Highly efficient blue organic electrophosphorescence devices using a trifluorine-replaced iridium complex

Article abstract of DOI:10.1016/j.orgel.2011.08.024

Highly efficient blue phosphorescent organic light-emitting devices have been achieved by using a new emitter Iridium complex, bis[(3,4,5- trifluorophenyl)-pyridinato-N,C^2′] picolinate (F ₃Irpic). For a mCP:4%F₃Irpic devic

Full text of DOI:10.1016/j.orgel.2011.08.024

Organic Electronics 12 (2011) 2061–2064
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Highly efficient blue organic electrophosphorescence devices using
a trifluorine-replaced iridium complex
Silu Tao a,b,c, , Shiu Lun Lai d, Chuan Wu a,c, Tsz Wai Ng c, Mei Yee Chan d, Weiming Zhao e,
⇑
Xiaohong Zhang f,
⇑
a School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu, PR China
b Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, PR China
c Center of Super-Diamond and Advanced Films (COSDAF) & Department of Physics and Materials Science, City University of Hong Kong, Hong Kong
Special Administrative Region
d Department of Chemistry, The University of Hong Kong, Hong Kong Special Administrative Region
e Dongguan LITEWELL (OLED) Technology Incorporation, Dongguan 523656, China
f Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics
and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2011
Received in revised form 27 July 2011
Accepted 27 August 2011
Available online 12 September 2011
Highly efficient blue phosphorescent organic light-emitting devices have been achieved by
0
using a new emitter Iridium complex, bis[(3,4,5-trifluorophenyl)-pyridinato-N,C2 ] picoli-
nate (F3Irpic). For a mCP:4%F3Irpic device, blue emission with a maximum power efficiency
of 18.1 lm/W has been realized, which is much higher than those of FIrpic based devices.
The emission peak was located at 480 nm with a subpeak at 511 nm. By using an optimized
device structure with a better host material, performance of the device can be further
increased to give a maximum power efficiency of 33.7 lm/W.
Keywords:
OLED
Blue electrophosphorescence
Iridium complex
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
of blue phosphorescent emitters. Although many blue phos-
phorescent emitters have been reported [6–15], till now,
0
White organic light-emitting devices (OLEDs) have
drawn much attention due to their applications as back-
lights for liquid-crystal displays and as solid-state light
sources [1–4]. For these applications, high efficiency and
long life-time are important. Phosphorescent OLEDs have
been shown to harvest 100% of the excitions generated by
electrical injection, corresponding to a four times increase
in efficiency compared to that achievable in singlet-
harvesting fluorescent OLEDs [5]. For this reason, white
phosphorescent OLED has been a focus of intense research.
Meanwhile, one bottleneck for further enhancing the effi-
ciency of white phosphorescent OLEDs is the performance
iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2 ] pico-
linate (FIrpic) is still the best and most widely used blue
phosphorescent emitter. In a typical device with FIrpic
doped in a 4,40-bis(9-carbazolyl)-2,20-biphenyl (CBP) host,
a maximum power efficiency of 7.7 lm/W with emission
peaks centered at 472 and 500 nm are usually achieved [6].
Due to the high triplet energy level of FIrpic, many new
materials with high triplet energies have been designed
and used as its host. Examples include N,N0-dicarbazolyl-
3,5-benzene (mCP), 4,40-bis(9-carbazolyl)-2,20-dimethyl-
biphenyl (CDBP), p-bis-(triphenylsilyly)benzene (UGH2)
[16–19]. By using mCP instead of CBP, power efficiency
has been increased to 8.9 lm/W [19]. However, efficiency
of blue phosphorescent OLEDs is still far below the require-
ments needed for solid state lighting applications. New
blue phosphorescent emitters of high performance are
obviously still in great demand.
⇑
Corresponding authors. Fax: +86 28 83201745 (S. Tao), +86 10
82543510 (X. Zhang).
E-mail addresses: silutao@uestc.edu.cn (S. Tao), xhzhang@mail.ipc.
ac.cn (X. Zhang).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.08.024

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S. Tao et al. / Organic Electronics 12 (2011) 2061–2064
In this paper, highly efficient blue phosphorescent
OLEDs were fabricated by vacuum deposition on ITO-
coated glass substrates with a sheet resistance of 30 X/
OLEDs have been achieved by using a new emitter, irid-
0
ium(III) bis[(3,4,5-trifluorophenyl)-pyridinato-N,C2 ] pico-
square. Before deposition, the ITO substrate was first care-
fully cleaned, dried in an oven at 120 °C, and then treated
with UV-ozone before loaded into a deposition chamber.
Organic layers were then sequentially deposited onto the
ITO substrate at a rate of 2–4 Å/s and a pressure below
5 ꢀ 10ꢁ6 mbar. Electroluminescence (EL) spectra and CIE
linate (F3Irpic). Using the typical mCP host, blue emission
with a maximum power efficiency of 18.1 lm/W has been
realized. The power efficiency can be further increased to
33.7 lm/W by using a host of UGH2. Performance of the de-
vice is among the best reported for blue OLEDs.
color coordinates were measured with
a Spectrascan
PR650 photometer. Current–voltage–luminescence charac-
teristics were measured with a computer-controlled Keith-
ley 236 source meter under ambient conditions.
2. Experimental details
The cyclometalating ligand 1-(3,4,5-trifluorophenyl)-
pyridine was prepared via a Suzuki coupling reaction be-
tween 1-bromopyridine and 3,4,5-trifluorophenyl boronic
acid. Synthesis of the final Ir complex involved two key
steps. In the first step, IrCl3 was reacted with excess of
the synthesized ligands to produce a chloro-bridged Ir di-
mer. This dimer can be readily converted to the mono-
meric complex F3Irpic by replacing the bridging
chlorides with Picolinic Acid in the presence of Na2CO3.
The synthetic pathway towards the preparation of F3Irpic
is outlined in Scheme 1. The identity was confirmed by
1H-nuclear magnetic resonance, elemental analysis, and
mass spectrometry. MS (m/z): 730; 1H NMR (Acetone,
300 MHz): d: 7.16–7.21 (t, 1H), d: 7.36–7.40 (t, 1H),
7.65–7.66 (d, 2H), 7.75–7.88 (m, 2H), 7.92–8.00 (m, 3H),
8.13–8.20 (m, 4H), 8.67–8.69 (d, 1H). Anal. Calcd. for
IrC28H14N3O2F6: C, 46.03; H, 1.93%; N, 5.75%. Found: C,
45.63; H, 2.00%; N, 5.70%.
Absorption and photoluminescence spectra were re-
corded, respectively with a Perkin–Elmer Lambda 2S UV–
Vis spectrophotometer and a Perkin–Elmer LS50B lumines-
cence spectrophotometer. The highest occupied molecular
orbital (HOMO) value of the Ir complex was measured via
ultraviolet photoelectron spectroscopy (UPS). The lowest
unoccupied molecular orbital (LUMO) value was deter-
mined from the difference between the HOMO energy
and the band gap determined from the optical absorption
edge.
3. Results and discussion
0
Iridium(III) bis[(3,4,5-trifluorophenyl)-pyridinato-N,C2 ]
picolinate (F3Irpic) was synthesized in house. The molecular
structure of F3Irpic (Scheme 1), which has been confirmed
with 1H nuclear magnetic resonance, mass spectrometry
and elemental analysis, is similar to that of FIrpic except
for the three fluorine atoms substituted at the 3,4,5-position
of the phenylpyridine ligand. Fig. 1 shows the absorption
and photoluminescence (PL) spectra of F3Irpic in dilute
dichloromethane solution. The PL spectrum of F3Irpic shows
a blue emission with a main emission peak centered at
479 nm and a smaller peak at 509 nm. The shape of the PL
spectrum is similar to that of FIrpic except for a red-shift
of 8 nm [18]. On the other hand, the photoluminescence
quantum yield of F3Irpic in chloroform solution was mea-
sured to be 0.67 using FIrpic (Uf: 0.50) as a calibration stan-
dard, which is higher than that of FIrpic. The higher PL
quantum yield of F3Irpic should be attributed to more fluo-
rine atoms substituted in the ligand than FIrpic [20].
OLEDs were made by doping F3Irpic in mCP with a device
structure of ITO/NPB (40 nm)/x% F3Irpic:mCP (30 nm)/BAlq
(40 nm)/LiF (0.5 nm)/Al (80 nm). In the devices, ITO (indium
tin oxide) and LiF/Al are the anode and the cathode, respec-
tively; 4,40-bis[N-(1-naphthyl)-N-phenyl amino] biphenyl
(NPB) is the hole transporting layer (HTL), and aluminum
Scheme 1. Synthetic route of F3Irpic.

S. Tao et al. / Organic Electronics 12 (2011) 2061–2064
2063
Fig. 3. I–V–L characteristics of the device, inset is the energy level
Fig. 1. Absorption and emission spectra of F3Irpic, inset is its molecular
diagram of the device.
structure.
the HOMO level of F3Irpic (6.1 eV) is lower than that of
mCP (5.9 eV). This suggests that the F3Irpic might not be
effective trap for the holes and thus decreases the chance
of forming exciton in the phosphorescent emitter.
(III) bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq)
is the electron transporting layer (ETL). The F3Irpic doped
mCP layer is the emission layer (EML).
The devices with different concentration of F3Irpic
shows almost the same EL spectra (Fig. 2) as the PL spec-
trum (Fig. 1) of the Ir complex. Current and power effi-
ciency of the best performing device (4% F3Irpic in mCP)
are shown in inset of Fig. 2. This device shows a maximum
external quantum efficiency of 9.8%. The corresponding
power (current) efficiency is 18.1 lm/W (25.9 cd/A), which
is much higher than that of a similarly structured device
using FIrpic dopant (7.5%, 8.9 lm/W) [19]. The good perfor-
mance of the device should be attributed to the higher PL
quantum yield of F3Irpic than FIrpic. Current–brightness–
voltage characteristics of this F3Irpic (4%):mCP device are
shown in Fig. 3. It has a turn-on voltage (at a brightness
of 1 cd/m2) of 4.3 V and delivers a maximum brightness
of 15000 cd/m2 at a voltage of 13.5 V and a current density
of 215 mA/cm2. A schematic energy level diagram for the
device is shown in the inset of Fig. 3. The HOMO level of
F3Irpic determined with UPS is 6.1 eV. The LUMO level as
determined from the HOMO level and the optical absorp-
tion edge is 3.4 eV. As shown in the energy level diagram,
To address this problem, the mCP host was replaced by
UGH2 which has a much larger energy gap. Chemical
structure and the corresponding energy level diagram for
the device are shown in Fig. 4. It can be noted that the
HOMO and LUMO levels of the Ir complex are entirely
bracketed by those of the UGH2 host. This suggests that
the Ir molecules would be effective traps for both electrons
and holes injected into the UGH2 layer. This is clear bene-
ficial for exciton formation at the Ir molecules. It was found
that the optimized device has a configuration of ITO/NPB
(40 nm)/mCP (10 nm)/8% F3Irpic:UGH2 (20 nm)/BAlq
(40 nm)/LiF (0.5 nm)/Al (80 nm). The mCP layer is used as
an interim layer to facilitate hole injection from the HTL
into EML to overcome the large energy difference (1.8 eV)
between NPB and UGH2.
Current efficiency–brightness–power efficiency charac-
teristics of F3Irpic:UGH2 device are shown in Fig. 5. This
device shows blue emission (see inset of Fig. 5) with a peak
centered at 482 nm and a sub-peak at 512 nm. The F3Irpi-
c:UGH2 device gives a maximum external quantum effi-
ciency of 14.3%. The maximum power (current) efficiency
of the device is 33.7 lm/W (37.6 cd/A), which is much high-
er than that of the F3Irpic:mCP device. The high efficiency
of the device is considered to arise from the following three
aspects. Firstly, the high PL quantum yield of the F3Irpic
contributes to the high device efficiency; secondly, the
good device structure can facilitate the injection and trans-
port of electrons and holes; thirdly, the energy levels of
F3Irpic are entirely bracketed by UGH2, which induced
the effective traps of the dopant for both electrons and
holes injected into the host. Similar to other phosphores-
cent OLEDs, efficiency of the device decreases as the
brightness increases. However, it is interesting to note that
even at a brightness of 1000 cd/m2, the efficiency of the de-
vice is still higher than 20 cd/A (9 lm/W). As far as we
know, performance of this device is among the best values
reported for blue OLEDs.
In summary, highly efficient blue phosphorescent OLEDs
have been achieved by using a new emitter iridium(III)
Fig. 2. EL spectra of the device at different doping concentrations, inset is
the current efficiency–current density–power efficiency characteristics.

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S. Tao et al. / Organic Electronics 12 (2011) 2061–2064
Fig. 4. The chemical structure of UGH2; the energy level diagram of device II.
for the Central Universities’’, ‘‘the Research Fund for
the Doctoral Program of Higher Education of China
(20100185120015)’’ and Jiangsu Key Laboratory for
Carbon-Based Functional Materials and Devices, Soochow
University, Suzhou, P.R. China.
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This work was supported by the Innovation and Tech-
nology Commission (Grant No. ITP/011/08NP) of Hong
Kong SAR. S.L. Tao would like to thank financial supports
from the National Natural Science Foundation of China
(NSFC Grant 21002011), ‘‘the Fundamental Research Funds