Green-emitting iridium(iii) complexes containing pyridine sulfonic acid as ancillary ligands for efficient OLEDs with extremely low efficiency roll-off

Article abstract of DOI:10.1039/c9tc03937f

A novel ancillary ligand of pyridine sulfonic acid (PySO₃) was developed for two green-emitting iridium(iii) compounds, Ir1 (λ_max = 496 nm) and Ir2 (λ_max = 504 nm), with trifluoromethyl-substituted 2-phenylpyridine derivatives as the main ligands. Due to the strong electron-withdrawing ability of PySO₃, both complexes have relatively low LUMO energy levels and good electron mobility, which benefit the charge balance in the organic light-emitting diodes (OLEDs) during the electroluminescence process. Therefore, all devices with double light-emitting layers exhibit good performances. In particular, the device using Ir2 as an emitter obtains a maximum luminance above 92000 cd m^-2, a maximum external quantum efficiency (EQE_max) of 25.5% with an extremely low efficiency roll-off, and the EQE still remains at 22.9% at the high luminance of 20000 cd m^-2. These results demonstrate that pyridine sulfonic acid is a potential and charming ligand for Ir(iii) complexes and high-performance OLEDs.

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ARTICLE
Green-emitting iridium(III) complexes containing pyridine sulfonic
acid as ancillary ligands for efficient OLEDs with extremely low
efficiency roll-off
、Cite this: DOI:
Lin Zhang,1 Zhi-Ping Yan,1 Zhen-Long Tu,1 Zheng-Guang Wu,1 You-Xuan Zheng1,2*
Received 00th January 2019,
Accepted 00th January 2019
A novel ancillary ligand of pyridine sulfonic acid (PySO3) was developed for two green-emitting
iridium(III) compounds, Ir1 (max = 496 nm) and Ir2 (max = 504 nm), with trifluoromethyl-
substituted 2-phenylpyridine derivatives as main ligands. Due to the strong electron-withdrawing
ability of PySO3, both complexes have relatively low LUMO energy levels and good electron
mobility, which benefit the charge balance in the organic light-emitting diodes (OLEDs) during the
electroluminescence process. Therefore, all devices with double light-emitting layers exhibit good
performances. Particularly, device using Ir2 as emitter obtains a maximum luminance above 92000 cd
m-2, a maximum external quantum efficiency (EQEmax) of 25.5% with an extremely low efficiency
roll-off, as well as the EQE still remains 22.9% at the high luminance of 20000 cd m-2. These results
demonstrate that pyridine sulfonic acid is a potential and charming ligand for Ir(III) complexes and
high-performance OLEDs.
DOI:
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Introduction
sulfonyl groups, it would show potential application in furnishing
electron injection (EI) and electron transporting (ET), which is
As a new-generation technology in flat-panel display, organic
light-emitting diode (OLED) has basked in the limelight, due to
its high brightness, fresh color and the potential in flexible
display.1-8 Among the various emitting materials utilized in
OLEDs, iridium(III) complexes are prominent due to the variable
ligand structure and tunable emission color. Moreover, the heavy
iridium atoms lead to strong spin-orbit coupling, allowing Ir(III)
complexes to capture both singlet and triplet excitons for light
emission with theoretically 100% internal quantum efficiency
(IQE).3,9
However, the efficiency roll-off is serious at high luminance
for most efficient OLEDs based on Ir(III) emitters, which is
mainly due to the unbalanced electron-hole injection, transport,
combination and some nonradioactive processes.10 Particularly,
the electron-hole balance in emissive layer is one of the most
important factors for achieving high-performance OLEDs.
Generally, using bipolar host materials and high electron
transport property dopant are two main methods to achieve
electron-hole balance in OLEDs.11,12 Owing to the excellent
performance of commercially available bipolar host material 2,6-
bis-(3-(carbazol-9-yl)phenyl)pyridine (2,6-DCzPPy) in practical
applications, it is highly desirable to develop new kinds of Ir(III)
complexes to achieve effective devices with low efficiency roll-
off ratios.
further used to promote the electroluminescence (EL)
performance. For example, Zhou et al. introduced phenylsulfonyl
substituent into the main ligands of the Ir(III) compound, and
proved that the SO2Ph group can improve the EI/ET ability of the
complexes, which show good device performances with a
maximum external efficiency (EQEmax) of 10.67%, a maximum
luminance (Lmax) of 48567 cd m-2 as well as a relatively low
efficiency-off with the EQE of 8.90% at the luminance of 6000
cd m-2.16 Wong and co-workers introduced aromatic sulfonyl
groups with different fluorine substituents into Ir(III) complexes
showing excellent EL performances afford by their EI/ET
abilities.19 All these results sufficiently indicate that sulfonyl
groups take advantages in electron-hole balance. As a result, it is
necessary to further study more Ir(III) complexes containing
sulfonyl groups for efficient OLEDs with low efficiency roll-off.
As is well known, pyridine sulfonic acid not only has similar
characteristics as SO2Ph group but also possess the advantages of
simple structure and easy synthesis. However, current research
fields on pyridine sulfonic acids as ligands are limited, and most
of them concern about oxidation catalysis,20,21 anticancer
metallodrugs22 and supramolecular framework.23,24 The
researches on phosphorescent Ir(III) complexes containing
pyridine sulfonic acid for OLEDs are rare. Herein, two green-
emitting complexes with 2-(4-(trifluoromethyl)phenyl)pyridine
(4-tfppy) or 2-(2-(trifluoromethyl)phenyl)pyridine (2-tfppy) as
main ligands and pyridine sulfonic acid (PySO3) as ancillary
At the same time, several Ir(III) complexes with sulfonyl group
in the main ligands have been reported and exhibited unique
advantages for OLEDs.13-19 According to the special structure of
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Journal Name
ligand were investigated. Compared with other ancillary ligands, for Ir-O and Ir-N, respectively, which are slightly longer than the
the strong electron-withdrawing PySO3 group reduces the LUMO reported bond lengths of 2.15 Å for analogous complexes with
energy levels of the complexes apparently and enhances their pyridine carboxylate (pic) as ancillary ligands.25 The longer bond
electron mobility, which would improve their EI/ET properties lengths resulting from strong polarity of S=O bonds indicate the
and finally maintain the balance of charge transport during the EL slighter weaker coordination ability of PySO3, which conforms to
process. Hence, the OLEDs exhibit excellent performances with a the reported results as well.Error! Bookmark not defined. The
Lmax of 92297 cd m-2 and an EQEmax of 25.5%. Notably, at the rest of Ir-N and Ir-C bond lengths range from 1.98-2.01 Å and
luminance of 10000 cd m-2 and 20000 cd m-2, the EQE still 2.03-2.04 Å, respectively, which are in agreement with other
remains as 24.5% and 22.9%, respectively, indicating an reported complexes.13,26 It is noticeable that the trifluoromethyl
extremely low efficiency roll-off.
group on the position 2 of phenyl ring causes more deformation
of main ligand because of the repulsion between the
trifluoromethyl group and pyridine ring. As the crystal data
proves, the torsion angles of C5-C6-C7-C8 and C13-C12-C17-C18 in
complex Ir2 are -9.3˚ and 1.5˚, respectively, which is larger than
torsion angle in Ir1 (7.1˚, 0.5˚). Besides, the trifluoromethyl
group in Ir2 may induce the C-H…F H-bond with the H atom on
position 3 of pyridine ring. According to the crystal structure of
Ir2, the C-H …F angles are in the range of 138.7˚-140.0˚ with
H … F distances of 2.21-2.24 Å, which satisfies the IUPAC
criteria for the hydrogen bond and would limit the trifluoromethyl
group rotation, molecular vibration and improve the
luminescence intensity. 27-29
Results and discussion
Synthesis and characterization
A1
A2
A2
A2
Br
Cl
Cl
A2
a
b
A1
A1
Ir
Ir
d
N
+
N
N
N
A1
B(OH)2
2
2
PySO3Na
A1
O
O
O
S
Na+
O
HO
N
-O
N
O
O
c
S
O
S
A2
Ir
N
N
The decomposition temperature (Td) of the complexes should
be high enough to avoid decomposition during the operation of
the OLEDs. Therefore, the thermal stability of Ir1 and Ir2 were
investigated through thermogravimetric analysis (TGA), which
2
PySO3Na
Ir 1: A1 = CF3, A2 = H
Ir 2: A1 = H, A2 = CF3
(a) Pd(PPh3)4, Na2CO3, THF/H2O, 80 oC, 10 h; (b) IrCl3, C2H5OCH2CH2OH, 115 oC,
12 h; (c) NaOH, CH3OH/C2H5OC2H5; (d) PySO3Na, C2H5OCH2CH2OH, 110 oC, 8 h.
o
o
gives Td as 384 C and 376 C, respectively (Fig. S7). Although
the coordination ability of PySO3 is slightly weaker than pyridine
carboxylate as mentioned, the thermal stability of complexes still
meets the requirement.
Scheme 1 Synthetic route of Ir(III) complexes.
The chemical structures and synthetic methods of the compounds
are shown in Scheme 1. The main ligands were synthesized using
traditional Suzuki coupling reactions, and the complexes were
prepared by common methods with good yields (see supporting
information for details). Gradient sublimation method was carried
out for further refinement. The resulting complexes were fully
characterized by 1H NMR, 13C NMR, 19F NMR and high
resolution electrospray ionization mass spectroscopy (HR ESI-
MS).
Photophysical property
1.0
0.8
0.6
0.4
0.2
0.0
Ir1
Ir2
1.0
0.8
0.6
0.4
0.2
0.0
Ir1
Ir2
300
400
500
600
700
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig.2 (a) UV-Vis absorption and emission spectra of the complexes in
CH2Cl2 at 298 K and (b) emission spectra in CH2Cl2 at 77 K.
The UV-Vis absorption and photoluminescence (PL) spectra of
Ir1 and Ir2 at room temperature are shown in Fig.2, and their PL
spectra in dichloromethane at 77 K are included as well. The
relevant data are listed in Table 1. The UV-Vis absorption spectra
of two complexes can be divided into three parts. The strongest
absorption peak under 320 nm comes from the spin-allowed π-π*
transition of the ligands. From 320 to 410 nm, the absorption
peaks become weaker and can be attributed to the spin-allowed
metal-ligand charge transfer (1MLCT), while the weakest 3MLCT
bands located between 410 and 470 nm.
Fig. 1 X-ray crystal structure of (a) Ir1 (CCDC: 1936819) and (b) Ir2
(CCDC:1937037).
Single crystal diffraction analysis was utilized to confirm the
molecular structures of Ir1 and Ir2 (Fig.1, Fig.S4), and their
detailed data are available in Table S1-S2. The bond lengths
between Ir(III) atoms and PySO3 ligands are 2.20 Å and 2.17 Å
2 | J. Name., 2012, 00, 1-3
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Complexes Ir1 and Ir2 exhibit maximum emission peaks at 496 photophysical property of the complexes slightly, as the emission
and 504 nm, which are in the green region with CIE coordinates spectrum of Ir2 is redshifted than that of Ir1. At 77 K in the
at (0.22, 0.59) and (0.24, 0.58), respectively. Compared with the frozen solutions, the PL spectra of both complexes are blue
complexes using same main ligands and common ancillary shifted and highly structured (Fig. 2(b)), which may due to the
ligands,
such
as
acetylacetonate
(acac)30
or increasing of the triplet ligand-centered transition (3LC).32 The
tetraphenylimidodiphosphinate (tpip)31, Ir1 exhibit a blue shift of triplet energy levels (T1) of Ir1 and Ir2 are 2.63 and 2.59 eV,
near 30 nm resulting from the impact of PySO3 group on the respectively, and the low-lying T1 could enhance energy transfer
electron cloud distribution of the complex. The position of the between the hosts and emitters as well.
trifluoromethyl group on the phenyl ring also affects the
Table 1. Photophysical and electrochemical properties of Ir1 and Ir2.
a
b
b
c
Td
λabs
λem
λem
CIEb
τb
Фd
HOMO/LUMOe
(eV)
complex
(oC)
(nm)
(nm)
(nm)
(x, y)
(μs)
(%)
Ir1
Ir2
384
376
258, 390
258, 395
496, 525
504, 529
493, 528
500, 534
(0.22, 0.59)
(0.24, 0.58)
1.45
1.66
41.84%
69.64%
(-5.79, -3.28)
(-5.78, -3.36)
a) Decomposition temperature; b) Measured in degassed CH2Cl2 solution (5 ×10-5 mol L-1) at room temperature; c) Measured in degassed CH2Cl2 solution
e)
(5 ×10-5 mol L-1) at 77 K; d) Absolute PLQY in co-deposited films with 2,6-DCzPPy; From the oxidation potentials of the cyclovoltammetry (CV)
diagram using ferrocene as the internal standard and the band gap from the absorption spectra in degassed solution. HOMO (eV) = -(Eox-E1/2,Fc)-4.8,
LUMO(eV)=HOMO+Ebandgap
.
The absolute photoluminescence quantum yields (PLQY) of
two complexes in CH2Cl2 and doped films were measured (Fig.
S5-S6, Table S4). Due to the strong interaction between the
complexes and solvent molecules, the vibrating movement of
the complexes is violent leading to a large non-radiative
constant (knr) and undesired PLQYs in CH2Cl2 solution (3.50%
for Ir1 and 5.87% for Ir2, respectively). However, in the co-
deposited films with widely used OLED host materials of 2,6-
DCzPPy or 4,4’,4’’-tri(9-carbazoyl)triphenylamine (TCTA), the
PLQYs become much higher. The PLQYs of Ir1 and Ir2 in co-
deposited films with 2,6-DCzPPy are up to 41.84% and 69.64%
respectively, while in doped films with TCTA, the PLQYs still
exceed 40%. In most cases, the photophysical properties of
emitters in doped films can better reflect the actual properties of
devices, therefore, it has important reference values for
exploring its device performances.33 Furthermore, the
phosphorescence lifetimes (τ) of Ir1 and Ir2 are 1.45 and 1.66 μs
in CH2Cl2, respectively, which are short enough to avoid severe
triplet-triplet annihilation (TTA) of the OLEDs.
of Ir1 and Ir2 are nearly the same, leading to the similar HOMO
energy levels (-5.79 and -5.78 eV, respectively). Due to the
longer UV cut-off wavelength of Ir2, the complex obtains
slightly lower LUMO energy level (-3.36 eV) than that of Ir1 (-
3.28 eV). Deserved to be mentioned, the strong electron-
withdrawing PySO3 shifts the HOMO/LUMO energy levels of
the complexes downwards distinctly. According to reported
complexes with acac or tpip as ancillary ligands,33,34 the LUMO
level of Ir1 reduced by about 0.25 eV, which would beneficial
for high electron mobility of two complexes.
Table 2 Theoretical calculation data of orbital distributions.
Calculated
Energy
levels / eV
-6.51
Composition (%)
Complex
Orbital
Ir
Main
Ligands
57.22
62.62
51.69
4.17
Ancillary
atom
39.10
26.23
44.41
93.12
81.03
14.19
49.96
65.83
51.56
4.41
ligand
3.68
HOMO-2
HOMO-1
HOMO
-6.36
11.16
3.90
-5.74
Ir1
LUMO
-1.95
2.71
LUMO+1
LUMO+2
HOMO-2
HOMO-1
HOMO
-1.84
4.94
14.03
82.55
3.55
Electrochemistry and theoretical calculation
-1.73
3.26
-6.49
46.49
20.02
44.78
92.74
92.97
3.34
The cyclic voltammetry (CV) is a common method to estimate
the energy levels of the complexes, and for reversible oxidation-
reduction potential the results are credible.34 The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels of these complexes
were calculated through redox potential data (Fig. S8, Table 1)
and the bandgaps (Eg) were derived from the UV-Vis spectra.
During the anodic scan, two complexes exhibit reversible redox
peaks at 1.0 and 1.1 V, respectively, which belong to the metal-
centered Ir3+/Ir4+ redox pairs. The position of trifluoromethyl
group has little effect on redox process, as the oxidation peaks
-6.37
14.15
3.66
-5.71
Ir2
LUMO
-1.97
2.84
LUMO+1
LUMO+2
-1.88
4.62
2.42
-1.73
3.19
93.38
Theoretical calculations were also carried out through time-
dependent density function theory (TD-DFT), and B3LYP with
6-31G (d, p) and LanL2DZ basis sets to obtain deep insight into
the orbital distributions of two complexes. The conductor-like
polarizable continuum model (C-PCM) method was carried out
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to calculate the solvent effect of CH2Cl2. The orbital distribution
data of several molecular orbitals (MOs) are listed in Table 2,
and their optimized structures and electron cloud distributions
are shown in Fig. S10. For most Ir(III) complexes, the main
ligands and Ir atom greatly affect the photophysical properties
of the complexes as the HOMO/LUMO levels mainly situate on
them. And the effects of ancillary ligands on electron cloud
distribution are significant as well (2.71%-93.38%), which leads
to the unusual HOMO/LUMO energy levels and the blueshift of
emission spectra.
character of the phenylsulfonyl group (S=O) could contribute to
the good electron mobility of two complexes as well, and finally
promote their device performances.
OLED performance
Devices D1 and D2 using Ir1 and Ir2 as emitters, respectively,
were fabricated to evaluate the EL performances with the
configuration
of
ITO/
HAT-CN
(hexa-
azatriphenylenehexacabonitrile, 6 nm)/ TAPC (60 nm)/ TCTA
(10 nm): Ir(III) complexes (8 wt%)/ 2,6-DCzPPy (10 nm): Ir(III)
complexes (8 wt%)/ TmPyPB (1,3,5-tri((3-pyridyl)-phen-3-
yl)benzene, 60 nm)/ LiF (1 nm)/ Al (100 nm). HAT-CN and LiF
were employed as interface modified materials for anode and
cathode, respectively. TAPC which has good hole mobility and
high LUMO level acted as hole-transport/electron-block layer,
and TCTA/2,6-DCzPPy were used as host materials, while
TmPyPB served as electron-transport/hole-block layer due to its
high electron mobility and low HOMO level.39-41 The energy
level diagram of the devices and chemical structures of the
relevant materials are listed in Fig. 4. The device performances
are shown in Fig. 5, including EL spectra, current efficiency (ηc)
and external quantum efficiency (EQE) verse luminance (L)
curves. The corresponding key data are listed in Table 3, and
other EL performances data can be found in Fig. S11.
Electron mobility
As the hole mobility of the hole transport materials are much
higher than the electron mobility of the electron transport
materials, the electron mobility of the dopant materials cause
great impact on the charge balance and affect the efficiency and
efficiency roll-off of the OLEDs greatly.35,36 Thus, emitters with
good electron mobility could benefit the transportation of the
electron leading to a more balanced distribution of hole/electron
Ir1
Ir2
6.0x10-6
4.0x10-6
2.0x10-6
1150
1200
1250
1300
F1/2 (V1/2 cm-1/2
)
and suppressing the annihilation of excitons, which helps to
promote the device performances.37
Fig. 3 Electron mobility of the complexes as a function of the square of
the electric field.
The transient electroluminescence (TEL) method is
a
convenient and efficient method to measure the transport
mobility of the materials.38 Therefore, it was carried out to
estimate the electron mobility of the complexes based on device
structures of ITO/ TAPC ((bis(4-(N,N-ditolylamino)phenyl)-
cyclohexane, 60 nm)/ Ir(III) complexes (60 nm)/ LiF (1 nm)/ Al
(100 nm). (Fig. S11) In which the TAPC acts as hole transport
layer, while the complexes act as emission/electron transport
layer. And the calculated electron mobility of Ir1 and Ir2 are in
the range of 4.00 - 4.50×10-6 and 3.88 - 4.74×10-6cm2 V-1 s-1
(Fig. 3), respectively, which are comparable to that of the
electron mobility of traditional electron transport material Alq3
(tris(8-hydroxyquinolinato)aluminum, 3.65 - 4.21×10-6 cm2 V-1
s-1, Fig. S12). The aromatic sulfonyl groups, as an electron-
withdrawing group without extended π conjugation, has been
used in many ligands to improve the electron mobility of the
complexes.13,17,19 Therefore, PySO3 which retains the structural
Fig. 4 The device structure and the energy level diagram of the HOMO
and LUMO levels of materials employed in this work and their chemical
structures.
Devices D1 and D2 exhibit green emissions with maximum
peaks at 496 and 507 nm with CIE coordinates of (0.22, 0.59)
and (0.24, 0.64), respectively, which are similar to their PL
spectra in solution indicating that the EL emission mainly
comes from the triplet excited states of the emitters. And there
are no residue emissions of host materials around 400 nm,
implying that the energy completely and effectively transfers
from the host to the complexes under the electric excitation.
Both devices exhibit pretty well performances. For device D1,
a Lmax of 44885 cd m-2, a maximum current efficiency (ηc, max
)
of 69.7 cd A-1, an EQEmax of 21.7% and a maximum power
efficiency (ηp,max) of 41.8 lm W-1 are obtained. And device D2
exhibits better EL performances with a a Lmax of 92297 cd m-2, a
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ηc,max of 91.4 cd A-1, an EQEmax of 25.5% and a ηp,max of 59.5 lm
W-1, respectively. As the emitters exhibit similar emission
spectra and electron mobility, the differences in the EL
performances mainly origin from the higher PLQY and lower
LUMO energy levels of Ir2, which benefited the electron
injection process and contributed to the charge balance of the
device D2. Moreover, for Ir2 in doped films, there might be
intramolecular hydrogen bond similar to the observation in
crystal structure, which could limit the rotation of the
trifluoromethyl group and molecular vibration, and slightly
alleviate the non-radiative transition process and benefit its EL
performances ultimately.
According to the recent reports, green-emitting OLEDs
whose EQEmax mainly located in the range of 16.6%-28.7%
always obtain sharply declined EQE-L curves at the high
luminance.42-47 For example, Tong et al. reported a green-
emitting device with the EQEmax of 25.2%, the Lmax of 27450 cd
m-2 and the efficiency roll-off radio reached 9.5% at the
luminance of 1000 cd m-2.43 Chen and coworkers fabricated
high performance devices with the lowest efficiency roll-off
value of 7.9% at the 10000 cd m-2 among the selected literatures,
but the efficiency is less impressive with the EQEmax of
20.0%.44 The devices in this case possess good efficiency and
extremely low efficiency roll-off with flat EQE-L curves (Fig.
5(b)). At the high luminance of 10000 cd m-2, the EQE of
device D1 can be kept at 20.5% with a small efficiency roll-off
ratio of 5.5% compared with the EQEmax. And this characteristic
becomes more outstanding for device D2, as the EQEs still
maintain at 24.5% and 22.9% when the luminance reach at
10000 and 20000 cd m-2, respectively, which only decrease by
3.2% and 9.5%. In theory, the good device performances can be
explained by the balanced electron-hole transportation and
subdued nonradiative quenching processes. Firstly, the double
light-emitting layers can widen the excitons recombination zone
efficiently, which alleviate the excitons annihilation resulting
from the high density of the excitons at the interface of the
organic layers.48 Secondly, the HOMO energy gap between
TAPC and 2,6-DCzPPy could be reduced because of another
emitting layer TCTA, which decreases the hole injection barrier
efficiently and finally promotes the device performances.
Thirdly, the emitters have their own superiority. The lifetimes of
the emitters are short enough to avoid serious triplet-triplet
annihilation and the bulky trifluoromethyl substituents in the
main ligands may increase the molecular distance which
Bookmark not
alleviate the self-quenching of the excitons.Error!
defined.
More importantly, the emitters have low LUMO energy
levels and good electron mobilities, which improve their EI and
ET ability, respectively, and finally enhance the charge balance
of the devices.
Table 3. EL performances of the devices D1 and D2.
a
b
c
d
e
f
Vturn-on
Lmax
ηc,max
ηc, L10000
EQEmax
(%)
ηp,max
Device
CIEg (x, y)
(V)
(cd m-2)
(cd A-1)
(cd A-1)
(lm W-1)
D1 (8 wt%)
D2 (8 wt%)
3.6
3.8
44885
92297
69.7
91.4
65.8
87.7
21.7%
25.5%
41.8
59.4
(0.22, 0.59)
(0.25, 0.64)
a The voltage at a luminance of 1 cd m-2. b Maximum luminance. c Maximum current efficiency. d The current efficiency at a luminance of 10000 cd m-2.
e Maximum external quantum efficiency. f Maximum power efficiency. g Calculated at 6 V.
100
80
60
40
20
0
30
25
20
15
10
5
(a)
(b)
(c)
1.0
0.8
0.6
0.4
0.2
0.0
D1
D2
D1
D2
D1
D2
0
5000
10000
15000
20000
400
500
600
700
0
5000
10000
15000
20000
Luminance (cd m-2)
Luminance (cd m-2)
Wavelength (nm)
Fig. 5 Characteristics of device D1 and D2: (a) electroluminescence spectra (6 V); (b) current-efficiency-luminance (ηc-L) curves; (c) external quantum
efficiency-luminance (EQE-L) curves.
Conclusions
strong polar PySO3 moiety lowers down the LUMO energy
levels and improves the electron mobility of emitters which
facilitates the electron injection possess and enhances the
balance of carries. Therefore, device with double light-emitting
layers using Ir2 as emitter obtains excellent performances with a
Lmax above 92000 cd m-2, a ηc, max of 91.4 cd A-1 and an EQEmax
In summary, the effect of pyridine sulfonic acid (PySO3) as
ancillary ligands in Ir(III) complexes for high-efficient OLEDs
was studied in details. Two green-emitting Ir(III) complexes Ir1
and Ir2 with emission peaks at 496 and 504 nm, respectively,
exhibit good thermal stability. Besides, the introduction of
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of 25.5%. Moreover, the EQE still remains 22.9% even at the
luminance of 20000 cd m-2 and the EQE curves decrease slowly
as the luminance increases exhibiting an extremely low
efficiency roll-off, which is rare among the reported green-
emitting OLEDs as well.
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Conflicts of interest
There are no conflicts to declare.
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Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, P. R.
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A pyridine sulfonic acid aas developed for green-emitting iridium(III) compounds,
which exhibit an EQEmax of 25.5% with extremely low efficiency roll-off, as well as
the EQE still remains 22.9% at 20000 cd m-2.