Highly efficient and solution-processed iridium complex for single-layer yellow electrophosphorescent diodes

Article abstract of DOI:10.1039/c2jm34462a

A novel electrophosphorescent material based on a biscyclometalated iridium(iii) complex of the N,N-diphenyl-4′′-(1-phenyl-1H-benzo[d] imidazol-2-yl)-[1,1′:4′,1′′-terphenyl]-4-amine ligand, named Ir(TPABPBI)₂(acac), has been synthesized and its application in organic light emitting devices (OLEDs) was studied. Highly efficient, solution-processed, single-layer, electrophosphorescent diodes utilizing the complex have been prepared and characterized. In these devices, the high triplet energy poly(9-vinylcarbazole) (PVK) is used as a host polymer doped with 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) employed for electron transport. When the doping concentration of Ir(TPABPBI) ₂(acac) is up to 4%, devices with a current efficiency of 30.0 cd A^-1 corresponding to an EQE of 15% can thus be achieved, along with the Commission de l'Eclairage 1931 chromaticity coordinates of (0.507, 0.486). More importantly, the yellow phosphorescence can be achieved without the generation of excimer emission. Such devices exhibit a very broad and featureless peak at ca. 568 nm with a wide full spectral width at half maximum (FWHM) of 83 nm. These results render Ir(TPABPBI)₂(acac) a promising organic phosphor for applications in yellow OLEDs, especially for large-area, highly efficient and cheap yellow PhOLEDs.

Full text of DOI:10.1039/c2jm34462a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 23005
www.rsc.org/materials
PAPER
Highly efficient and solution-processed iridium complex for single-layer
yellow electrophosphorescent diodes
a
b
a
a
b
Xinhua Ouyang, Dongcheng Chen, Shuiming Zeng, Xingye Zhang, Shijian Su and Ziyi Ge
a
*
*
Received 9th July 2012, Accepted 7th September 2012
DOI: 10.1039/c2jm34462a
A novel electrophosphorescent material based on a biscyclometalated iridium(III) complex of the N,N-
diphenyl-400-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,10:40,100-terphenyl]-4-amine ligand, named
Ir(TPABPBI)2(acac), has been synthesized and its application in organic light emitting devices
(OLEDs) was studied. Highly efficient, solution-processed, single-layer, electrophosphorescent diodes
utilizing the complex have been prepared and characterized. In these devices, the high triplet energy
poly(9-vinylcarbazole) (PVK) is used as a host polymer doped with 2-(4-biphenylyl)-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole (PBD) employed for electron transport. When the doping concentration of
Ir(TPABPBI)2(acac) is up to 4%, devices with a current efficiency of 30.0 cd Aꢀ1 corresponding to an
EQE of 15% can thus be achieved, along with the Commission de l’Eclairage 1931 chromaticity
coordinates of (0.507, 0.486). More importantly, the yellow phosphorescence can be achieved without
the generation of excimer emission. Such devices exhibit a very broad and featureless peak at ca. 568 nm
with a wide full spectral width at half maximum (FWHM) of 83 nm. These results render
Ir(TPABPBI)2(acac) a promising organic phosphor for applications in yellow OLEDs, especially for
large-area, highly efficient and cheap yellow PhOLEDs.
research efforts towards the design and synthesis of high effi-
ciency yellow organic phosphors are so far limited.
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) based
on iridium (Ir) complexes have been attracting great attention
due to their capability to harvest both triplet and singlet excited
states to realize 100% theoretical internal quantum efficiency.1–3
Numerous significant advances have been reported for the
application of full-color OLEDs and solid-state lighting. For
example, power efficiencies of over 30 lm Wꢀ1 have been acces-
sible in red, green, and blue (RGB) PhOLEDs.4,5 However,
almost all of these high-performance devices are fabricated by
multilayer thermal vacuum deposition technology with compli-
cated device structures, and relatively high cost and time-
consumption as well as low process yields. In order to overcome
these drawbacks, the development of solution-processed fabri-
cation and single-layer PhOLEDs will offer an attractive alter-
native to obtain low-cost and large-area flat panel display
technology, if high efficiency can also be simultaneously
provided. However, it will be a huge challenge to researchers. On
the other hand, although yellow phosphors are of importance for
fabricating white light and full-color display applications,
Yellow light, especially pure yellow, is difficult to achieve
since the yellow-light chromaticity region is very narrow in the
Commission International de l’Eclairage (CIE) chromaticity
diagram. Moreover, in multilayer devices, the color purity will
be easily affected by unexpected emission from the other layers.
To date, there are relatively few studies on yellow PhO-
LEDs.6–10 The efficiencies and color purity of a large proportion
of them are still low. In 2003, Tsuzuki et al.11 fabricated a high
efficiency and low turn-on voltage yellow-emitting OLEDs
based on p-PF–pyiridium derivatives. However, they did not
disclose the CIE coordinates and the spectral bandwidth was
rather narrow, which would inevitably lower the spectral
coverage of white OLEDs in combination with blue-emitting
materials, resulting in low color rendering indexes. Subse-
quently, a series of yellow phosphorescent OLEDs12–14 with
wide bandwidths were reported. Unfortunately, these devices
exhibited unsatisfactorily low efficiencies. Recently, Ma et al.15
introduced a novel yellow-emission phosphorescent complex as
a dopant material, and the optimized device showed stable pure
yellow electrophosphorescence with the CIE coordinates of
(0.46, 0.53), and the spectra were largely insensitive to the
driving voltages. However, the device was fabricated with a
sophisticated structure, which was deposited by co-evaporated
phosphor and host materials and rendered the device of low
reproducibility and increased manufacturing cost. In view of
this regard, it is highly desirable to design and synthesize highly
aNingbo Institute of Material Technology and Engineering (NIMTE),
Chinese Academy of Sciences (CAS), Ningbo, 315201, P. R. China.
E-mail: geziyi@nimte.ac.cn
bState Key Laboratory of Luminescent Materials and Devices (South
China University of Technology), Institute of Polymer Optoelectronic
Materials and Devices, South China University of Technology,
Guangzhou, 510641, P. R. China. E-mail: mssjsu@scut.edu.cn
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 23005–23011 | 23005

View Article Online
efficient yellow phosphors with excellent solubility in common
organic solvents. Very recently, some multilayer and solution-
processed devices have been realized based on PVK and
Ir(mppy)3.16 In these devices, the means to avoid the intermis-
cibility of the interfaces between the upper and lower layer
materials is considered to be a big problem. An alternative
approach is to fabricate single-layer and solution-processed
devices. Suzuki et al.17 reported single-layer, high-efficiency,
green devices with a solution-processed technique, showing a
maximum EQE of 11.8% and power efficiency of 38.6 lm Wꢀ1
.
However, to the best of our knowledge, solution-processed
yellow PhOLEDs with a single-layer structure have not been
reported to date.
In this work, triphenylamine and benzimidazole groups were
chosen to prepare target molecules owing to their excellent
charge transfer and emission properties; they have been used
extensively to construct OLED materials.18–21 In addition, the
benzimidazole configuration was adopted to increase the solu-
bility of the iridium complex Ir(TPABPBI)2(acac). The synthetic
route and the structure of the target molecule were shown in
Scheme 1. The complex was used to fabricate highly efficient,
solution-processed, single-layer devices employing the high
triplet energy material poly(9-vinylcarbazole) (PVK) as a host
polymer with PBD as the electron-transport molecule, and
showed a maximum current efficiency 30 cd Aꢀ1 and an EQE of
15% with CIE coordinates of (0.507, 0.486).
Fig. 1 The electron densities of HOMO, HOMOꢀ1, LUMO and
LUMO+1 for Ir(TPABPBI)2(acac).
were determined in CDCl3 with a Bruker DRX 400 MHz spec-
trometer. Chemical shifts (d) were given relative to tetrame-
thylsilane (TMS). The coupling constants (J) were reported in
Hz. Elemental analyses were recorded with a Perkin-Elmer 2400
analyzer. MALDI-TOF-MS spectra were acquired using a
Voyager-DE STR MALDI-TOF mass spectrometer (PerSeptive
Biosystems) in the linear positive mode with delayed extraction.
Samples were analyzed using a dichloromethane solution of the
sample (1 mL) mixed with 10 mL of dithranol matrix (10 mg
mLꢀ1 in 70 : 30 CH2Cl2–EtOH) before loading onto a metal
sample plate. The course of the experiment was monitored by
TLC. Column chromatography was carried out on silica gel
(100–200 mesh).
Experimental section
Characterization
Melting points were determined using an Electrothermal IA 900
apparatus and the thermometer was uncorrected. FT-IR spectra
were recorded on a Perkin-Elmer Fourier transform infrared
spectrometer and measured as KBr pellets. 1H NMR spectra
Materials
N1-phenylbenzene-1,2-diamine,diphenylamine,
4,40-dibromo-
1,10-biphenyl, 4-bromobenzoyl chloride, 4,4,40,40,5,5,50,50-octa-
methyl-2,20-bi(1,3,2-dioxaborolane), pentane-2,4-dione, diphe-
nylphosphineferrocene palladium dichloride (PdCl2(dppf)) and
Pd(PPh3)4 were purchased from TCI. KOAc, K2CO3, Et4NOH,
Fig. 2 Optical absorption and photoluminescence spectra of Ir(T-
Scheme 1 The synthetic route to the target molecule.
23006 | J. Mater. Chem., 2012, 22, 23005–23011
PABPBI)2(acac) in dilute dichloromethane solution and film.
This journal is ª The Royal Society of Chemistry 2012

View Article Online
Fig. 3 The CV measurements of Ir(TPABPBI)2(acac) and ferrocene.
Fig. 5 The EL spectra of Ir(TPABPBI)2(acac) with different doping
concentrations and CIE diagram.
Scheme 2 Schematic energy diagram of the charge-carrier dynamics and
LUMO/HOMO. The vertical arrows represent the direct trapping of the
electrons and holes.
Fig. 6 The current efficiency and external quantum efficiency of Ir(T-
PABPBI)2(acac) with different doping concentrations.
mmol), 4-bromobenzoyalchloride (12.1 g, 55 mmol), dried THF
(160 mL), and triethylamine (8 mL) was stirred for 32 h at room
temperature. The solution was poured into 200 mL water and
extracted with dichloromethane. After the solvent was removed, the
crude product was recrystallized from THF and methanol. A gray
solid was obtained with a yield of 78.5% (15.85 g). EI-MS (m/z):
366.1, 367.9(M+), whichwasin good agreement withthe literature.22
Fig. 4 The current density–voltage–luminance (I–V–L) characteristics
of Ir(TPABPBI)2(acac).
Synthesis of 2-(4-bromophenyl)-1-phenyl-1H-benzo[d]imidazole
(BPBI-Br)
IrCl3$H2O and 2-ethoxyethanol were obtained from J&K
Chemical company. The other solvents were from Sinopharm
Chemical Reagent Co. Ltd. The solvents were dried using stan-
dard procedures. All other reagents were used as received from
commercial sources, unless otherwise stated.
The synthetic procedure followed the method reported in litera-
ture.22 BPPA (7.34 g, 20 mmol) was dissolved in 80 mL acetic acid
and heated at reflux for about16 h. Following this, the solvent was
removed to afford a gray solid. Yield: 6.63 g, 95%. EI-MS (m/z):
348.2, 350.1 (M+), which was in agreement with the literature.22
Synthesis of 4-bromo-N-(2-(phenylamino)phenyl)benzamide
(BPPA)
Synthesis of 1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-1H-benzo[d]imidazole (BPBE)
The synthetic procedure followed the method reported in litera-
ture.22 The mixture of N1-phenylbenzene-1,2-diamine (10.16 g, 55
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) (3.15
g, 9 mmol), PdCl2(dppf) (0.22 g, 0.27 mmol) and potassium
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 23005–23011 | 23007

View Article Online
Table 1 Characteristics of yellow PhOLED with various dopant concentrations of Ir(TPABPBI)2(acac) hC, hP, and hE are the maximum current, power
and external quantum efficiencies, respectively. Von is the voltage required to obtain a luminance of 1 cd mꢀ2
hC@1000 hP@1000 hC@100 hP@100
mA cmꢀ2 mA cmꢀ2
hC
hP
hE
Von
(V)
V@1000
V@20 mA
V@100
CIE
cd mꢀ2
(cd Aꢀ1
cd mꢀ2
Conc.% (cd Aꢀ1) (lm Wꢀ1) (%)
cd mꢀ2 (V) cmꢀ2 (V)
mA cmꢀ2 (V) (x, y)
)
(lm Wꢀ1
)
(cd Aꢀ1
)
(lm Wꢀ1
)
1.0
2.0
4.0
6.0
8.0
10.0
22.0
22.5
30.0
20.4
23.5
14.4
4.96
5.09
6.80
4.61
5.22
3.25
11.7 4.00
12.0 3.75
15.0 4.25
10.9 4.00
12.5 4.00
7.7 5.00
6.9
6.7
8.2
7.3
8.7
8.4
8.8
8.5
11.0
10.8
13.6
12.6
13.7
12.1
0.507,
0.486
0.507,
0.486
0.507,
0.486
0.507,
0.486
0.507,
0.486
0.507,
0.486
21.9
20.9
29.4
20.1
21.7
13.6
4.92
4.66
6.12
4.54
4.61
3.06
5.83
1.31
5.74
1.31
10.8
9.5
9.34
2.84
6.56
1.48
10.9
9.9
6.69
1.52
5.67
1.29
acetate (2.52 g, 27 mmol) were placed into a two-neck round-
bottom flask equipped with a stirrer and reflux condenser.
DMSO (60 mL) was added and the mixture was stirred under
nitrogen for 10 min. After that BPBI-Br (3.14 g, 9.0 mmol) was
added and the mixture was kept at 80 ꢁC for 24 h. After cooling
to room temperature, the mixture was extracted with ethyl
acetate followed by purification by column chromatography on
silica gel with ethyl acetate–hexane (v/v, 1 : 3) as the eluent to
Synthesis of N,N-diphenyl-400-(1-phenyl-1H-benzo[d]imidazol-2-
yl)-[1,10: 40,100-terphenyl]-4-amine (TPABPBI)
BPBE (1.58 g, 4.0 mmol), TPAB-Br (1.80 g, 4.5 mmol) and
Pd(PPh3)4 (20 mg, 0.04 mmol) were suspended in toluene (16 mL)
and Et4NOH (8 mL of a 20% aqueous solution) and the reaction
was stirred at 90 ꢁC for 18 h. The orange solution was extracted
with CH2Cl2 (40 mL), washed with water (2 ꢂ 50 mL), dried over
MgSO4 and evaporated to dryness. After drying under vacuum,
it was purified by silica gel column chromatography using ethyl
acetate–petroleum ether (60–90 ꢁC) (1 : 10) as an eluent to afford
1
offer a white solid. Yield: 2.58 g, 72.5%. H NMR (CDCl3, 400
MHz, ppm) d: 7.90 (d, 1H, J ¼ 8.4 Hz), 7.73 (d, 1H, J ¼ 8.0 Hz),
7.56 (d, 1H, J ¼ 8.0 Hz), 7.51–7.44 (m, 3H), 7.36–7.24 (m, 5H),
1.33 (s, 12H). 13C NMR (CDCl3, 100 MHz, ppm) d: 152.2, 143.1,
137.3, 137.0, 134.6, 132.4, 129.9, 128.6, 128.5, 127.4, 123.5, 123.0,
119.9, 110.5, 84.0, 24.9. EI-MS (m/z): 396 (M+).
1
a white solid, yield: 1.53 g, 65%. H NMR (CDCl3, 400 MHz,
ppm) d: 7.92 (d, 1H, J ¼ 8.4 Hz), 7.69–7.65 (m, 6H), 7.61–7.57
(m, 3H), 7.55–7.50 (m, 5H), 7.39–7.36 (m, 3H), 7.31–7.28 (m,
5H), 7.15 (d, 6H, J ¼ 8.0 Hz), 7.04 (t, 2H, J ¼ 7.6 Hz). 13C NMR
(CDCl3, 100 MHz, ppm) d: 152.1, 147.6, 147.4, 143.1, 141.5,
140.1, 138.4, 137.4, 137.1, 134.2, 129.9, 129.8, 129.3, 128.8, 128.6,
127.6, 127.5, 127.4, 127.0, 126.7, 124.5, 123.8, 123.4, 123.0, 119.8,
110.4. EI-MS (m/z): 589.3 (M+).
Synthesis of 40-bromo-N,N-diphenyl-[1,10-biphenyl]-4-amine
(TPAB-Br)
The synthetic procedure followed the method reported in liter-
ature.23 A mixture of 4,40-dibromobiphenyl (8.46 g, 27.4 mmol),
Pd(OAc)2 (105 mg, 0.18 mmol), dppf (253 mg, 0.46 mmol),
diphenylamine (1.54 g, 9.13 mmol), then sodium tert-butoxide
(1.32 g, 13.7 mmol) in dry toluene was placed in a three-necked
Synthesis of bis[N,N-diphenyl-400-(1-phenyl-1H-benzo[d]
imidazol-2-yl)-[1,10:40,100-terphenyl]-4-amine](acetyl-acetonate)
(Ir(TPABPBI)2(acac))
ꢁ
flask under a nitrogen atmosphere and was stirred at 90 C for
15 h. After cooling, the reaction was quenched by adding water
and then was extracted with ethyl acetate. The organic layer was
dried over anhydrous MgSO4 and evaporated under vacuum.
The products were purified by silica gel column chromatography
eluting with hexane. A white solid of TPAB-Br was obtained in
65% yield (2.37 g). EI-MS (m/z): 399.1, 401.3 (M+), which was in
agreement with the literature.23
IrCl3$H2O (1 mmol) and the ligands (2.5 mmol) were added in a
16 mL of mixture of 2-ethoxyethanol and water (v/v ¼ 3 : 1). The
mixture was refluxed under nitrogen for 24 h and cooled to room
temperature. The orange precipitate was collected by filtration
and washed with water, ethanol and hexane. The solid was then
pumped dry completely to give the crude chloro-bridged dimer
complex. Without further purification, the dimer was added to a
Table 2 Performance summary for recently reported yellow PhOLEDs
Brightness
(cd mꢀ2
Current efficiency
(cd Aꢀ1
Power efficiency
(lm Wꢀ1
Emitter
Von (V)
EL (nm)
)
)
)
CIE (x, y)
Ref.
Ir[2-(Cz-30-yl)-Py]3
Ir(Flpy)3
(fbi)2Ir(acac)
4(C^N)3Ir
Ir(TPABPBI)2(acac)
9.8
5.4
4.0
3.8
4.2
560
548
560
568
568
9800
7047
31 072
23 790
16 500
22
29.5
30
11.2
30
6.5
13.2
7.8
2.7
6.8
(0.51, 0.48)
(0.45, 0.54)
(0.51, 0.48)
(0.52, 0.47)
(0.507, 0.486)
14
27
6
26
This work
23008 | J. Mater. Chem., 2012, 22, 23005–23011
This journal is ª The Royal Society of Chemistry 2012

View Article Online
mixture of Na2CO3 (5 mmol), acetyl acetone (1.5 mmol), and 2-
ethoxyethanol (10 mL). After refluxing under nitrogen for 24 h,
the solution was cooled to room temperature. The orange
precipitate was filtered off and washed with water, ethanol and
hexane. The crude product was purified by chromatography on
silica gel using petroleum ether–dichloromethane as the mobile
Results and discussion
Synthesis
Scheme 1 displays the molecular structure and synthetic route to
the desired complex. It can be seen that the molecule contains a
triphenylamine and benzimidazole unit. Our design of the
benzimidazole-type iridium complex Ir(TPABPBI)2(acac) was
inspired by the structure of benzimidazole derivatives, materials
widely used for fluorescent OLEDs that exhibit good solubility
and high electron mobility. Moreover, the benzimidazole-type
iridium complexes are limited. To expand the family of iridium
complexes with excellent solubility, we adopted benzimidazole as
the electron accepting group, because we have previously used
this successfully as an ET material.18,19 In addition, a phenyl ring
structure is introduced between the benzimidazole and the tri-
phenylamine groups to extend the p-conjugation length and
adjust the HOMO and LUMO energy levels. The key interme-
diate for the synthesis of Ir(TPABPBI)2(acac) was TPABPBI,
which was synthesized by Suzuki couplings of BPBE and TPAB-
Br using Pd(PPh3)4 as catalyst. BPBE was prepared by mixing
BPBI-Br and bis(pinacolato)diboron in the presence of
PdCl2(dppf), K2CO3 and DMSO. The TPAB-Br and BPBI-Br
were synthesized according to the literature procedures.22,23
Further reaction with IrCl3$H2O in refluxing 2-ethoxyethanol–
H2O (3 : 1, v/v) affords the chloride-bridged dimer complex in
high yield. Then, the dimer complex reacted easily with the
acetylacetone ligand to obtain the complex Ir(TPABPBI)2(acac).
1H and 13C NMR, IR, mass spectrometry, and elemental analysis
were employed to confirm the chemical structures of the above-
mentioned compounds, as described in the Experimental section.
1
phase and afforded a yellow solid, yield: 0.37 g, 25%. H NMR
(CDCl3, 400 MHz, ppm) d: 7.89 (d, 2H, J ¼ 8.4 Hz), 7.78 (d, 2H,
J ¼ 8.4 Hz), 7.73 (d, 2H, J ¼ 8.4 Hz), 7.68–7.54 (m, 16H), 7.41–
7.33 (m, 22H), 7.18–7.14 (m, 6H), 7.01–6.98 (m, 10H), 5.17 (s,
1H), 1.76 (s, 6H). 13C NMR (CDCl3, 100 MHz, ppm) d: 188.2,
184.3, 150.1, 146.3, 146.1, 142.8, 140.7, 139.6, 137.7, 136.9, 136.3,
133.6, 130.1, 129.2, 128.8, 128.4, 128.3, 127.1, 126.9, 126.3, 125.9,
125.4, 123.8, 122.9, 122.5, 122.1, 119.1, 112.4, 101.4, 54.3, 28.2.
FAB-MS (m/z): 1467.9 (M++1). Anal. calcd for C91H67IrN6O2:
C, 74.41, H, 4.60, N, 5.72; found: C, 74.27, H, 4.46, N, 5.56%.
Fabrication of OLEDs
Indium-tin oxide (ITO) coated glass with a sheet resistance of 15–20
U per square was used as the substrate. The ITO coated glass
substrates were cleaned in an ultrasonic bath with acetone, deter-
gent, deionized water, and isopropanol in sequence, and then dried
in an oven. After oxygen plasma cleaning for 4 min, a 40 nm thick
poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate) (PEDOT:
PSS) (Bayer Baytron 4083) layer was first spin-coated onto the ITO
substrate from water solution with a speed of 3000 rpm for 3 min
then baked in a vacuum oven at 160 ꢁC for 30 min. The electrolu-
minescent (EL) properties of the new gust Ir(TPABPBI)2(acac) was
spin-coated onto the PEDOT:PSS with a speed of 2500 rpm for 30 s.
The single-layer devices were fabricated using PVK and PBD with
different molar ratios of Ir(TPABPBI)2(acac) as the emissive layer.
Theoretical calculations
ꢁ
Then, the film was annealed at 80 C for 20 min under an atmo-
sphereofN2. ThethicknessofELlayerwasaboutꢃ80nm. Finallya
1.5 nm thick cesium fluoride (CsF) film and a 120 nm thick
aluminum (Al) film were evaporated with a shadow mask to form
the top electrode, at a base pressure of 3 ꢂ 10ꢀ4 Pa. The device
fabrication, exceptforthePEDOT:PSScoating, wascarriedoutina
nitrogen atmosphere dry-box (Vacuum Atmosphere Co.) contain-
ing less than 10 ppm oxygen and moisture. The current–voltage (I–
V) characteristics were measured by a Keithley 236 source
measurement unit. The EL spectra, luminance and CIE coordinate
characteristics were measured using a PR-655 Spectrascan spec-
trometer. The corresponding thin-film photoluminescence (PL) and
absorption spectra measurements were conducted using a model
UV-3100 spectrophotometer and a model RF-5301PC spectro-
photometer on quartz substrates, respectively.
The three-dimensional geometries and the frontier molecular
orbital energy levels of Ir(TPABPBI)2(acac) were calculated using
density functional theory. The electronic densities of HOMO,
HOMOꢀ1, LUMO and LUMO+1 levels are depicted in Fig. 1. As
shown in Fig. 1, the spatial distributions of electron densities
shows HOMOꢀ1 for the Ir(TPABPBI)2(acac) is mostly localized
on the triphenylamine moieties, and the LUMO is distributed on
benzimidazole moieties. This implies that the absorption and
emission process may mainly be attributed to the p–p* transition.
It is noted that the electronic densities of HOMO, HOMOꢀ1,
LUMO and LUMO+1 are not distributed on the acac moiety,
showing the absorption and emission are not affected by it. In
addition, the calculated geometries indicate the non-planarity of
the ground state molecular structure. These characteristics can
effectively prevent intermolecular interactions between p-systems
and, thus, suppress molecular recrystallization in the film. The
triplet energy gap DE(T1 ꢀ S0) of Ir(TPABPBI)2(acac) was pre-
dicted as 2.12 eV.
Computational details
The geometric and electronic structure of Ir(TPABPBI)2(acac)
were optimized by density functional theory (DFT) level of
theory with the three-parameter Becke-style hybrid functional
(B3LYP). The HOMO and LUMO energy levels and the triplet
energy gap DE(T1 ꢀ S0) are predicted by the 6-31G(d,p) basis set.
All calculations on the molecule have been performed on the
large computer Origin 2000 server centre using the Gaussian03
program package.24 The compositions of molecular orbitals were
analyzed using the GaussView 3.0 program.
Photophysical properties
The absorption spectra of Ir(TPABPBI)2(acac) in solution and
thin film are given in Fig. 2. Obviously, the absorption of Ir(T-
PABPBI)2(acac) is almost the same in solution and in the film.
There appears only one strong peak at 352 nm and two shoulders
at 302 nm and 460 nm. The strongest absorption bands in the
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 23005–23011 | 23009

View Article Online
violet and ultraviolet regions are assigned to the spin-allowed
1
charge carrier dynamics (injection, transport, trapping, and
emission) is illustrated in Scheme 2. Electrons and holes are
injected mainly into the electron-transport material (PBD) and
hole transport material (PVK), respectively, given that they are
the dominant species. The electrode interfaces and their LUMO
and HOMO energy levels best match those of the CsF/Al cathode
and PEDOT:PSS work functions, respectively. Direct injection
from the electrodes into the dopant is a minor contributor because
of the low dopant concentration at the electrode interfaces. Also,
the isolated dopants are effective charge traps that suppress the
continuity of carrier transport in the bulk material. Electron and
hole trapping in these dopants, therefore, occurs mainly by direct
trapping from PBD and PVK, respectively.
intraligand p–p* transitions. At lower energies a shoulder of
460 nm can be seen, related to 3p–p* transitions, and weak broad
bands extending into the visible region, which can be associated
with an admixture of the typical spin-allowed 1MLCT, spin–
orbit coupling enhanced 3p–p*, and spin-forbidden 3MLCT
transitions. These features are analogous to other (C^N)2Ir(acac)
complexes as reported previously.25 The strong spin–orbit
coupling is seen in the photoluminescence (PL) spectra, wherein
the oscillator strengths for singlet and triplet MLCT are similar
(i.e. difference in their extinction coefficients of less than a factor
of 2), and thus lead to efficient phosphorescence. In Fig. 2,
Ir(TPABPBI)2(acac) emits a yellow light with broad and
featureless PL peaking at 569 nm with a shoulder at 610 nm in
film, while the PL in solution shows a peak at 566 nm and a
shoulder at 607 nm, suggesting that the lowest excited triplet
state is dominated by the 3MLCT excited state. A relatively small
Stokes shift of 70 nm between 3MLCT absorption and PL peaks
also suggests the emission is obtained from a predominantly
The current density–voltage–luminance (J–V–L) is shown in
Fig. 4 and the current density–efficiencies are in Fig. 6. Simple
devices utilizing different doping concentrations of Ir(TPABP-
BI)2(acac) exhibit a maximum brightness of 9450 (1%), 10 020
(2%), 16 500 (4%), 10 750 (6%), 11 120 (8%), and 11 160 (10%)
cd mꢀ2, luminous efficiency of 22.0 (1%), 22.5 (2%), 30.0 (4%),
20.4 (6%), 23.5 (8%), 14.4 (10%) cd Aꢀ1, respectively. As
expected, all of these devices show low threshold voltages of 4.0,
3.75, 4.25, 4.0, 4.0, 4.0, 5.0 V, respectively, which can be
attributed to the fact that the reasonable work function value of
the CsF reduces the electron injection barrier at the interface
between the cathode aluminium and the emissive layer. The EL
spectra of Ir(TPABPBI)2(acac) shows yellow light emission with
CIE coordinates of (0.507, 0.486), and the peaks are located at
568 nm with a shoulder at 608 nm (Fig. 5). As shown in the CIE
diagram, all of the coordinates with different doping concen-
trations lie within the yellow region. Changes in CIE coordi-
nates are not observed in these devices, revealing the complex
emits stable yellow light. Fig. 5 shows the normalized EL
spectra for yellow OLEDs viewed at the normal direction with
different Ir(TPABPBI)2(acac) concentrations. By comparing the
EL with the PL spectra, the profiles are the same apart from the
intensity of the shoulder peak. For dopant concentrations
ranging from 1% to 10%, an intense yellow light emission at
568 nm is observed, which is independent of the doping
concentration of Ir(TPABPBI)2(acac). On the contrary, as the
concentration increases from 1% to 4%, the intensity of emis-
sion peak at 608 nm undergoes significant spectral improve-
ment. While concentration improved beyond 4%, the intensity
of the shoulder is reduced gradually. It can be confirmed that
complete energy transfer took place between the PVK host and
the guest dopant and/or the effective charge trapping upon
sufficiently high concentrations of Ir(TPABPBI)2(acac). We can
highlight the fact that our present devices using Ir(TPABP-
BI)2(acac) phosphor exhibit EL with a wide FWHM of 83 nm,
and, more importantly, this high value in FWHM does only rely
on the emission of the phosphor. Our results demonstrated no
excimer emission to broaden the EL spectrum of yellow-emit-
ting OLEDs , and the EL spectra and the corresponding CIE
are little influenced by the dopant concentration. Furthermore,
the CIE coordinates of these yellow-emitting OLEDs fall within
the extremely narrow yellow chromaticity region, and this is of
benefit to obtain a good EL purity.
3MLCT state. The solution luminescence quantum yield (FPL
)
was measured to be 0.33 by using Rhodamine 6G as a standard.
Electrochemical properties
The electrochemical properties are considered to be a very
important parameter for evaluating materials in optoelectronic
applications. In this work, the electrochemical behaviors of the
compounds Ir(TPABPBI)2(acac) and ferrocene, in solutions of
chloroform and acetonitrile (v/v, 4 : 1) at room temperature with
0.1 M tetrabutylammonium hexafluorophosphate (TBPAPF6) as
the electrolyte, were examined by cyclic voltammetry (CV) using a
standard three-electrode system as shown in Fig. 3. The onset
oxidation peak (Eox) for the Ir(TPABPBI)2(acac) is 1.27 V, and
the oxidation andreduction peaks of ferrocene are 0.497 and 0.393
V, respectively. The HOMO of the complex was determined to be
ꢀ5.625 eV. The LUMO level was deduced by combining the
optical bandgap and the HOMO level, the value was ꢀ3.262 eV. It
should be pointed out that the LUMO level of the (C^N)2Ir(acac)
complex is contributed by the p*-orbitals of the ligand (benzoi-
midazolphenylamine derivative) and independent of the ancillary
ligand (acetylacetonate), while the HOMO corresponds to the 5d-
orbital of Ir with substantial mixing with the p-orbitals of the
benzoimidazolphenylamine ligand. In addition, the value of the
LUMO level is close to the work function of CsF/Al, the lower
energy barrier between the emitting layer and the cathode will
facilitate electron injection into the emission layer.
Electroluminescence
To investigate the EL performances of Ir(TPABPBI)2(acac) as
yellow phosphors, the single-layer devices were fabricated with a
typical configuration as follows: ITO/PEDOT:PSS (40 nm)/
PVK:Ir(TPABPBI)2(acac):PBD (80 nm)/CsF (1.5 nm)/Al
(120 nm) using solution-processed technique, where indium tin
oxide (ITO) is used as the anode, PEDOT:PSS is used as the hole
injection layer, PVK as the host material and 2-([1,10-biphenyl]-4-
yl)-5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazole (PBD) as the elec-
tron-transporting material in emitter layer, and CsF is used as the
electron injecting layer. The schematic energy diagram of the
From Table 1 and Fig. 6, the efficiencies in terms of hC, hP, and
hE are maximized at a doping concentration of 4% Ir(TPABP-
BI)2(acac), in which they are 30.0 cd Aꢀ1, 6.8 lm Wꢀ1, and 15%,
23010 | J. Mater. Chem., 2012, 22, 23005–23011
This journal is ª The Royal Society of Chemistry 2012

View Article Online
respectively. The increase in efficiencies with the change in doping
concentration from 1 to 4% is due to the improvement in lumi-
nescence sites for exciton recombination. However, at higher
concentrations (>4%), device efficiencies reduce due to triplet–
triplet annihilation as well as concentration-induced quenching.
China (21074144), Qianjiang Talent Project, and a visiting
professorship for Senior International Scientists from the
Chinese Academy of Sciences.
Notes and references
Moreover, theVon andvoltagesrequiredtoachieve1000 cdm mꢀ2
,
1 C. H. Chen, F. I. Wu, Y. Y. Tsai and C. H. Cheng, Adv. Funct.
Mater., 2011, 21, 3150.
20 and 100 mA cmꢀ2 are 4.25, 8.1, 10.8, and 13.6 V, respectively,
and the correspondingcurrentefficiencies are29.3, 20.2, 9.2cdAꢀ1
and power efficiencies are 5.9, 5.3, 2.8 lm Wꢀ1, respectively. These
comparatively low Von and operating voltages show that effective
charge transport exists throughout the device. It is worth noting
that, for devices doped with lower amount of Ir(TPABPBI)2(acac)
(1–4%), the maximal brightness and current efficiency increase
gradually. Itcanbeseenthephenomenamaybeassociatedwiththe
charge trapping nature which commonly exists in yellow phos-
phorescent dopants, and is particularly obvious at relatively low
concentrations of 1% and 2%. Subsequently, on further increasing
the concentration of Ir(TPABPBI)2(acac) up to 10%, the fraction
of holes and electrons which are directly injected onto Ir(T-
PABPBI)2(acac) is increased greatly, the injection of which will not
correspond with the HOMO and the LUMO levels of the host
material. This yields a decrease in the brightness and current effi-
ciency of the devices. Additionally, the relatively low power effi-
ciency(lmWꢀ1)ofthesedevicesisclearlyrelatedtotwoparameters
that should be optimized in order to raise the power efficiency.
These optimizations include enhancing the carrier injection to
reduce the turn-on voltage by selecting appropriate electrodes, and
the optimization of the device microcavity.14 In the meanwhile, the
operating voltage and low contact resistance should be considered
to increase the power efficiency. These parameters are not intrinsic
to the emissive material but rather to the device engineering, which
is now under investigation. The Ir(TPABPBI)2(acac) also shows
good performance compared with yellow-emitting PhOLEDs
reported in the literature, as summarized in Table 2.6,14,26,27
2 Z. Ma, L. Chen, J. Ding, L. Wang, X. Jing and F. Wang, Adv. Mater.,
2011, 23, 3726.
3 D. H. Kim, N. S. Cho, H.-Y. Oh, J. H. Yang, W. S. Jeon, J. S. Park,
M. C. Suh and J. H. Kwon, Adv. Mater., 2011, 23, 2721.
€
4 N. Seidler, S. Reineke, K. Walzer, B. Lussem, A. Tomkeviciene,
J. V. Grazulevicius and K. Leo, Appl. Phys. Lett., 2010, 96, 093304.
5 X. Qi, M. Slootsky and S. Forrest, Appl. Phys. Lett., 2008, 93, 193306.
6 W. S. Huang, J. T. Lin, C. H. Chien, Y. T. Tao, S. S. Sun and
Y. S. Wen, Chem. Mater., 2004, 16, 2480.
7 H. C. Ting, Y. M. Chen, H. W. You, W. Y. Hung, S. H. Lin,
A. Chaskar, S. H. Chou, Y. Chi, R. H. Liu and K. T. Wong, J.
Mater. Chem., 2012, 22, 8399.
8 G. J. Zhou, Q. Wang, C. L. Ho, W. Y. Wong, D. G. Ma, L. X. Wang
and Z. Y. Lin, Chem.–Asian J., 2008, 3, 1830.
9 Y. T. Tao, Q. A. Wang, C. L. Yang, C. Zhong, J. G. Qin and
D. G. Ma, Adv. Funct. Mater., 2010, 20, 2923.
10 B. P. Yan, C. C. C. Cheung, S. C. F. Kui, V. A. L. Roy and C. M. Che,
Appl. Phys. Lett., 2007, 91, 063508.
11 T. Tsuzuki, N. Shirasawa, T. Suzuki and S. Tokito, Adv. Mater.,
2003, 15, 1455.
12 X. M. Yu, G. J. Zhou, C. S. Lam, W. Y. Wong, X. L. Zhu, J. X. Sun,
M. Wong and H. S. Kwok, J. Organomet. Chem., 2008, 693, 1518.
13 J. H. Yao, C. Zhen, K. P. Loh and Z. K. Chen, Tetrahedron, 2008, 64,
10814.
14 S. L. Lai, S. L. Tao, M. Y. Chan, M. F. Lo, T. W. Ng, S. T. Lee,
W. M. Zhao and C. S. Lee, J. Mater. Chem., 2011, 21, 4983.
15 T. Fei, C. L. Wang, S. F. Xue, X. Gu, F. Li and T. G. Ma, Semicond.
Sci. Technol., 2009, 24, 105019.
16 H. A. Al-Attar, G. C. Griffiths, T. N. Moore, M. Tavasli, M. A. Fox,
M. R. Bryce and A. P. Monkman, Adv. Funct. Mater., 2011, 21, 2376.
17 M. Suzuki, S. Tokito, F. Sato, T. Igarashi, K. Kondo, T. Koyama and
T. Yamaguchi, Appl. Phys. Lett., 2005, 86, 103507.
18 Z. Y. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M. A. Kakimoto, Chem. Mater., 2008, 20, 2532.
19 Z. M. Wang, P. Lu, S. M. Chen, Z. Gao, F. Z. Shen, W. S. Zhang,
Y. X. Xu, H. S. Kowok and Y. G. Ma, J. Mater. Chem., 2011, 21,
5451.
Conclusions
20 S. L. Gong, Y. H. Chen, J. J. Luo, C. L. Yang, C. Zhong, J. G. Qin
and D. G. Ma, Adv. Funct. Mater., 2011, 21, 1168.
We have designed, synthesized and characterized a new Ir(III)
complex, named Ir(TPABPBI)2(acac), based on the N,N-
diphenyl-400-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,10:40,100-ter-
phenyl]-4-amine ligand, which is an extremely promising phos-
phorescent emitter for solution-processed PhOLEDs. The
electroluminescent properties of Ir(TPABPBI)2(acac) as an
emissive dopant in PhOLEDs were examined. The demonstration
of bright, efficient OLEDs in a spin-coated single-layer devices
promises the possibility of highly efficient, solution-processable,
single-layer, large-area, and cheap devices. Such devices exhibit
EL with a relatively broad (FWHM 83 nm) yellow emission. At
the doping level of 4%, the devices show high peak forward
viewing, hC, hP, and hE of 30.0 cd Aꢀ1, 6.8 lm Wꢀ1, and 15%,
respectively, along with CIE coordinates of (0.507, 0.486). In
essence, the present results reveal that Ir(TPABPBI)2(acac) is a
good candidate to serve as a phosphorescent material for appli-
cations in large-area, highly efficient and cheap yellow PhOLEDs.
21 W. J. Xu, S. J. Liu, H. B. Sun, X. Y. Zhao, Q. Zhao, S. Sun, S. Cheng,
T. C. Ma, L. X. Zhou and W. Huang, J. Mater. Chem., 2011, 21,
13999.
22 Z. Y. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M. A. Kakimoto, Adv. Funct. Mater., 2008, 18, 584.
23 Y. J. Chang and T. J. Chow, Tetrahedron, 2009, 65, 4726.
24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi,
V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli,
C. Adamo, S. Clifford, J. Ochterski, G. A. Peterson, P. Y. Ayala,
Q. Cui, K. Morokuma, D. K. Malik, A. D. Rabuk,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,
R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Chalacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong,
J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople,
GAUSSIAN 2003, Gaussian, Inc., Pittsburgh, PA, 2003.
25 S. Amansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am.
Chem. Soc., 2001, 123, 4304.
Acknowledgements
26 C. L. Ho, W. Y. Wong, G. J. Zhou, B. Yao, Z. Xie and L. Wang, Adv.
Funct. Mater., 2007, 17, 2925.
27 X. M. Yu, G. J. Zhou, C. S. Lam, W. Y. Wong, X. L. Zhu, J. X. Sun,
M. Wong and H. S. Kwok, J. Organomet. Chem., 2008, 693, 1518.
This work was financially supported by the NIMTE Foundation
(Y10821QF04), the National Natural Science Foundation of
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 23005–23011 | 23011