Efficient red phosphorescent Ir(iii) complexes based on rigid ligands with high external quantum efficiency and low efficiency roll-off

Article abstract of DOI:10.1039/c9tc06940b

The rigid ligands in red phosphorescent Ir(iii) complexes can suppress the vibration and rotation around metal ions to maximize the utilization of low energy singlet and triplet states, thus greatly reducing the probability of non-radiative decay. In this paper, the coordination groups (phenyl and quinolinyl) are fused by a cyclohexyl group to produce rigidified ligands (5,6-dihydro-benzo[c]acridine and 9-fluoro-5,6-dihydro-benzo[c]acridine). A bulky ancillary ligand (2,2,6,6-tetramethylheptane-3,5-dione) was applied for the synthesis of Ir(iii) complexes (Ir-DHBA and Ir-F-DHBA) to minimize the intimate interactions between Ir(iii) molecules and consequently to reduce triplet-triplet annihilation and triplet-polaron annihilation. When the two Ir(iii) complexes were applied as dopants in organic light-emitting diode (OLED) devices, they both showed excellent electroluminescence (EL) performance. The device based on Ir-DHBA demonstrated a low efficiency roll-off with a maximum external quantum efficiency (EQE) of 26.0%, 25.3% EQE at a brightness of 1000 cd m-2 and 22.7% EQE at 10?000 cd m-2. The device based on Ir-F-DHBA (at a doping ratio of 3%) with an exciplex as a co-host showed a maximum EQE of over 28% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.61, 0.39) and an EL emission peak at 600 nm.

Full text of DOI:10.1039/c9tc06940b

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Efficient red phosphorescent Ir(III) complexes
based on rigid ligands with high external
Cite this:DOI: 10.1039/c9tc06940b
quantum efficiency and low efficiency roll-off†
b
Sheng-Nan Liu,‡a Kai-Ning Tong,‡a Yue Zhao,
Jin-Feng Cheng,a
ac
ac
Man-Keung Fung
*
and Jian Fan
*
The rigid ligands in red phosphorescent Ir(III) complexes can suppress the vibration and rotation around
metal ions to maximize the utilization of low energy singlet and triplet states, thus greatly reducing the
probability of non-radiative decay. In this paper, the coordination groups (phenyl and quinolinyl) are
fused by a cyclohexyl group to produce rigidified ligands (5,6-dihydro-benzo[c]acridine and 9-fluoro-
5,6-dihydro-benzo[c]acridine).
A bulky ancillary ligand (2,2,6,6-tetramethylheptane-3,5-dione) was
applied for the synthesis of Ir(III) complexes (Ir-DHBA and Ir-F-DHBA) to minimize the intimate
interactions between Ir(III) molecules and consequently to reduce triplet–triplet annihilation and triplet-
polaron annihilation. When the two Ir(III) complexes were applied as dopants in organic light-emitting
diode (OLED) devices, they both showed excellent electroluminescence (EL) performance. The device
based on Ir-DHBA demonstrated a low efficiency roll-off with a maximum external quantum efficiency
(EQE) of 26.0%, 25.3% EQE at a brightness of 1000 cd mꢀ2 and 22.7% EQE at 10 000 cd mꢀ2. The device
based on Ir-F-DHBA (at a doping ratio of 3%) with an exciplex as a co-host showed a maximum EQE of
over 28% with Commission Internationale de l’Eclairage (CIE) coordinates of (0.61, 0.39) and an EL
emission peak at 600 nm.
Received 19th December 2019,
Accepted 19th March 2020
DOI: 10.1039/c9tc06940b
rsc.li/materials-c
research community.3 The last thirty years have witnessed an
accelerated development of emissive materials from fluores-
Introduction
Since C. W. Tang and S. A. VanSlyke invented OLEDs in 1987,1 cent emitters4 to phosphorescent complexes5 to thermally
there is significant progress in the understanding of its device activated delayed fluorescent (TADF) molecules,6 which is
physics and the related practical application from PMOLED mainly stimulated by the demand for efficient and reliable
(passive matrix organic light emitting diode) panels to foldable/ OLED device performance. Although the recent advances in the
flexible displays and lighting.2 It is worth noting that OLED design, synthesis and application of TADF materials in
materials play an important role in the development of OLED OLEDs,7–9 particularly in TADF-sensitizing-fluorescent OLEDs
technology. As compared with hole/electron-injecting and hole/ (TSF-OLEDs),10 have boosted interest in this topic, fluorescent
electron-transporting materials, the light-emitting materials in compounds and phosphorescent complexes have been exten-
the emission layer (EML) have received great attention in both sively studied and widely used by now. Phosphors such as
the information display/lighting industries and the academic Ir(III)11 and Pt(II) complexes12 can harvest both singlet and
triplet excitons to achieve 100% internal quantum efficiency,
among which Ir(III) complexes have been the mainstream
a Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory
emitters due to their high photoluminescence quantum yield
(PLQY), short radiative lifetime, and good stability.13
for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,
Jiangsu 215123, China. E-mail: mkfung@suda.edu.cn, jianfan@suda.edu.cn
So far many red phosphorescent Ir(III) complexes have been
reported with high electroluminescence performance.14–16 For
example, by employing a thianthrene 5,5,10,10-tetraoxide
moiety, Wu and Zhou demonstrated an efficient deep-red
OLED (lEL = 640 nm) with a peak EQE of 25.8%.17 By using
Ir–S–P–S four membered ring structures, Zheng et al. reported
red phosphorescent emitters with a maximum current effi-
ciency (CE) of 57.98 cd Aꢀ1 and a maximum EQE of 24.9%.18
b Coordination Chemistry Institute, State Key Laboratory of Coordination
Chemistry, School of Chemistry and Chemical Engineering, Nanjing National
Laboratory of Microstructures, Nanjing University, Nanjing 210023, China
c Institute of Organic Optoelectronics (IOO) of the Jiangsu Industrial Technology
Research Institute (JITRI), Suzhou, Jiangsu 215212, China
† Electronic supplementary information (ESI) available. CCDC 1971605 and
1971475. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9tc06940b
‡ These two authors contributed equally to this work.
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Scheme 1 Synthetic routes of ligands and Ir(III) complexes.
Red emitters generally suffer from non-emissive decay since the spectrophotometer. Matrix-assisted laser desorption/ionization
radiative and non-radiative rate constants strongly depend on time of flight mass spectrometry (MALDI-TOF-MS) was per-
the energy gap between the excited state and the ground state. formed on a Bruker ultrafleXtreme MALDI-TOF spectrometer.
Herein, we used the rigid ligand to reduce the non-radiative Thermogravimetric analysis (TGA) was carried out on a TA SDT
decay, where the coordination motifs, the phenyl group and 2960 instrument at a heating rate of 10 1C minꢀ1 under
the quinolinyl group, were fused together by a cyclohexane. In nitrogen. Cyclic voltammetry (CV) was carried out on a CHI600
Ir-F-DHBA, a fluorine atom was introduced to tune its frontier voltammetric analyzer at room temperature with ferrocenium–
orbital energy levels and the emission wavelength. In order to ferrocene (Fc/Fc+) as the internal standard at a scan rate of
reduce the self-quenching effect, alkyl groups were generally 50 mV sꢀ1. The three-electrode configuration consisted of a Pt-
incorporated into the chromatic ligand and the ancillary ligands wire electrode (counter electrode), an Ag/AgCl electrode (reference
in Ir(III) complexes. For example, 2,2,6,6-tetramethylheptane-3,5- electrode) and a platinum electrode (working electrode). Degassed
dione19 and 3,7-diethyl-nonane-4,6-dione20 are often used as DMF was used as a solvent with tetrabutylammonium hexafluor-
non-chromatic ancillary ligands in red Ir(III) emitters. So the rigid ophosphate (TBAPF6) (0.1 M) as the supporting electrolyte.
C^N cyclometalating ligands and bulky ancillary ligand are com-
Synthesis of compound 1. A mixture of 3,4-dihydronaphthalen-
bined together in Ir-DHBA and Ir-F-DHBA to enhance the device 1(2H)-one (600 mg, 4.104 mmol) and 2-nitrobenzaldehyde
performance. In addition, an exciplex was used as a co-host for (682 mg, 4.515 mmol) in H2O (10 mL) was stirred at 0 1C for a
device fabrication to reduce the driving voltage and to balance the few seconds, then triethylamine (2.5 mL) was added dropwise.
charge carrier transport in the emissive region, which could lead The reaction was monitored by TLC. After the completion of the
to the enhancement in efficiency.21 When the two complexes were reaction, cold water (20 mL) and dichloromethane (100 mL) were
used as dopants, all devices showed remarkable EL performances. added into the mixture. The organic phase was concentrated
The OLEDs based on Ir-DHBA exhibited a peak EQE, CE, and under reduced pressure. The crude product was further purified
power efficiency (PE) of 26.0%, 46.7 cd Aꢀ1, and 61.5 lm Wꢀ1
,
by column chromatography (PE : EA = 6 : 1, v/v) and was used
respectively. Furthermore, the device showed low efficiency roll- directly in the next step (708 mg, yield = 58%).
off with an EQE of 25.3% at 1000 cd mꢀ2 and 22.7% at a high
Synthesis of compound 2. Compound 2 was prepared using
luminance of 10 000 cd mꢀ2. When Ir-F-DHBA was dispersed in a the same procedure as that of compound 1 (light brown solid,
co-host, the maximum EQE, CE and PE were 28.2%, 51.4 cd Aꢀ1
and 63.2 lm Wꢀ1, respectively.
Experiment
General information
All the chemicals and solvents were purchased commercially and
were used without further purification. The 1H NMR and 19F NMR
spectra were acquired on a Bruker AVANCE III HD 400 NMR
spectrometer at room temperature. UV-vis absorption spectra
were recorded on a PerkinElmer Lambda 750 spectrophotometer.
PL spectra were recorded on a Hitachi F-4600 fluorescence
Fig. 1 Crystal structures of compounds Ir-DHBA (left, CCDC 1971605)
and Ir-F-DHBA (right, CCDC 1971475).†
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concentrated under reduced pressure. The crude product was
further purified by column chromatography (DCM) to afford a
pale yellow solid (450 mg, yield = 89%). 1H NMR (400 MHz,
CDCl3) d 8.57 (d, J = 7.5 Hz, 1H), 8.13 (d, J = 8.4 Hz, 1H), 7.91
(s, 1H), 7.73 (d, J = 8.0 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H), 7.50–7.40
(m, 2H), 7.36 (t, J = 7.2 Hz, 1H), 7.27 (d, J = 7.3 Hz, 1H), 3.15–
3.08 (m, 2H), 3.04–2.96 (m, 2H).
Synthesis of compound 4. Compound 4 was prepared with
compound 2 as the precursor (white solid, yield = 90%).
1H NMR (400 MHz, CDCl3) d 8.54 (d, J = 7.4 Hz, 1H), 8.12
(dd, J = 9.1, 5.4 Hz, 1H), 7.86 (s, 1H), 7.53–7.31 (m, 4H), 7.33–
7.24 (m, 1H), 3.21–3.06 (m, 2H), 3.08–2.93 (m, 2H).
Synthesis of Ir-DHBA. Under a nitrogen atmosphere, com-
pound 3 (400 mg, 1.729 mmol) and IrCl3ꢂnH2O (249 mg,
0.786 mmol) were mixed in 2-ethoxyethanol and water
(80 mL, v/v = 3 : 1). The reaction mixture was heated to reflux
for 2 days. A red solid was collected by filtration. This crude
chloro-bridged dimer, 2,2,6,6-tetramethyl-heptane-3,5-dione
(326 mg, 1.769 mmol), and sodium carbonate (417 mg,
3.933 mmol) in 2-ethoxyethanol (20 mL) were refluxed for
2 days under nitrogen conditions. After the removal of solvent,
the residue was extracted by dichloromethane (3 ꢁ 100 mL).
The organic phase was washed with water and dried over
anhydrous sodium sulfate. The crude product was purified by
silica column chromatography (PE : EA = 12 : 1, v/v) to afford a
red solid (510 mg, yield = 35%). 1H NMR (400 MHz, CDCl3)
d 8.29 (d, J = 8.7 Hz, 2H), 7.83 (s, 2H), 7.67 (d, J = 7.9 Hz, 2H),
7.38 (t, J = 7.4 Hz, 2H), 7.31–7.20 (m, 2H), 6.66 (d, J = 7.1 Hz,
2H), 6.57 (t, J = 7.4 Hz, 2H), 6.48 (d, J = 7.5 Hz, 2H), 4.93 (s, 1H),
3.19 (dd, J = 48.2, 37.3 Hz, 8H), 0.64 (s, 18H). MALDI-TOF-MS:
m/z: calcd for C45H43IrN2O2: 836.2954, found: 836.2159.
Synthesis of Ir-F-DHBA. Compound Ir-F-DHBA was prepared
with compound 6 as the starting material (red solid, yield =
42%). 1H NMR (400 MHz, CDCl3) d 8.29 (dd, J = 9.5, 5.4 Hz, 2H),
7.77 (s, 2H), 7.31 (dd, J = 8.8, 2.6 Hz, 2H), 7.01 (t, J = 10.2 Hz,
2H), 6.67 (d, J = 7.2 Hz, 2H), 6.59 (t, J = 7.4 Hz, 2H), 6.43 (d, J =
7.5 Hz, 2H), 4.98 (s, 1H), 3.35–3.00 (m, 8H), 0.65 (s, 18H). 19F
NMR (400 MHz, CDCl3) d ꢀ115.98. MALDI-TOF-MS: m/z: calcd
for C45H41F2IrN2O2: 872.2765, found: 872.3045.
Fig. 2 TGA curves of Ir-DHBA and Ir-F-DHBA at a heating rate of
10 1C minꢀ1 under a nitrogen atmosphere.
Fig. 3 Cyclic voltammograms of Ir-DHBA and Ir-F-DHBA measured in
DMF solution with a scan rate of 50 mV sꢀ1
.
yield = 54%). 1H NMR (400 MHz, CDCl3) d 8.15 (dd, J = 8.9,
5.0 Hz, 1H), 8.05 (d, J = 7.7 Hz, 1H), 7.67 (dd, J = 9.7, 2.0 Hz, 1H),
7.48 (t, J = 7.2 Hz, 1H), 7.31 (t, J = 7.5 Hz, 1H), 7.22 (d, J = 7.5 Hz,
1H), 7.14 (t, J = 7.8 Hz, 1H), 6.43 (s, 1H), 3.16–3.03 (m, 2H), 2.93
(d, J = 15.7 Hz, 2H), 2.30–2.15 (m, 1H), 1.79 (d, J = 16.0 Hz, 1H).
Synthesis of compound 3. A mixture of compound 1
(650 mg, 2.186 mmol) and iron powder (612 mg, 10.93 mmol)
in HOAc (15 mL) was heated at 70 1C for 5 hours. After cooling
down to room temperature, the reaction mixture was extracted
with dichloromethane (3 ꢁ 100 mL). The organic phase was
Results and discussion
Preparation and characterization
The synthetic routes of rigid ligands and Ir(III) complexes are
illustrated in Scheme 1. Compounds 1 and 2 were synthesized
by an addition reaction of enolate to aldehyde. During the
second step toward the synthesis of ligands, three types of
Table 1 Photophysical and electrochemical data for Ir-DHBA and Ir-F-DHBA
PLQYb
HOMO/LUMOc (eV)
HOMO/LUMOd (eV)
a
a
Complex
labs (nm)
lem (nm)
Eg (eV)
Ir-DHBA
Ir-F-DHBA
349/413/439/477/518/564
353/412/441/479/525/567
596
598
2.10
2.09
87.8%
88.3%
ꢀ4.78/ꢀ1.69
ꢀ4.91/ꢀ1.85
ꢀ5.10/ꢀ3.00
ꢀ5.02/ꢀ2.93
a
b
Measured in anhydrous toluene solution (10ꢀ5 mol Lꢀ1) at room temperature. Measured in degassed DCM solution (10ꢀ5 mol Lꢀ1) at room
c
d
temperature. Calculated by TD-DFT at the B3LYP/6-31G(d) level. HOMO levels were calculated from CV data, and LUMO levels were calculated
from the HOMOs and Egs.
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Fig. 4 Absorption spectra of Ir-DHBA and Ir-F-DHBA in toluene at room
temperature.
Fig. 6 Frontier molecular orbital distributions and energy levels of
Ir-DHBA and Ir-F-DHBA.
ligand in Ir-F-DHBA is slightly bent with the dihedral angle
between phenyl and quinolinyl motifs of 11.801, and it is 15.101
in Ir-DHBA (Fig. S7, ESI†).
A high thermal decomposition temperature (Td, the temperature
at 5% weight loss) is essential for the fabrication and stability of
OLED devices. The TGA curves of Ir-DHBA and Ir-F-DHBA measured
under nitrogen at a heating rate of 10 1C minꢀ1 are shown in Fig. 2.
The Tds of Ir-DHBA and Ir-F-DHBA were 323 1C and 320 1C,
respectively, which allowed device fabrication via a vacuum deposi-
tion method. Differential scanning calorimetry (DSC) measurement
was also performed, but unfortunately the glass transition behavior
was not observed for both compounds.
Fig. 5 PL spectra of Ir-DHBA and Ir-F-DHBA in toluene at room temperature.
reactions took place to finish the cyclization: (1) the reduction
of the nitro group to amine; (2) the condensation reaction
between the amino and keto groups; and (3) the acid catalyzed
Electrochemical properties
dehydration reaction to generate the pyridine ring. The synth- The electrochemical properties of the two Ir(III) complexes were
esis of these two Ir(III) complexes was similar to the reported studied by CV measurement. The HOMO and LUMO energy
procedure: (1) the treatment of ligands with IrCl3 in the mixture levels of the materials in each layer should be aligned properly to
of 2-ethoxyethanol and water gave the chloro-bridged dimer favor the efficient electron/hole transfer from electrodes to the
and (2) the coordination interaction of the dimer with an emitter layer and the confinement of excitons solely on the
ancillary dione ligand produced the target complexes with good dopants, which laid the basis for the setup of the OLED device
yields (Fig. S1–S6, ESI†).
configuration. The HOMO values of Ir-DHBA and Ir-F-DHBA were
The structure of each Ir(III) compound is confirmed by single- determined from the oxidization potential onset (Fig. 3). Since
crystal X-ray diffraction analysis (Fig. 1). Each Ir(III) metal ion is ferrocene was used as the internal standard, their HOMO energy
octahedrally coordinated by two rigid ligands and one dione with levels were calculated using the equation, EHOMO = ꢀ(Eox + 4.8) eV.
two nitrogen atoms opposite to each other with respect to the Ir(III) Ir-DHBA and Ir-F-DHBA showed similar HOMO energy levels due
ion. In Ir-F-DHBA, the Ir–C bond lengths are 1.983–1.990 Å and to their same electron donating group (phenyl). Ir-F-DHBA
the Ir–N bonds lengths are 2.076–2.078 Å, which are slightly demonstrated a slightly higher HOMO (ꢀ5.02 eV) than Ir-DHBA
shorter than the Ir–O bonds (2.138–2.139 Å). The angles of N–Ir–N (ꢀ5.10 eV), which could be due to the efficient conjugation
and O–Ir–O are 175.461 and 85.721, respectively. A fluorine atom is between the donor and a stronger acceptor (fluorinated quinolinyl
involved in the hydrogen bonding with the neighboring methy- group) in Ir-F-DHBA. The LUMO energies (Table 1) were calcu-
lene group (dHꢂꢂꢂF = 2.411 Å), which could favor the packing of lated using the equation, ELUMO = Eg + EHOMO, where Eg is the
Ir-F-DHBA molecules in the solid state. The C^N cyclometalating optical bandgap (Eg = 1240/ledge).
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These two complexes principally displayed similar photophysical
properties. Since the energy of the rigid C^N cyclometalating
ligand is higher than that of the ancillary ligand, the absorption
and luminescence are dominated by the rigid ligand and the
related metal–ligand charge-transfer (MLCT) state. The absorp-
tion band below 400 nm was assigned to the p–p* transitions
localized on rigid ligands. A set of weak bands above 400 nm are
attributed to MLCT transitions and the low energy absorption
band around 570 nm could arise from the spin-forbidden 3MLCT
absorptions and partial contribution from 3p–p* transitions.22
As compared with Ir-DHBA, the electron-withdrawing fluoro sub-
stituent in Ir-F-DHBA led to a small red-shift from 596 nm to
598 nm in their PL spectra, which is consistent with the observa-
tion of the bathochromic shift of p–p* transitions in Ir-F-DHBA.
Notably, Ir-DHBA and Ir-F-DHBA both showed intense featureless
MLCT emission23 with high PLQYs in DCM solution (87.8% for
Ir-DHBA and 88.3% for Ir-F-DHBA, respectively).
Fig. 7 The natural transition orbital distributions of Ir-DHBA and Ir-F-DHBA
based on optimized T1 geometries.
To investigate the electronic structures of both Ir(III) com-
plexes, density functional theory calculations were carried out
at the B3LYP/6-31G(d) level. As shown in Fig. 6, the HOMOs of
Ir-DHBA and Ir-F-DHBA mostly located on the phenyl group and
the Ir(III) metal ion with a very small amount on the oxygen atoms
Photophysical properties
The ultraviolet-visible (UV-vis) absorption and photolumines-
cence (PL) spectra of Ir-DHBA/Ir-F-DHBA recorded in toluene
at room temperature are shown in Fig. 4 and 5, respectively.
Fig. 8 Device configurations with energy band diagrams and their chemical structures.
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Fig. 9 (a) Current density and luminance as a function of voltage. (b) EQE versus luminance. (c) Current efficiency and power efficiency versus current
ꢀ2
density. (d) EL spectra of devices at 0.2 mA cm
.
of the ancillary ligand. Their LUMOs were mostly distributed over were utilized as the hole transport layer (HTL) and electron-
quinolinyl groups and a small portion over the phenyl group and blocking layer (EBL), respectively. 4,6-Bis(3,5-di(pyridin-4-yl)phenyl)-
the Ir(III) ion. Therefore, the ancillary ligand does not make a 2-methylpyrimidine (B4PyMPM) was applied as the electron
significant contribution to the frontier molecular orbitals. These transport layer (ETL) and hole-blocking layer (HBL). The OLEDs
results agreed well with the observation of MLCT transition fabricated using Ir-DHBA were designated as A1 (EML configu-
in their absorption spectra. The calculated LUMO energy level ration: CBP:B4PYMPM:3 wt% Ir-DHBA), B1 (EML configu-
of Ir-F-DHBA is slightly deeper than that of Ir-DHBA, which could ration: TCTA:3 wt% Ir-DHBA) and C1 (EML configuration:
be ascribed to the introduction of the fluorine atom in Ir-F-DHBA. TCTA:B4PYMPM:3 wt% Ir-DHBA), in which CBP stands for
The distribution of the hole and particle natural transition orbitals 4,40-di(9H-carbazol-9-yl)-1,10-biphenyl. The EML configurations
(NTOs) of Ir-DHBA and Ir-F-DHBA is shown in Fig. 7. In these two in devices A2, B2 and C2 with Ir-F-DHBA as the dopant are
complexes, the hole/particle orbitals are mainly located on the similar to those containing the Ir-DHBA emitter. The device
cyclometalating ligands and the Ir(III) ion, indicating their hybrid T1 structures and the alignment of HOMO/LUMO energy levels of
characteristics between MLCT and ligand-centered p–p* transition. these materials are shown in Fig. 8.
The curves of current density–voltage–luminance ( J–V–L), EQE
versus luminance, CE/PE versus current density and EL spectra of
Electroluminescence properties
To study the EL properties of Ir-DHBA and Ir-F-DHBA, OLEDs all devices are illustrated in Fig. 9. The device data are summar-
based on these dopants were fabricated with structures: ITO/HAT- ized in Table 2. The turn-on voltages (defined as the voltage at
CN (10 nm)/TAPC (45 nm)/TCTA (20 nm)/EML (20 nm)/B4PyMPM 5 cd mꢀ2) of these devices were found to be below 2.5 V. It is
(50 nm)/Liq (2 nm)/Al (120 nm). Hexaazatriphenylenehexa cabo- notable that devices C1 and C2 with TCTA and B4PyMPM as co-
nitrile (HAT-CN) and 8-hydroxyquinolinolato-lithium (Liq) were hosts show the lowest turn-on voltages among all our device
used as the hole-injecting layer (HIL) and electron-injecting layer configurations. This result could be explained by the low charge
(EIL), respectively. Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane injection barriers between the EML and the neighboring HTL/
(TAPC) and 4,40,400-tri(9-carbazolyl) triphenylamine (TCTA) ETL. Kwon et al. reported highly efficient red phosphorescent
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Table 2 Summary of electroluminescence performances for Ir-DHBA and Ir-F-DHBA devices
EQEc (%)
CEc (cd Aꢀ1
)
PEc (lm Wꢀ1
)
CIEd (x, y)
a
b
Device
Guest
Von (V)
lEL (nm)
A1e
B1f
C1g
A2h
B2i
C2 j
Ir-DHBA
Ir-DHBA
Ir-DHBA
Ir-F-DHBA
Ir-F-DHBA
Ir-F-DHBA
2.34
2.31
2.29
2.48
2.43
2.38
596
596
596
600
600
600
28.0/25.9/17.4
25.8/23.5/13.8
26.0/25.3/22.7
28.2/22.6/13.9
27.8/22.2/11.2
28.0/26.2/17.4
50.0/46.1/31.1
50.0/45.4/26.7
46.7/45.5/40.7
51.4/41.3/25.4
47.1/39.9/20.2
49.5/46.7 31.0
67.2/41.1/17.0
67.9/42.1/15.5
61.5/42.4/21.9
63.2/33.6/12.2
64.6/34.7/10.2
65.9/41.1/15.9
(0.61, 0.39)
(0.60, 0.40)
(0.61, 0.39)
(0.61, 0.39)
(0.61, 0.39)
(0.61, 0.39)
a
b
c
Turn on voltage measured at 5 cd mꢀ2
.
Emission wavelength measured at 0.2 mA cmꢀ2
.
Efficiencies in the order of at maximum, at 1000 cd mꢀ2
and at 10000 cd mꢀ2
.
Commission Internationale de l’Eclairage (CIE) coordinates. e EML configuration: CBP:B4PyMPM:3 wt%Ir-DHBA. EML
d
f
g
h
configuration: TCTA:3 wt%Ir-DHBA. EML configuration: TCTA:B4PyMPM:3 wt%Ir-DHBA. EML configuration: CBP:B4PyMPM:3 wt%Ir-F-DHBA.
i
j
EML configuration: TCTA:3 wt%Ir-F-DHBA. EML configuration: TCTA:B4PyMPM:3 wt%Ir-F-DHBA.
OLEDs based on Ir(III) complex Ir(mphmq)2(tmd) with a non- 21871199). The authors would like to thank Mr Kai Zhang for
fused ligand.24 The maximum CE, PE and EQE of the device with the DFT calculation and helpful discussion. This work was also
this highly substituted complex Ir(mphmq)2(tmd) were 30.1 cd Aꢀ1
,
supported by the Collaborative Innovation Centre of Suzhou
32.0 lm Wꢀ1 and 24.6%, respectively. In our work, by applying the Nano Science and Technology (Nano-CIC), the Priority Academic
rigidified ligand the device based on Ir-DHBA with TCTA as the Program Development of Jiangsu Higher Education Institutions
single host achieved much higher efficiency with maximum CE, PE (PAPD), and the ‘‘111’’ Project of the State Administration of
and EQE values of 50.0 cd Aꢀ1, 67.9 lm Wꢀ1 and 25.8%, respectively, Foreign Experts Affairs of China.
and 47.1 cd Aꢀ1, 64.6 lm Wꢀ1 and 27.8%, respectively, for the device
based on Ir-F-DHBA with TCTA as the host. When a co-host system
was applied for device fabrication, a slightly better device performance
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Acknowledgements
The authors are grateful for the financial support from the
National Key R&D Program of China (No. 2016YFB0400703)
and the National Natural Science Foundation of China (No.
J. Mater. Chem. C
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