Yellow organic light-emitting diodes based on phosphorescent iridium(III) pyrazine complexes: Fine tuning of emission color

Article abstract of DOI:10.1016/j.ica.2008.10.001

The synthesis and luminescence of a series of new iridium pyrazine complexes were investigated. The phosphorescent peak wavelength can be fine-tuned in yellow color range of 553-588 nm. Complexes 4 and 6 were used as representative examples for fabricatio

Full text of DOI:10.1016/j.ica.2008.10.001

Inorganica Chimica Acta 362 (2009) 2231–2236
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Yellow organic light-emitting diodes based on phosphorescent iridium(III)
pyrazine complexes: Fine tuning of emission color
b
b,
Guoping Ge a, Guolin Zhang a,1, Haiqing Guo a, , Yutao Chuai , Dechun Zou
*
*
a Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
b Key Laboratory of Polymer Chemistry and Physics of Education Ministry, Department of Polymer Science and Engineering,
College of Chemistry and Molecular Engineering, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2008
Received in revised form 2 October 2008
Accepted 6 October 2008
Available online 15 October 2008
The synthesis and luminescence of a series of new iridium pyrazine complexes were investigated. The
phosphorescent peak wavelength can be fine-tuned in yellow color range of 553–588 nm. Complexes 4
and 6 were used as representative examples for fabrication of multilayered OLEDs. The two OLEDs all
exhibited high efficiency. The devices based on complex 4 exhibited an external quantum efficiency of
4.6% (power efficiency 9.7l m/W) and a maximum brightness of 32700 cd/m2. The device also showed
high color purity with an emission maximum at 561 nm and Commission Internationale de l’Eclairage
(CIE) coordinates of (x = 0.45, y = 0.54).
Keywords:
Yellow phosphor
Iridium pyrazine complex
3MLCT
Ó 2008 Elsevier B.V. All rights reserved.
OLED
Fine-tuning of color
1. Introduction
Modifying the skeletal arrangement as well as the substituent
groups of the cyclometalating ligand (C^N) afford significant tun-
Much attention has been paid to the phosphorescent materials
in recent years for their potential application as highly efficient
electroluminescent (EL) emitters in organic light-emitting devices
(OLEDs), since the first demonstration of highly efficient phospho-
rescent OLEDs with a maximum EQE of 4% were reported by Baldo
et al. in 1998 [1]. Unlike the devices based on fluorescent materials
that can only make use of singlet exciton and whose internal effi-
ciency is limited to about 25%, phosphorescent materials can har-
vest both singlet and triplet excitons and endow OLEDs with the
potential of reaching an internal efficiency of 100% [2,3]. Heavy-
metal complexes, particularly those containing Os(II) [4–8], Pt(II)
[9–12] and Ir(III) [13–19], where strong spin orbit coupling leads
to singlet–triplet state mixing, can result in high efficient elec-
trophosphorescence in OLEDs. Among the research of phosphores-
cent materials, rationally tuning the emission wavelength of
heavy-metal phosphorescent emitters over the entire visible range
has emerged as an important ongoing research task [17]. Irid-
ium(III) complexes are highly tunable in the emission color [20].
ing of phosphorescence [15,16,20,21]. Using appropriate ancillary
ligands also cause shifting of the emission from the red to the blue
region [22–24]. OLEDs based on iridium complexes containing two
hetero-atoms in the ligands such as thiazole, azine and thiazine
have high efficiency and long luminance half-life [25–30]. How-
ever, rationally tuning the emission wavelength of iridium com-
plexes containing two hetero-atoms in the ligands is seldom
reported. Bright yellow phosphorescent materials can be directly
used as emitting materials in OLEDs. They can be also used in
the fabrication of white OLEDs combined with a blue light-emis-
sion layer [31]. We got succeeded in preparation of two efficient
yellow OLEDs using the phosphorescent iridium(III) pyrazine
complexes of Ir(DPP)2(acac) (DPP = 2,3-diphenylpyrazine, acac =
acetylacetone) (1) [29] and Ir(MDPP)2(acac) (MDPP = 5-methyl-
2,3-diphenylpyrazine) (2) [30], both exhibiting high color purity
with featureless emission peak at 588 and 580 nm, respectively.
In this paper, we will report a systematic design, synthesis, and
characterization of yellow-emitting Ir(III) pyrazine complexes
based on (1) and (2) via variation of cyclometalating and ancillary
ligands. The phosphorescent peak wavelength can be fine-tuned in
yellow range (over the 553–588 nm range). Complexes 4 and 6
were used as representative examples for fabrication of multilay-
ered OLEDs. The two OLEDs all exhibited high efficiency.
* Corresponding authors. Tel./fax: +86 10 62755702 (H. Guo).
E-mail address: guohq@pku.edu.cn (H. Guo).
1
Present address: College of Chemistry, Liaoning University, Shenyang 110036,
China.
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.10.001

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G. Ge et al. / Inorganica Chimica Acta 362 (2009) 2231–2236
and phenyl pyrazine to replace acetylacetonate should cause hyp-
2. Results and discussion
2.1. Synthesis and characterization
sochromic shifts of emission wavelengths as in the case in phenyl
pyridine based organometallic complexes [15,21,23]. Thus,
Ir(DPP)3 (3), Ir(DPP)2(pic) (pic = picolinic acid) (4), Ir(MDPP)3 (5),
Ir(DPPF)2(acac) (6) and Ir(MDPPF)2(acac) (7) were prepared
successfully according to the synthesis routes shown in Scheme
1. 2,3-Diphenylpyrazine (DPP), 5-methyl-2,3-diphenylpyrazine
A serials of yellow-emitting Ir(III) pyrazine complexes were de-
signed and synthesized by using three different ways to tune the
phosphorescence. Inserting an electron-donating CH3 group at
the pyrazinyl group, replacing the phenyl fragment with a 4-fluo-
rophenyl substituent in the cyclometalating ligand, or using ancil-
lary ligands with less electron donor ability such as picolinic acid
(MDPP) and the complex of [Ir(DPP)2(l-Cl)]2 were prepared
according to the procedure previously reported [29]. 4,40-Di-
fluoro-2,3-diphenylpyrazine (DPPF) and 4,40-difluoro-5-methyl-
X
N
X
X
X
N
N
O
N
N
CO3
acetylacetone, Na2
N
N
Cl
Cl
Ir
2
IrCl3
3H2O
N
Ir
Ir
O
2-ethoxyethanol, reflux
Y
Y
Y
2-ethoxyethanol
Y
2
2
Y
Y
Y
Y
X=H,Y=H,
X=H,Y=H, (Ir(DPP)2Cl)2
X=H,Y=H, DPP
X=CH3 ,Y=H, (Ir(MDPP)2Cl)2
X=H,Y=F, (Ir(DPPF)2Cl)2
X=CH3 ,Y=F, (Ir(MDPPF)2Cl)2
X=CH3,Y=H, MDPP
X=H,Y=F, DPPF
X=H,Y=H, Ir(DPP)2(acac) (1)
X=CH3 ,Y=H, Ir(MDPP)2(acac) (2)
X=H,Y=F, Ir(DPPF)2(acac) (6)
X=CH3 ,Y=F, Ir(MDPPF)2(acac) (7)
X=CH3 ,Y=F, MDPPF
N
N
N
N
N
IrCl
3 3H2O
N
N
N
N
Cl
Cl
picolinic acid, Na CO3
2
Ir
Ir
Ir
2-ethoxyethanol
C=O
O
2-ethoxyethanol, reflux
2
2
2
DPP
(Ir(DPP)2Cl)2
Ir(DPP)2(pic) (4)
X
X
N
N
AgO2CCF3
N
Ir
N
IrCl3
+
200ºC,8h,solvent-free
3
X=H, Ir(DPP)3 (3)
X=CH3, Ir(MDPP)3 (5)
Scheme 1. Synthesis routes for the iridium complexes.
0.12
0.10
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
250
1
0.08
2
0.06
3
4
0.04
5
0.02
6
7
0.00
-0.02
440
460
480
500
520
540
560
580
600
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 1. UV–Vis absorptions of the iridium complexes in CH2Cl2.

G. Ge et al. / Inorganica Chimica Acta 362 (2009) 2231–2236
2233
2,3-diphenylpyrazine (MDPPF) were obtained from direct conden-
sation of ethylenediamine or 1,2-diaminopropane with 4,40-diflu-
orobenzil in refluxing ethanol, respectively. Cyclometalated Ir(III)
with bidentate picolinic acid ligands give the desired products,
(pz)2Ir(pic) (pic = picolinic acid). Ir(DPP)3 (3) and Ir(MDPP)3 (5)
were prepared according to the procedure previously reported
[33]. The complexes are all fully characterized by NMR spectros-
copy and elemental analysis. The 1HNMR spectra of Ir(DPP)3 (3)
and Ir(MDPP)3 (5) are consistent with a facial geometry around
the Ir centre, which indicates that the number of coupled spins is
equal to that of protons on one ligand because the three cyclo-
metalating ligands are magnetically equivalent due to the inherent
C3 symmetry of the complexes [34]. In addition, only fac configura-
tion was observed in all cases when tris-cyclometalated arylpyri-
dine complexes were synthesized in one-step procedure
involving the reaction of IrCl3 ꢀ 3H2O with excess of phenylpyridine
ligand, in the presence of silver trifluoroacetate reported by Gru-
shin et al. [33].
l-chloro-bridged dimers of general formula (pz)2Ir(l-Cl)2Ir(pz)2,
where pz represents the cyclometalated pyrazine, were synthe-
sized by the same method reported by Nonoyama [32]. The crude
products of these dimers were used for subsequent preparation of
(pz)2Ir(LX). Replacement of bridging chlorides with bidentate
b-diketonate ligands give the desired products, (pz)2Ir(acac)
(acac = acetylacetonate) and replacement of bridging chlorides
1.1
1.0
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
2
3
4
5
6
7
2.2. Photophysical data
The UV–Vis absorption bands of the iridium complexes are sim-
ilar in shape and can be divided to three regions: below 375, be-
tween 375–460 and above 460 nm (Fig. 1). The absorptions can
be assigned to the spin-allowed 1
p–p transition of the cyclometa-
*
lated pyrazine ligands, spin-allowed metal to ligand charge-trans-
fer (1MLCT) transition, and spin–orbit coupling enhanced 3p
–p
*
and the spin-forbidden 3MLCT bands, respectively [17].
The absorption bands above 460 nm are correlated to the pho-
toluminescence (PL) of the complexes (Fig. 2). It is notable that
the absorption edges from complex 1 to 7 are blue-shifted gradu-
ally (the inset of Fig. 5). The PL peaks of the complexes are also
blue-shifted gradually from 1 to 7 that are in good consistent with
the absorption spectra.
500
550
600
650
700
Wavelength (nm)
Fig. 2. PL spectra of the iridium complexes in CH2Cl2.
N
N
N
O
N
D1
O
O
Ir
Ir
O
F
F
2
2
F
F
7
553nm
6
561nm
A3
B2
N
N
N
A1
A4
N
N
N
O
O
O
N
O
O
Ir
Ir
Ir
C
O
2
2
2
588nm
1
2 580nm
4
569nm
B1
A2
N
N
N
N
C1
Ir
Ir
3
3
571nm
3
5
564nm
Fig. 3. Structure and emission kmax of the iridium complexes.

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G. Ge et al. / Inorganica Chimica Acta 362 (2009) 2231–2236
2000
1800
2.3. Tuning of the emission properties
100000
10000
1000
100
a
b
The insertion of an extra electron-donating CH3 group at the
pyrazinyl group results in a blue shift (see route A1, C1 and D1
in Fig. 3). DFT calculations indicate that the highest occupied
1600
1400
1200
1000
800
600
400
200
0
molecular orbital (HOMO) is mainly contributed from d
p states
a
b
of the iridium atom and the relatively electron rich phenyl group
of the cyclometalated ligand, whereas the lowest unoccupied
molecular orbital (LUMO) is predominantly located at the N-het-
erocyclic part of the cyclometalated ligand [17,35]. When elec-
tron-donating groups were incorporated onto the pyrazinyl ring,
it will raise the LUMO energy and thereby increase the HOMO–
LUMO energy gap [36]. Replacing the phenyl fragment with a 4-
fluorophenyl substituent in the cyclometalating ligand blue-
shifted the emission wavelengths of the complexes from 588 and
580 nm to 561 and 553 nm, respectively (route A3 and B2), by low-
ering the HOMO energy level of the cyclometalated ligand using an
electronegative F atom [17]. Using picolinic acid instead of acetyl-
acetonate as ancillary ligands caused a hypsochromic shift of emis-
sion wavelengths from 588 to 569 nm (route A4). The picolinic acid
has less electron donor ability than acetylacetonate, so introducing
it will increase the HOMO–LUMO energy gap, resulting in a blue
shift of the lowest MLCT and the emission maximum [17]. The sim-
ilar phenomenon was also observed by using corresponding phenyl
pyrazine instead of acetylacetonate as ancillary ligands. Thus, com-
plexes 3 and 5 exhibited a hypsochromic shift in comparison to
complexes 1 and 2, respectively (route A2 and B1).
10
1
-2
0
2
4
6
8
10
12
14
16
18
20
22
Voltage (V)
Fig. 5. The current–voltage–luminance characteristics of device I(a) and device I(b).
20
15
10
5
20
10
0
a
b
a
b
2.4. Electroluminescent devices
Devices I and II are fabricated by using 4 and 6 as emitters,
respectively. The configurations of the devices are as ITO/
NPB(30 nm)/NPB:7% Ir complexes(25 nm)/BCP(10 nm)/Alq3(30
nm)/Mg:Ag(10:1, 120 nm)/Ag(10 nm), in which 4,4-bis[N-(1-naph-
thyl)-N-phenyl-amino] biphenyl (NPB) served as host for iridium
complexes and the hole transporter, 2,9-dimethyl-4,7-diphenyl-
1,10-phenanthroline (BCP) as the hole-blocker, and tri-(8-hydroxy-
quinoline) aluminum(III)(Alq3) as the electron-transporter,
respectively.
0
0
400
800
1200
1600
2000
Current Density (mA/cm2)
Fig. 6. The current efficiency–current–power efficiency characteristics of device I(a)
and device II(b).
Devices I and II emitted yellow light with emission maxima at
569 and 561 nm, respectively (Fig. 4). The EL spectrum is consis-
tent with the PL spectrum, indicating that the EL is originated from
the triplet states of the iridium complexes [37]. There are no char-
acteristic emission peaks from NPB or Alq3, indicating an effective
energy transfer from the host exciton to the Ir dopant. The CIE
chromaticity coordinates for device I and II at 7 V are x = 0.48,
y = 0.50 and x = 0.45, y = 0.54, respectively.
From Figs. 5–7, it can be seen that device I with a turn-on volt-
age of 2.9 V shows its highest brightness of 52500 cd/mꢁ2 at
16.3 V, a maximum external quantum efficiency of 3.7% and a
highest power efficiency of 8.1 lm/Wꢁ1 and device II shows its
highest brightness of 32700 cd/mꢁ2 at 18.0 V, a maximum external
5
4
1.0
a
b
0.8
a
b
3
2
1
0
0.6
0.4
0.2
0.0
0
500
1000
1500
2000
Current Density (mA/cm2)
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 7. The external quantum efficiency–current characteristics of device I(a) and
Fig. 4. The electroluminescence spectra of device I(a) and device II(b) at 7 V.
device II(b).

G. Ge et al. / Inorganica Chimica Acta 362 (2009) 2231–2236
2235
quantum efficiency of 4.6% and a highest power efficiency of
4.3.1. Synthesis of Ir(MDPPF)2(acac) (7)
Cyclometalated Ir(III)-chloro-bridged dimers [(MDPPF)2Ir(l-
9.7 lm/Wꢁ1. The devices showed a gradual decrease in gext with
increasing current density, which is attributed to increasing trip-
let–triplet annihilation of the phosphor-bound excitons [16].
Cl)]2 were synthesized according to the Nonoyama route, by reflux-
ing IrCl3 ꢀ 3H2O with 2.2 equiv of MDPPF in a 3:1 mixture of 2-eth-
oxyethanol and water [32]. The reaction mixture was refluxed
under nitrogen atmosphere for 24 h. After cooling to room temper-
ature, a precipitate was collected by suction and washed with eth-
anol and then dried in vacuum to give a cyclometallated Ir-chloro-
bridged dimer as orange powder. Yield: 57%.
The dimer complex (0.72 g), 0.12 mL of acetyl acetone and
0.31 g of sodium carbonate were refluxed under nitrogen atmo-
sphere in 8 mL of 2-ethoxyethanol for 12 h. After the reaction mix-
ture was cooled to room temperature, the precipitate was filtered
off and washed with water, ethanol, and ether to afford orange-yel-
low powdery Ir(MDPPF)2(acac) in 26% yield. Calc. for C39H29F4Ir-
N4O2: C, 54.85; H, 3.42; N, 6.56. Found: C, 54.62; H, 3.68; N,
6.45%. 1H NMR (CDCl3, 300 MHz) d: 1.89 (s, 6H), 2.70 (s, 6H),
5.31 (s, 1H), 5.92 (m, 2H), 6.25 (m, 2H), 6.79 (m, 2H), 7.22 (t,
4H), 7.65 (m, 4H), 8.21 (s, 2H).
3. Conclusions
In summary, we synthesized a series of Ir(III) pyrazine com-
plexes. The phosphorescent peak wavelength could be fine-tuned
in yellow color range of 553–588 nm by using appropriate ancillary
and cyclometalating ligands containing different substituent
groups. The results suggest that the strategies for tuning emission
color used in phenyl pyridine based organometallic complexes are
also suitable to phenyl pyrazine based Ir(III) complexes. Complexes
4 and 6 were selected as examples in application in OLEDs. The two
OLEDs all exhibited high efficiency and high color purity. This im-
plies that the iridium pyrazine complexes may find potential appli-
cations in yellow OLEDs or white OLEDs when combined with a
blue light-emission layer.
4.3.2. Ir(DPPF)2(acac) (acac = acetylacetone) (6)
4. Experimental
These orange-yellow powders were prepared similarly to the
synthesis of Ir(MDPPF)2(acac) in 45% yield. Anal. Calc. for
C37H25F4IrN4O2: C, 53.81; H, 3.05; N, 6.78. Found: C, 53.49; H,
3.30; N, 6.52%. 1H NMR (CDCl3, 300 MHz) d: 1.89 (s, 6H), 5.31 (s,
1H), 5.95 (m, 2H), 6.28 (m, 2H), 6.89 (m, 2H), 7.25 (t, 4H), 7.69 (t,
4H), 8.35 (d, 2H), 8.41 (d, 2H).
4.1. Measurement
1H NMR spectra were measured on a Mercury Plus 300 spec-
trometer in CDCl3 using tetramethylsilane as an internal reference.
Elemental analyses were performed on an Elementary Vario EL
instrument. Mass spectra were measured on a Bruker BIFLEX III
mass spectrophotometer. UV–vis absorption spectra were recorded
using a Shimadzu UV-2550 spectrophotometer. Emission and exci-
tation spectra were measured on a Hitachi F-4500 fluorescence
spectrophotometer.
4.3.3. Synthesis of Ir(DPP)2(pic) (4)
The dimer complex of [Ir(DPP)2(l-Cl)]2 (0.89 g), 0.198 g of
picolinic acid and 0.43 g of sodium carbonate were refluxed un-
der nitrogen atmosphere in 10 mL of 2-ethoxyethanol for 12 h.
After the reaction mixture was cooled to room temperature,
the precipitate was filtered off and washed with water, ethanol,
and ether to give the desired complex Ir(DPP)2(pic) (4) as red-
dish orange powder. Yield: 18%. 1H NMR (CDCl3, 300 MHz) d:
6.26 (d, 1H), 6.53 (m, 2H), 6.62 (t, 1H), 6.73–6.76 (t, 1H),
6.78–6.82 (t, 1H), 6.87–6.92 (m, 2H), 7.43–7.52 (m, 8H), 7.58–
7.71 (m, 5H), 7.97–8.01 (t, 1H), 8.15 (d, 1H), 8.33 (d, 1H), 8.43
(d, 1H), 8.82 (d, 1H).
4.2. Preparation of ligands
4.2.1. Synthesis of 4,40-difluoro-5-methyl-2,3-diphenylpyrazine
(MDPPF)
1,2-Di-aminopropane (1.26 mL) was added slowly to a solution
of 3.00 g of 4,40-difluorobenzil and 25 mL of ethanol at room tem-
perature under stirring. The mixed solution was refluxed for 0.5 h.
After cooling to room temperature, 100 mL of H2O was added to
the mixed solution and a yellow precipitate was collected by suc-
tion and washed with water to afford 4,40-difluoro-2-methyl-2,3-
dihydro-5,6-diphenylpyrazine (MDPPFH) in 73% yield.
A solution of 3.14 g of 4,40-difluoro-2-methyl-2,3-dihydro-5,6-
diphenylpyrazine and 2.74 g of chloranil in 30 mL of xylene was re-
fluxed for 7 h. After cooling to room temperature, the solution was
diluted with ether, extracted with NaOH solution to remove tetra-
chlorohydroquinone. The solution was further extracted with con-
centrated hydrochloric acid. Neutralization of the acidic extract
gave 4,40-difluoro-5-methyl–2,3-diphenylpyrazine as brown pow-
ders. Yield: 63%. M.p. 87–89 °C. Anal. Calc. for C17H12F2N2: C,
72.33; H, 4.28; N, 9.92. Found: C, 72.16; H, 4.45; N, 9.80%.
DPPF: These brown powders were prepared similarly to the syn-
thesis of MDPPF in 81% yield. M.p. 121–122 °C. Anal. Calc. for
C16H10F2N2: C, 71.63; H, 3.76; N, 10.44. Found: C, 71.47; H, 3.89;
N, 10.40%.
4.3.4. Synthesis of Ir(DPP)3 (3)
A mixture of IrCl3 ꢀ 3H2O (0.1 mmol), the ligand DPP (0.6 mmol)
and silver trifluoroacetate AgOOCCF3 (0.3 mmol) was stirred for 8 h
at 200 °C, under a nitrogen atmosphere. After cooling to room tem-
perature, the reaction mixture was diluted with dichloromethane
and filtered to remove inorganic salts and insoluble by-products.
After concentrating the solution, 50 mL of CH3OH was added and
a yellow precipitate was collected by suction and washed with
CH3OH to afford Ir(DPP)3 in 12% yield. Anal. Calc. for C48H33IrN6:
C, 65.07; H, 3.75; N, 9.48. Found: C, 64.68; H, 3.62; N, 9.49%. 1H
NMR (CDCl3, 300 MHz) d: 6.59 (m, 3H), 6.83 (m, 6H), 7.02 (d,
3H), 7.51 (m, 9H), 7.65 (m, 9H), 8.23 (d, 3H).
4.3.5. Ir(MDPP)3 (5)
These yellow powders were prepared similarly to the synthesis
of Ir(DPP)3 in 11% yield. Anal. Calc. for C51H39IrN6: C, 66.01; H, 4.24;
N, 9.04. Found: C, 65.78; H, 4.31; N, 8.99%. 1H NMR (CDCl3,
300 MHz) d: 2.51 (s, 9H), 6.55 (m, 3H), 6.77 (m, 6H), 6.91 (d, 3H),
7.45 (s, 3H), 7.50 (m, 9H), 7.61 (m, 6H).
4.3. Preparation of Ir complexes
All synthetic procedures involving IrCl3 ꢀ 3H2O and other Ir(III)
species were carried out in an inert gas atmosphere despite the
air stability of the compounds, the main concern being the oxida-
tive stability of intermediate complexes at the high temperatures
used in the reactions [16].
4.4. OLED fabrication and measurements
The device was fabricated with a standard vacuum–vapor depo-
sition process. The emitting area was 2 ꢂ 2 mm and all OLEDs were
formed on the same glass substrate. 4,4-Bis[N-(1-naphthyl)-N-

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deposited on the indium tin oxide (ITO)-coated glass. The NPB
layer was followed by a layer of NPB doped with Iridium com-
plexes. Layers of 2,9-dimethyl–4,7-diphenyl-1,10-phenanthroline
(BCP)—to serve as the hole-blocker; tri-(8-hydroxyquinoline) alu-
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as the cathode, were successively deposited. The EL spectra were
measured with a spectrofluorometer FP-6200 (JASCO), a source-
measure unit R6145(Advantest), multimeter 2000(Keithley) and
luminance was directly detected by using a multifunctional optical
meter 1835-C(Newport). All the measurements were automatically
controlled by a computer system, and only about 400 ms is needed
to get one L–J–V curve by using our measuring system. The quan-
tum efficiency was calculated by using the luminescence, emission
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Acknowledgements
The work was supported by the National Nature Science Foun-
dation of China (Nos. 20474003 and 90401028) and 973 Program,
PR China (2002CB613405).
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