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Pure Red Iridium(III) Complexes Possessing Good Electron Mobility with 1,5-Naphthyridin-4-ol Derivatives for High-performance OLEDs with EQE Over 31% Guang-Zhao Lu, Qi Zhu, Liang Liu, Zheng-Guang Wu, Youxuan Zheng, Liang Zhou, Jing-Lin Zuo, and Hongjie Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02558 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Applied Materials & Interfaces

Pure Red Iridium(III) Complexes Possessing Good Electron Mobility with 1,5Naphthyridin-4-ol Derivatives for High-performance OLEDs with EQE Over 31% Guang-Zhao Lu,1# Qi Zhu,2# Liang Liu,1 Zheng-Guang Wu,1 You-Xuan Zheng,1,* Liang Zhou,2,* Jing-Lin Zuo, Hongjie Zhang 1State

Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering,

Nanjing University, Nanjing 210093, P. R. China E-mail: [email protected] 2State

Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, P.R. China E-mail: [email protected] #Lu and Zhu have same contribution to this paper.

ABSTRACT:

Three

iridium(III)

complexes

(Ir(4tfmpq)2mND,

Ir(4tfmpq)2mmND

and

Ir(4tfmpq)2mpND) with 4-(4-(trifluoromethyl)phenyl)quinazoline (4tfmpq) main ligand, 1,5naphthyridin-4-ol derivatives (mND: 8-methyl-1,5-naphthyridin-4-ol, mmND: 2,8-dimethyl-1,5naphthyridin-4-ol, mpND: 8-methyl-2-phenyl-1,5-naphthyridin-4-ol) ancillary ligands were studied. The complexes (Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND) emit pure red emissions of 626-630 nm with high PLQYs (photoluminescence quantum yields) of 85.2%-93.4% in CH2Cl2 and better electron mobilities than that of AlQ (tri(8-hydroxyquinoline)aluminum). Employing three pure red emitters, all the PHOLEDs exhibited superior performances with an EQEmax (maximum external quantum efficiency) of 31.48% and the efficiency roll-off is very mild, which are among the best results ever reported for pure red OLEDs using Ir(III) complexes. In addition, CIE(x, y) coordinates of (0.670, 0.327) are also close to the standard red emission required by the National Television System Committee (NTSC).

Keywords: organic light-emitting diode, pure red iridium(III) complex, 5-naphthyridin-4-ol derivative, electron mobility, high efficiency

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Introduction Due to the excellent optoelectronic properties, iridium(III) complexes are widely used as emitters in OLEDs (organic light-emitting diodes).1-3 Compared with the well-developed green iridium(III) emitters, the efficient red emitters are limited for high performance red PHOLEDs.4-11 Therefore, it is still important to achieve efficient red Ir(III) complexes for devices with high performances.12-15 But in the energy gap theory, for the materials with longer wavelength emission, the nonradiative rates (knr) would increase with the decreasing of radiative rates (kr).16 To solve the above problem, it is useful to prepare the red Ir(III) complexes containing rigid ligands, such as nitrogen-containing heterocycles, to decrease the nonradiative rates. Over the past years, some efficient red Ir(III) phosphors have been developed for high performance devices. Suh et al. reported high-performance PHOLEDs containing an Ir(III) complex with the CIE(x, y) coordinates of (0.65, 0.35) and an EQEmax of 24.3%.17 Yang group reported a complex of Ir(ptq)2(acac), and its devices displayed the CIE(x, y) coordinates of (0.61, 0.36) with an EQEmax of 22.9%.18 Cho et al. and Wang et al. reported an Ir(Th-PQ)3 complex, and its devices achieved an electroluminescence peak at 612 nm with an EQEmax of 21.3%.19,20 Recently, Chi et al. reported efficient PHOLEDs with an EQEmax of 27.4% and CIE(x, y) coordinates of (0.63, 0.38) containing a bis-tridentate Ir(III) complex.21 Despite much efforts, there are only few studies realized high device EQE values over 30%, especially for pure red PHOLEDs with mild efficiency roll-off. Furthermore, in consideration of the much higher carrier transporting ability of hole transporting material than that of electron transporting material, the design of the emitters with good electron transporting property is also important for efficient PHOLEDs. Some groups reported high performance devices used several Ir(III) complexes containing heterocyclic ligands for good electron mobility.22 Our group recently found the 4-(4-(trifluoromethyl)phenyl) quinazoline (4tfmpq) ligand is very useful for red Ir(III) complexes, which always show higher photoluminescence quantum yield (PLQY) and better electron mobility than those containing the widely used phenylquinoline (piq) ligand.23,24 Furthermore, it is well known that the metal complexes based on 8-hydroxyquinoline (Q) are usually good electron-transporting materials (such


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as AlQ), and the heterocycle structures with more nitrogen atoms would further increase their electron affinity and transport property. Combining these characters together, three Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND (Scheme 1) complexes containing 4-hydroxy-1,5-naphthyridine derivatives (mND: 8-methyl-1,5naphthyridin-4-ol,

mmND:

2,8-dimethyl-1,5-naphthyridin-4-ol,

mpND:

8-methyl-2-phenyl-1,5-

naphthyridin-4-ol) as ancillary ligands by introducing another nitrogen atom in the position 5 of 8hydroxyquinoline were developed in this paper. Therefore, three red complexes show high PLQY up to 93.4% in CH2Cl2 and better electron mobility than AlQ, and their double-emitting-layer devices exhibit superior performances with an EQEmax of 31.48% and the efficiency roll-off is very mild.

Results and Discussion Synthesis and characterization of three complexes CF3 CF3

CF3

CF3

Cl N

a

Ir

Ir

N

N

B(OH)2

Cl

b

Cl

N

N

N

4tfmpq

N N

2

[(4tfmpq)2Ir(-Cl)]2 N

2 CF3

H N

N OH

NH2 N

O O

H N

O

(EtO)3CR

N

O O

Ir(4tfmpq)2mND

N

CH3

c

O

d

N

CH3

2

Ir(4tfmpq)2mmND Ph

CF3

(b) IrCl3, EtOCH2CH2OH, H2O,115 °C,12h;

N

N

Ir

OH

(d) EtOCH2CH2OH, 115 °C, 6h;

N

O

N

mmND N

N

Ir N

OH

(a) Pd(PPh3)4, Na2CO3, THF-H2O, 70 °C, 12h;

(c) Ph2O, 220 °C, 1h;

H

2

CF3

R

O

N

O

N

mND O

N

Ir

mpND

N N

O

N Ph

2

Ir(4tfmpq)2mpND

Scheme 1. The synthesis routes for the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes.

As shown in Scheme 1, the mND, mmND, mpND ancillary ligands were synthesized firstly by the nucleophilic substitution reaction and then dehydration reaction in a diphenyl ether solution. And the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes were prepared from [(C^N)2Ir(µCl)]2 dimmer and 1,5-naphthyridin-4-ol derivatives. The crude products were refined by silica chromatography and vaccum sublimation.



Molecular structures by the ORTEP drawing and crystallographic data of the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes are shown in Figure 1 and in Table S1, Table S2 (SI section), respectively. The distorted octahedral coordination geometry formed with iridium center and three chelating ligands is shown in the three complexes. The Ir-C (1.975-1.997 Å), Ir-N (2.023-2.147 Å) and Ir-O (2.149-2.158 Å) bond lengths are in good agreement with the other similar constituted Ir(III) complexes.25,26

Figure 1. ORTEP diagram of three complexes: Ir(4tfmpq)2mND (CCDC No. 1832341), Ir(4tfmpq)2mmND (CCDC No. 1832330) and Ir(4tfmpq)2mpND (CCDC No. 1832352).

Furthermore, observed from Figure S1, all Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND phosphors have good thermal stability and their decomposition temperatures (5% weight loss) are as high as 347 oC, 396 oC and 406 oC, respectively.

Electrochemical property and theoretical calculations To calculate the HOMO and LUMO energy levels and electron distributions of all Ir(III) molecules, the density functional theory (DFT) calculations for them were performed. From Figure 2 it can be observed that the HOMO orbitals mostly locate on the 1,5-naphthyridin-4-ol derivatives (64.92-71.17%) together with d orbitals of iridium atom (20.94-24.19%) with a very small portion of the 4tfmpq (7.89-10.89%), but their LUMO orbitals are mostly distributed over the main ligand (94.34-94.56%) and a small amount on iridium d orbitals (4.00-4.09%) and ancillary ligand (1.44-1.58%), respectively. These results illustrate that the attachment of 1,5-naphthyridin-4-ol derivatives as ancillary ligands significantly affect the frontier molecular orbitals, which means that the electron cloud densities of the HOMOs and LUMOs are divided completely over the different parts of three iridium(III) emitters. The little differences between the


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calculated LUMOs energy levels (-2.63 eV - 2.65 eV) fit well with the variation trend of the experimental ones (-3.32 eV - -3.36 eV) due to the same main ligand. And the little differences between the calculated HOMOs energy levels (-5.61 eV - -5.65 eV) also correlate well with the variation tendency of the experimental ones (-5.60 eV - -5.63 eV) owing to the small changes in the 1,5-naphthyridin-4-ol derivatives by tuning the substituents from hydrogen, methyl to phenyl. (Figure 2, Figure S3, Table 1).

LUMOs -2.63 eV

energy gap

-2.63 eV

2.98

3.02

-5.61 eV

-5.65 eV

-2.65 eV

2.99

-5.64 eV

HOMOs

Ir(4tfmpq)2mND

Ir(4tfmpq)2mmND Ir(4tfmpq)2mpND

Figure 2. The isodensity surface plots and HOMO/LUMO orbital levels of the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes.

Photophysical property The

ultraviolet-visible

absorption

and

photoluminescence

spectra

of

Ir(4tfmpq)2mND,

Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND in CH2Cl2 are displayed in Figure 3, and Table 1 collected their photophysical data. The absorption bands below 400 nm originate from the intraligand π→π* transitions, and the bands in the 400-630 nm region are belong to the mixed 1MLCT and 3MLCT (metal-to-ligand charge-transfer) or LLCT (ligand-to-ligand charge-transfer) transitions due to the iridium atom caused spinorbit coupling.27 In comparsion with the complex Ir(4tfmpq)2mND showing emission peak at 626 nm, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes exhibit a slightly bathochromic emission bands at 630 nm and 629 nm (Figure 3(b)), respectively, which coincide with similar energy gaps of the complexes by the electrochemical analysis and theoretical calculation.



(a)

Normalized PL Intensity (a.u.)

Ir(4tfmpq)2mND

Normalized Absorption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ir(4tfmpq)2mmND Ir(4tfmpq)2mpND

300

400 500 Wavelength (nm)

600

700

(b)

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Ir(4tfmpq)2mND Ir(4tfmpq)2mmND Ir(4tfmpq)2mpND

500

550

600


750

800

Figure 3. The UV-vis absorption (a) and emission (b) spectra of the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes in degassed CH2Cl2 (5  10-5 M).

Table 1. Photophysical data of the Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes. Compound

k rd

(%)

(µs)

× 10 S

626

93.4b/85.3c

1.86a/0.34c

2.47

630

88.4b/89.5c

1.95a/0.68c

629

85.2b/78.6c

2.04a/0.75c

Absorptiona

Emissiona

( C)

 (nm)

max (nm)

Ir(4tfmpq)2mND

347

Ir(4tfmpq)2mmND

396

Ir(4tfmpq)2mpND

406

a:

298 K

Td o

285/344/359/ 395/485 282/344/357/ 391/484 283/341/360/ 399/481

P

k nrd

HOMO/LUMOe

CIEa

(eV)

(x, y)

4.26

-5.63/-3.35

(0.652, 0.351)

1.31

1.54

-5.60/-3.32

(0.661, 0.342)

1.05

2.84

-5.63/-3.36

(0.671, 0.331)

6

-1

× 10 S 5

-1

Measured in degassed CH2Cl2; b: Absoluted emission quantum yields measured in degassed CH2Cl2 solution; c: Measured

in 5 wt% doped TCTA films; d): Radiative decay rate kr = Φ/τ, and nonradiative decay rate knr = (1 − Φ)/τ, measured in 5 wt% doped TCTA films; e): HOMO (eV) = -(Eox - E1/2,Fc) - 4.8, LUMO (eV) = HOMO + Ebandgap.

Due to the application of 4-(4-(trifluoromethyl)phenyl)quinazoline and the relatively rigid structure of 1,5-naphthyridin-4-ol derivatives, all complexes show high PL quantum yields in CH2Cl2 solution (93.4% for Ir(4tfmpq)2mND, 88.4% for Ir(4tfmpq)2mmND and 85.2% for Ir(4tfmpq)2mpND, respectively), suggesting the PHOLEDs with these emitters would show high performances. Their excited state lifetimes in CH2Cl2 solution (1.86 μs for Ir(4tfmpq)2mND, 1.95 μs for Ir(4tfmpq)2mmND and 2.04 μs for Ir(4tfmpq)2mpND, respectively) are in the range of microseconds (Figure S2). Besides, the phosphorescence lifetimes (0.34 - 0.75 μs) and PLQYs (78.6% - 89.5%) of the three complexes in the 5 wt% doped 4,4',4''-tris(carbazol-9-yl)triphenylamine (TCTA) films are very close to the values measured in CH2Cl2 solution (Figure S3). Moreover, the radiative decay rates (kr) of all iridium complexes in the range from 1.05 × 106 to 2.47× 106 s−1 in doped films are very high for iridium complexes, which may be beneficial for OLED performances because the triplet excitons by electrical excitation can decay quickly


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through the radiative pathway. And the small changes of the substituents on 1,5-naphthyridin-4-ol ligands has a slight effect on the CIE(x, y) coordinates of (0.652, 0.351) for Ir(4tfmpq)2mND, (0.661, 0.342) for Ir(4tfmpq)2mmND and (0.671, 0.331) for Ir(4tfmpq)2mpND measured in CH2Cl2 solution, respectively, which are just helpful to obtain the pure red emission (Table 1).

-1

4

Ir(tfmpq)2mND

-1

)

Electron-transport Ability and OLED Performance

Ir(tfmpq)2mmND Ir(tfmpq)2mpND

2

Electron Mobility (10 cm V s

3

AlQ

-6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

1150

1200

1250

Electric Field (V cm

)

-1 1/2

1300

Figure 4. Electric field dependence of charge electron mobility in the thin films of Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND, Ir(4tfmpq)2mpND and AlQ complexes.

As mentioned above, to get the efficient PHOLEDs the dopants with good electron mobility are very important. In order to investigate the effect of the introduction of 1,5-naphthyridin-4-ol derivatives on the electron mobility of Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND complexes, the transient electroluminescence (TEL) measurements with the device structure of ITO / TAPC (di-(4-(N,Nditolylamino)phenyl)cyclohexane, 25 nm) / Ir(III) complexes or AlQ (30 nm) / LiF (1 nm) / Al (100 nm) were conducted.28 To make a comparison, the electron mobility of tri(8-hydroxyquinolato)aluminum (AlQ) was also measured as a reference.29,30 The transient EL signals of these devices are listed in SI, and the electric field dependence of charge electron mobility curves are shown in Figure 4. It can be calculated that the electron mobilities (e) of Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND layers are 2.0102.812×10-6, 2.493-3.214×10-6 and 1.707-1.973 cm2 V-1 s-1, respectively, under an electric field of 1150 1300 (V cm-1)1/2, while that of AlQ are 1.147-1.450×10-6 cm2 V-1 s-1. Therefore, the electron mobilities of three compounds are all higher than that of AlQ. In addition, as the substituents in the ancillary ligands



vary from phenyl, hydrogen to methyl, the electron mobilities of the corresponding Ir(III) complexes Ir(4tfmpq)2mpND, Ir(4tfmpq)2mND and Ir(4tfmpq)2mmND are improved gradually. It turns out that the introduction of 1,5-naphthyridin-4-ol ligands is helpful to improve the Ir(III) complexes’ electron mobility, which validates that PHOLEDs with these red Ir(III) complexes may have good performances. To illustrate their electroluminescence (EL) properties, PHOLEDs using three emitters were fabricated with different configutions. Firstly, the single-emitting-layer PHOLEDs consist of ITO / HAT-CN (dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile, 6 nm) / HAT-CN (0.2 wt%): TAPC (50 nm) / Ir(III) complexes (x wt%): 2,6DCzPPy (2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, 10 nm) / Tm3PyP26PyB (1,3,5-tris(6-(3-(pyridin-3-yl)phenyl)pyridin-2-yl)benzene, 60 nm) / LiF (1 nm) / Al (100 nm) with Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND and Ir(4tfmpq)2mpND are named as S1-S3, respectively (Scheme 2). All devices were optimized and the details of optimizing the doping concentration of Ir(III) emitters are included in the SI. The characteristics of the devices are shown in Figure 5, and the key data are collected in Table 2. NC

-1.80 eV

N

CN N N

-2.60 eV

-2.70 eV

NC N

-6.10 eV

N

N CN

HAT-CN Tm3PyP26PyB (60 nm)

-5.70 eV

N

CN

NC

-5.50 eV

N

N

N

-2.40 eV

Ir: 2,6DCzPPy (10 nm)

-5.50 eV

N

Ir: TCTA (10 nm)

ITO

HAT-CN (0.2 wt%):TAPC (50 nm)

-5.10 eV

HAT-CN (6 nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-4.30 eV

TAPC CF3

CF3

LiF/Al

N

N

N

Ir N

Ir N

O N

2

N

N

O 2

CF3 N

Ir

-6.70 eV N N

N

N

N

N N

N

O

Ph

Tm3PyP26PyB

2

Ir(tfmpq)2mpND

-9.50 eV

N N

Ir(tfmpq)2mmND

Ir(tfmpq)2mND N

N

TCTA

N

2,6DCzPPy

Scheme 2. Energy level diagrams and chemical molecular structures of materials used.

As shown in Scheme 2, the HOMO (around -5.60 eV) and LUMO (around -3.35 eV) levels of three dopants were embedded between the HOMO (-6.10 eV) and the LUMO of (-2.60 eV) of 2,6DCzPPy, suggesting the carriers can be injected to the host material or directly trapped by the Ir(III) emitters. Besides, the obvious different energy levels between the dopants and host suggest efficient energy transfer from the 2,6DCzPPy to three emitters. The normalized EL spectra at 1 mA (Figure 5(a)) show peaks at about 628, 628, 620 nm for three devices S1-S3, respectively. And the CIE(x, y) color coordinates (0.667,


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0.331) and (0.668, 0.328) of S1 and S2, respectively, are closest to the standard red demanded of (0.67, 0.33) from the NTSC (National Television System Committee), and the CIE(x, y) of (0.651, 0.334) of S3 is also quite close to the pure red color.31 Because three complexes have similar ancillary ligands, with the changes of the substituents in the ancillary ligands from phenyl and hydrogen to methyl, the resulting complexes from Ir(4tfmpq)2mpND and Ir(4tfmpq)2mND to Ir(4tfmpq)2mmND exhibit very close photoluminescence quantum efficiency and better electron-transporting properties, which would leads to

0.4 0.2 0.0

-1

25 (c) 20

S1 S2 S3

S1 S2 S3

15

10

10

5

5

0

0

2000 4000 6000 -2 8000 Luminance (cd m )

0 10000

-2

3

10

2

20

1

10

0

0

30 25

4

10

10

0

20

15

-2

800

40

2

4 6 Voltage (V)

8

30 (d) -1

600 Wavelength (nm)

60

Luminance (cd m )

0.6

S1 S2 S3 S1 S2 S3

(b)

10 10

S1 S2 S3

25

EQE (%)

0.8

80 Current density (mA cm )

S1 S2 S3

1.0 (a)

Power Efficiency (lm W )

Normalized EL. Intensity (a.u.)

the gradually enhanced OLEDs performances.

Current Efficiency (cd A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10 5 0

100

1000 -2 Luminance (cd m )

10000

Figure 5. (a) EL spectra, (b) J - V - L curves, (c) c - L - p curves and (d) EQE - L curves of S1 - S3 devices.

Respectively, the device S3 (P = 85.2%, e = 1.707-1.973 cm2 V-1 s-1 of Ir(4tfmpq)2mpND) at a 2 wt% doping concentration shows lowest EL performances with a maximum luminance (Lmax) over 43000 cd m-2, a maximum current efficiency (ηc,max) of 21.90 cd A-1, a maximum power efficiency (ηp,max) of 20.85 lm W1

and a maximum external quantum efficiency (EQEmax) of 23.54%. Relatively, the device S1 (P = 93.4%,

e = 2.010-2.812×10-6 cm2 V-1 s-1 of Ir(4tfmpq)2mND) at the doping concentration of 4 wt% exhibits better performances with a Lmax, a ηc,max and EQEmax of 48960 cd m-2, 23.87 cd A-1 and 28.22%, respectively. Due to the high P (88.4%) and the best electron-transporting ability (e = 2.493-3.214×10-6 cm2 V-1 s-1) of



Ir(4tfmpq)2mmND, the device S2 at a doping concentration of 4 wt% exhibits the best characteristics with a Lmax over 38300 cd m-2, a ηc,max of 23.46 cd A-1 and an EQEmax of 28.85%, respectively. To optimize their device performances, the second host material TCTA was also introduced as a “hole ladder” layer, which has matched HOMO level (-5.70 eV) between TAPC (-5.50 eV) and 2,6DCzPPy (2.70 eV, Scheme 2). Therefore, the double emissive layers contained devices with ITO / HAT-CN (6 nm) / HAT-CN (0.2 wt%): TAPC (50 nm) / iridium(III) dopants (x wt%): TCTA (10 nm) / iridium(III) dopants (x wt%) : 2,6DCzPPy (10 nm) / Tm3PyP26PyB (60 nm) / LiF (1 nm) / Al (100 nm) using Ir(4tfmpq)2mND, Ir(4tfmpq)2mmND, Ir(4tfmpq)2mpND emitters are named as D1-D3, respectively (Scheme 2). Their EL

80 -2

0.6 0.4 0.2 0.0

60 40

D1 D2 D3

35 30

30

15

20 15 10

5 0

5 0

2000 4000 6000 -2 8000 Luminance (cd m )

0

2

35 25

10

0

40

20

0 10000

2

10

1

-1

25

D1 D2 D3

3

10

10

0

Power efficiency (lm W )

-1

(c)

10

20


30

4

D1 D2 D3 D1 D2 D3

-2

0.8

(b)

Luminance (cd m )

D1 D2 D3

4 6 Voltage (V)

8

10 10

(d)

25

EQE (%)

1.0 (a)

Current Density (mA cm )

Normalized EL. Intensity (a.u.)

characteristics and the key values are shown in Figure 6 and Table 2, respectively.

Current efficiency (cd A )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 17

20 D1 D2 D3

15 10 5 0

100 1000-2 Luminance (cd m )

10000

Figure 6. (a) EL spectra, (b) J - V - L, (c) c - L - p and (d) EQE - Lcurves of D1 - D3 devices.

The stepwise changed HOMO energy levels of TAPC (-5.50 eV), TCTA (-5.70 eV), 2,6DCzPPy (-6.10 eV) and LUMO energy levels of Tm3PyP26PyB (-2.70 eV), 2,6DCzPPy (-2.60 eV), TCTA (-2.40 eV) are beneficial for the carrier injection and transport. From Figure 6(a) it can be observed that the emissions only from the phosphor dopants, suggesting efficient energy transfer from TCTA and 2,6DCzPPy to the


Page 11 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ir(III) guests. The device D1 based on Ir(4tfmpq)2mND (4 wt%) shows good characteristics with a Lmax of 54080 cd m2,

a ηc,max of 25.59 cd A-1, a ηp,max of 18.54 lm W-1 and a EQEmax of 30.12%. The device D2 containing

Ir(4tfmpq)2mmND at 4 wt% doped concentration displays the best EL performances with the ηc,max, EQEmax and ηp,max of 25.24 cd A-1, 31.48% and 19.29 lm W-1, respectively. The EQE of 31.48% is one of the best characteristics reported, especially for pure red OLEDs.17 Device D3 based on Ir(4tfmpq)2mpND (2 wt%) exhibits the ηc,max, EQEmax and ηp,max of 26.95 cd A-1, 27.70% and 22.80 lm W-1, respectively,. Furthermore, the efficiency roll-off ratios of all devices are mild, and the EQE values can still be obtained as 29.52%, 26.00% and 24.87% when the luminance reached 1000 cd m-2 for D1-D3, respectively. Theoretically, these good device properties of three complexes perhaps due to the following reasons. Firstly, the units of quinazoline and -CF3 were introduced into the main ligand to achieve the pure red emission with high PLQYs and good electron mobility for all complexes. And the -CF3 unit is also beneficial to increase steric hindrance of Ir(III) complexes and suppress triplet - triplet annihilation in the film. Secondly, the 1,5-naphthyridin-4-ol derivatives were introduced to boost the electron-transporting ability of Ir(III) complexes. Thirdly, double light-emitting layers can realize balanced hole/electron distribution and broad exciton recombination zone simultaneously.32,33

Table 2. The key data of S1-S3 and D1-D3 devices. Vturn-on

Lmax

ηc,max

ηext,max

ηp,max

ηca

ηexta

CIEa

(V)

(cd m-2)

(cd A-1)

(%)

(lm W-1)

(cd A-1)

(%)

(x, y)

S1

3.1

48960

23.87

28.22

17.85

19.19

22.69

(0.667, 0.331)

S2

3.0

38300

23.46

28.85

18.49

15.74

19.11

(0.668, 0.328)

S3

3.1

43130

21.90

23.54

20.85

14.10

14.21

(0.651, 0.334)

D1

3.4

54080

25.59

30.12

18.54

25.14

29.52

(0.668, 0.330)

D2

3.2

44300

25.24

31.48

19.29

21.01

26.00

(0.670, 0.327)

D3

3.2

51160

26.95

27.70

22.80

24.20

24.87

(0.656, 0.335)

Device

a

Measured at 1000 cd m-2.



Page 12 of 17

Conclusion In conclusion, three red Ir(III) phosphors using 1,5-naphthyridin-4-ol derivatives as ancillary ligands were prepared with pure red emission peaks from 626 to 630 nm, good photoluminescence quantum efficiency up to 93.4% and good electron mobility. All devices show good performances, and the doubleemitting-layer pure red device fabricated using Ir(4tfmpq)2mmND dopant displayed exceedingly performances with a maximum luminance over 44300 cd m-2 and an EQE of 31.48% with mild efficiency roll-off. The device performances are among the highest results with the pure red color ever reported.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. General information of the materials, measurement, X-ray crystallography, electrochemical tests and theoretical calculation, PHOLEDs fabrication and measurements; The synthesis details, crystallographic data, TGA curves, lifetime curves, cyclic voltammogram curves, 1H NMR,

13C

NMR and MALDI-TOF-

MS spectra of the complexes; The emission spectra transient EL signals of the doped films; The current efficiency versus luminance curves of the devices with different doped concentrations.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51773088, 21771172).

Author information Corresponding Authors [email protected] [email protected]

Notes The authors declare no competing financial interest.

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