude. On the other hand, the k value of 3a is 1.5ꢀ
10 dm mol s , which is only slightly lower than that of 1.
This finding shows that the hindered adamantyl group is
q
8
3
ꢀ1 ꢀ1
only slightly more effective in the suppression of k than the
q
tert-butyl group. It is noted that the k value of 4 is 1.0ꢀ
q
7
3
ꢀ1 ꢀ1
10 dm mol s , which is lower than that of 1 by 69-fold,
and is even lower than that of 2a. This shows that the nor-
bornene moiety is effective in mitigating emission self-
quenching of platinum(II) Schiff base complexes.
[19]
complexes. On the other hand, by comparing the spectral
Table 1. Photophysical data for complexes 1, 2a, 2b, 3a, 3b, and 4 in dichloromethane.
3
ꢀ1
ꢀ1
8
3
ꢀ1 ꢀ1
l
abs [nm] (e [dm mol cm ])
q em
lem [nm] t [ms] k (10 dm mol s ) F
1
2
2
3
3
4
253 (41600), 318 (24900), 366 (36100), 382 (34100), 462 (9300), 503sh (8600), 535 (9900)
263 (36100), 302 (12900),327 (16800), 370 (31500), 388 (34600), 478 (6900), 526sh (6600), 560 (8100) 643
259 (39600), 304sh (15400), 329 (21100), 368 (34600), 387 (36400), 476 (9000), 519 (7900), 555 (8600) 642
261 (42800), 320 (21700), 367 (38800), 385 (43900), 470 (8500), 519sh (7300), 546 (8300)
259 (39800), 322 (20000), 366 (35300), 384 (40400), 470 (8100), 515sh (6800), 544 (7800)
262 (27300), 319 (14200), 368 (28500), 386 (32000), 465 (10500), 534 (7200)
618
3.6
5.0
5.5
3.5
5.9
6.3
6.9
0.2
0.2
1.5
0.08
0.1
0.20
0.14
0.06
0.06
0.10
0.20
a
b
a
b
632
631
624
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Chi-Ming Che et al.
phenylamine (TcTa) and 26DCzPPy were chosen as the host
materials for EML1 and EML2, respectively. The device
structure and energy level diagram of the designed OLEDs
and chemical structure of the host materials used are depict-
ed in Figure 2. The stepwise HOMO levels of TAPC
Emission Quantum Yields
Despite the diminished k value of 2a, the emission quan-
q
tum yield of 2a drops to 0.14. Moreover, the emission quan-
tum yield of 3a is just 0.06. This may be due to the addition-
al nonradiative decay channel arising from the modes of vi-
bration of the adamantane moiety.
On the other hand, the emission quantum yield of 4 can
be kept at the same level as that of 1 (0.20 in CH Cl ). The
2
2
low k value of 4 shows that the norbornene group is bulky
q
enough to reduce self-aggregation, while its rigidity does not
lead to a decrease in the emission quantum yield.
The emission quantum yields of thin films of 1 and 4 in
the mCP host are 0.18 and 0.29, respectively. The higher
thin-film emission quantum yield of 4 is attributed to re-
duced self-quenching and intermolecular aggregation in the
solid state.
Electrochemical Properties
The energy levels of the platinum(II) complexes were esti-
mated by cyclic voltammetry and the results are shown in
Table 2. The HOMO levels of 3a, 3b and 4 are at a higher
energy relative to 1. The energy gaps of these complexes
range from 2.25 to 2.28 eV, which are smaller than that of
1
(2.48 eV). Moreover, the HOMO level of 4 is ꢀ5.15 eV,
which is the highest among all complexes examined herein.
The higher HOMO level and narrower band gap of 4 are
conceived to account for its improved hole-trapping ability;
thus leading to enhanced EL efficiency.
Table 2. Electrochemical properties of complexes 1, 2a, 2b, 3a, 3b, and
4
4 6
measured in 0.1m nBu NPF in N,N-dimethylformamide (DMF). All
o
+/0
potentials are referenced to the ferrocenium/ferrocene (E
Cp=cyclopentadienyl) couple.
A( FeCp
H
U
G
R
N
U
2
);
HOMO [eV]
LUMO [eV]
E
g
[eV]
1
2
2
3
3
4
ꢀ5.37
ꢀ5.37
ꢀ5.36
ꢀ5.28
ꢀ5.23
ꢀ5.15
ꢀ2.89
ꢀ2.93
ꢀ2.96
ꢀ3.00
ꢀ2.98
ꢀ2.89
2.48
2.41
2.40
2.28
2.25
2.26
a
b
a
b
Figure 2. a) Chemical structures of TAPC, TcTa, 26DCzPPy, and
TmPyPB. b) Double EML device structure and the proposed energy
levels diagram of the designed double EML OLEDs. ITO=indium tin
oxide.
(
ꢀ5.3 eV), TcTa (ꢀ5.8 eV), and 26DCzPPy (ꢀ6.1 eV) are
[9,32,33]
EL Performances
The device performance of 4 was examined by fabricating
a series of single or double EML devices. Di-[4-(N,N-ditolyl-
amino)phenyl]cyclohexane (TAPC) was used as a hole-
transport/electron-blocking layer (HTL/EBL) for its high
beneficial for the injection and transport of holes,
whereas the stepwise LUMO levels of TmPyPB (ꢀ2.7 eV),
AHCTUNGTRENNUNG
26DCzPPy (ꢀ2.6 eV), and TcTa (ꢀ2.4 eV) are beneficial for
[7,9,33]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
the injection and transport of electrons.
Balanced injec-
tion of carriers (holes and electrons) and a wide recombina-
tion zone are therefore expected. The width of the recombi-
nation zone can be adjusted by controlling the thicknesses
of EML1 and EML2. Because the HOMO and LUMO
levels of 4 (ꢀ5.15 and ꢀ2.89 eV) are within those of TcTa
and 26DCzPPy, carrier trapping is conceived to be the domi-
ꢀ
2
2
ꢀ1 ꢀ1
hole mobility (1ꢀ10 cm V s ) and high-lying LUMO
[
32]
level (ꢀ1.8 eV).
1,3,5-Tri(m-pyrid-3-ylphenyl)benzene
(
TmPyPB) was used as hole-blocking/electron-transport
layer (HBL/ETL) for its low-lying HOMO level (ꢀ6.7 eV)
ꢀ
3
2
ꢀ1 ꢀ1 [7]
and high electron mobility (1ꢀ10 cm V s ).
[34]
In the single EML devices, 2,6-bis[3-(9H-carbazol-9-yl)-
phenyl]pyridine (26DCzPPy) was used as the host material.
In the double EML devices, 4,4’,4’’-tris(carbazole-9-yl)tri-
nant EL mechanism for these devices.
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Chi-Ming Che et al.
Preliminary Testing of the Red OLEDs
A series of single EML devices with the structure of ITO/
TAPC (40 nm)/4 (x wt%):26DCzPPy (10 nm)/TmPyPB
(
40 nm)/LiF (1 nm)/Al (100 nm) were fabricated and exam-
ined to optimize the doping concentration of 4. The dopant
here 4) concentration dependence of the EL efficiency and
(
brightness–voltage–current density characteristics of these
devices are depicted in Figure S1a in the Supporting Infor-
mation. Upon increasing the dopant concentration, the EL
efficiency increased to a maximum value and then decreased
gradually. As shown in Table S1 in the Supporting Informa-
tion, the 4 wt% doped device gave the highest brightness of
ꢀ
2
ꢀ1
9
198 cdm , the highest current efficiency of 10.43 cdA ,
ꢀ
1
and the highest power efficiency of 9.36 LmW . At a bright-
ness of 1000 cdm , the 5 wt% doped device showed the
ꢀ
2
ꢀ
1
highest EL current efficiency of 4.36 cdA . The optimum
doping concentration of 4 for a single EML device was
therefore taken as 4 wt%, which was higher than that of
[19]
1
(1.5%). The increase in allowed doping concentration
without deteriorating the device performance of 4 is attrib-
uted to the reduced intermolecular aggregation. Thus, the
incorporation of a norbornene group into the periphery of
a Schiff base ligand is an effective way to mitigate bimolecu-
lar self-quenching of emission. However, the EL efficiencies
of these devices showed rapid roll-off with increasing cur-
rent density. This may be attributed to an unbalanced carrier
distribution on 4 and narrow recombination zone.
The normalized EL spectra of these devices and the pho-
toluminescence (PL) spectrum of 26DCzPPy are given in
Figure S1b in the Supporting Information. Apart from the
emission of 4 (peak at l=628 nm), devices with a low
dopant (here 4) concentration also showed another emission
Figure 3. a) EL efficiency–current density characteristics of the double
EML devices with 4 at different doping concentrations. Inset: Bright-
ness–voltage–current density characteristics of the double EML devices
with 4 at different doping concentrations. b) PL spectra of 26DCzPPy
and TcTa, and normalized EL spectra of the double EML devices with 4
with
a
maximum at l=395 nm originating from
[30]
26DCzPPy. The emission of 26DCzPPy diminished rapid-
ꢀ
2
at different doping concentrations operating at 10 mAcm
.
ly upon increasing the dopant concentration. The EL of de-
vices with a dopant (4) concentration of 4 wt% or higher
only displayed the emission of 4. This indicates that energy
transfer from the host material to 4 can go nearly to comple-
tion when the concentration of 4 reaches 4 wt% or above.
cy. The brightness–voltage–current density characteristics of
these devices are depicted in the inset of Figure 3a. With in-
creasing dopant concentration, the EL efficiency increased
to a maximum value and then decreased gradually. As
shown in Table 3, the 4 wt% doped device gave the highest
Double EML Device Architecture
ꢀ
2
A
of
series of double EML devices with the structure
ITO/TAPC (40 nm)/4 (x wt%):TcTa (10 nm)/4
(10 nm)/
brightness of 10810 cdm , the highest current efficiency of
ꢀ1
10.82 cdA , and the highest power efficiency of 9.00 Lm
(
x wt%):26DCzPPy
TmPyPB (40 nm)/LiF (1 nm)/
Al (100 nm) were also fabricat-
ed. These devices were con- Doping of 4
Table 3. The key properties of double EML devices with 4 at different doping concentrations.
[
a]
[b]
[c]
[d]
ꢀ1
V
[V]
turn-on
B
h
c max
h
p
h
c 1000 (cdA at
hc roll-off at
1000 cdm
ꢀ
2
ꢀ1
ꢀ1
ꢀ2
ꢀ2[e]
[%]
[cdm
]
[cdA
]
[lmW
]
1000 cdm
)
[%]
ceived to have a broadened re-
combination zone and more
balanced carrier injection. In
a recent paper, the efficacy of
this device design strategy was
2
3
4
5
6
7
3.5
3.5
3.7
3.5
3.5
3.5
6712
9512
10810
9834
7424
6414
7.75
9.92
10.82
8.27
8.72
5.77
6.35
7.83
9.00
5.81
7.20
5.18
5.28
6.79
7.02
5.83
5.90
3.32
32
32
35
30
32
42
[30]
demonstrated.
Figure 3 de-
picts the dopant concentration
dependence of the EL efficien- at 1000 cdm . [e] hc roll-off =1ꢀ(hc 1000/hc max).
[
a] Maximum brightness. [b] Maximum current efficiency. [c] Maximum power efficiency. [d] Current efficiency
ꢀ2
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Chi-Ming Che et al.
ꢀ
1
W . Compared with the single EML devices, these double
EML devices showed slower roll-off of EL efficiencies. At
ꢀ
2
a brightness of 1000 cdm , the 4 wt% doped device re-
ꢀ
1
tained a high EL current efficiency of 7.02 cdA , which was
9% higher than that of the devices fabricated with 1 under
4
[30]
the same device architecture. This enhancement in current
efficiency at high luminance is attributed to the improved
carrier trapping ability of 4. This current efficiency of the
double EML device was also 61% higher than that of the
single EML devices fabricated with 4. This may be due to
broadening of the recombination zone.
ꢀ
2
The current efficiency roll-off at 1000 cdm of the double
EML device fabricated with 4 was 35%, which was lower
than that of the device based on 1. This can be attributed
Figure 4. Proposed energy level diagram of the codoped devices investi-
gated herein. Molecular structure of FIrpic is also shown.
[30]
to the mitigated triplet–triplet annihilation as a result of the
reduced k value of 4.
q
2
[9]
The EL spectra of these double EML devices and the PL
spectra of TcTa and 26DCzPPy were compared. As shown
in Figure 3b, there is weak emission at l=380–460 nm,
apart from the emission of 4, which has been shown to come
N,C ’]picolinate (FIrpic), which has a band gap of 2.68 eV,
was codoped into the EML at a low concentration (ꢁ
0.5 wt%) in an attempt to further delay the roll-off of EL
efficiency. As depicted in Figure 4b, the LUMO level of
[30]
[9]
from TcTa and 26DCzPPy.
Compared with the single
FIrpic (ꢀ3.47 eV)
is 0.58 eV lower than that of 4
EML devices, the double EML devices showed much
weaker host emission at low dopant concentration, which in-
dicated more efficient energy transfer from the host materi-
al to 4.
(ꢀ2.89 eV). Therefore, the codoped FIrpic in EML2 is ex-
pected to function as a deep electron trap. On the other
hand, the HOMO level (ꢀ6.15 eV) of FIrpic is lower than
that of 26DCzPPy (ꢀ6.1 eV); therefore, the transport of
holes to FIrpic molecules could be insignificant.
A series of codoped single EML devices with the struc-
Optimization of the Hole-Blocking-Layer Thickness
ture
of
ITO/TAPC
(30 nm)/FIrpic
(z wt%):4
To further improve the EL performance of 4, four devices
with the structure of ITO/TAPC (30 nm)/4 (4 wt%):TcTa
(4 wt%):26DCzPPy (10 nm)/TmPyPB (60 nm)/LiF (1 nm)/
Al (100 nm) were fabricated. A control device with a similar
structure, but without codoping of FIrpic was fabricated.
The EL efficiency increased to a maximum value and then
decreased gradually with increasing concentration of the co-
dopant FIrpic (Figure S4a in the Supporting Information).
As depicted in Table 4, the 0.3 wt% codoped device gave
(
(
10 nm)/4 (4 wt%):26DCzPPy (10 nm)/TmPyPB (y nm)/LiF
1 nm)/Al (100 nm) were fabricated by adjusting the thick-
ness of HBL/ETL to be 50, 60, 70, and 80 nm, respectively.
The EL spectra of these devices are given in Figure S2b in
the Supporting Information. All of these devices displayed
close to pure red emission. Moreover, as depicted in Fig-
ure S2a in the Supporting Information, all of the EL effi-
ciencies were significantly enhanced upon increasing the
thickness of HBL/ETL. This is attributed to the improved
balance of holes and electrons. As shown in Table S2 in the
Supporting Information, the device with 70 nm HBL/ETL
gave the highest EL current and power efficiencies of
ꢀ
2
the highest brightness of 13184 cdm , the highest current
ꢀ
1
efficiency of 17.79 cdA , and the highest power efficiency
ꢀ
1
ꢀ2
of 16.92 LmW . At a brightness of 1000 cdm , the same
ꢀ
1
device retained an EL efficiency of 10.04 cdA , which was
28.1% higher than that of the control device. The PL spec-
trum of FIrpic and the normalized EL spectra of these devi-
ces are given in Figure S4b in the Supporting Information.
Apart from the emission of 4, a weak emission of FIrpic was
also observed from the codoped devices. With increasing co-
doping concentration of FIrpic, the intensity of FIrpic emis-
sion increased monotonously.
ꢀ
1
ꢀ1
1
6.91 cdA and 13.97 LmW , respectively, whereas the
ꢀ
2
highest brightness of 12645 cdm was obtained for the
device with the HBL/ETL thickness of 60 nm. At a bright-
ness of 1000 cdm , the device with 80 nm HBL/ETL re-
tained a high EL current effi-
ciency of 10.87 cdA . Howev-
er, the EL efficiency roll-off in
these devices was still very
severe.
ꢀ
2
ꢀ
1
Table 4. The key performance parameters of single EML codoped devices with FIrpic at different doping con-
centrations.
ꢀ
1
Doping of FIrpic
%]
V
[V]
turn-on
B
[cdm
h
c max
h
p
h
c 1000 (cdA at
hc roll-off at
1000 cdm [%]
ꢀ2
ꢀ1
ꢀ1
ꢀ2
ꢀ2
[
]
[cdA
]
[lmW
]
1000 cdm
)
0
0
0
3.3
3.2
3.3
3.3
3.3
3.2
10996
11600
12102
13184
11308
11045
12.90
14.61
15.92
17.79
16.06
11.85
11.06
13.90
15.15
16.92
15.26
10.33
7.84
8.16
8.51
10.04
7.92
7.95
39
44
47
44
51
33
.1
.2
Effect of FIrpic as a Codopant
A commonly used iridium
A
H
U
G
R
N
U
G
0
0
.4
.5
complex, iridium(III)bis[(4,6-di-
ACHTUNGTRENNUNG
fluorophenyl)pyridinato-
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Chi-Ming Che et al.
A series of codoped double Table 5. The key performance parameters of double EML codoped devices with FIrpic at different doping
concentrations.
EML devices with the structure
ꢀ
1
Doping of FIrpic
%]
V
[V]
turn-on
B
[cdm
h
c max
h
p
h
c 1000 (cdA at
hc roll-off at
1000 cdm [%]
of
ITO/TAPC
(30 nm)/4
ꢀ
2
ꢀ1
ꢀ1
ꢀ2
ꢀ2
[
]
[cdA
]
[lmW
]
1000 cdm
)
(
(
(
(
4 wt%):TcTa
(10 nm)/FIrpic
0
0
0
0
3.4
3.4
3.4
3.4
3.4
3.3
13038
13967
16532
15013
12575
11768
12.00
14.50
16.55
15.43
13.82
11.89
8.17
11.22
11.59
10.08
9.72
9.96
10.89
14.56
14.69
8.48
17
25
12
4.8
39
9.0
z wt%):4 (4 wt%):26DCzPPy
10 nm)/TmPyPB (60 nm)/LiF
1 nm)/Al (100 nm) were fabri-
.1
.2
.3
cated. A control device with 0.4
a similar structure, but without 0.5
7.80
10.82
ꢀ
2
ꢀ1
1
6532 cdm , the highest current efficiency of 16.55 cdA ,
ꢀ
1
and the highest power efficiency of 11.59 LmW . At
a brightness of 1000 cdm , the 0.3 wt% codoped double
EML device retained a high EL efficiency of 14.69 cdA ,
which was 48% higher than that of the control device. Thus,
codoping FIrpic into the EML is an effective strategy to im-
prove the EL device performance.
ꢀ
2
ꢀ1
As shown in Figure 5b, all of the EL emissions of these
codoped double EML devices are identical to the PL of 4.
The 0.2 wt% codoped device had Commission Internatio-
nale de lꢂEclairage (CIE) coordinates of (0.612, 0.380) at
ꢀ
2
a current density of 10 mAcm . With increasing codoping
concentration of FIrpic, no marked FIrpic emission was ob-
served from these codoped double EML devices.
Other Iridium
Two other iridium complexes, iridium
,6-diuoro-2,3-bipyridine]acetylacetonate
HOMO and LUMO levels of ꢀ6.30 and ꢀ3.60 eV, respec-
tively) and iridium(III)bis[(3,5-difluoro-4-cyanophenyl)pyri-
dine]picolinate (FCNIrpic; with HOMO and LUMO levels
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
2
[12]
AHCTUNGTRENNUNG
[
13]
of ꢀ5.72 and ꢀ2.98 eV,
respectively), were also used as
codopants to give a series of single and double EML codop-
ed devices.
The codoped devices fabricated with FK306 gave better
EL performance than the codoped devices fabricated with
FIrpic. As shown in Table 6 and Figure S5 in the Supporting
Information, EL current, power, and external quantum effi-
ꢀ
1
ꢀ1
ciencies as high as 20.43 cdA , 18.33 LmW , and 11.7%,
respectively, have been realized by optimizing the codoping
concentration of FK306 to 0.2%. No discernable FK306
Figure 5. a) EL efficiency–current density characteristics of the double
EML codoped devices with FIrpic at different doping concentrations.
Inset: Brightness–voltage–current density characteristics of the double
EML codoped devices with FIrpic at different doping concentrations.
b) PL spectrum of FIrpic and normalized EL spectra of the double EML
codoped devices with FIrpic at different doping concentrations operating
emission was observed, even at a codoping concentration of
.5%. At a current density of 10 mAcm , the CIE coordi-
nates of the 0.3% FK306 codoped device were (0.621,
ꢀ2
0
ꢀ2
at 10 mAcm
.
Table 6. Key performance parameters of double EML codoped devices with FK306 at different doping con-
centrations.
codoping of FIrpic was also fab-
ricated. As shown in Figure 5a,
the EL efficiency increased to
a maximum value and then de-
creased gradually with increas-
ꢀ
1
Doping of FK306
%]
V
[V]
turn-on
B
[cdm
h
c max
h
p
h
c 1000 (cdA at
hc roll-off at
1000 cdm [%]
ꢀ2
ꢀ1
ꢀ1
ꢀ2
ꢀ2
[
]
[cdA
]
[lmW
]
1000 cdm
)
0
0
3.6
3.6
3.4
3.5
3.5
3.5
12679
15229
16673
17635
15478
14900
11.38
17.33
20.43
18.15
17.29
16.75
9.84
12.10
18.33
15.57
15.11
14.03
5.82
13.50
11.94
12.23
11.78
11.60
49
22
42
33
32
31
.1
ing codoping concentration of 0.2
FIrpic. As shown in Table 5, the 0.3
0
0
.4
.5
0.2% codoped device gave the
highest brightness of
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Chi-Ming Che et al.
0
.373). However, all of the EL performances of the devices
nificant FCNIrpic emission together with no improvement
in the performance of devices codoped with FCNIrpic was
observed.
codoped with FCNIrpic were inferior. As shown in Fig-
ure S6 in the Supporting Information, these devices gave
a maximum EL efficiency of 15.56 cdA and displayed sig-
nificant emission from FCNIrpic.
ꢀ
1
Enhancement of Carrier Balance by Using Codopant
To examine the mechanism for the improved performance
of these codoped devices, the distributions of holes and elec-
trons in the double EML devices and in the codoped double
EML devices were analyzed. Figure 6 shows a schematic
Device Lifetime
To evaluate the operation stability, devices were encapsulat-
ed and operated continuously at an initial brightness of
000 cdm , and the residual brightness of the operating
ꢀ2
1
device was tested once a day. The lifetime (LT ) was ob-
5
0
[35,36]
tained by extrapolating the curve at lower brightness.
As shown in Figure S3 in the Supporting Information, the
double EML device with 60 nm HBL/ETL exhibited a life-
time of 12500 h, which was 30% higher than the corre-
sponding value of 9600 h recorded for the devices fabricated
[30]
with 1 under similar conditions. This is attributed to the
enhanced current efficiency of devices fabricated with 4.
Moreover, a longer device lifetime of 18000 h has been real-
ized with the 0.3 wt% FIrpic codoped double EML device.
This device lifetime data is superior to those of efficient, red
OLEDs fabricated with platinum(II) emissive dopant re-
[21]
ported in the literature.
Figure 6. Schematic representation of carrier distribution in the EMLs of
double EML and codoped double EML devices. Symbols ꢀ and + repre-
sent electrons and holes, respectively.
Electroluminescent Mechanisms of the Codoped
Devices
representation of distribution of carriers in the double EML
devices. For the control devices, holes and electrons were
the major carriers in EML1 and EML2, respectively. As
a result, the holes and electrons were unevenly distributed
to molecules of 4 within both EML1 and EML2 despite the
balanced injection of carriers from the anode and cathode.
For the codoped double EML devices, some electrons
within EML2 were trapped by FIrpic or FK306 molecules,
whereas most holes were trapped by 4. With increasing co-
doping concentration of FIrpic or FK306, more electrons
within EML2 would be trapped by FIrpic or FK306, leading
to a decreased density of electrons distributed to molecules
of 4. Therefore, improved carrier balance within the mole-
cules of 4 in EML2 could be realized by optimizing the co-
doping concentration of FIrpic or FK306. This accounts for
the significant enhancement in EL efficiency for the FIrpic
or FK306 codoped device. Furthermore, the FK306 codoped
devices displayed even higher EL efficiency than those of
the FIrpic codoped devices. This is attributed to the LUMO
level (ꢀ6.30 eV) of FK306 being lower than that of FIrpic,
which endows the FK306 molecules with a stronger elec-
tron-trapping ability. The lower HOMO level of FK306 also
makes the transfer of holes to FK306 more difficult; thus re-
sulting in the undiscernible FK306 emission.
Effect of Energy Levels of the Codopant
The EL mechanisms of these codoped devices were exam-
ined. Carrier injection and transport processes in these devi-
ces were first analyzed. As shown in Figure S7 in the Sup-
porting Information, holes were injected from the anode
into EML1 through the HTL. Some holes could be injected
into EML2 through hopping of holes among molecules of 4.
On the other hand, electrons were injected from the cathode
into EML2 through the HBL/ETL. Some electrons could be
injected into EML1 through electron hopping among the
molecules of 4. The LUMO levels (ꢀ3.47 and ꢀ3.60 eV) of
FIrpic and FK306 are lower than that of 4 (ꢀ2.89 eV); this
means that the codoped FIrpic or FK306 molecules in
EML2 function as a deep electron trapper. Therefore, elec-
trons in EML2 would be preferentially trapped by FIrpic or
FK306 molecules, whereas holes in EMLs would be trapped
by 4 owing to its higher HOMO level. The observation of
2
6DCzPPy emission in the EL spectra revealed that it was
possible for holes to transfer from TcTa to 26DCzPPy,
whereas the weak FIrpic emission and undiscernible FK306
emission altogether suggested that it was difficult for holes
to transfer from 26DCzPPy to FIrpic or FK306. On the
other hand, the LUMO level of FCNIrpic (ꢀ2.96 eV) is
only slightly lower than that of 4 (ꢀ2.89 eV). Therefore, co-
doped FCNIrpic would not function as a deep electron trap-
per. Transfer of holes from 26DCzPPy to FCNIrpic is facile
owing to the HOMO level (ꢀ5.70 eV) of FCNIrpic being
higher than that of 26DCzPPy (ꢀ6.10 eV). As a result, sig-
In the electron-dominant EML2 of these codoped double
EML devices, electron hopping between the molecules of
FIrpic or FK306 was inefficient owing to the low doping
concentration, whereas electron transport through the
26DCzPPy molecules would be slow because electrons were
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Chi-Ming Che et al.
trapped by the molecules of 4, FIrpic, or FK306. Therefore,
electron hopping between the molecules of 4 would be the
dominant electron-transport mechanism within EML2. Be-
cause electron mobility is proportional to the density of
electrons involved in the transport process, it is proposed
that the codoping of FIrpic or FK306 would delay the trans-
port of electrons in EML2 as trapping of electrons by FIrpic
or FK306 molecules would reduce the amount of electrons
trapped by 4.
To validate this proposition, a series of electron-only devi-
ces with the structure of ITO/TmPyPB (30 nm)/FIrpic
(z wt%):4 (4 wt%):26DCzPPy (50 nm)/TmPyPB (30 nm)/
LiF (1 nm)/Al (100 nm) were designed and fabricated. Here,
TmPyPB acted as a hole blocker at the anode and an elec-
tron transporter at the cathode. Therefore, only electrons
would be transported in these devices. Six devices with
FIrpic doping concentrations of 0, 0.1, 0.2, 0.3, 0.4, and
0.5% were fabricated. Current density–voltage (J–V) char-
acteristics of these devices are depicted in Figure 7. As
shown in the inset of Figure 7, the delay of electron trans-
port caused broadening of the recombination zone, resulting
in decreased exciton density and accounting for mitigating
the annihilation of excitons and delaying the roll-off of EL
efficiency.
Figure 8. a) Normalized PL spectra of 4 wt% 4-doped 26DCzPPy, 0.3%
FIrpic-doped 26DCzPPy, and 0.3% FIrpic- and 4 wt% 4-codoped
26DCzPPy films. b) Normalized PL spectra of 4 wt% 4-doped
26DCzPPy, 0.3 wt% FK306-doped 26DCzPPy, and 0.3 wt% FK306- and
4 wt% 4-codoped 26DCzPPy films.
This can be attributed to significant spectral overlap be-
tween the emission of 26DCzPPy and absorption of
[30,37]
FIrpic.
On the other hand, the emissions of 4 and
2
6DCzPPy were observed to have similar intensity in the 4
doped film, which indicated that energy transfer from
26DCzPPy to 4 was inefficient. This is because the emission
spectrum of 26DCzPPy does not have a large spectral over-
Figure 7. Current density–voltage characteristics of the electron-only de-
vices with FIrpic at different doping concentrations. Inset: Schematic rep-
resentation of the recombination zone in double EML devices and co-
doped double EML devices.
[30]
lap with the absorption spectrum of 4. However, the PL
spectrum of the film of 4 codoped with FIrpic displayed an
intense emission of 4 accompanied by a weak 26DCzPPy
emission and a very weak FIrpic emission. A similar phe-
nomenon has also been observed upon replacement of
FIrpic with FK306 in these 26DCzPPy thin films, as shown
in Figure 8b. These results revealed that the codoped FIrpic
or FK306 molecules acted as an energy-transfer ladder to fa-
cilitate energy transfer from 26DCzPPy to 4. This is support-
ed by the good spectral overlap between the emission spec-
tra of FIrpic or FK306 and the absorption of 4. Efficient
energy transfer from the host to the dopant is highly desira-
ble in EL devices because this can harvest exciton energy at
a high current density, improve the color purity, and delay
the roll-off of EL efficiency.
FIrpic as an Energy-Transfer Ladder between Host and
Dopant
The photophysical properties of these codoped systems have
been examined. The PL spectrum of 26DCzPPy thin film
doped both 4 wt% of 4 and 0.3 wt% of FIrpic was com-
pared with that of the 26DCzPPy thin films doped with
4
wt% of 4 and 0.3 wt% of FIrpic alone. In each case, the
film thickness was around 50 nm. As shown in Figure 8a,
the film doped with FIrpic showed an intense FIrpic emis-
sion accompanied by a weak 26DCzPPy emission, which
suggested fast energy transfer from 26DCzPPy to FIrpic.
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Chi-Ming Che et al.
Conclusion
with a Quanta Ray GCR 150-10 pulsed Nd:YAG laser system (pulse
output l=355 nm, 5–6 ns).
Luminescence quantum yields were determined by using the method of
A series of sterically hindered red-light-emitting platinu-
m(II) Schiff base complexes with narrow band gaps were
synthesized. The emission self-quenching rate constant of 4
is significantly mitigated owing to the presence of a rigid
norbornene moiety. With 4, a high doping concentration of
[
39]
Demas and Crosby
3 2
with [Ru ACHTUNTGRNNEG(U bpy) ]Cl (bpy=2,2’-bipyridine) in de-
gassed acetonitrile as a standard reference solution (F
r
=0.062) and cal-
culated according to Equation (1):
2
F
s
¼ F
r
ðB
r
=B
s
Þðn
s
=n
r
Þ ðD
s
=D
r
Þ
ð1Þ
4
wt% could be used in EL devices without deteriorating
in which the subscripts s and r refer to sample and reference standard so-
lution, respectively; n is the refractive index of the solvents; D is the inte-
grated intensity; and F is the luminescence quantum yield. The quantity
B was calculated by B=1–10 , in which A is the absorbance at the ex-
citation wavelength and L is the optical path length.
the device performance; thus allowing more complete
energy transfer from the host to the emitter.
ꢀ
AL
A series of double EML EL devices were fabricated by
doping 4 into host materials with stepwise energy levels.
Red emission with a high EL efficiency was realized. This is
attributed to reduced self-aggregation, improved carrier
trapping ability of 4, and optimization of the double EML
device structure. At a brightness of 1000 cdm , the EL cur-
rent efficiency of the device fabricated with 4 was 49%
higher than that of the device fabricated with 1 under the
same double EML device architecture.
Cyclic voltammetry was performed on a Princeton Applied Research
Model 273A potentiostat/galvanostat coulometer and Model 270/250 uni-
versal programmer by using a three-electrode cell system with a glassy
3
carbon disk as the working electrode, an Ag/AgNO (0.1m) electrode in
ꢀ
2
CH CN as the reference electrode, and a platinum wire as the counter
3
electrode. Tetrabutylammonium hexafluorophosphate (0.1m) was used as
the supporting electrolyte. Ferrocenium/ferrocene was used as an internal
+
/0
reference and all potentials were quoted with respect to Cp
2
Fe
.
Synthesis
Codoping low levels of FIrpic or FK306 into the electron-
dominant EML was effective in improving the EL perfor-
mance. This is attributed to enhanced carrier balance,
broadening of the recombination zone, and facilitating the
transfer of energy from the host to the emitter molecules.
Compared with control devices, the codoped devices dis-
played higher EL efficiency, slower roll-off of EL efficiency,
higher brightness, and improved device lifetimes. The co-
doped devices exhibited high EL current, power, and exter-
The synthetic schemes for all of the platinum(II) Schiff base complexes
are given in Scheme 1. A mixture of the corresponding ligand and
sodium acetate (2 equiv) was dissolved in a minimum amount of hot
DMF. Potassium tetrachloroplatinate (1 equiv) in hot DMSO was added
and the reaction mixture was kept at 808C overnight.
For 1, a red precipitate was collected by filtration and the crude product
was recrystallized in hot DMF. Characterization data for 1 matched that
reported in the literature.
[
19]
For 2a, 2b, 3a, 3b, and 4, water was added to the reaction mixture and
the precipitate was collected by filtration. The crude product was purified
by chromatography on an alumina column (CH Cl /hexane 1:4). Com-
2 2
pound 4 was further purified by recrystallization in hexane. The platinu-
ꢀ
1
ꢀ1
nal quantum efficiencies up to 20.43 cdA , 18.33 LmW ,
and 11.7%, respectively. At high brightness of
000 cdm , EL current efficiency and EQE as high as
a
ꢀ2
1
1
m(II) complexes were obtained as red solids.
ꢀ1
4.69 cdA and 8.3%, respectively, was retained. Further-
1
Compound 2a: Yield: 60%; H NMR (300 MHz, CDCl
2H), 8.00 (m, 2H), 7.67 (s, 2H), 7.31 (m, 4H), 1.58 (s, 18H), 1.35 ppm (s,
3
): d=8.90 (s,
more, this device showed a long projected lifetime (LT ) of
5
0
1
8000 h; this finding was better than that of efficient, plati-
18H).
[21]
1
num-based, red OLEDs reported in the literature.
Compound 2b: Yield: 57%; H NMR (400 MHz, CDCl
3
): d=8.85 (d, J=
1
3.8 Hz, 2H), 7.93 (d, J=1.8 Hz, 1H), 7.89 (d, J=8.9 Hz, 1H), 7.66 (dd,
J=4.4, 2.6 Hz, 2H), 7.38–7.33 (m, 2H), 7.32 (d, J=2.5 Hz, 1H), 1.58 (s,
9
H), 1.56 (s, 9H), 1.46 (s, 9H), 1.37 (s, 9H), 1.35 ppm (s, 9H).
Experimental Section
1
Compound 3a: Yield: 23%; H NMR (300 MHz, CDCl
3
): d=8.86 (s,
2
6
H), 7.99 (s, 2H), 7.64 (d, J=8.2 Hz, 2H), 7.51–7.29 (m, 6H), 2.12 (s,
H), 1.93 (s, 12H), 1.79 ppm (s, 12H).
Materials
1
All chemicals and solvents (AR grade) were used as received. HPLC-
grade solvent was used for photophysical measurements. TAPC, TcTa,
Compound 3b: Yield: 75%; H NMR (400 MHz, CDCl ): d=8.82 (d, J=
3
14.4 Hz, 2H), 7.92 (s, 1H), 7.87 (d, J=8.8 Hz, 1H), 7.68–7.56 (m, 2H),
2
6DCzPPy, TmPyPB, and FIrpic were purchased from Luminescence
Technology Corp. All of these materials were used as received. Complex
was purified by gradient sublimation before use.
7.44 (s, 1H), 7.42–7.28 (m, 4H), 2.12 (s, 6H), 1.94 (d, J=8.2 Hz, 12H),
1.87–1.70 (m, 12H), 1.45 ppm (s, 9H).
4
1
Compound 4: Yield: 32%; H NMR (500 MHz, CD
2
Cl
2
): d=8.76 (s, 2H),
7
7
8
.97 (dd, J=6.0, 3.2 Hz, 2H), 7.32 (dd, J=6.1, 3.2 Hz, 2H), 7.28 (s, 2H),
.08 (s, 2H), 3.36 (d, J=6.4 Hz, 4H), 1.97 (d, J=8.3 Hz, 4H), 1.80 (d, J=
.0 Hz, 2H), 1.62 (d, J=8.8 Hz, 2H), 1.31 ppm (d, J=7.5 Hz, 4H).
Measurement
Positive-ion-mode electron ionization (EI) and fast atom bombardment
(
FAB) mass spectra were recorded by using a Finnigan MAT 95 mass
Device Fabrication and Characterization
spectrometer or a Thermo Scientific DFS high resolution magnetic sector
1
13
ꢀ1
mass spectrometer. The H (300, 400, or 500 MHz) and C (126 MHz)
NMR spectrosca were recorded by using Bruker Avance DPX-300, 400
or DRX-500 spectrometer with chemical shifts (in ppm) relative to tetra-
methylsilane (TMS) as a reference. UV/Vis spectra were recorded on
a Hewlett Packard 8453 UV/Vis spectrophotometer. Steady-state emis-
sion and excitation spectra were recorded on a Fluorolog-3 Model FL3-
ITO-coated glass with a sheet resistance of 12 Wsq was used as the
anode substrate. Prior to film deposition, patterned ITO substrates were
cleaned with detergent, rinsed in deionized water, dried in an oven, and
finally treated with oxygen plasma for 5 min at a pressure of 15 Pa to en-
[
38]
hance the surface work function of ITO anode (from 4.7 to 5.1 eV).
All of the organic layers were deposited at a rate of 0.2 nms under high
vacuum conditions (ꢁ2ꢀ10 Pa). The doped and codoped layers were
prepared by coevaporating dopant(s) and host material from two or
three individual sources, and the doping concentration was modulated by
controlling the evaporation rate of dopant. LiF and Al were deposited in
ꢀ1
ꢀ
5
2
1 spectrofluorometer. Solution samples for measurements were de-
gassed with five freeze–pump–thaw cycles. The emission spectra were
corrected for monochromator and photomultiplier efficiency and for
xenon lamp stability. Emission lifetime measurements were performed
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Chi-Ming Che et al.
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another vacuum chamber (ꢁ8.0ꢀ10 Pa) with the rates of 0.01 and
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ꢀ
1
1
nms , respectively, without being exposed to the atmosphere. The
thicknesses of these deposited layers and the evaporation rate of individ-
ual material were monitored under vacuum condition with quartz crystal
monitors. A shadow mask was used to define the cathode and to make
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2
four 9 mm devices on each substrate. Current density–voltage–brightness
(
J–V–B) characteristics of OLEDs were measured by using a Keithley
source measurement unit (Keithley 2400 and Keithley 2000) with a silicon
photodiode. The absorption spectra were measured by using a Hamamat-
su R928 photomultiplier tube. EL and PL spectra were measured with
a Fluorolog-3 Model FL3-21 spectrofluorometer.
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Acknowledgements
We are grateful for support from the “Hong Kong Scholars Program”
(
(
no. XJ2011036), the National Natural Science Foundation of China
grant no. 21201161), the China Postdoctoral Science Foundation (grant
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Lee, J. Am. Chem. Soc. 2004, 126, 4958–4971.
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Research Scheme (project no. T23-713/11), and the Innovation and Tech-
nology Commission of the HKSAR Government (GHP/043/10). This
work was also supported by Jilin Provincial Science and Technology De-
velopment Program of China (20130522125JH), Guangdong Special Proj-
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Received: June 5, 2014
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436–1441.
Published online: && &&, 0000
Chem. Asian J. 2014, 00, 0 – 0
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

FULL PAPER
Energy Transfer
Seeing red: Red-light-emitting plat
AHCTUNGTRENGNnUN um(II) Schiff base complexes with
ACHTUNGTRENNUNGi -
Liang Zhou, Chun-Lam Kwong,
Chi-Chung Kwok, Gang Cheng,
Hongjie Zhang,
bulky groups are described. This
enhanced bulkiness mitigates self-
quenching. Codoping of wide band gap
iridium(III) complexes into the elec-
Chi-Ming Che*
&&&& — &&&&
ACHTUNGTRENNUNG
tron dominant light-emitting layer
results in efficient red electrolumines-
cent devices with slow efficiency roll-
off (see figure).
Efficient Red Electroluminescent
Devices with Sterically Hindered
Phosphorescent Platinum(II) Schiff
Base Complexes and Iridium Complex
Codopant
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Chem. Asian J. 2014, 00, 0 – 0
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!