High efficiency electroluminescence of orange-red iridium(III) complexes for OLEDs with an EQE over 30%

Article abstract of DOI:10.1016/j.dyepig.2021.109733

In this study, three pyrazol-pyridine ligands, 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy), 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (cf3pzpy), and 2-(3-phenyl-1H-pyrazol-5-yl)pyridine (phpzpy) were successfully synthesized for three orange-red iridium (III) complexes Ir1, Ir2, and Ir3, respectively, in which (2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline (BTPIQ) was applied as main ligand. Their single crystals were obtained by vacuum sublimation. They showed distinct photoluminescent emissions at 583 nm with a shoulder peak at 624 nm, with high phosphorescence quantum yields of up to 89%. Organic light-emitting devices (OLEDs) with these complexes as emitters exhibits good performances. Especially, the device with Ir3 complex achieves the best performance with a maximum luminance of 24,188 cd m^?2, and a highest external quantum efficiency of 30.65%. This research proved that Ir(III) complexes show highly enhanced performance by the modification of electro-donating benzene ring group into ancillary ligands, which also offers us an efficient strategy to obtain high efficiency orange-red Ir(III) complexes for OLEDs.

Full text of DOI:10.1016/j.dyepig.2021.109733

Dyes and Pigments 195 (2021) 109733
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
High efficiency electroluminescence of orange-red iridium(III) complexes
for OLEDs with an EQE over 30%
a
,
1
b
,
1
c,
*
a
a,
***
b,**
Ning Su , Shuaibing Li , Kun Yang , Feifan Zhou , Jun Song
, Liang Zhou
,
a,d
Junle Qu
a
Center for Biomedical Optics and Photonics (CBOP) & College of Physics and Optoelectronic Engineering, Key Laboratory of Optoelectronic Devices and Systems,
Shenzhen University, Shenzhen 518060, PR China
b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
c
Department of Materials Science and Engineering, Southern University of Science and Technology (SUSTech), No. 1088, Xueyuan Road, Shenzhen, Guangdong, 518055
China
d
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409, Moscow, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, three pyrazol-pyridine ligands, 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy), 2-(3-(tri-
fluoromethyl)-1H-pyrazol-5-yl)pyridine (cf3pzpy), and 2-(3-phenyl-1H-pyrazol-5-yl)pyridine (phpzpy) were
successfully synthesized for three orange-red iridium (III) complexes Ir1, Ir2, and Ir3, respectively, in which
Organic light-emitting diode
Orange-red iridium(III) complexes
Electroluminescence
High efficiency
(
2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline (BTPIQ) was applied as main ligand. Their single crystals were
obtained by vacuum sublimation. They showed distinct photoluminescent emissions at 583 nm with a shoulder
peak at 624 nm, with high phosphorescence quantum yields of up to 89%. Organic light-emitting devices
(
OLEDs) with these complexes as emitters exhibits good performances. Especially, the device with Ir3 complex
ꢀ 2
achieves the best performance with a maximum luminance of 24,188 cd m , and a highest external quantum
efficiency of 30.65%. This research proved that Ir(III) complexes show highly enhanced performance by the
modification of electro-donating benzene ring group into ancillary ligands, which also offers us an efficient
strategy to obtain high efficiency orange-red Ir(III) complexes for OLEDs.
1
. Introduction
Organic light-emitting diodes (OLEDs) are generally considered as an
(piq) derivatives and selenium atom contained structures [21–23] are
usually employed as main ligands, and common acetylacetone (acac)/-
pyridinic acid (pic) units were utilized as ancillary ligands, respectively
[24,25]. However, these obtained orange-red/red Ir(III) complexes
usually suffer from relatively low external quantum efficiency (EQE)
values (less than 30%), which originates from the unbalanced carrier
transportation and enhanced non-radioactive quenching [26–29].
Consequently, it is important to develop novel orange-red/red Ir(III)
complexes to enable excellent device performance.
important technologies for novel flat-panel displays and solid-state
lighting sources, which is due to their unique properties, including
high efficiency, good color quality, and low manufacturing costs [1–9].
Phosphorescent iridium (III) complexes are one of the most potential
candidates in OLEDs due to their unique properties, such as excellent
performance, nearly 100% internal quantum efficiency as well as easily
controlled emissions [10–15].To date, the majority of molecular design
tactics have been developed for highly efficient phosphorescent Ir(III)
complexes to fine-tune their emissions covering entire regions and to
further satisfy the demand for OLEDs in future [16–20]. In terms of
In our previous work, we successfully developed two bluish-green Ir
2 2
(III) complexes namely Ir(BTBP) mepzpy and Ir(BTBP) phpzpy [30], in
which 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy), and 2-(3-phe-
nyl-1H-pyrazol-5-yl)pyridine (phpzpy) were utilized as ancillary li-
gands, respectively, and their devices achieved EQE values of 25.33%
orange-red/red Ir(III) complexes, large -extended 1-phenylisoquinoline
π
*** Corresponding author.
*
*
* Corresponding author.
Corresponding author.
E-mail addresses: yangk3@sustech.edu.cn (K. Yang), songjun@szu.edu.cn (J. Song), zhoul@ciac.ac.cn (L. Zhou).
1
Su and Li contributed equally to this paper.
https://doi.org/10.1016/j.dyepig.2021.109733
Received 7 July 2021; Received in revised form 16 August 2021; Accepted 17 August 2021
Available online 19 August 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

N. Su et al.
Dyes and Pigments 195 (2021) 109733
and 24.31%, respectively. It is proved that utilizing pyrazolpyridine
derivatives ancillary ligands with more nitrogen atoms and a large
red residue was dissolved in dichloromethane (DCM) (50 mL), and then
2 4
washed with brine and distilled water (200 mL), dried over Na SO , and
spatial structure of –CF
3
groups into Ir(III) complexes has a positive
concentrated to obtain crude orange-red products, which were purified
by column chromatography and then washed with methanol. High pu-
effect on strengthening the injection and transportation of the carriers,
leading to highly efficient device performances [31–33].
rity orange-red crystals were obtained by vacuum sublimation.
1
To further extend the advantage of this type of Ir(III) complexes, in
Ir1 (yield: 78%). H NMR (500 MHz, CDCl
3
) δ 8.80 (d, J = 8.6 Hz,
this work, a novel
π-extended main ligand 1-(2,6-bis(trifluoromethyl)
1H), 8.71 (d, J = 8.3 Hz, 1H), 8.56 (s, 1H), 8.48 (s, 1H), 7.89–7.73 (m,
6H), 7.70 (d, J = 5.7 Hz, 1H), 7.62–7.58 (m, 1H), 7.46–7.38 (m, 2H),
7.31 (d, J = 6.6 Hz, 2H), 7.21 (d, J = 6.6 Hz, 1H), 6.88–6.85 (m, 1H),
pyridin-4-yl)isoquinoline (BTPIQ) was designed for three pyrazol
pyridine-based Ir(III) complexes (Ir1, Ir2, and Ir3), in which 2-(3-
methyl-1H-pyrazol-5-yl)pyridine (mepzpy), 2-(3-(trifluoromethyl)-1H-
pyrazol-5-yl)pyridine (cf3pzpy), and 2-(3-phenyl-1H-pyrazol-5-yl)pyri-
dine (phpzpy) were applied as ancillary ligands, respectively. The
combination of an isoquinoline unit would take good effect on the red
shift of their absorption and emission [34,35].Multiple trifluoromethyl
6.20 (s, 1H), 2.09 (s, 3H). 19F NMR (376 MHz, CDCl
) δ ꢀ 62.95 (s),
3
ꢀ 63.91 (s), ꢀ 67.23 (d,
J
= 5.7 Hz). HRMS (m/z): calcd for
+
C
41
H
22
F
12IrN
7
[M+H] 1034.1453, found 1034.1447.
1
Ir2 (yield: 76%). H NMR (500 MHz, CDCl
3
) δ 8.75 (t, J = 7.5 Hz,
2H), 8.61 (s, 1H), 8.46 (s, 1H), 7.90–7.74 (m, 7H), 7.70 (t, J = 7.8 Hz,
1H), 7.55–7.45 (m, 2H), 7.33 (d, J = 6.6 Hz, 1H), 7.19 (d, J = 6.6 Hz,
(
-CF
3
) groups in the main ligand (BTPIQ) would significantly restrain
π-π
1
9
stacking, and more nitrogen atoms in Ir(III) complexes would efficiently
improve the electron affinity and mobilities of Ir(III) complexes. Espe-
cially, by employing three Ir(III) complexes as emitters, the devices
achieve good performance levels with a maximum luminance of 24,188
1H), 7.02 (dd, J = 17.1, 10.8 Hz, 2H), 6.63 (s, 1H). F NMR (376 MHz,
CDCl
3
) δ ꢀ 60.82 (d, J = 1.8 Hz), ꢀ 63.01 (s), ꢀ 63.85 (d, J = 4.1 Hz),
+
ꢀ 67.26 (d, J = 3.8 Hz). HRMS (m/z): calcd for C41
H
19
F
15IrN
7
[M+H]
1088.1170, found 1088.1164.
ꢀ
2
ꢀ 1
1
cd m , a highest current efficiency of 41.70 cd A , a maximum power
efficiency of 37.43 lm W 1, and a best external quantum efficiency of
Ir3 (yield: 80%). H NMR (500 MHz, CDCl
3
) δ 8.79 (d, J = 8.9 Hz,
ꢀ
1H), 8.74 (d, J = 8.6 Hz, 1H), 8.61 (s, 1H), 8.53 (s, 1H), 7.85–7.68 (m,
7H), 7.64–7.61 (m, 1H), 7.58–7.47 (m, 4H), 7.38 (d, J = 6.2 Hz, 1H),
7.25–7.19 (m, 4H), 7.13 (t, J = 7.4 Hz, 1H), 6.94–6.87 (m, 1H), 6.71 (s,
3
0.65% [33,36–39].This work would have a significant effect on the
design of highly efficient orange-red OLEDs.
1
9
1
H). F NMR (376 MHz, CDCl
3
) δ ꢀ 62.93 (s), ꢀ 64.07 (s), ꢀ 67.05 (s),
2
. Experimental section
ꢀ 67.19 (s). HRMS (m/z): calcd for C46
H
24
F
12IrN
7
[M+H]+ 1096.1609,
found 1096.1604.
2
.1. General synthesis of Ir(III) complexes
3
. Results and discussion
The suspension of main ligand BTPIQ (1.00 g, 2.92 mmol) and IrCl
3
(
2 2 2
0.44 g, 1.46 mmol) was refluxed overnight in EtOCH CH OH: H O (40
3.1. Synthesis and characterization
mL, V/V = 3:1). After cooling to RT, distilled water (100 mL) was added
to the system, and the orange precipitates were formed and then filtered
As shown in Scheme 1, main ligand, BTPIQ, was successfully ob-
tained based on a previously reported literature [35].The -chloro--
bridged product [(BTPIQ) Ir ( -Cl)] was synthesized by mixing IrCl (1
equiv.) with BTPIQ (3 equiv.) in 2-ethoxyethanol (EtOCH CH OH) and
to obtain the chloro-bridged products [(BTPIQ)
mixture of ancillary ligands (3a, 3 b, or 3c, Scheme 1) (3 eq), [(BTPIQ)
-Cl)] (1 eq), and Na CO (5 eq) was then refluxed for 12 h in
EtOCH CH OH (15 mL). The solution was slowly cooled to RT, and then
the redundant solution was removed under reduced pressure, the dark-
2
Ir (
μ-Cl)]
2
. And then a
μ
2
Ir
2
μ
2
3
(
μ
2
2
3
2
2
2
2
water (v/v = 3/1) under high temperature. The ancillary ligands (3a, 3
b, and 3c) were obtained based on our previous research [30,32,40].As
Scheme 1. Synthetic route of Ir1, Ir2, and Ir3 complexes.
2

N. Su et al.
Dyes and Pigments 195 (2021) 109733
shown in Scheme 1, Ir1, Ir2, and Ir3 complexes were synthesized by
Table 1
Crystal information of Ir1, Ir2, and Ir3 complexes.
heating
μ
-chloro-bridged product [(BTPIQ)
2
Ir (
μ
2
-Cl)] with different
ancillary ligands and Na
2
CO in EtOCH CH OH, respectively. They were
3
2
2
Ir1 Ir2
Ir3
further confirmed by 1H NMR, F NMR, high-resolution mass spec-
trometry (HRMS) (Fig. S8-S16, Supporting Information (SI)), and X-ray
crystallography (SI). It is worth noting that single crystals of three Ir(III)
complexes were successfully obtained under vacuum sublimation with
19
Formula
C
41
H
22
F
12 IrN
7
C
41
H
19
F
15IrN
7
46 24 7
C H F12IrN
Formula weight
T (K)
1032.85
296 (2)
0.71073
1086.83
296 (2)
0.71073
1094.92
296 (2)
0.71073
Wavelength (Å)
Crystal system
Space group
a (Å)
Monoclinic
P2 /c
Monoclinic
P2 /c
Monoclinic
P2/c
good yields (>90%) due to the presence of more –CF
3
groups in their
1
1
structures, which is also beneficial for OLED fabrication and applications
with vacuum sublimation.
9.128 (2)
14.114 (3)
30.228 (7)
90.00
9.186 (3)
14.305 (5)
30.008 (10)
90.00
15.7884 (12)
9.0675 (7)
28.543 (2)
90.00
b (Å)
c (Å)
α
(deg)
3
.2. X-ray crystallographic analysis thermal stability
β (deg)
γ (deg)
96.667 (5)
90.00
97.459 (7)
90.00
90.895 (2)
90.00
The single crystals of the Ir1, Ir2, and Ir3 complexes are shown in
3
V (Å )
3868.0 (16)
4
3910 (2)
4
4085.8 (5)
4
Fig. 1, and collected information and sectional bond lengths/angles data
are summarized in Table 1 and Table S1 (SI), respectively. The Ir–N
bond lengths in ancillary ligand range from 2.038 (4) to 2.176 (5) Å,
which is longer than that of the Ir–N (2.044(4)–2.157(4)) in the main
ligand. Moreover, the Ir–C bond lengths were 2.023(5)–2.067(5) Å,
which is within reasonable bounds [41].The C–Ir–N angles in the Ir2
Z
3
ρ
calcd (g/cm )
(Mo K ) (mm
F (000)
1.774
1.846
1.780
ꢀ
1
μ
α
)
3.551
3.529
3.368
2008
2104
2136
Range of transm factors
(deg)
1.356–27.497
1.579–25.008
1.290–25.009
Reflns collected
26,009
21,431
22,411
◦
◦
Unique (Rint
)
8826 (0.0609)
0.0439, 0.0936
0.0785, 0.1079
0.992
6882 (0.0475)
0.0375, 0.0834
0.0534, 0.0897
1.067
7217 (0.0446)
0.0343, 0.0806
0.0476, 0.0967
1.042
complex were between 103.9 (2) and 168.7 (2) , while those in the Ir1
a
b
R
R
1
, wR
, wR
2
[I > 2s(I)]
(all data)
◦
and Ir3 complexes were approximately 80.0 . Moreover, Ir1 and Ir3
a
b
1
2
◦
complexes also exhibited similar N–Ir–N angles (76.82 (19) and 76.74
2
GOF on F
CCDC No.
◦
(
(
17) , whereas the Ir2 complex showed much larger N–Ir–N angles
1,822,135
b
1,822,165
2
1,822,134
R1 = Σ||F | ꢀ |F ||/ΣF |. wR = [Σw (Fo2 ꢀ Fc2) /Σw (Fo )]
a
2
1/2
◦
92.76 (18) ), which originated from the large spatial –CF
3
group into
o
c
o
2
ancillary ligand of Ir2 complex.
Thermal stability is an important factor in OLEDs applications,
especially with the vacuum sublimation method. Thermogravimetric
analysis (TGA) curves was shown in Fig. 2. As a result, Ir1, Ir2, and Ir3
complexes displayed decomposed temperatures (T
d
: 5% weight loss) of
◦
3
85, 408, and 390 C, respectively. (Table 2). Therefore, all Ir(III)
complexes would be successfully applied in OLEDs fabrication
employing a vacuum sublimation process owing to their excellent
◦
thermal stabilities (>380 C).
3
.3. Photophysical property
Fig. 3a shows the UV/vis absorption and emission spectra of Ir1, Ir2,
ꢀ 5
and Ir3 complexes in a dilute DCM (10 M) at room temperature (RT).
Three Ir(III) complexes have two apparent absorption bands, in which
the absorptions below 330 nm are assigned to the ligand-centered (LC)
transitions, while the absorptions around 380 nm is attributed to metal-
to-ligand charge transfer (MLCT) transitions [42].Moreover, Ir2 com-
plex displays slightly blue-shifted absorption bands compared to the
Fig. 2. TGA curves of Ir1, Ir2, and Ir3 complexes.
approximately 623 nm, exhibiting orange-red emissions. It is worth
noting that the different ancillary ligands of Ir(III) complexes have little
effect on their emissions; for example, Ir2 complex has a 1 nm
blue-shifted PL emission peak than Ir1 complex, whilst a 2 nm
others due to electron-withdrawing –CF
3
group into ancillary ligand.
Fig. 3a also displays the photoluminescent (PL) spectra of three Ir(III)
ꢀ 5
complexes in dilute DCM (10 M). They show similar emissions with
one main peak located at about 583 nm and one shoulder peak at
Fig. 1. ORTEP diagram of Ir1 (CCDC No. 1822135), Ir2 (CCDC No. 1822165) and Ir3 (CCDC No. 1822134) complexes with the atom-numbering schemes. Hydrogen
atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
3

N. Su et al.
Dyes and Pigments 195 (2021) 109733
Table 2
Photophysical data of Ir1, Ir2, and Ir3 complexes.
o
a
c
d
p
e
f
Complexes
T
d
( C)
UV (nm)
PL (nm)
Φ
PL (%)
τ
(μs)
EHOMO (eV)
ELUMO (eV)
a
b
RT
77 K
Ir1
Ir2
Ir3
385
408
390
261, 351, 374
261, 295, 356
261, 287,374
584,624
583,622
586,624
584, 635, 693
583,634, 693
586,636, 693
72
60
89
2.77
2.64
2.70
ꢀ 5.59
ꢀ 5.62
ꢀ 5.57
ꢀ 3.46
ꢀ 3.47
ꢀ 3.47
a
ꢀ 5
ꢀ 5
Measured in dilute DCM (10 .M) at RT.
b
c
d
f
Measured in dilute DCM (10 .M) at 77 K.
ꢀ 5
2
2
Measured in dilute DCM (10 .M) in N
2
atmosphere according to the equation of Φ
s
= Φ
r s r s r s r
(ɳ A I /ɳ A I ).
ꢀ 5
e
Measured in dilute DCM (10 .M) in N
2
atmosphere; Calculated from empirical equation: EHOMO = - (Eox+ 4.8) eV.
opt
Calculated from ELUMO = E
g
+ EHOMO.
ꢀ 5
Fig. 3. Normalized UV/vis absorption and emission spectra of Ir1, Ir2, and Ir3 complexes in dilute DCM (10 M) at RT (a) and 77 K (b).
red-shifted PL emission is observed in Ir3 complex than that of Ir1
(Table 2), indicating they would have potential applications in OLEDs.
complex. Furthermore, PL emissions at 77 K are also investigated
(
Fig. 3b). Three Ir(III) complexes show one more emission peak located
at 693 nm at 77 K than their counterparts measured at RT, which in-
dicates that all Ir(III) complexes are emitted from a mixture of
3.4. Electrochemical property and theoretical calculation
3
3
3
MLCT/ LLCT dominating character with a little
43].Moreover, phosphorescent lifetime ( ) values of all complexes are
.77, 2.64, and 2.70 s, (Fig. S1 (SI), Table 2), respectively, which is
π
-
π
* contribution
The highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels (EHOMO/ELUMO) of Ir(III)
complexes are vital for OLEDs structure design. Cyclic voltammetry (CV)
curves were shown in Fig. S2 (SI) to assess their electrochemical prop-
erties. They exhibit different oxidation potentials, which is due to
different substituent groups in ancillary ligands of Ir(III) complexes.
[
τ
p
2
μ
similar to reported orange-red/red Ir(III) complexes [44]. Furthermore,
they display good phosphorescence quantum yields (ΦPL) of 0.60–0.89
Fig. 4. Theoretical HOMO/LUMO energy levels of the Ir(III) complexes.
4

N. Su et al.
Dyes and Pigments 195 (2021) 109733
According to EHOMO = ꢀ [Eox - E (Fc/Fc+) + 4.80] eV, the HOMO levels
are ꢀ 5.59, ꢀ 5.62, and ꢀ 5.57 eV, respectively. Their ELUMO values are
(Scheme 2), respectively. As shown in Scheme 2, HATCN, with unique
hole-injecting properties, served as the hole-injecting layer. TAPC and
Tm3PyP26PyB acted as the hole- and electron-transport media,
respectively. Both TCTA and CzSi were used as host materials due to
their compatible energy levels and excellent carrier transport properties
[46].To optimize the electroluminescent properties of each Ir(III) com-
plex, different dopant concentrations were utilized in each device. The
electroluminescent (EL) spectra, current density (J)-voltage (V)-lumi-
ꢀ
3.46, ꢀ 3.47, and ꢀ 3.47 eV, respectively. The corresponding data is
^
listed in Table 2. Generally, reduction potential is related to the C N
main ligand, while oxidation is associated with both iridium atom and
ancillary ligands [45].Therefore, three Ir(III) complexes exhibit quite
similar ELUMO values (ꢀ 3.46, ꢀ 3.47, and ꢀ 3.47 eV) as they have same
^
C N main ligand BTPIQ, however, their EHOMO values change from
ꢀ
5.57 to ꢀ 5.62 eV, originating from different ancillary ligands of Ir(III)
complexes.
Moreover, density functional theory (DFT) calculations were further
researched (Fig. 4). Their LUMOs are mostly located on BTPIQ with
nance (L), current efficiency (
η
c
η
p
) versus luminance, power efficiency ( )
versus luminance, and external quantum efficiency (EQE) characteristics
of D1, D2, and D3 devices with different doping concentrations are
depicted in Fig. S3–S5 (SI), and key EL data are presented in
Tables S3–S5 (SI). The best EL performances of each device is shown in
Fig. 5, and important data is listed in Table 3.
9
4.01%–95.67%, and distribute minimally on Ir atom (2.37%–4.12%)
and ancillary ligands (1.84%–1.95%). However, the HOMOs are
dependent on ancillary ligands of Ir(III) complexes. For example, in Ir1
and Ir3 complexes, their HOMOs distributed on the ancillary ligands
As shown in Fig. 5a, the EL emissions of the three devices peak at
583 nm and 625 nm, which matches well with their PL emissions of
three Ir(III) complexes, indicating their triplet excited states nature. It is
found that EL emissions are not sensitive to the doping concentration. As
shown in Fig. S3a, D1 device has the same emissions with doping con-
centrations at 2, 3, 4, 5, and 6 wt%. Although the D2 and D3 devices
have higher doping concentrations ranging from 3 to 8 wt%, their
emissions also have no clear difference (Fig. S4a and Fig. S5a). No
apparent emission peaks from TCTA and CzSi in each device is found,
suggesting more productive energy transfer. All Ir(III) complexes ach-
ieve orange-red emissions with CIE coordinates of (0.56, 0.39), (0.55,
(
65.83% and 82.83%), and Ir center (12.47% and 23.29%), but there is
little distribution of BTPIQ (4.70% and 10.88%). For Ir2 complex, the
HOMOs were largely located on BTPIQ (49.00%) and Ir center
(
41.04%), while there is little distribution on ancillary ligand (9.96%),
which is related to the withdrawing –CF unit in ancillary ligand.
3
Finally, the desirable bipolar characteristics of Ir1 and Ir3 complexes
indicate that such multifunctional phosphors would play an important
role in conducting both holes and electrons in addition to emitting light,
leading to highly efficient orange-red OLEDs [38,42]. (vide infa).
0
.38), and (0.54, 0.37), respectively (Table 3).
Moreover, D1–D3 devices display low turn-on voltages (Von) of 3.3,
3
.5. Electroluminescent property
3.3, and 3.5 V, respectively (Table 3). To optimize the device perfor-
mance, different doping concentrations are used in each device. In terms
of D1, the doping concentration has a positive effect on the electrolu-
minescent performance to a certain extent. For example, the EL per-
To further study their applications into organic light-emitting diodes
(
OLEDs), double emissive-layer OLEDs with the structure of ITO/
HATCN (hexaazatriphenylenehexacabonitrile, 8 nm)/HATCN (0.3 wt
): TAPC (bis(4-(N,N-ditolylamino)phenyl)-cyclohexane, 40 nm)/TAPC
formances are relatively low with a maximum luminance (L ) of
max
ꢀ 2
ꢀ 1
%
14,823 cd m , a maximum current efficiency (ηc,max) of 29.23 cd A , a
′
′′
ꢀ 1
(
10 nm)/Ir(III) complexes: TcTa (4,4 ,4 -tris(carbazol-9-yl)-triphenyl-
amine) (x wt%, 10 nm)/Ir(III) complexes: CzSi (9-(4-tert-butylphenyl)-
,6-bis(triphenylsilyl)9H-carbazole) (x wt%, 10 nm)/Tm3PyP26PyB
1,3,5-tris(6-(3-(pyridin-3-yl)phenyl) pyridine-2-yl)benzene, 60 nm)/
LiF (1 nm)/Al (100 nm) were fabricated and named as D1, D2 and D3
maximum power efficiency (ηp,max) of 27.83 lm W , and an external
quantum efficiency (EQEmax) of 16.46% at 2 wt% doping concentration
3
(Fig. S3 and Table S3, SI). When increasing to 4 wt%, the performances
ꢀ 2
(
improve considerably, resulting in an Lmax of 15,448 cd m
,
ηc,max of
Scheme 2. The OLEDs device structure and their molecular structures used in this work.
5

N. Su et al.
Dyes and Pigments 195 (2021) 109733
c
Fig. 5. Device characteristics of D1-D3: (a) EL spectra; (b) J-V-L curves; (c) η -L curves and (d) EQE curves.
a Lmax of 24,188 cd m 2, a
ꢀ
η
c,max of 41.70 cd A , a p,max of 37.43 lm
ꢀ 1
η
Table 3
ꢀ 1
W
, and an EQEmax of 30.65%. It is concluded that ancillary ligands
Key EL data of devices D1-D3.
significantly affect the EL properties. In particular, the D3 device with
Ir3 complex as guest has the highest device performance, which is due to
the electron-donating group of the benzene ring into Ir3 complex.
However, these fabricated devices show a degree of efficiency roll-off,
Device
dopant
V
on
a
L
max (cd
η
c,max
η
p,max
EQEmax
CIE (x,y)
ꢀ 2
b
e
(
(V)
m
)
(cd
(lm
(%) /L (cd
ꢀ
1
c
ꢀ 1
d
ꢀ 2
f
con.)
A
)
W
)
m )
D1 (4 wt
3.3
3.3
3.5
15,448
16,350
24,188
29.28
29.13
41.70
31.66
27.73
37.43
21.42/
1.39
(0.56,0.39)
(0.55,0.38)
(0.54,0.37)
for example, the EL efficiency with an EQE of 16.70%, a
η
c
of 28.81
%
)
ꢀ 1
ꢀ 1
cd A , a of 17.75 lm W are presented in device D3 (5 wt%) at 1000
η
p
D2 (6 wt
20.86/
1.06
ꢀ 2
cd m , and some research works based on how to obtain high efficiency
with low efficiency roll-off are in progress. Additionally, we investigated
the PL spectra and PL decays of each blend flim (TcTa: 4 wt% Ir1, CzSi: 4
wt% Ir1, TcTa: 6 wt% Ir2, CzSi: 6 wt% Ir2, TcTa: 5 wt% Ir3, CzSi: 5 wt%
Ir3) to support OLED results (Fig. S6-S7, Table S6, SI). As shown in
Fig. S6, the blend films show little difference emissions, and they match
well with the EL emissions of their counterparts. Fig. S7 shows the PL
decays of six blend films, and the lifetimes of each blend film are 1.83,
%
)
D3 (5 wt
30.65/
1.02
%
)
a
b
c
d
e
f
ꢀ 2
.
Turn-on voltage, the driving voltage at 1 cd m
Maximum luminance.
Current efficiency.
Power efficiency.
External quantum efficiency calculated within visible spectrum region.
The luminance when the device obtained the maximum external quantum
2
.01, 1.83, 0.90, 0.92, 1.72 s, respectively (Table S6, SI). It is apparent
μ
efficiency.
that each complex has different lifetimes in distinct host materials. As a
result, Ir3 complex based blend films display the relatively short life-
times than the others, which is beneficial for achieving good EL per-
formance. Moreover, we find that device performance is related to ΦPL
of dopants. For example, device D3 shows the best EL performance with
Ir3 complex as the emitter, which has the highest ΦPL of 89% among
three complexes. Devices D1 and D2 display poorer OLEDs properties
than D3, which are consistent with their ΦPL of 72% and 60%,
respectively.
3.25 cd A 1
ꢀ
η
ꢀ 1
, p,max of 31.66 lm W , and EQEmax of 21.42%. At 5 wt%
3
and 6 wt%, the EL performance gradually diminishes with EQEmax
values of 18.85% and 16.89%, respectively. In addition, the optimiza-
tion of D2 and D3 devices with different doping concentrations is shown
in Fig. S4–S5 and Tables S4–S5 in SI. Finally, the D2 and D3 devices
obtain their best performance at 6 wt% and 5 wt%, respectively. (Fig. 5,
Table 3).
By further modifying the ancillary ligand of the Ir1 complex with
trifluoromethyl and benzene ring groups, respectively, device D2 with
Ir2 complex as dopant and device D3 employing Ir3 complex as an
emitter exhibit slightly reduced and sharply increased EL properties
compared to device D1. For example, device D2 at 6 wt% displays
Compared to the reported literatures (Table S7, SI), this work shows
one of the highest performances in orange-red/red OLEDs. For example,
Zheng et al. reported a series of four-membered orange-red/red Ir(III)
complexes with Ir–S–P–S structures in ancillary ligands and the corre-
sponding OLEDs achieved the EQEmax of 24.90% with CIE coordinates of
ꢀ
2
inferior EL performances with an Lmax of 16,350 cd m
,
η
c,max of 29.13
p,max of 27.73 lm W , and EQEmax of 20.86%. Device D3, at 5
wt% with Ir3 complex as a dopant, achieves the best performances with
(
0.59, 0.39) [36]. Afterwards, by further modification the ancillary li-
gands structures from Ir–S–P–S to Ir–S–C–S, an improved EQEmax of
6.10% with CIE coordinates of (0.61, 0.36) was successfully achieved
ꢀ 1
ꢀ 1
cd A
,
η
2
6

N. Su et al.
Dyes and Pigments 195 (2021) 109733
[
37]. Liu et al. developed Ir(III) complex BZQPG and the corresponding
[3] Rao J, Yang L, Li X, Zhao L, Wang S, Ding J, Wang L. Meta junction promoting
efficient thermally activated delayed fluorescence in donor-acceptor conjugated
polymers. Angew Chem Int Ed 2020;59:17903–9.
OLEDs achieved an EQEmax of 27.90% with CIE coordinates of (0.57,
0
.42) [38]. Afterwards, Zheng et al. also reported a series of Ir–S–C–S
[
[
[
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configuration fixation with C–H⋅⋅⋅F hydrogen bonding for thermally activated
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four-membered red Ir(III) complexes and the device achieved the EQE-
max of 30.54% with CIE coordinates of (0.63, 0.36) [33]. Moreover, the
same research group also developed three Ir(III) complexes Ir
(
4tfmpq)
4tfmpq)
2
2
mND, Ir (4tfmpq)
2 2
mmND, and Ir (4tfmpq) mpND), with Ir
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(
mmND as emitter, its device showed the EQEmax of 31.48%
with CIE coordinates of (0.67, 0.32) [39]. Therefore, in this work, the
obtained EQEmax of 30.65% is among the highest one in orange-red/red
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the electronic mobility. Secondly, multiple -CF3 groups in the main
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Zhu W, Chou PT. Exploiting racemism enhanced organic room-temperature
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ligand can more effectively depress
π
- packing and reduce the annihi-
π
lation effect of triplet state, thus significantly improving the device
performance. Finally, TCTA and CzSi were utilized in OLEDs structures,
which can further enhance the hole/electron injection and excitons
transportation, resulting in better device performance.
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4
. Conclusion
In summary, three orange-red Ir(III) complexes Ir1–Ir3 were suc-
[
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cessfully obtained in high yields (>76%) and vacuum sublimation yields
(
>90%). They show distinct photoluminescent emissions at approxi-
mately 583–586 nm with shoulder peaks at 624 nm, with high phos-
phorescence quantum yields of 60%–89%. With three Ir(III) complexes
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ꢀ 2
nance of 24,188 cd m , and a highest external quantum efficiency of
0.65%.
3
Author contribution statement
[
[
N. S. designed and synthesized three iridium complexes, measured
their photophysical, thermal stability, electrochemical properties, and
wrote the paper.S. L. fabricated OLEDs, characterized the EL perfor-
mance, and measured the PL spectra, PL decays of iridium complexes
based blend films.K. Y. characterized three iridium complexes, and
measured the X-ray crystallographic analysis.F. Z. carried out the
theoretical calculations of three iridium complexes.J. S. supervised this
project.L. Z. designed the OLED structures and gave some positive sug-
gestions to optimize the OLED results.J. Q. participated in the discussion
of the synthesis and characterizations of three iridium complexes.
16] Ryu CH, Lim J, Kim MB, Lee JH, Hwang H, Lee JY, Lee KM. Tris(5-phenyl-1 h
-
1,2,4-triazolyl)iridium(iii) complex and its use in blue phosphorescent organic
light-emitting diodes to provide an external quantum efficiency of up to 27.8%.
Adv Opt Mater 2021;9:2001957.
[17] Yan ZP, Luo XF, Liao K, Zheng YX, Zuo JL. Rational design of the platinahelicene
enantiomers for deep-red circularly polarized organic light-emitting diodes. Front
Chem 2020;8:501.
[
18] Wang S, Zhao L, Zhang B, Ding J, Xie Z, Wang L, Wong WY. High-energy-level blue
phosphor for solution-processed white organic light-emitting diodes with efficiency
comparable to fluorescent tubes. iScience 2018;6:128–37.
[
19] Liao X-J, Zhu J-J, Yuan L, Yan Z-P, Luo X-F, Zhang Y-P, Lu J-J, Zheng Y-X. Efficient
organic light-emitting diodes based on iridium(iii) complexes containing indolo
[
3,2,1-jk]carbazole derivatives with narrow emission bandwidths and low
Declaration of competing interest
There are no conflicts of interest to declare.
Acknowledgments
efficiency roll offs. J Mater Chem C 2021. https://doi.org/10.1039/d1tc01449h.
20] Yang X, Zhou G, Wong WY. Functionalization of phosphorescent emitters and their
host materials by main-group elements for phosphorescent organic light-emitting
devices. Chem Soc Rev 2015;44:8484–575.
[
[
21] Petrenko A, Leitonas K, Volyniuk D, Baryshnikov GV, Belyakov S, Minaev BF,
Agren H, Durgaryan H, GraZulevicius JV, Arsenyan P. Benzoselenophenylpyridine
platinum complexes: green versus red phosphorescence towards hybrid oleds.
Dalton Trans 2020;49:3393–7.
This work was supported by the National Natural Science Foundation
of China (61620106016/61835009/61775145/22005133); Guangdong
Province Key Area R&D Program (2019B110233004).
[
22] Thiyagarajan MD, Balijapalli U, Leitonas K, Volyniuk D, Simokaitiene J,
Keruckas J, Jatautien e˙ E, Pathak M, Iyer SK, Gra ˇz ulevi ˇc ius JV. Human-eyes-
friendly white electroluminescence from solution-processable hybrid oleds
exploiting new iridium (iii) complex containing benzoimidazophenanthridine
ligand. Dyes Pigments 2020;174:108068.
Appendix A. Supplementary data
[
[
23] Arsenyan P, Petrenko A, Leitonas K, Volyniuk D, Simokaitiene J, Klinavicius T,
Skuodis E, Lee JH, Grazulevicius JV. Synthesis and performance in oleds of
selenium-containing phosphorescent emitters with red emission color deeper than
the corresponding ntsc standard. Inorg Chem 2019;58:10174–83.
Supplementary data related to this article can be found at https
:
//doi.org/10.1016/j.dyepig.2021.109733.
24] Chen L, Wang S, Yan Z, Ding J, Wang L. An oligocarbazole-encapsulated
heteroleptic red iridium complex for solution-processed nondoped phosphorescent
organic light-emitting diodes with over 10% external quantum efficiency. J Mater
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