Efficient OLEDs with low efficiency roll-off using iridium complexes possessing good electron mobility

Article abstract of DOI:10.1039/c5tc00073d

Two bis-cyclometalated iridium complexes (Ir1 and Ir2) with trifluoromethyl substituted bipyridine (2′,6′-bis(trifluoromethyl)-2,3′-bipyridine (L1) and 2′,6′-bis(trifluoromethyl)-2,4′-bipyridine (L2)) as the main ligands and tetraphenylimidodiphosphinate

Full text of DOI:10.1039/c5tc00073d

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Materials Chemistry C
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Efficient OLEDs with low efficiency roll-off using
iridium complexes possessing good electron
mobility†
Cite this: J. Mater. Chem. C, 2015,
3, 3694
Qiu-Lei Xu, Xiao Liang, Song Zhang, Yi-Ming Jing, Xuan Liu, Guang-Zhao Lu,
You-Xuan Zheng* and Jing-Lin Zuo
Two bis-cyclometalated iridium complexes (Ir1 and Ir2) with trifluoromethyl substituted bipyridine (20,60-bis-
(trifluoromethyl)-2,30-bipyridine (L1) and 20,60-bis(trifluoromethyl)-2,40-bipyridine (L2)) as the main ligands
and tetraphenylimidodiphosphinate as the ancillary ligand were prepared, and their X-ray crystallography,
photoluminescence, electrochemistry properties were investigated. The Ir1 and Ir2 complexes show green
emissions at about 500 and 502 nm with high quantum efficiencies of 0.63 and 0.93, respectively.
Moreover, they also exhibit higher electron mobility than that of Alq3 (tris-(8-hydroxyquinoline)aluminium).
The organic light emitting diodes (OLEDs) with the structure of ITO/TAPC (1,1-bis[4-(di-p-tolylamino)-
phenyl]cyclohexane, 40 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 10 nm)/Ir complex (8 wt%): PPO21
(3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-
yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) showed excellent performances, partly due to their
high quantum efficiency and high electron mobility. For the devices G1 and G2, the maximum current
efficiency (Zc) values are as high as 101.96/99.97 cd Aꢀ1 and the maximum external quantum efficiencies
of 31.6% and 30.5% with low electroluminescence efficiency roll-off. The Zc data still remain over 90 cd Aꢀ1
even at the luminance of 10 000 cd mꢀ2, which proves that the complexes have potential applications as
efficient green emitters in OLEDs.
Received 9th January 2015,
Accepted 15th February 2015
DOI: 10.1039/c5tc00073d
www.rsc.org/MaterialsC
substituent effects, which leads to a wide flexible emission
color range.
Introduction
Due to the high quantum efficiency, short lifetime of triplet
However, for many OLEDs with high efficiency based on
excited states and broad range of emission colors, phosphorescent Ir(III) complexes, the device efficiency roll-off ratios are serious,
iridium(III) complexes have been widely used as dopants in which can mainly be attributed to the deterioration of charge
organic light-emitting diodes (OLEDs).1 The strong spin–orbit carrier balance and the increase of nonradioactive quenching
coupling (SOC) introduced by the central heavy atom can processes, including triplet–triplet annihilation (TTA), triplet-
promote the triplet to ground radiative transition, resulting in polaron annihilation (TPA), and electric field induced dissociation
unusually high phosphorescence quantum yields at room of excitons at high current density.2 Therefore, the balanced
temperature. On the other hand, since the phosphorescence injection and transport of the electron–hole is a crucial factor
of Ir(III) complexes primarily originates from the metal-to- for high efficient OLEDs. As we know, because the hole mobility
ligand charge transfers (MLCT) and the ligand-centered (LC) of most hole transport materials is roughly 2–3 orders of
transitions,1a the energy level of the excited state can be magnitude higher than the electron mobility of the electron
controlled by tuning the energy levels of the ligands through transport materials, the efficiency and efficiency roll-off of
OLEDs rely on the capability of electron transport. Thus, it is
necessary to use the ambipolar host materials and synthesize
State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center
Ir(III) dopants with outstanding electron mobility3 to obtain
of Advanced Microstructures, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, P. R. China. E-mail: yxzheng@nju.edu.cn
† Electronic supplementary information (ESI) available: For the the ORTEP
diagrams of L2, the crystallographic data for L1, Ir1 and Ir2, the orbital distribu-
tions of complexes Ir1, Ir2, IrA and IrB, the transient EL signals for the device
based on Ir1, the lifetime curves and normalized emission spectra of Ir1 and Ir2
in degassed solution. CCDC 996478–996480. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5tc00073d
phosphorescent OLEDs with low efficiency roll-off.
It is well-known that Ph2PQO has been widely used to construct
electron transport and ambipolar host materials because of its
ability to improve electron injecting and transporting capabilities
due to its strongly electron-deficient nature.4 In our group, we
found that tetraphenylimidodiphosphinate (tpip) derivatives,5
3694 | J. Mater. Chem. C, 2015, 3, 3694--3701
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Fig. 1 Oak Ridge thermal ellipsoidal plot (ORTEP) diagrams of the complexes
Ir1 (left) and Ir2 (right) with the atom-numbering schemes. Hydrogen atoms
are omitted for clarity. Ellipsoids are drawn at 30% probability level.
of the mixture by silica chromatography provided the products Ir1
and Ir2, which were further purified by vacuum sublimation. All
the new compounds were fully characterized by 1H NMR and high
resolution mass spectrometry; moreover, the crystal structures
further confirmed the identity of both the complexes.
Scheme 1 Synthetic routes of ligands and complexes.
X-ray crystallography
which have two diphenyl phosphoryl (Ph2PQO) groups, are actually
useful ancillary ligands for Ir(III) complexes and corresponding
devices. Besides, Htpip has four bulky aromatic groups, which
may lead to a larger spatial separation of the neighboring
molecules of the Ir(III) complexes to suppress the TTA and TPA
effectively. Moreover, fluorination can enhance the electron
mobility and result in a better balance of charge injection
and transfer, the lower vibrational frequency of the C–F bond
can reduce the rate of radiationless deactivation, the bulky CF3
substituents can affect the molecular packing and the steric
protection around the metal can suppress the self-quenching
behavior.6 In addition, nitrogen heterocycle will increase the
electron affinity and a more negative framework of ligand C4N
such as bipyridine may improve the electron mobility. Thus, in
this article we synthesized two iridium complexes (Ir1 and Ir2,
Scheme 1) with –CF3 substituted bipyridine as the main ligand
and tpip as the ancillary ligand, expecting to get efficient
phosphorescent Ir(III) complexes with high electron mobility
for high efficient OLEDs. To study the influence of introducing
the bipyridine ligand on the properties of iridium complexes,
we set our former reported –CF3 substituted phenylpyridine
complexes (IrA and IrB, Scheme 1) as references.5c
Single crystals of L1 (Fig. S1, ESI†) and Ir2 were obtained from
petroleum ether and CH2Cl2 solutions, and Ir1 was grown from
vacuum sublimation. Fig. 1 shows the Oak Ridge thermal
ellipsoidal plot (ORTEP)7 diagrams of Ir1 and Ir2 given by
X-ray analysis. The selected parameters of the molecular structures
and atomic coordinates were collected in the Tables S1 and S2
(ESI†). The iridium center adopts a distorted octahedral coordina-
tion geometry with two C4N cyclometalated ligands and one O4O
ancillary ligand. For the Ir1, the range of dihedral angles between
two pyridine rings of each ligand are 7.3(9)–19.31, indicating that
the two pyridine ring are not coplanar for the steric effect of the
trifluoromethyl group. The Ir–N bond lengths ranging from
2.004(6) to 2.006(5) Å and Ir–C bond length ranging from
1.868(7) to 1.977(6) Å are slightly shorter than values reported for
IrA (Ir–N = 2.016(3)–2.025(4) Å and Ir–C = 1.974(9)–1.981(8) Å). The
bond length ranges of Ir–N and Ir–C in Ir2 are 2.031(8)–2.061(8) Å
and 2.012(9)–2.039(9) Å, respectively. Furthermore, the C–C
and C–N bond lengths and angles are in agreement with the
corresponding parameters described in other similarly consti-
tuted complexes.
Thermal stability
The thermal stability of emitters is very important for efficient
OLEDs. If a complex is suitable for application in OLEDs, the
melting point (Tm) and decomposition temperature (Td) should
be high enough to guarantee that the complex could be
Results and discussion
Preparation and characterization of compounds
Scheme 1 shows the synthetic routes for the new iridium deposited onto the solid face without any decomposition on
complexes Ir1 and Ir2. The reaction of 2,6-bis(trifluoromethyl)- sublimation. The differential scanning calorimetry (DSC) and
pyridine with LDA (lithium diisopropylamide) and B(OPr-i)3 thermogravimetric analysis (TGA) curves of Ir1 and Ir2 are
gave two crude aryl boronic acids. The corresponding two listed in Fig. 2. The melting points of Ir1 and Ir2 are 315 1C
trifluoromethyl fluorinated bipyridine ligands (L1 and L2) were and 309 1C, respectively. From the TGA curves it can be
synthesized using a Suzuki coupling reaction. The tetraphenyl- observed that no loss of weight was observed below 360 1C,
imidodiphosphinate acid (Htpip) and potassium salt (Ktpip) and the decomposition temperature (5% loss of weight) is
were prepared according to our previous publications.5 The 363 1C for Ir1 and 368 1C for Ir2, indicating that the complexes
iridium complexes were obtained in two steps. The purification are suitable for application in OLEDs.
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shorten the emission decay time and depress the triplet–triplet
annihilation. The emission spectra in CH2Cl2 exhibit maximum
peaks in the range of 500 nm to 502 nm generated from the
electronic 0–0 transition between the lowest triplet excited state
and the ground state, making them green phosphors. The
emission at lower energy range around 533 nm might stem
from the overlapping vibrational satellites.8 In general, the
emission bands from the MLCT states are broad and feature-
less, whereas a highly structured emission band mainly originates
from the 3p–p* state. Accordingly, all of the complexes emit
from a mixture of MLCT states and the dominant ligand-based
3p–p* state. This indicates that the MLCT characters involved
in the emitting T1 states of different complexes are various, but
significant since a dominant MLCT character in T1 usually
leads to large inhomogeneity and low-energy lying metal-
ligand vibrational satellites, smearing out the spectrum below
the electronic original emission.9
Fig. 2 The DSC and TGA curves of Ir1 and Ir2 complexes.
Similar emission wavelength suggests that the position of
the nitrogen atom of the pyridine and trifluoromethyl groups
on the pyridine ring have no obvious effect on the wavelength
of the Ir(III) complexes. Furthermore, the complexes based on
bipyridine show much higher quantum efficiency (Fp) as 0.63
and 0.93 for Ir1 and Ir2 (Fp of IrA and IrB are only 0.06 and
0.10), respectively. These results indicate that the introduction
Fig. 3 The UV-vis absorption (a) and emission (b) spectra of complexes
Ir1, Ir2, IrA and IrB in degassed dichloromethane (5 ꢁ 10ꢀ5 M) at room
temperature.
Photophysical and electrochemical property
The UV-vis absorption and emission spectra of the two complexes of another pyridine in the main ligand will greatly improve the
Ir1, Ir2 and the referring complexes IrA, IrB in CH2Cl2 (5 ꢁ efficiency of iridium complexes and they have potential application
10ꢀ5 M) are shown in Fig. 3 and Fig. S4 (ESI†), and the in the OLEDs.
photophysical data are collected in Table 1. The absorption
The redox properties and highest occupied molecular orbital
spectra of these complexes show broad and intense bands (HOMO), lowest unoccupied molecular orbital (LUMO) energy
below 320 nm, which can be assigned to the spin-allowed levels of the dopants are relative to the charge transport ability
intraligand 1LC (p - p*) transition of cyclometalated main and the OLED structure. To calculate the HOMO and LUMO
ligands and tpip ligands. The weak bands last to 520 nm can be energy levels of the heteroleptic complexes, cyclic voltammetry
assigned to the spin-allowed metal-ligand charge transfer band experiments of complexes Ir1 and Ir2 were carried out using
(1MLCT), partially overlapped by the broad LC absorptions, and ferrocene as the internal standard (Fig. 4). During the anodic
spin forbidden 3MLCT transition bands caused by the large scan in CH2Cl2, Ir1 and Ir2 exhibit a quasi-reversible redox with
spin orbital coupling (SOC) that was introduced by the iridium the oxidation potential in the region of 1.45–1.47 V, which is
center, indicating an efficient spin–orbit coupling is a prerequisite attributed to the metal-centered IrIII/IrIV redox couple in accor-
for the phosphorescent emission. No distinct difference is observed dance with the reported cyclometallated Ir(III) systems.10 It is
in the absorption profiles for any of the complexes, indicating the noteworthy that the complexes Ir1 and Ir2 show higher oxida-
similar electronic and vibrational structures of the ground states tion potentials than that of IrA and IrB (Table 1), which can be
(S0) and the first excited states (S1).
ascribed to the electronegative effect of the pyridine ring
For the phosphorescent emitters used in OLEDs, significant making the complexes more difficult to lose electron. The
mixing of the lowest triplet and higher lying singlet excited states HOMO levels of the complexes were calculated from the oxida-
caused by the efficient SOC is in favor of high phosphorescent tion potentials and the LUMO levels were calculated from the
quantum efficiency. Moreover, strong SOC effect can drastically HOMO and band gap obtained from the UV-vis absorption
Table 1 Photophysical data of Ir(III) complexes Ir1, Ir2, IrA and IrB
Absorptionb l (nm)
Emission lem (nm) 298 K
t298 K (ms)
FP (%)
Eox (V)
HOMO/LUMOd (eV)
a
b
b
c
Compound
Tm/Td (1C)
Ir1
IrA
Ir2
IrB
315/363
266/336
309/368
334/358
349/404/484
365/418/484/520
346/394/486
342/383/471
500/534
542/548
502/533
480/487
2.12
3.61
3.13
1.83
63
6
93
10
1.47
1.16
1.45
1.26
ꢀ5.93/ꢀ3.05
ꢀ5.82/ꢀ3.46
ꢀ5.93/ꢀ3.04
ꢀ5.92/ꢀ3.29
a
b
c
Tm: melting temperature, Td: decomposed temperature. Absorption, emission spectra and lifetime were taken in degassed CH2Cl2. F:
d
emission quantum yields were measured using the integrating-sphere system. From the onset of oxidation potentials of the cyclovoltammetry
(CV) diagram using ferrocene as the internal standard and the optical band gap from the absorption spectra in degassed CH2Cl2.
3696 | J. Mater. Chem. C, 2015, 3, 3694--3701
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contribution from the tpip ligand (5.89–9.22%), while the LUMOs
are mainly localized on the main ligands (93.50–94.78%) with
minor contribution from Ir d orbitals (3.08–3.74%) and tpip
ligand (1.88–2.98%). Therefore, replacing the phenyl ring with
the pyridine ring changes both the HOMO and the LUMO
energy. Compared with the IrA and IrB complexes, the orbital
distributions of HOMO for Ir1 and Ir2 have more contributions
from Ir d orbitals and tpip ligand and less contribution from
the bipyridine ligand. The calculation results indicated that
the replacement of the phenyl group with pyridine affected the
HOMO/LUMO levels of the iridium complexes.
Fig. 4 Cyclic voltammograms of complexes Ir1 and Ir2.
Electron mobility
The good electron mobility of the phosphorescent emitters
would facilitate the injection and transport of electrons, which
will broaden the recombination zone, balance the distribution
of hole–electron and reduce leakage current, leading to suppressed
TTA and TPA effects,16 improved recombination probability, high
device efficiency, and low efficiency roll-off. To measure the
electron mobility of the two complexes, we conducted the transient
electroluminescence (TEL) measurement based on the device of
ITO/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane,
50 nm)/Ir complexes (60 nm).17 The TAPC is the hole-transport
layer, whereas the Ir complexes perform as both the emissive and
electron-transport layer. To check the accuracy of our measure-
ments, we also measured the electron mobility of Alq3 (tris-(8-
hydroxyquinoline)aluminum, Fig. S, ESI†), which is the typically
well-known electron transport material, whose electron mobility
has been reported in many references.18 The experimental results
(Fig. 6, Fig. S, ESI†) showed that the electron mobilities in 60 nm
Ir1 and Ir2 layers are between 7.00–7.20 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and
spectra (Table 1).11 The HOMO levels of the two group complexes
do not show much difference, while the LUMO levels of Ir1 and
Ir2 complexes are much higher than that of the IrA and IrB ones,
which would be helpful to the electron transport ability.
The density functional theory (DFT) calculations for Ir(III)
complexes are conducted to gain insights into the electronic
states and the orbital distribution employing the Gaussian09
software12 with B3LYP functional.13 The basis set used for C, H,
N, O, F and P atoms was 6-31G(d,p), while the LanL2DZ basis
set were employed for Ir atoms.14 The solvent effect of CH2Cl2
was taken into consideration using a conductor like polarizable
continuum model (C-PCM).15 The relative energies of the
HOMO/LUMO for all complexes are shown in Fig. 5 and the
orbital distributions are summarized in Table S3 (ESI†). The
results are helpful for the assignment of the electron transition
characteristics and the discussion on the photophysical variations.
For these complexes, the HOMOs correspond to a mixture of
the phenyl or pyridine group attached to the pyridine ring
(34.39–42.38%) and Ir d orbitals (51.73–56.39%) with minor
5.11–5.29 ꢁ 10ꢀ6 cm2 Vꢀ1
s
ꢀ1, respectively, under an electric
to 1300 (V cm
ꢀ1 1/2, while that of
field from 1150 (V cmꢀ1 1/2
)
)
Alq3 is between 4.74–4.86 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1. The results suggest
that Ir1 and Ir2 complexes have higher mobility than that of Alq3
and IrA, IrB.5c In addition, the electron mobility of Ir1 is better
than that of Ir2 suggesting that the N atom in the 5-position has
higher effect on the improvement of the electron flow than in
4-position. The good electron transport ability of Ir1 and Ir2 will
strengthen the recombination probability of electrons and
holes, which indicate that OLEDs based on Ir1 and Ir2 may
have good performances.
Fig. 6 (a) The transient EL signals for the device structure of ITO/TAPC
(50 nm)/Ir2 (60 nm) under different applied fields, and (b) electric field
dependence of charge electron mobility in the thin films of Ir1, Ir2
and Alq3.
Fig. 5 Theoretical (black) and experimental (red, determined by cyclic
voltammetry) HOMO/LUMO energy levels of complexes Ir1, Ir2, IrA
and IrB.
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OLEDs performance
To verify our conjecture, devices G1 and G2 using Ir1 and Ir2 as
the emitters, respectively, with the structure of ITO/TAPC (40 nm)/
mCP (N,N0-dicarbazolyl-3,5-benzene, 10 nm)/Ir complex (8 wt%):
PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-
9H-carbazole, 25 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene,
50 nm)/LiF (1 nm)/Al (100 nm) were investigated to evaluate the
electroluminescence (EL) performances. Scheme 2 shows the
energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this study and their
molecular structures. Because of the low HOMOs of Ir1 and Ir2
(ꢀ5.93 eV), we chose PPO21 (HOMO = 6.21 eV)19 as the host
material. mCP was also added as another hole transport material
to lower the HOMO energy barrier between TAPC20 and PPO21.
TmPyPB21 was used as an electron transport material.
The electroluminescence spectra, current density–lumi-
nance ( J–L), current efficiency (Zc)–luminance (Zc–L) and exter-
nal quantum efficiency–luminance (ZEQE–L) curves for G1 and
G2 are shown in Fig. 7, voltage–luminance (V–L) and power
efficiency–luminance (Zp–L) curves are list in Fig. S5 (ESI†), and
the key EL data are summarized in Table 2. The optimal device
performances are achieved at the doping level of 8 wt% and
both the devices display excellent performances. All devices
emit green light with the EL emission peaks at 505 and 506 nm
for G1 and G2, respectively, and the emission spectra are
almost invariant of the current density and also do not show
any concentration dependence. As shown in Fig. 7(a), the EL
spectra are very close to the PL spectra of the complexes in
CH2Cl2 solution, indicating that the EL emissions of the devices
originate from the triplet excited states of the phosphors. No
emission from TAPC, mCP and TmPyPB suggests that the
exciton was only formed in the emissive layers. The absence
of the PPO21 emission demonstrates that the energy and/or
charge transfer from the host exciton to the phosphor is com-
plete upon electrical excitation. The Commission Internationale
Fig. 7 Characteristics of devices of G1 and G2 with configuration ITO/
TAPC (40 nm)/mCP (10 nm)/Ir complex (8 wt%): PPO21 (25 nm)/TmPyPB
(50 nm)/LiF (1 nm)/Al (100 nm): (a) electroluminescence spectra; (b) current
density–luminance (J–L) curves; (c) current efficiency–luminance (Zc–L)
curves; (d) external quantum efficiency–luminance (ZEQE–L) curves.
de l’Eclairage (CIE) colour coordinates are x = 0.32, y = 0.59 for
both G1 and G2 devices.
For device G1, the maximum current efficiency (Zc,max) is
101.96 cd Aꢀ1 obtained at 7.8 V with a maximum external
quantum efficiency of 31.6%, the maximum power efficiency
(Zp,max) is 43.34 lm Wꢀ1 (7.2 V) and the maximum luminance
(Lmax) is 35 178 cd mꢀ2. Device G2 has a maximum luminance of
52 515 cd mꢀ2 at 15.3 V and displays a maximum current efficiency
of 99.97 cd Aꢀ1 with a maximum external quantum efficiency
of 30.5%, and a maximum power efficiency of 43.60 lm Wꢀ1
(7.2 V). Furthermore, both the devices keep high efficiency at
relative high luminance and the roll-off of EL efficiency is very
low. For example, the current efficiencies for devices G1 and G2
still remain as high as 90.5 and 92.02 cd Aꢀ1 even at the
brightness of 10 000 cd mꢀ2, which indicate that the complexes
have application potential in OLEDs.
All the device performances are higher than that of IrA and
IrB used as emitters. The high performances perhaps can be
attributed to the high quantum efficiency, electron mobility of
the Ir(III) complexes and suitable energy level. The small energy
barrier (0.02 eV) between TmPyPB and PPO21 make it easy for
electron injection. The low-lying HOMO level (6.7 eV) and high
triplet energy level (2.78 eV) of TmPyPB will achieve a well
confinement of hole within the emissive layer. Moreover, each
material has high carrier mobility (mh = 1 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1
for TAPC, mh = 5.0 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for mCP and me = 1 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1 for TmPyPB).19–21 The emitters even have
higher electron mobility than that of Alq3, which leads to a well-
balanced charge carrier transport and efficient recombination.
The dopants act as the hole and electron traps to retard the
motion of both types of carriers. The good electron mobility of
the dopants is particularly important for the reason that the
hole mobility of the TAPC is higher than the electron mobility
of the TmPyPB; moreover, the excitons accumulation is expected
Scheme 2 Energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this study and their molecular
structures.
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Table 2 EL performances of the devices G1 and G2
Vturn-on
(V)
Lmax(voltage)b
Z
c,max(voltage)c
Zc,L10000
Z
p,max(voltage)e
a
d
Device
Emitter
[cd mꢀ2 (V)]
[cd Aꢀ1 (V)]
(cd Aꢀ1
)
ZEQE,max (%)
[lm Wꢀ1 (V)]
CIE (x,y)
G1
G2
Ir1
Ir2
4.1
4.9
35 178(17.4)
52 515(15.3)
101.96(7.8)
99.97(7.2)
90.52
92.02
31.6
30.5
43.34(7.2)
43.60(7.2)
0.32, 0.59
0.32, 0.59
a
b
c
d
Vturn-on: turn-on voltage recorded at a luminance of 1 cd mꢀ2
.
Lmax: maximum luminance.
Z
c,max: maximum current efficiency. Zc,L10000
:
current efficiency at 10 000 cd mꢀ2
.
Z
p,max: maximum power efficiency.
e
in hole blocking layer near the interface of the emitting layer spectrophotometer, respectively. The decay lifetimes and absolute
(Ir complexes: PPO21) and TmPyPB due to the high energy barrier photoluminescent quantum yields were measured with an
between PPO21 and TmPyPB.19–21 The accumulation of excitons Edinburgh Instrument FLS-920 fluorescence spectrometer equipped
is expected to cause the serious TTA and TPA of the iridium with an integrating sphere in degassed CH2Cl2 solution at room
complexes, and consequently high efficiency roll-off. In our case, temperature. Cyclic voltammetry measurements were conducted on
the good electron mobility of the phosphorescent emitters would a MPI-A multifunctional electrochemical and chemiluminescent
improve the device efficiency and lower the efficiency roll-off.
system (Xi’an Remex Analytical Instrument Ltd Co., China) at room
It can also be observed that the turn on voltages of the two temperature with a polished Pt plate as the working electrode,
devices are high (4.1 V for G1 and 4.9 V for G2) though the dopants platinum thread as the counter electrode and Ag–AgNO3 (0.1 M)
have good electron mobility, and the charge transport of TAPC/ in CH3CN as the reference electrode, tetra-n-butylammonium
TmPyPb are also high. One reason is that the carrier mobility of the perchlorate (0.1 M) was used as the supporting electrolyte, using
host material PPO21 is very low (mh = 9 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1 and me = Fc+/Fc as the internal standard, the scan rate was 0.1 V sꢀ1
.
3.0 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1).19 Furthermore, the insertion of the mCP
also would cause energy loss during the current flow. These two
factors may lead to the high turn-on voltages of the devices.
X-ray crystallography
X-ray crystallography of the single crystals of the complexes and
ligand were carried out on a Bruker SMART CCD diffractometer
using monochromated Mo Ka radiation (l = 0.71073 Å) at room
temperature. Cell parameters were retrieved using SMART soft-
ware and refined using SAINT22 on all observed reflections.
Data were collected using a narrow-frame method with scan
widths of 0.301 in o and an exposure time of 10 s per frame. The
highly redundant data sets were reduced using SAINT and
corrected for Lorentz and polarization effects. Absorption correc-
tions were applied using SADABS23 supplied by Bruker. The
structures were solved by direct methods and refined by full-
matrix least-squares on F2 using the program SHELXS-97.24 The
positions of metal atoms and their first coordination spheres
were located from direct-methods E-maps; other non-hydrogen
atoms were found in alternating difference Fourier syntheses
and least-squares refinement cycles and during the final cycles
refined anisotropically. Hydrogen atoms were placed in calculated
Conclusions
In summary, using CF3 substituted bipyridine as the main
ligand and tpip as the ancillary ligand, we prepared two green
phosphorescent iridium complexes (Ir1 and Ir2). Compared
with the phenylpyridine based complexes (IrA and IrB), the
bipyridine contained complexes Ir1 and Ir2 show higher quantum
efficiency and electron mobility. The OLEDs based on the Ir1
and Ir2 complexes (ITO/TAPC (40 nm)/mCP (10 nm)/Ir complex
(8 wt%): PPO21 (25 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al
(100 nm)) show excellent performances with maximum current
efficiencies of 101.96/99.97 cd Aꢀ1 and maximum external
quantum efficiencies of 31.6% and 30.5% along with low efficiency
roll-off. Even at the brightness of 10 000 cd mꢀ2, the current
efficiencies still reach as high as 90.52 and 92.02 cd Aꢀ1, suggesting
that the improved electron mobility of the emitters would lead to a
better carrier balance, and consequently to high efficiency with low
efficiency roll-off.
position and refined as riding atoms with a uniform value of Uiso
.
OLEDs fabrication and measurement
All OLEDs with the emission area of 0.1 cm2 were fabricated on
the pre-patterned ITO-coated glass substrate with a sheet
resistance of 15 O sqꢀ1. All chemicals used for EL devices were
sublimed in vacuum (2.2 ꢁ 10ꢀ4 Pa) prior to use. The deposi-
tion rate for organic compounds is 1–2 Å sꢀ1. The phosphor
Experimental
Materials and measurements
All reagents and chemicals were purchased from commercial and PPO21 host were co-evaporated to form the 25 nm emitting
sources and used without further purification. 1H NMR spectra layer from two separate sources. The cathode consisting of
were measured on a Bruker AM 500 spectrometer. Mass spectra LiF/Al was deposited by the evaporation of LiF with a deposition
(MS) were obtained with an ESI-MS (LCQ Fleet, Thermo Fisher rate of 0.1 Å sꢀ1 and then by evaporation of Al metal with a rate
Scientific). High resolution mass spectra (Agilent 6540 UHD of 3 Å sꢀ1. The effective area of the emitting diode is 0.1 cm2.
Accurate-Mass Q-TOF LC/MS) were recorded for the complexes. The characteristics of the devices were measured with a computer
Absorption and photoluminescence spectra were measured on a UV- controlled KEITHLEY 2400 source meter with a calibrated
3100 spectrophotometer and a Hitachi F-4600 photoluminescence silicon diode in air without device encapsulation. On the basis
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Journal of Materials Chemistry C
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1
of the uncorrected PL and EL spectra, the CIE coordinates were
Ir2. Yield: 59.7% H NMR (500 MHz, CDCl3) d 8.90 (d, J =
calculated using a test program of the spectra scan PR650 5.4 Hz, 1H), 7.94 (s, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.66 (dd, J =
spectrophotometer.
11.9, 7.5 Hz, 2H), 7.53–7.31 (m, 4H), 7.27–7.13 (m, 3H), 7.08
(td, J = 7.6, 3.1 Hz, 2H), 6.66 (t, J = 6.5 Hz, 1H). ESI-MS: 1192.75
[M]+ (C48H30F12IrN5O2P2). HR EI-MS Calcd: m/z 1190.9295 for
Syntheses
All reactions were performed an under nitrogen atmosphere. [M]+ (C48H30F12IrN5O2P2), found: m/z 1192.1364 [M + H]+.
Solvents were carefully dried and distilled from appropriate
drying agents prior to use for the syntheses of ligands.
Acknowledgements
General syntheses of ligands. A stirred solution of 2,6-bis-
(trifluoromethyl)pyridine (0.215 g, 10 mmol) in diethyl ether (20 mL)
This study was supported by the National Natural Science Founda-
was cooled to ꢀ78 1C. LDA (lithium diisopropylamide, 6.0 mL,
tion of China (21371093, 91433113), the Major State Basic Research
10 mmol) was added over 20 min and stirred for 1 h, and then
Development Program (2011CB808704, 2013CB922101) and the
B(OPr-i)3 (2.89 mL, 12.4 mmol) was added. The mixture was warmed
Natural Science Foundation of Jiangsu Province (BK20130054).
to room temperature and stirred for another 1 h. The pH was
adjusted to 10 by the slow addition of 10% aqueous NaOH
solution (20 mL). After 1 hour, the organic phase was acidified
to pH = 4 by the dropwise addition of 3 N HCl. The extraction
References
with ethyl acetate and evaporation of the organic phase gave
the crude corresponding aryl boronic acids. 2-Bromopyridine
(1 mL, 10 mmol) and tetrakis(triphenylphosphine)palladium0
(0.34 g, 0.3 mmol) and the boronic acids were added in 50 mL
THF. After 20 mL of aqueous 2 N K2CO3 was delivered, the
reaction mixture was heated at 70 1C for 1 day under an nitrogen
atmosphere. The mixture was poured into water and extracted
with CH2Cl2 (10 mL ꢁ 3 times). Finally, silica column purifica-
tion (n-hexane : EtOAc = 7 : 1 as eluant) gave the colorless liquid
20,60-bis(trifluoromethyl)-2,30-bipyridine (L1) and white solid
20,60-bis(trifluoromethyl)-2,40-bipyridine (L2).
20,60-Bis(trifluoromethyl)-2,30-bipyridine (L1). 10% yield.
1H NMR (500 MHz, CDCl3) d 8.74 (d, J = 4.2 Hz, 1H), 8.14
(d, J = 8.0 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.85 (t, J = 7.2 Hz, 1H),
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m/z 292.18 for [M]+ (C12H6F6N2), found: m/z 293.25 [M + H]+.
20,60-Bis(trifluoromethyl)-2,40-bipyridine (L2). 30% yield.
1H NMR (500 MHz, CDCl3) d 8.87–8.77 (m, 1H), 8.54 (s, 2H),
7.99–7.91 (m, 2H), 7.49 (ddd, J = 6.6, 4.7, 2.0 Hz, 1H). MS(ESI):
Calcd: m/z 292.18 for [M]+ (C12H6F6N2), found: m/z 293.33
[M + H]+.
General syntheses of iridium complexes. A mixture of IrCl3ꢂ
3H2O (1 mmol) and L1 or L2 (2.5 mmol) in 2-ethoxyethanol
and water (20 mL, 3 : 1, v/v) was refluxed for 24 h. After cooling,
the yellow solid precipitate was filtered to give the crude
cyclometalated Ir(III) chloro-bridged dimer. Then, the slurry of
crude chloro-bridged dimer (0.2 mmol) and Ktpip (0.5 mmol) in
2-ethoxyethanol (20 mL) was refluxed for 24 h. The solvent was
evaporated at low pressure and the crude product was washed
with water, and then chromatographed using CH2Cl2 to give
complexes Ir1 and Ir2, which were further purified by sublimation
in vacuum.
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