Efficient bluish green electroluminescence of iridium complexes with good electron mobility

Article abstract of DOI:10.1039/c8nj01726c

Two novel iridium(iii) complexes (Ir1 and Ir2), synthesized using 2′-(trifluoromethyl)-2,5′-bipyrimidine and 5-fluoro-2′-(trifluoromethyl)-2,5′-bipyrimidine as the main ligands, and tetraphenylimidodiphosphinate (tpip) as an ancillary ligand, were investigated. The introduction of a nitrogen heterocycle and CF₃ substituent improves the electron mobility of the Ir(iii) complexes, which is beneficial for device performance. Both of the complexes emit bluish green photoluminescence with very high quantum efficiency yields (Ir1: λ_max: 485/516 nm, η_PL: 89%; Ir2: λ_max: 482/513 nm, η_PL: 95%) and good electron mobility. Organic light-emitting diodes (OLEDs) constructed with an ITO (indium tin oxide)/MoO₃ (molybdenum oxide, 3 nm)/TAPC (di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane, 50 nm)/mCP (1,3-bis(9H-carbazol-9-yl)benzene, 5 nm)/Ir complex (6 wt%):PPO21 (3-(diphenylphosphoryl)-9-(4-(diphenyl-phosphoryl)phenyl)-9H-carbazole, 10 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 50 nm)/LiF (1 nm)/Al (100 nm) structure showed good device performances. Device G1 based on Ir1 showed a η_c,max value of 62.99 cd A^-1 with an EQE_max value of 23.5%. Owing to the slightly higher PL efficiency and lower LUMO levels of Ir2, which are beneficial for electron injection, the device based on Ir2 displayed a slightly better performance with a η_c,max value of 71.18 cd A^-1 and an EQE_max value of 27.7%. Even at a practical brightness of 1000 cd m², values of 57.39 cd A^-1 and 22.3% could still be reached.

Full text of DOI:10.1039/c8nj01726c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Efficient bluish green electroluminescence of iridium
complexes with good electron mobility
Cite
this:
DOI:
10.1039/x0xx00000x
HuaꢀBo Han,1 ZhengꢀGuang Wu,1 YouꢀXuan Zheng1,2*
Two novel iridium(III) complexes (Ir1 and Ir2) using 2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀbipyrimidine and 5ꢀ
fluoroꢀ2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀbipyrimidine as main ligands, tetraphenylimidodiphosphinate (tpip) as
ancillary ligand, were investigated. The introducing of nitrogen heterocycle and CF3 substituent
would improve the electron mobility of Ir(III) complex, which is beneficial for the device
performances. Both complexes emit bluish green photoluminescence with very high quantum
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
efficiency yields (Ir1: λmax: 485/516 nm, ηPL: 89%; Ir2: λmax: 482/513 nm, ηPL: 95%) and good
electron mobility. The organic lightꢀemitting diodes (OLEDs) with the structure of ITO (indiumꢀtinꢀ
oxide) / MoO3 (molybdenum oxide, 3 nm) / TAPC (diꢀ[4ꢀ(N,Nꢀditolylꢀamino)ꢀphenyl]cyclohexane,
50 nm) / mCP (1,3ꢀbis(9Hꢀcarbazolꢀ9ꢀyl)benzene, 5 nm) / Ir complexes (6 wt%) : PPO21 (3ꢀ
(diphenylphosphoryl)ꢀ9ꢀ(4ꢀ(diphenylꢀphosphoryl)phenyl)ꢀ9Hꢀcarbazole, 10 nm) / TmPyPB (1,3,5ꢀ
tri(mꢀpyridꢀ3ꢀylꢀphenyl)benzene, 50 nm) / LiF (1 nm) / Al (100 nm) showed good device
performances. The device G1 based on Ir1 showed a ηc,max of 62.99 cd Aꢀ1 with an EQEmax of 23.5%.
Owing to the little higher PL efficiency and lower LUMO levels of Ir2, which is beneficial the
electron injection, the device based on Ir2 displayed slightly better performances with a ηc,max of
71.18 cd Aꢀ1 and an EQEmax of 27.7%. Even at the practical brightness of 1 000 cd m2, the values still
can be kept at 57.39 cd Aꢀ1 and 22.3%.
Introduction
electron mobility, 2,5'ꢀbipyrimidine was introduced to replace the
widely used 2ꢀphenylpyridine (ppy). What’s more, from our
Organic lightꢀemitting diodes (OLEDs) have attracted enormous
interest in last decades for application in largeꢀsize, flexibleꢀpanel
display technologies and solidꢀstate lighting due to their
advantages as color tunability and low power consumption.1
Notably, among all kind transitionꢀmetal complexes, iridium(III)
complexes are the most attractive and widely investigated
phosphorescent emitting materials for highly efficient OLEDs
owing to their encouraging properties including thermal stability,
short excited lifetime, high quantum efficiency yields and tunable
emission wavelength over the whole visible region.2
It is wellꢀknown that the hole mobility of holeꢀtransporting
materials is always much higher than the electron mobility of
electronꢀtransporting materials. Thus, to avoid the serious
efficiency rollꢀoff caused by the charge carrier balance
deterioration and nonradioactive quenching processes, the
synthesis of Ir(III) complexes with excellent electron mobility,
which will be beneficial to the balance of electronꢀhole injection
and transport, is essential to gain phosphorescent OLEDs with
low efficiency rollꢀoff. 3
previous work we found out that tetraphenylimidodiphosphinate
(tpip) is a good ancillary ligand for Ir(III) complexes due to their
four bulky aromatic groups, leading to an increase of the spatial
separation from neighboring molecules to suppress tripletꢀtriplet
annihilation and tripletꢀpolaron annihilation effectively,5 as well
as the Ph2P=O to improve the electron injection/transportation
property.6 Besides that, the bulky CF3 substituents in the main
ligands can affect the molecular packing and the steric protection
surrounding the metal would restrain the selfꢀquenching impact. 7
Thus, in this work, Ir1 and Ir2 using 2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀ
bipyrimidine and 5ꢀfluoroꢀ2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀbipyrimidine
as main ligands, tpip ancillary ligand, were investigated, which
display emissions peak at 485 and 482 nm, respectively.
Compared with the Ir1 complex, one more fluorine atom was
introduced to the main ligand in Ir2 because the CꢀF bond with
low vibrational frequency can reduce radiationless deactivation
rate, which is beneficial for the efficiency of both complex and
device.8 The devices based both complexes showed good
characteristics. The device G1 based on Ir1 showed a ηc,max of
62.99 cd Aꢀ1 with an EQEmax of 23.5%. Owing to the little higher
PL efficiency and lower LUMO levels of Ir2, which is beneficial
the electron injection, the device based on Ir2 displayed slightly
Nitrogen heterocycle would enhance the electron affinity and
the electron mobility of the Ir(III) complexes.4 Aiming to get
emitters and devices with high efficiency by improving the
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better performances with a ηc,max of 71.18 cd Aꢀ1 and an EQEmax
of 27.7%. Even at the practical brightness of 1 000 cd m2, the
values still can be kept at 57.39 cd Aꢀ1 and 22.3%.
All the reagents were used with commercial grade. The ligands
and complexes were synthesized under nitrogen atmosphere and
the synthetic routes were listed in Scheme 1.
Experimental section
Syntheses
Scheme 1. The synthetic routes of the ligands and complexes.
L2. 69% yield. MS (ESI): calcd. for M+ (C9H4F4N4 ) m/z =
+
General syntheses of ligands.
1
244.15, found 245.16. H NMR (400 MHz, CDCl3) δ 9.81 (S,
5ꢀBromoꢀ2ꢀ(trifluoromethyl)pyrimidine (800 mg), KOAc (690
mg, 2.0 e.q), Bis(pinacolate)diboron ((Bpin)2, 1.10 g) and
2H), 8.77 (S, 2H).
General syntheses of iridium complexes.
Pd(dppf)Cl2
(bis(diphenylphosphion)ferrocene palladium(II)
A mixture of IrCl3 (1 mmol) and L1/L2 (2.5 mmol) in 2ꢀ
ethoxyethanol and water (20 mL, 3:1, v/v) was refluxed for 24 h.
After cooling, the solid precipitate was filtered to give the crude
cyclometalated Ir(III) chloroꢀbridged dimer [(C^N)Ir(µꢀCl)]2.
Then, the slurry of crude chloroꢀbridged dimer (0.2 mmol) and
Ktpip (0.5 mmol) in 2ꢀethoxyethanol (20 mL) was refluxed for 24
h. The solvent was evaporated and the mixture was poured into
water, extracted with CH2Cl2. Then, silica gel chromatography
gave complexes Ir1 and Ir2, which were further purified by
sublimation in vacuum.
dichloride) (50~80 mg) were added in dioxane (10 mL). The
solution was stirred at 90 C for 3 h. The extraction with ethyl
o
acetate and evaporation of the organic phase gave the crude
corresponding aryl borate. 2ꢀChloropyrimidine or 2ꢀchloroꢀ5ꢀ
fluoropyrimidine (10 mmol), Pd(dppf)Cl2 (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
oC for 1 day. The mixture was poured into water and extracted
with CH2Cl2 (10 mL × 3 times). Finally, silica column
purification (PE:EA
respectively.
=
10:1
)
gave L1 and L2 ligands,
1
Ir1. 48% yield. H NMR (400 MHz, CDCl3) δ 8.98 (dd, J =
5.8, 2.2 Hz, 2H), 8.82 (s, 2H), 8.55 (dd, J = 4.8, 2.2 Hz, 2H), 7.83
(ddd, J = 12.5, 7.7, 1.7 Hz, 4H), 7.44 – 7.31 (m, 10H), 7.19 (td, J
= 7.5, 1.3 Hz, 2H), 7.00 (td, J = 7.8, 3.1 Hz, 4H), 6.76 (dd, J =
5.8, 4.9 Hz, 2H). MS(ESI) m/z calcd for C42H28F6IrN9O2P2:
1058.90 [M]+, found 1059.97 [M+H]+. Anal. Calcd. For
C42H28F6IrN9O2P2: C 47.64, H 2.67, N 11.91. Found: C 47.62, H
2.85, N 12.31.
+
L1. 72% yield. MS (ESI): calcd. for M+ (C9H5F3N4 ) m/z =
226.16, found 227.18. 1H NMR (400 MHz, CDCl3) δ 9.86 (s, 2H),
8.91 (d, J = 4.9 Hz, 2H), 7.39 (t, J = 4.9 Hz, 1H).
2 | J. Name., 2012, 00, 1-3
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1
Ir2. 51% yield. H NMR (400 MHz, CDCl3) δ 8.85 (t, J = 3.0 the thermogravimetric analysis (TGA) curves of Ir1 and Ir2 in
Hz, 2H), 8.76 (s, 2H), 8.45 (d, J = 3.1 Hz, 2H), 7.90 – 7.81 (m, Fig. 2 it can be figured out that there is no loss observed below
4H), 7.47 – 7.36 (m, 10H), 7.23 (dd, J = 7.4, 1.3 Hz, 2H), 7.06 (td, 300 °C in weight. And the decomposition temperatures (5% loss
J = 7.8, 3.1 Hz, 4H). MS(ESI) m/z calcd for C42H26F8IrN9O2P2: of weight) are 368 °C for Ir1 and 329 °C for Ir2, respectively,
1094.88 [M]+, found 1095.97 [M+H]+. Anal. Calcd. For suggesting that both complexes have the potential application in
C42H26F8IrN9O2P2: C 46.07, H 2.39, N 11.51. Found: C 45.87, H OLEDs.
2.57, N 11.27.
100
Ir1
Ir2
Results and discussion
80
Preparation and characterization of compounds
60
As shown in Scheme 1, the main ligands L1 and L2 were
synthesized using a Suzuki coupling reaction. The iridium
40
complexes were prepared by the reaction of corresponding
chlorideꢀbridged dimmers [(C^N)Ir(µꢀCl)]2 with potassium salt of
tpip (ktpip). All new compounds were fully characterized by 1H
20
NMR spectrometry, and the crystal structures of Ir1 and Ir2
further confirmed their identity.
0
0
150
300
450
600
Temperature (oC)
Fig. 2. The TGA curves of Ir1 and Ir2.
Electrochemical property and theoretical calculation
40
(a)
Ir1
Ir2
30
20
10
0
Fig. 1. Oka Ridge Thermal Ellipsoidal plot (ORTEP) diagrams of Ir1
(CCDC No. 1584485) and Ir2 (CCDC No. 1584486) complexes with the
atomꢀnumbering schemes. Hydrogen atoms are omitted for clarity.
Ellipsoids are drawn at 30% probability level.
-10
0.0
0.5
1.0
1.5
2.0
Voltage (V)
Fig. 3. (a) The cyclic voltammogram curves of Ir1 and Ir2; (b)
Contour plots of Ir1 and Ir2
.
Single crystals of Ir1 and Ir2 were obtained by vacuum
sublimation. The structures of Ir1 and Ir2 were proved via the Xꢀ
ray diffraction analysis and the crystal diagrams are displayed in
Fig. 1. The molecular parameters and atomic coordinates are
showed 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 L1/L2 or tpip, with the twisted octahedral
coordination geometry. For Ir1 and Ir2, angles of [OꢀIrꢀO] are
80.93(12) ꢀ 81.17(12)°, and the angles of [CꢀIrꢀN] are 80.01(2) ꢀ
106.6(2)°. The lengths of IrꢀC bonds range from 1.956(5) Å to
1.965(5) Å. The IrꢀN bonds have the lengths of 2.039(4) ꢀ
2.046(4) Å. And the lengths of IrꢀO bonds are longer a bit, which
are 2.195(2) ꢀ 2.212(3) Å. These results are similar to the
parameters of the cyclometalated Ir(III) complexes that have been
reported.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of the
dopants are important for the design of the OLED structures. In
order to determine the HOMO/LUMO values of the Ir1 and Ir2,
their electrochemical properties were investigated by cyclic
voltammetry in deaerated CH3CN (Fig. 3(a)). The HOMO levels
were calculated from the oxidation peak potential (Eox) and the
band gap (Eg) was calculated from the UVꢀvis absorption edges.9
Then the LUMO levels were determined according to the
equation LUMO = HOMO + Eg. During the progress of anodic
oxidation, the oxide peaks can be observed for both complexes
with the oxidation potentials with peaks at 1.46 and 1.60 V,
respectively, which can be ascribed to the metalꢀcentered
Ir(III)/Ir(IV) oxide couples, consistent with the cyclometallated
Ir(III) system reported.10 Ir1 exhibits an oxidation potential (1.46
V) and HOMO/LUMO energy levels are calculated as ꢀ6.10/ꢀ3.56
eV. Due to one more F atom in the main ligand, the oxidation
potential of Ir2 increases to 1.60 V and the HOMO/LUMO
energy levels decrease to ꢀ6.23/ꢀ3.70 eV. The lower LUMO level
of Ir2, which benefits to trapping the electron and broadening the
The thermal stability of the materials is crucial for efficient
OLEDs application. 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 deposited onto the
solid face without any decomposition during sublimation. From
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recombination zone, may be in favour of the balance of the solvent effect of CH2Cl2 was taken into consideration using
electronꢀhole injection and transport. As a result, the device based conductor like polarizable continuum model (CꢀPCM). It can be
on Ir2 will show better performances. What’s more, the band observed from the theoretical calculation that nearly all LUMOs
gaps (Eg) of the complexes which contribute to the variations of are on L1/L2 ligands (95.23%/94.85%) with minor contributions
the PL spectra are almost the same. Thus, the introduction of the from iridium
special one more F atom has almost no effect to the emission (2.75%/2.23%). HOMOs are mostly situated on L1/L2 ligands
spectra for the two complexes. (30.85%/33.30%) and orbitals of iridium atom
d orbitals (2.02%/2.92%) and tpip ligand
d
In order to gain insights into the electronic state and the orbital (55.67%/53.14%) with minor contributions from the tpip ligand
distribution, the density functional theory (DFT) calculations11 for (13.49%/13.56%). From both electrochemical and theoretical
both Ir(III) complexes were conducted employing Gaussian09 calculation results it can be found that the modification of the
software with B3LYP function.12 Plots of the HOMO/LUMO and main ligand has great effects on orbital energy of the Ir(III)
the molecular orbital energy levels are presented in Fig. 3(b). The complexes and more nitrogen atoms introduced would decrease
basis set used for C, H, N, O and F atoms was 6ꢀ31G(d, p) while both the HOMO and LUMO levels by comparison with those
the LanL2DZ basis set was employed for iridium atoms.13 The complexes based on ppy ligand.
Table 1. Photophyscial date of Ir(III) complexes.
a)
b)
c)
Td
λabs
λem
Φ d)
(%)
HOMO/LUMO e)
(eV)
Complex
(°C)
368
329
(nm)
(nm)
Ir1
233/258/300
228/263/302
485/516
482/513
89
95
ꢀ6.10/ꢀ3.56
Ir2
ꢀ6.23/ꢀ3.70
a)
b) c)
d)
Td: decomposition temperature;
Measured in degassed CH2Cl2 solution at a concentration of 5 × 10ꢀ5 mol·Lꢀ1 at room temperature; emission
quantum yields were measured relative to Ir(ppy)3 (Φ = 0.4) in degassed CH2Cl2 solution at room temperature. e) 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.
Photophysical property
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(b)
The UVꢀvis absorption and photoluminescence spectra of the
complexes Ir1 and Ir2 in CH2Cl2 (5×10ꢀ5 M) are shown in Fig.
4 and two complexes show similar photophysical properties.
The absorption spectra of two complexes show broad and
intense bands below 320 nm, assigned to the spinꢀallowed
intraligand 1LC (π→π*) transition of cyclometalated L1/L2 and
tpip ligands. The weak bands last to 500 nm can be assigned to
spinꢀallowed metalꢀligand charge transfer band (1MLCT) and
spin forbidden 3MLCT transition bands caused by the large spin
orbital coupling (SOC) that was introduced by Ir(III) center
indicating an efficient spinꢀorbit coupling that is prerequisite for
phosphorescent emission.14 The emissions peak at 485 and 482
nm with obvious shoulders at 516 and 513 nm in CH2Cl2,
produced by the electronic transition between the lowest triplet
excited state and the ground state, which makes Ir1 and Ir2
bluish green phosphors with Commission Internationale de
1’Eclairage (CIE) color coordinates of (0.214, 0.512) and
(0.201, 0.492), respectively. Furthermore, the complexes show
much high quantum efficiencies as 0.89 and 0.95 for Ir1 and
Ir2, respectively. Due to one more F atom in the main ligand,
Ir2 has a little higher quantum efficiency than Ir1, suggesting
that the device based on Ir2 may show better performances.
Ir1
Ir2
Ir1
Ir2
300
400
Wavelength (nm)
500
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 4. (a) UVꢀvis absorption and (b) emission spectra of Ir1 and Ir2
complexes in degassed CH2Cl2 solutions (5.0 × 10ꢀ5 mol Lꢀ1) at room
temperature.
Electron mobility
8
6
4
Ir1
Alq3
2
0
1160
1200
1240
1280
Electric Field ( )
V cm-1
1/2
Fig. 5. Electric field dependence of charge electron mobility in the thin
films of Ir1 and Alq3.
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According to our previous studies, the excellent electron
mobility of emitters could benefit electron transport and
improve the device efficiency.15 To determine the electron
mobility of the complex, the transient electroluminescence
(TEL) measurement was carried out via the devices with a
simple structure of ITO (indiumꢀtinꢀoxide) / TAPC (diꢀ[4ꢀ(N,Nꢀ
ditolylꢀamino)ꢀphenyl]cyclohexane, 50 nm) / Ir1 (60 nm) / LiF
(1 nm)/ Al (100 nm).16 The Ir(III) complex acts as not only the
emissive layer (EML) but also electronꢀtransport layer. The
experimental results (Fig. 5 and Fig. S1) indicated that the
electron mobilities of Ir1 are 6.82ꢀ7.20 × 10ꢀ6 cm2 Vꢀ1 sꢀ1, under
was used as hole transport/electron block layer (HTL/EBL),
while TmPyPB was used as electron transport/hole block layer
(ETL/HBL) with high electron mobility (1 × 10ꢀ3 cm2 Vꢀ1 sꢀ1)
and lowꢀlying HOMO level (ꢀ6.7 eV). Due to the low HOMOs
of the dopants, PPO21 was chosen as the host (EHOMO = 6.21
eV). Furthermore, to reduce the energy barrier between the
TAPC and PPO21 layers, another hole transport layer of mCP
(EHOMO = 5.90 eV) was added as the energy “ladder”. The
chemical structures of the materials mentioned above as well as
the device structure and energy level diagrams are depicted in
Fig. 6.
the electric field from 1040 (V cmꢀ1)1/2 to 1300 (V cmꢀ1)1/2
,
Theoretically speaking, the stepwise HOMO levels of TAPC
(ꢀ5.50 eV), mCP (ꢀ5.90 eV), and PP021 (ꢀ6.21 eV) are
beneficial for the injection and transport of holes, while the
stepwise LUMO levels of TmPyPB (ꢀ2.70 eV), PPO21 (ꢀ2.68
eV) and mCP (ꢀ2.30 eV) are beneficial for the injection and
transport of electrons. Therefore, balanced distribution of
carriers (holes and electrons) and wide recombination zone
could be expected. More importantly, the LUMO level of TAPC
is 0.3 eV higher than that of mCP while the HOMO level of
TmPyPB is 0.49 eV lower than that of PPO21. Thus, holes and
electrons are well confined within EMLs and the triplet excitons
quenching of the dopants will be effectively avoided. In the
devices, the optimized Ir(III) complexes with 6 wt% doped
concentrations for Ir1 and Ir2.
higher than that of the widely used electron transport material
Alq3 (aluminum 8ꢀhydroxyquinolinate, 4.74ꢀ4.86 ×10ꢀ6 cm2 Vꢀ1
sꢀ1).17 The results suggest that the introducing of more nitrogen
atoms in the complexes can improve their electron mobility,
which is also beneficial for the device performances.
OLEDs performance
The EL spectra, luminanceꢀvoltageꢀcurrent density (LꢀVꢀJ),
current efficiencyꢀluminance (ŋcꢀL) and power efficiencyꢀ
luminance (ŋpꢀL) curves for G1 and G2 are shown in Fig. 7,
respectively, and the crucial EL data are shown in Table 2.
From the Fig. 7(a), it can be seen that the peaks of EL emission
are 485 and 482 nm with the obvious shoulders 514 and 513 nm
for G1 and G2, respectively. The emission spectra are almost
invariant of the current density and there is no dependence on
concentration.The EL spectra are almost identical to the PL
spectra of the complexes, suggesting that the devices’ EL
emissions come from the triplet excited states of the phosphors
and the CIE color coordinates of G1 and G2 are (0.199, 0.496)
and (0.182, 0.474), respectively.
Fig. 6. Energy level diagram of HOMO and LUMO levels (relative to
vacuum level) for materials investigated in this work and their
molecular structures.
Both devices show good performances. For the device G1, a
maximum current efficiency (ηc,max) of 62.99 cd Aꢀ1 with a
maximum external quantum efficiency (EQEmax) of 23.5%, a
maximum power efficiency (ηp,max) of 37.02 lm Wꢀ1 and a
maximum luminance (Lmax) of 33819 cd mꢀ2 were obtained. At
the practical luminance of 1 000 cd m2, the current efficiency
still reaches to 52.97 cd Aꢀ1 with an EQE of 19.7%. Perhaps due
to the little higher PL efficiency and lower LUMO levels of Ir2
(which is beneficial the electron injection), device G2 exhibits
better device performances with a Lmax of 33922 cd mꢀ2, a ηc,max
of 71.18 cd Aꢀ1, an EQEmax of 27.7% and a ηp,max of 35.48 lm Wꢀ
1, respectively. At the luminance of 1 000 cd m2, the current
efficiency is still as high as 57.39 cd Aꢀ1 with an EQE of 22.3%.
To evaluate the EL performances of the complexes, two devices
named G1 and G2 with Ir1 and Ir2 as the emitters,
respectively, were fabricated with the structure of ITO / MoO3
(molybdenum oxide, 3 nm) / TAPC (50 nm) / mCP (1,3ꢀbis(9Hꢀ
carbazolꢀ9ꢀyl)benzene, 5 nm) / Ir complex (6 wt%) : PPO21 (3ꢀ
(diphenylphosphoryl)ꢀ9ꢀ(4ꢀ(diphenylꢀphosphoryl)phenyl)ꢀ9Hꢀ
carbazole, 10 nm) / TmPyPB (1,3,5ꢀtri(mꢀpyridꢀ3ꢀylꢀphenyl)
benzene, 50 nm) / LiF (1 nm) / Al (100 nm). In the devices
MoO3 and LiF served as holeꢀ and electronꢀinject interface
modified materials, respectively. Owning high hole mobility (1
× 10ꢀ2 cm2 Vꢀ1 sꢀ1) and highꢀlying LUMO level (ꢀ2.0 eV), TAPC
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These results suggest that the complexes are promising
materials for OLEDs.
200
160
120
80
1.0
104
103
102
101
100
(b)
G1
G2
(a)
0.8
G1
G2
0.6
0.4
0.2
0.0
40
0
200
300
400
500
600
700
3
6
9
12
15
18
Voltage (V)
Wavelength (nm)
10
(c)
10
G1
G2
(d)
G1
G2
1
0.1
1
2000
4000
6000
8000
10000
0
2000
4000
6000
8000
10000
-2
Luminance (cd m-2)
Luminance (cd m )
Fig. 7. Characteristics of devices of G1 and G2: (a) electroluminescence spectra at 8 V; (b) luminance – voltage – current density (L – V ꢀ J) curves; (c)
current efficiency – luminance ( c – L) curves; (d) power efficiency – luminance ( p – L) curves.
η
η
Table 2. EL performances of devices G1and G2.
c)
e)
ηc,max
ηc,L1000
a)
b)
g)
Vturnꢀon
(V)
Lmax
ηp,max
CIEh)
(x,y)
Device
(cd Aꢀ1)
(cd Aꢀ1)
(EQEL1000 )
(cd mꢀ2)
(lm Wꢀ1)
d)
f)
(EQEmax
)
4.1
4.2
33819
33922
62.99 (23.5%)
71.18 (27.7%)
52.97 (19.7%)
57.39 (22.3%)
37.02
35.48
0.199, 0.496
0.182, 0.474
G1
G2
a) turnꢀon voltage recorded at a luminance of 1 cd mꢀ2, b) maximum luminance, c) maximum current efficiency, d) maximum external quantum efficiency
e)
f)
g)
h)
(EQE); current efficiency at 1 000 cd mꢀ2; EQE at 1 000 cd mꢀ2; maximum power efficiency, Commission Internationale de l'Eclairage
coordinates (CIE).
efficient Ir(III) complexes with good electron mobility for
OLEDs with good performances.
Conclusions
In conclusion, two bisꢀcyclometalatted iridium complexes Ir1
and Ir2 with 2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀbipyrimidine and 5ꢀ
Acknowledgements
fluoroꢀ2'ꢀ(trifluoromethyl)ꢀ2,5'ꢀbipyrimidine as main ligands
were reported. Both complexes emit bluish green
phosphorescence with high quantum efficiency and good
electron mobility, which are beneficial for the fabrication of
efficient OLEDs. Through the comparison with the device using
Ir1, the device with Ir2 emitter displays better EL performances
with a Lmax of 33922 cd mꢀ2, a ηc,max of 71.18 cd Aꢀ1, a ηp,max of
35.48 lm Wꢀ1 and an EQEmax of 27.7%, respectively. The
research results indicate that the modification of the ligands
with nitrogen atoms is an useful strategy for the design of
This work was supported by the National Natural Science
Foundation of China (51773088) and the Natural Science
Foundation of Jiangsu Province (BY2016075ꢀ02).
Conflicts of interest
There are no conflicts to declare.
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Graphical abstract
G1
G2
10
1
0.1
2000
4000
6000
8000
10000
Luminance (cd m-2)
Two efficient bluish green iridium complexes with good electron mobility
were applied in efficient OLEDs showing a maximum current efficiency
of 71.18 cd A-1 and a maximum external quantum efficiency of 27.7%.