Efficient green photoluminescence and electroluminescence of iridium complexes with high electron mobility

Article abstract of DOI:10.1039/C8DT02961J

Aiming to balance the injection and transport of electrons and holes, nitrogen heterocycle and 1,3,4-oxadiazole derivatives were introduced in iridium(iii) complexes to obtain organic light-emitting diodes (OLEDs) with high performances. Thus, two novel Ir(iii) complexes (Ir(tfmphpm)₂(pop) and Ir(tfmppm)₂(pop)) with green emissions using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometalating ligands, and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (pop) as an ancillary ligand were synthesized. Both emitters show high photoluminescence efficiencies up to 94% and good electron mobility. The devices using two emitters with the structure of ITO (indium-tin-oxide)/MoO₃ (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 30 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir(iii) complexes (6 wt%):PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm) display good electroluminescence performances with a maximum luminance of 48981 cd m², a maximum current efficiency of 92.79 cd A¹ and a maximum external quantum efficiency up to 31.8%, respectively, and the efficiency roll-off ratio is low, suggesting that they have potential application in OLEDs.

Full text of DOI:10.1039/C8DT02961J

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Efficient green photoluminescence and
electroluminescence of iridium complexes
with high electron mobility†
Cite this: Dalton Trans., 2018, 47,
16543
Hua-Bo Han,a Zheng-Guang Wu,a Zhi-Ping Yan,a Yue Zhao a and
a,b
You-Xuan Zheng
*
Aiming to balance the injection and transport of electrons and holes, nitrogen heterocycle and 1,3,4-oxa-
diazole derivatives were introduced in iridium(III) complexes to obtain organic light-emitting diodes
(OLEDs) with high performances. Thus, two novel Ir(III) complexes (Ir(tfmphpm)2(pop) and
Ir(tfmppm)2(pop)) with green emissions using 2-(3,5-bis(trifluoromethyl)phenyl)pyrimidine (tfmphpm) and
2-(2,6-bis(trifluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometalating ligands, and 2-(5-phenyl-
1,3,4-oxadiazol-2-yl)phenol (pop) as an ancillary ligand were synthesized. Both emitters show high
photoluminescence efficiencies up to 94% and good electron mobility. The devices using two emitters
with the structure of ITO (indium-tin-oxide)/MoO3 (molybdenum oxide, 5 nm)/TAPC (di-[4-(N,N-ditolyl-
amino)-phenyl]cyclohexane, 30 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir(III) complexes
(6 wt%) : PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole, 10 nm)/
TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene, 40 nm)/LiF (1 nm)/Al (100 nm) display good electro-
luminescence performances with a maximum luminance of 48 981 cd m−2, a maximum current
efficiency of 92.79 cd A−1 and a maximum external quantum efficiency up to 31.8%, respectively, and the
efficiency roll-off ratio is low, suggesting that they have potential application in OLEDs.
Received 20th July 2018,
Accepted 24th October 2018
DOI: 10.1039/c8dt02961j
rsc.li/dalton
considered as the most fascinating candidates because of their
tunable emission energy, excellent chemical stability and high
Introduction
Since Tang and VanSlyke fabricated the first high efficiency photoluminescence efficiency.3
thin-film organic light-emitting diodes (OLEDs) by the vacuum
However, the problem of imbalance between the injection
deposition method in 1987,1 many types of materials have and transport of electrons and holes still exists and deeply
been extensively investigated because of their great varieties influences the performances of OLEDs, because the hole mobi-
and high emission efficiency. Compared to fluorophores, the lity of the majority of hole-transporting materials is much
phosphorescent transition metal complexes can not only make higher than the electron mobility of the electron-transporting
use of singlet excitons but also harvest triplet excitons to materials. To avoid a serious efficiency roll-off caused by the
remarkably promote the characteristics of the devices.2 Among charge carrier balance deterioration and nonradioactive
all these complexes, cyclometalated iridium(III) complexes are quenching processes, the synthesis of dopants with excellent
electron mobility is essential to gain efficient phosphorescent
OLEDs.4
aState Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of
Advanced Organic Materials, Collaborative Innovation Center of Advanced
Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, P. R. China. E-mail: yxzheng@nju.edu.cn
As is well known, the introduction of a nitrogen heterocycle
in the ligands would enhance the electron mobility of Ir(III)
complexes.5 Meanwhile, OLEDs based on Ir(III) complexes with
1,3,4-oxadiazole derivatives as the ancillary ligand always
exhibit good performances due to their high electron mobility,
high photoluminescence quantum yield and good thermal
and chemical stabilities.6 Besides that, bulky trifluoromethyl
(–CF3) substituents can affect the molecular packing and the
steric protection surrounding the metal would restrain the
bMaAnShan High-Tech Research Institute of Nanjing University, MaAnShan, 238200,
P. R. China
†Electronic supplementary information (ESI) available: The transient EL signals
for the device structure of ITO/TAPC (50 nm)/Ir complexes (60 nm)/LiF (1 nm)/Al
(100 nm) under different applied fields of Ir(tfmphpm)2(pop) and Ir
(tfmppm)2(pop). The crystallographic data, selected bonds and angles of com-
plexes Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop). CCDC 1830696 and 1830699.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt02961j
self-quenching impact, and the C–F bond with
vibrational frequency can reduce the radiationless deactivation
a low
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Scheme 1 The synthetic routes of ligands and the complexes.
rate.7 On this basis, as shown in Scheme 1, two new hetero- 2H), 7.99 (s, 1H), 7.32 (t, J = 4.8 Hz, 1H). 19F NMR (377 MHz,
leptic Ir(III) complexes with 2-(3,5-bis(trifluoromethyl)phenyl) DMSO) δ −61.68 (s). 13C NMR (101 MHz, DMSO) δ 160.65 (s),
pyrimidine (tfmphpm) and 2-(2,6-bis(trifluoromethyl)pyridin- 158.61 (s), 139.94 (s), 131.40 (q, J = 33.1 Hz, 3H), 127.99 (q, J =
4-yl) pyrimidine (tfmppm) as cyclometalating ligands and 3.3 Hz), 124.67 (m), 123.65 (q, J = 273.71 Hz), 121.83 (s).
2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol (pop) as the ancillary
ligand were synthesized and investigated.
Synthesis of 2-(2,6-bis(trifluoromethyl)pyridin-4-yl) pyrimi-
dine (tfmppm). A stirred solution of 2,6-bis-(trifluoromethyl)
Two complexes show green emissions with peaks at 502 pyridine (0.22 g, 10 mmol) in diethyl ether (20 mL) was cooled
and 505 nm with photoluminescence quantum efficiencies of to −78 °C. LDA (lithium diisopropylamide, 6 mL, 10 mmol)
87% and 94%, respectively. They also have higher electron was added over 20 min and stirred for 1 h, and then B(OPr-i)3
mobility than the widely used electron transport material of (2.89 mL, 12.4 mmol) was added. The mixture was warmed to
Alq3 (aluminum 8-hydroxyquinolinate). Therefore, for the device room temperature and stirred for another 1 h. The pH was
G1 based on Ir(tfmphpm)2(pop), a maximum current efficiency adjusted to 10 by the slow addition of 10% aqueous NaOH
(ηc,max) of 59.82 cd A−1 and a maximum external quantum solution (20 mL). After 1 hour, the organic phase was acidified
efficiency (EQEmax) of 27.3% were obtained while the device G2 to pH = 4 by the dropwise addition of 3 N HCl. The extraction
using Ir(tfmppm)2(pop) as the emitter shows better EL perform- with ethyl acetate and evaporation of the organic phase
ances with a ηc,max and an EQEmax up to 92.79 cd A−1 and gave the corresponding crude aryl boronic acids.
31.8%, respectively, and the efficiency roll-off is rather low.
2-Chloropyrimidine (10 mmol, 1.15 g), bis(diphenylpho-
sphion)ferrocene palladium(II) dichloride (0.3 mmol, 0.22 g)
and boronic acids (12 mmol, 3.52 g) were added into 50 mL
THF. After 20 mL of aqueous 2 N K2CO3 was added, the
reaction mixture was heated at 70 °C for 1 day. The mixture
was poured into water and extracted with CH2Cl2 (10 mL × 3
Experimental section
Syntheses
All the reagents used were of commercial grade. The ligands times). Finally, silica column purification (PE : EA = 10 : 1) gave
and complexes were synthesized under a nitrogen atmosphere 2-(2,6-bis(trifluoromethyl)pyridin-4-yl) pyrimidine in 71% yield
+
and the synthetic routes are listed in Scheme 1. The pop (7.1 mmol, 2.10 g). MS (ESI): calcd for M+ (C11H5F6N3 ) m/z =
1
ligand was prepared according to our former publications.6
293.17, found 294.19. H NMR (400 MHz, CDCl3) δ 8.94 (d, J =
Synthesis of 2-(3,5-bis(trifluoromethyl)phenyl) pyrimidine 4.9 Hz, 2H), 8.93 (s, 2H), 7.43 (t, J = 4.9 Hz, 1H). 19F NMR
(tfmphpm). 2-Chloropyrimidine (10 mmol, 1.15 g), bis(diphe- (377 MHz, DMSO) δ −66.88 (s). 13C NMR (101 MHz, DMSO)
nylphosphion) ferrocene palladium(II) dichloride (0.3 mmol, δ 159.38 (s), 158.92 (s), 149.84 (s), 148.50 (q, J = 35.4 Hz),
0.22 g) and (3,5-bis(trifluoromethyl)phenyl)boronic acid 123.15 (s), 122.04 (q, J = 2.1 Hz), 121.76 (q, J = 275.73 Hz).
(12 mmol, 3.10 g) were added into 50 mL THF. After 20 mL of
General syntheses of Ir(III) complexes. A mixture of IrCl3
aqueous 2 N K2CO3 was added, the reaction mixture was (1 mmol, 0.30 g) and tfmphpm/tfmppm (2.5 mmol, 0.73 g) in
heated at 70 °C for 1 day. The mixture was poured into water 2-ethoxyethanol and water (20 mL, 3 : 1, v/v) was refluxed for
and extracted with CH2Cl2 (10 mL × 3 times). Finally, silica 24 h. After cooling, the solid precipitate was filtered to give the
column purification (PE : EA = 10 : 1) gave 2-(3,5-bis(trifluoro- crude cyclometalated Ir(III) chloro-bridged dimer. Then, the
methyl)phenyl)pyrimidine in 89% yield (8.9 mmol, 2.61 g). MS slurry of the crude chloro-bridged dimer (0.2 mmol, 0.32 g)
(ESI): calcd for M+ (C12H6F6N2+) m/z = 292.18, found 293.17. and pop potassium salt (0.5 mmol, 0.14 g) in 2-ethoxyethanol
1H NMR (400 MHz, CDCl3) δ 8.97 (s, 2H), 8.88 (d, J = 4.9 Hz, (20 mL) was refluxed for 24 h. The solvent was evaporated and
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the mixture was poured into water, extracted with CH2Cl2, and
then chromatographed to give complexes Ir(tfmphpm)2(pop)
(0.30 mmol, 0.31 g) and Ir(tfmppm)2(pop) (0.28 mmol, 0.29 g),
respectively, which were further purified by sublimation under
vacuum (4.5 × 105 Pa, 260 °C).
Ir(tfmphpm)2(pop). 76% yield. 1H NMR (400 MHz, CDCl3)
δ 8.76 (d, J = 5.8 Hz, 1H), 8.72 (dd, J = 4.7, 2.1 Hz, 3H), 8.64 (d,
J = 1.6 Hz, 1H), 7.99–7.95 (m, 1H), 7.74–7.71 (m, 2H), 7.69 (d,
J = 3.7 Hz, 2H), 7.59 (dd, J = 8.1, 1.8 Hz, 1H), 7.53 (ddd, J = 6.5,
3.8, 1.3 Hz, 1H), 7.45 (dd, J = 10.3, 4.6 Hz, 2H), 7.17 (ddd, J =
8.7, 6.9, 1.8 Hz, 1H), 7.03 (dd, J = 5.8, 4.8 Hz, 1H), 6.86 (dd, J =
5.9, 4.7 Hz, 1H), 6.66 (dd, J = 8.7, 0.6 Hz, 1H), 6.52 (ddd, J =
8.0, 6.9, 1.0 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 175.24,
174.59, 167.06, 163.29, 159.29, 157.98, 157.61, 156.93, 156.43,
148.97, 135.91, 134.57, 132.63, 129.23, 127.12, 126.78, 125.15,
122.62, 122.13, 117.78, 117.26, 115.25, 106.20. 19F NMR
(377 MHz, DMSO) δ −58.16 (s), −58.90 (s), −60.94 (s),
−61.13 (s). MS (ESI) m/z calcd for C38H19F12IrN6O2: 1011.81
[M]+, found 1012.93 [M + H]+. Anal. calcd For C38H19F12IrN6O2:
C 45.11, H 1.89, N 8.31. Found: C 45.08, H 2.16, N 8.39.
Fig. 1 Oak Ridge Thermal Ellipsoidal Plot (ORTEP) diagram of Ir
(tfmphpm)2(pop) (CCDC No. 1830696†) and Ir(tfmppm)2(pop) (CCDC
No. 1830699†). Hydrogen atoms are omitted for clarity. Ellipsoids are
drawn at the 30% probability level.
2.117(3) Å. These results are similar to the parameters of the
cyclometalated Ir(III) complexes that have been reported.
Thermal stability
Good thermal stability of the materials is a necessary prerequi-
site for efficient and stable OLEDs. If a complex can be applied
in practical OLEDs, the decomposition temperature (Td) needs
to be high enough to guarantee that the complex could be de-
posited onto the solid face without any decomposition on sub-
limation. The thermogravimetric analysis (TGA) curves of Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) are shown in Fig. 2. It
can be figured out that there is no loss observed below 290 °C
in weight from the curves of TGA for Ir(tfmphpm)2(pop) and
the decomposition temperature (5% loss of weight) is 312 °C
for Ir(tfmphpm)2(pop). However, there is no loss in weight
observed below 320 °C from the curves of TGA for
Ir(tfmppm)2(pop), and the decomposition temperature (5%
loss of weight) improves to 354 °C for Ir(tfmppm)2(pop). The
good thermal stability suggests that both complexes have
potential application in OLEDs.
Ir(tfmppm)2(pop). 71% yield. 1H NMR (400 MHz, CDCl3)
δ 8.88–8.79 (m, 3H), 8.60 (s, 1H), 8.52 (s, 1H), 8.13–8.05 (m,
1H), 7.76–7.69 (m, 2H), 7.61 (dd, J = 8.1, 1.7 Hz, 1H), 7.59–7.52
(m, 1H), 7.47 (t, J = 7.5 Hz, 2H), 7.21 (qd, J = 3.4, 1.9 Hz, 2H),
7.05 (dd, J = 5.9, 4.8 Hz, 1H), 6.67 (d, J = 8.2 Hz, 1H), 6.61–6.51
(m, 1H). 13C NMR (101 MHz, acetone) δ 174.97, 174.25, 168.08,
165.42, 161.05, 160.62, 160.22, 159.03, 153.47, 146.87, 142.01,
136.23, 134.36, 131.04, 129.56, 128.38, 125.88, 125.36, 124.37,
122.57, 122.01, 120.40, 119.73, 117.29, 108.55. 19F NMR
(377 MHz, DMSO) δ −62.74 (s), −63.42 (s), −66.20 (s), −66.37
(s). MS (ESI) m/z calcd for C36H17F12IrN8O2: 1013.78 [M]+,
found 1014.89 [M + H]+. Anal. calcd For C36H17F12IrN8O2: C
42.65, H 1.69, N 11.05. Found: C 42.49, H 1.99, N 11.34.
Results and discussion
Preparation and characterization of compounds
Electrochemical properties and theoretical calculation
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of the
dopants will guide in designing the structure of the OLED.
Cyclic voltammetry was used to test the oxidation peak poten-
Scheme
1
shows the synthetic route to ligands, Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) complexes. Both the
new compounds were further purified by vacuum sublimation
and fully characterized by 1H NMR spectrometry, and the
crystal structures further confirmed the identity of the
complexes.
Single crystals of Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop)
were obtained by vacuum sublimation and the crystal dia-
grams are displayed in Fig. 1. The molecular parameters and
atomic coordinates are shown in Table S1.† From the structure
diagrams of crystals it can be found that the iridium atom is
embraced by C, N and O atoms from tfmphpm/tfmppm and
pop, with
a twisted octahedral coordination geometry.
For Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop), the angles of
[N–Ir–O] are 85.8(2)°–86.78(12)°, and the angles of [C–Ir–N] are
80.2(2)°–81.38(14)°. The lengths of Ir–C bonds range from
2.034(4) to 2.064(4) Å. The Ir–N bonds have the lengths of
Fig. 2 The TGA curves of Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop) (the
2.026(4)–2.107(3) Å. The lengths of Ir–O bonds are 2.105 Å– heating rate is 20 °C per minute).
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Fig. 3 (a) The cyclic voltammogram curves and (b) contour plots of Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop).
Fig. 4 (a) The UV-vis absorption and (b) emission spectra of Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) complexes in degassed CH2Cl2
solutions (5.0 × 10−5 mol L−1) at room temperature.
tial (Eox) of Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop). The
HOMO levels were calculated from the Eox and the band gaps
(Eg), which were estimated from the UV-vis absorption edges.8
Then the LUMO levels were determined according to the
equation LUMO = HOMO + Eg. The oxidation potentials of Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) (Fig. 3(a)) were found
to be 0.97 V and 1.12 V, respectively, which can be ascribed to
the metal-centered Ir(III)/Ir(IV) oxide couple, consistent with the
cyclometalated Ir(III) system reported.9 However, the reduction
peaks of Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop) are not
obvious, demonstrating that the redox process is not completely
reversible, which is also observed in related Ir(III) complexes
containing oxadiazole units.10 The HOMO/LUMO energy levels
are calculated as −5.60/−3.13 eV and −5.67/−3.23 eV, respect-
ively, for Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop). With a
lower LUMO level which can benefit the trapping of electrons
and broaden the recombination zone, the device based on Ir
(tfmppm)2(pop) would show better performances. What’s more,
the difference between the Eg of the complexes, which contrib-
utes to the variations of the PL spectra, is small. Thus, the intro-
duction of the special one more nitrogen atom has almost no
effect on the emission spectra for the two complexes.
To provide deep understanding about the nature of the
excited states of these complexes, the density functional theory
(DFT) calculations11 for both were conducted employing the
Gaussian09 software with the B3LYP functional.12 Plots of the
HOMO/LUMO and the molecular orbital energy levels are pre-
sented in Fig. 3(b). The basis set used for C, H, N, O and F
atoms was 6-31G(d, p) while the LanL2DZ basis set was
employed for iridium atoms.13 The solvent effect of CH2Cl2
was taken into consideration using a conductor like polariz-
able continuum model (C-PCM). It can be observed from the
theoretical calculation that nearly all LUMOs are on the cyclo-
metalating ligands (90.57/94.84%) and HOMOs are mostly situ-
ated on the ancillary ligand (80.62/81.78%). The rising compo-
sitions of HOMOs on the ancillary ligand make the electro-
chemical oxidation processes occur on both metal centered
orbitals and the ancillary ligand, leading to the irreversible
redox processes of the Ir(III) complexes.
The absorption spectra of the two complexes show broad and
intense bands below 340 nm, assigned to the spin-allowed
intraligand LC (π → π*) transitions of tfmphpm/tfmppm and
1
pop ligands. The weak bands lasting to 520 nm can be assigned
to spin-allowed metal–ligand charge transfer (1MLCT) and
spin forbidden 3MLCT transition bands caused by the large
spin orbital coupling (SOC) that was introduced by the Ir(III)
center.14 The strongest emission peaks at 494 nm and 506 nm
in CH2Cl2, respectively, were produced by the electronic tran-
sition between the lowest triplet excited state and the ground
state, which make Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop)
green phosphors. From Fig. S1 (ESI†) and Table 1, it can be
seen that the PL spectra of Ir(tfmphpm)2(pop) and
Ir(tfmppm)2(pop) at 77 K become structured compared with
those at room temperature. In general, the emission bands
from the MLCT states are broad and featureless, whereas a
highly structured emission band mainly originates from the
3π–π* state. Accordingly, the complexes Ir(tfmphpm)2(pop) and
Ir(tfmppm)2(pop) emit from a mixture of MLCT states and the
3
ligand-based π–π* state. This indicates that the MLCT charac-
ters 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. Although the
structures are different, the emissions of Ir(tfmphpm)2(pop)
and Ir(tfmppm)2(pop) are analogous, which are consistent
with their electrochemical results.
Both complexes show high quantum efficiencies up to 0.87
and 0.94 for Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop), respect-
ively, suggesting that they are potential emitters for efficient
devices. Furthermore, the phosphorescence lifetime (τ) is the
crucial factor that determines the rate of triplet–triplet annihil-
ation in OLEDs. Longer τ values usually cause greater triplet–
triplet annihilation. The lifetimes of the complexes Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) are in the range of
microseconds (1.00 and 0.93 μs in CH2Cl2 solution, respectively)
at room temperature (Fig. S2† and Table 1) and are indicative of
the phosphorescence origin for the excited states in each case.
Photophysical properties
Electron mobility
The UV-vis absorption and photoluminescence spectra of two
complexes Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop) in An excellent electron mobility of the emitter could benefit elec-
CH2Cl2 (5 × 10−5 M) at room temperature are shown in Fig. 4. tron transport, which would balance the injection and trans-
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Table 1 Photophysical data of Ir(III) complexes
λem c (nm)
Complex
Td a (°C)
λabs b (nm)
298 K
77 K
Φd (%)
τe (μs)
HOMO/LUMOf (eV)
Ir(tfmphpm)2(pop)
Ir(tfmppm)2(pop)
312
354
250/393
238/391
494
506
486/507/521
512
87
94
1.00
0.93
−5.60/−3.13
−5.67/−3.23
a Td: decomposition temperature. b Measured in degassed CH2Cl2 solution at a concentration of 5 × 10−5 mol L−1 at room temperature.
c Measured in degassed CH2Cl2 solution at a concentration of 5 × 10−5 mol L−1 at 298 K and 77 K, respectively. d Emission quantum yields were
measured relative to Ir(ppy)3 (Φ = 0.4) in degassed CH2Cl2 solution at room temperature. e Measured in degassed CH2Cl2 solution at a concen-
tration of 5 × 10−5 mol L−1 at room temperature. f 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 CH3CN solution.
port of electrons and holes, in favour of the enhancement of OLED performance
the device efficiency.15 To determine their electron
mobility values, the transient electroluminescence (TEL)
measurement was carried out via the devices with a structure
of ITO (indium–tin–oxide)/TAPC (1,1-bis[4-[N,N-di(p-tolyl)
To evaluate the EL performances of the two complexes, single
emitting layer (EML) devices named G1 and G2 using Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) as the emitters,
respectively, were fabricated with the structure of ITO/MoO3
amino]pyridin-4-yl] cyclohexane, 50 nm)/Ir(III) complexes
(molybdenum oxide,
5 nm)/TAPC (30 nm)/Ir complexes
(60 nm)/LiF (1 nm)/Al (100 nm).16 The Ir(III) complexes act as
not only the emissive layers (EML) but also electron-transport
layers. The experimental results (Fig. 5) indicated that the
electron mobility values of Ir(tfmphpm)2(pop) and
Ir(tfmppm)2(pop) are 6.05–6.21 × 10−6 cm2 V−1 s−1 and
(x wt%) : PPO21 (10 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al
(100 nm). MoO3 and LiF served as hole- and electron-injection
interface modified materials, respectively. TAPC possessing
high hole mobility (1 × 10−2 cm2 V−1 s−1) and a high-lying
LUMO level (−2.0 eV) was used as the hole transport/electron
block layer (HTL/EBL), while TmPyPB with high electron mobi-
lity (1 × 10−3 cm2 V−1 s−1) and a low-lying HOMO level
(−6.7 eV) was used as an electron transport/hole block layer
(ETL/HBL). The chemical structures of the materials
mentioned above as well as the device structure and energy
level diagrams are depicted in Fig. 6.
Theoretically speaking, the stepwise HOMO levels of TAPC
(−5.50 eV), mCP (−5.90 eV) and PPO21 (−6.21 eV) are ben-
eficial for the injection and transport of holes, while the step-
wise LUMO levels of TmPyPB (−2.70 eV) and PPO21 (−2.68 eV)
are beneficial for the injection and transport of electrons.
Therefore, balanced distribution of carriers (holes and
electrons) and a wide recombination zone could be expected.
More importantly, the LUMO level of TAPC is 0.3 eV higher
than that of mCP which was added to lower the HOMO energy
barrier between TAPC and PPO21. The HOMO level of TmPyPB
6.67–6.90 × 10−6 cm2 V−1
field from 1040 (V cm−1 1/2
that of the electron transport material Alq3 (aluminum
8-hydroxyquinolinate, 4.74–4.87
10−6 cm2 V−1 −1).17
s
−1, respectively, under the electric
)
to 1300 (V cm
)
−1 1/2, higher than
×
s
Additionally, the electron mobility of Ir(tfmppm)2(pop) is
higher than that of Ir(tfmphpm)2(pop) by introducing one
more nitrogen heterocycle to the cyclometalating ligand. These
results exactly fit our design intended to improve the electron
transport ability of the emitter. The good electron transport
ability of Ir(tfmphpm)2(pop) and Ir(tfmppm)2(pop) will
reinforce the recombination chance of electrons and holes, so
that their devices may show good performances, especially for
the device based on Ir(tfmppm)2(pop).
Fig. 6 Energy level diagram of HOMO and LUMO levels (relative to the
vacuum level) for materials investigated in this work and their molecular
structures.
Fig. 5 Electric field dependence of charge electron mobility in the thin
films of Ir(tfmphpm)2(pop), Ir(tfmppm)2(pop) and Alq3.
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Fig. 7 Characteristics of devices G1 and G2: (a) electroluminescence spectra at 8 V; (b) luminance–voltage–current density (L–V–J) curves; (c)
current efficiency–luminance (ηc–L) curves; and (d) power efficiency–luminance (ηp–L) curves.
is 0.49 eV lower than that of PPO21. Thus, holes and electrons
From Fig. 7 and Table 2, it can be found that the devices
are well confined within EMLs and the triplet exciton based on both complexes with pop as an ancillary ligand show
quenching of the dopants will be effectively avoided. In the good performances with low roll-off. For device G1,
devices, the optimized Ir(III) complex doping concentrations maximum current efficiency (ηc,max
of 59.82 cd A−1
are
Ir(tfmppm)2(pop).
a
a
)
,
6
wt% for both complexes Ir(tfmphpm)2(pop) and maximum external quantum efficiency (EQEmax) of 27.3%, a
maximum power efficiency (ηp,max) of 22.91 lm W−1 and a
The EL spectra, luminance–voltage–current density (L–V–J) maximum luminance (Lmax) of 15 003 cd m−2 were obtained,
curves, current efficiency–luminance (ηc–L) curves, and power while device G2 using Ir(tfmppm)2(pop) as the emitter shows
efficiency–luminance (ηp–L) curves for G1 and G2 are shown in better EL performances with a Lmax, a ηc,max, a ηp,max and an
Fig. 7, and the crucial EL data are shown in Table 2. The peaks EQEmax up to 48 981 cd m−2, 92.79 cd A−1, 39.46 lm W−1 and
of EL emission are 478 nm and 499 nm for G1 and G2, respect- 31.8%, respectively. Moreover, G2 can retain its high efficiency
ively, which are almost invariant of the current density and even at relatively high luminance and the EL efficiency roll-off is
there is no dependence on doped concentrations. From rather low. For instance, the current efficiencies for the device
Fig. 7(a), it can be seen that the EL spectra are almost identical G2 still remain as high as 89.20 cd A−1 at the brightness of 1000
to the PL spectra of the complexes, suggesting that the EL cd m−2 and 63.53 cd A−1 at the brightness of 10 000 cd m−2
emissions of the devices arise from the triplet excited states of respectively.
,
the phosphors. The Commission Internationale de 1′Eclairage
Considering the difference between the structures of both
(CIE) color coordinates of G1 and G2 are (0.152 and 0.384) and emitters, the introduction of one more nitrogen heterocycle to
(0.210 and 0.560), respectively.
the cyclometalating ligand does affect the nature of Ir
Table 2 EL performances of devices G1 and G2
Lmax b (cd m−2
)
ηc,max c (cd A−1) (EQEmax
)
ηc,L1000 e (cd A−1) (EQEL1000
)
ηp,max g (lm W−1
)
CIEh (x,y)
d
f
Device
Vturn-on a (V)
G1
G2
4.1
4.3
15 003
48 981
59.82 (27.3%)
92.79 (31.8%)
52.56 (24.0%)
89.20 (30.5%)
22.91
39.46
0.152, 0.384
0.210, 0.560
a Turn-on voltage recorded at a luminance of 1 cd m−2 b Maximum luminance. c Maximum current efficiency. d Maximum external quantum
.
efficiency (EQEmax). e Current efficiency at 1000 cd m−2
l’Eclairage coordinates (CIE).
.
f EQE at 1000 cd m−2
.
g Maximum power efficiency. h Commission Internationale de
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of electrons, which would broaden the recombination zone
and balance the distribution of holes and electrons, particu-
larly at a high doping concentration, leading to the suppressed
triplet–triplet annihilation (TTA) and triplet-polaron annihil-
ation (TPA) effects, resulting in excellent EL performances,
consistent with the design intention.
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Conclusions
In conclusion, two bis-cyclometalated Ir(III) complexes Ir
(tfmphpm)2(pop) and Ir(tfmppm)2(pop) with 2-(3,5-bis(tri-
fluoromethyl)phenyl)pyrimidine (tfmphpm) and 2-(2,6-bis(tri-
fluoromethyl)pyridin-4-yl)pyrimidine (tfmppm) as cyclometa-
lating ligands and 2-(5-phenyl-1,3,4-oxadiazol-2-yl)phenol
(pop) as the ancillary ligand were reported. Both complexes
emit green phosphorescence with high quantum efficiencies
up to 94%. By introducing one more nitrogen heterocycle to
the cyclometalating ligand, Ir(tfmppm)2(pop) attains better
electron mobility and a lower LUMO level, which would facili-
tate the injection and transport of electrons to broaden the
recombination zone and balance the distribution of holes and
electrons. As a result, device G2 based on Ir(tfmppm)2(pop) as
the emitter shows better EL performances with the maximum
luminance, current efficiency, power efficiency and external
quantum efficiency up to 48 981 cd m−2, 92.79 cd A−1, 39.46
lm W−1 and 31.8%, respectively. Moreover, the ηc and EQE of
the device G2 can still be retained as high as 89.20 cd A−1 and
30.5%, respectively, at the practical luminance of 1000 cd m−2
.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51773088), the Natural Science
Foundation of Jiangsu Province (BY2016075-02) and the
Fundamental Research Funds for the Central Universities
(020514380131).
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