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Cyclometalated Iridium(III) Carbene Phosphors for Highly Efficient Blue Organic Light-Emitting Diodes Zhao Chen, Liqi Wang, Sikai Su, Xingyu Zheng, Nianyong Zhu, Cheuk-Lam Ho, Shuming Chen, and Wai-Yeung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09172 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017
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Cyclometalated Iridium(III) Carbene Phosphors for Highly Efficient Blue Organic Light‐Emitting Diodes Zhao Chen,1 Liqi Wang,1 Sikai Su,2 Xingyu Zheng,2 Nianyong Zhu,1 Cheuk-Lam Ho,1,3* Shuming Chen,2* Wai-Yeung Wong1,3* 1
Institute of Molecular Functional Materials and Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P.R. China; HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen, 518057, P.R. China E-mail:
[email protected] 2
Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, P.R. China 3
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hong Hom, Hong Kong, P.R. China E-mail:
[email protected];
[email protected] KEYWORDS: blue phosphor, carbene, iridium, organic-lighting emitting diode, single-doped OLED ABSTRACT: Five deep blue carbene-based iridium(III) phosphors were synthesized and characterized. Interestingly, one of them can be fabricated into deep blue, sky blue and white organic light-emitting diodes (OLEDs) through changing the host materials and exciton blocking layers. These deep and sky blue devices exhibit Commission Internationale de l'Éclairage (CIE) coordinates of (0.145, 0.186) and (0.152, 0.277) with external quantum efficiency (EQE) of 15.2% and 9.6%, respectively. The EQE of the deep blue device can be further improved up to 19.0% by choosing a host with suitable energy level of its lowest unoccupied molecular orbital (LUMO).
1. INTRODUCTION Highly efficient deep blue phosphorescent organic lightemitting diodes (PHOLEDs) are the most difficult and challenging target to achieve among different colors in the full color spectrum because of two reasons.13 Firstly, the energy gap (Eg) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the metallophosphors needs to be wide enough.4,5 Secondly, the host and electron/hole transporting layer (ETL/HTL) should be compatible with such high-triplet energy phosphor in terms of HOMO, LUMO and triplet energy (ET) levels.68 To achieve wide Eg of the iridium(III) complexes, the common approaches include adding electron-withdrawing groups, typically fluoro substituent (F), on the phenyl ring of 2phenylpyridine to decrease the HOMO or adding electrondonating groups on the pyridine to raise the LUMO.9,10 The blue PHOLEDs using fluorination approach can achieve maximum EQE of 25%, however, they suffer from severe efficiency roll-off.11 Therefore, highly efficient blue PHOLEDs with luminance (L1/2) higher than 3,000 cd m2 at the half-maximum EQE are still very rare.12,13 Recently, tris(cyclometalated) N-heterocyclic carbene (NHC, CˆC:) based iridium(III) complexes (Ir(CˆC:)3) have received considerable attention.14,15 The stronger ligand field of CˆC: compared to traditional N-heterocycle based ligands
(CˆN) increases the strength of iridium-carbene (Ir-Ccarbene) bond, remarkably elevating the LUMO levels. Thus, deep blue or near-UV phosphorescence can be attained by widening the energy gap.16,17 Moreover, the thermal and photo stabilities of these complexes are impressively high due to the shorter IrCcarbene bond, which reduces or eliminates the decay rate through non-radioactive ligand-field state.16,18 Therefore, Ir(CˆC:)3 complexes are promising materials for robust and efficient blue PHOLEDs.17 Thompson and Forrest et al. reported facial (fac-) and meridional (mer-) Ir(pmp)3 [tris-(Nphenyl,N-methyl-pyridoimidazol-2-yl)iridium(III), 2' in Scheme 1], which showed emission at 418 and 465 nm in 2methyltetrahydrofuran (2-MeTHF) at 77 K with nearly 100% quantum yield. By comparison with their analogues [Scheme 1, tris-(N-phenyl,N-methyl-benzimidazol-2-yl)iridium(III), facand mer-Ir(pmb)3 whose emission peaks appear at around 380 nm],16 the triplet states in Ir(pmp)3 are stabilized. Another difference is the steric interference between the H-atoms at the 1,7 phenyl and benzimidazole group positions on pmb (Scheme 1, blue circle). Removal of such steric hindrance by using a pyridyl ring afforded red-shifted emissions and higher PLQYs in fac- and mer-Ir(pmp)3. Meanwhile, to confine the excitons within the whole emissive layer (EML), the phosphor itself can be used as the exciton blocking layer (EBL) due to the similar ET. As a result, the devices of fac- and merIr(pmp)3 with graded doping exhibit CIE coordinates of (0.16, 0.09) and (0.16, 0.15) with maximum EQE of 10.1 ± 0.2% and
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14.4 ± 0.4%, respectively.19 Furthermore, these devices still keep unusually high brightness values of 7,800 ± 400 and 22,000 ± 1,000 cd m2 when the EQE dropped to half of their maximum values. Hence, the efficiency roll-off was found to be remarkably restricted.19 However, problems arise from the extremely high HOMO/LUMO and high ET level of blue emissive Ir(CˆC:)3 complexes. The former builds a substantial energy barrier to hinder the charge transportation to dopant, leading to a nonbalanced charge recombination in EML.20 The latter also makes the dopant to use excitons inefficiently.21 Alternatively, if the recombination can directly occur on the phosphor by managing the holes and electrons on the HOMO and LUMO of the phosphor itself, highly efficient blue PHOLEDs can be achieved.22,23 Herein, we design five deep blue mer-Ir(CˆC:)3 phosphors, in which their HOMO/LUMO levels and hole conductivities can be varied by electron-withdrawing/donating group substitution on the ligand. All the metallophosphors display blue emission within 420450 nm in solution with high photoluminescence quantum yield (PLQY). Diphenyl-4triphenylsilylphenyl phosphine oxide (TSPO1) was used as the host and Ir(CF3-pei)3 (5) as the EBL (Scheme 1). Emitter 4 successfully transfers the hole from EBL 5 to recombine with the electron from TSPO1 and the phosphor itself, which achieves CIE of (0.145, 0.186) and maximum EQE of 15.2%. By replacing the host with bis[2-(diphenylphosphino)phenyl ether oxide (DPEPO) with much shallower LUMO level, the EQE of the device can be further improved up to 19.0%. Moreover, the current density at the half-maximum EQE (J1/2) is 83 mA cm2, corresponding to a brightness of 7,489 cd m2. Such results are comparable to those highly efficient deep blue PHOLEDs reported in the literature.2,19 Interestingly, the emission colors of devices made from blue-emissive dye 4 can also be tuned into sky blue and white if the emitter itself was used as the EBL and a new carbazole-based host 6 was employed (supporting information, SI).
Scheme 1. Chemical structures and synthetic routes of metallophosphors 15 and the known compound 2'.
large extinction coefficients (ε) of 16,000120,000 M1 cm1, while the absorptions at the lower energy side are ascribed to metal-to-ligand charge-transfer (MLCT) (ε < 16,000 M-1 cm1 14 ). On the other hand, strong deep blue emissions at 420450 nm were detected for 15 in THF with high PLQY at low temperatures. Phosphor 4 exhibits a relatively lower PLQY (75%) as compared to phosphors 13 because of the stronger rotation of tert-butyl substituent on phosphor 4, which may slightly quench the excited state of 4 in solution.26 Replacing the tert-butyl substituent by hydrogen, fluoro or methyl unit, the PLQYs of 1, 2
2. RESULTS AND DISCUSSION
Figure 1. UV-visible absorption and low temperature (77 K) photoluminescence spectra of phosphors 15.
All the Ir(III) complexes were synthesized by the singlestep reaction between [Ir(μ-Cl)(COD)]2 (COD = 1,5cyclooctadiene) and NHC ligands24,25 in the presence of silver oxide (Schemes 1 and S1).19 All these tris(cyclometalated) Ir(III) phosphors prefer adopting mer-configuration, making the mer-Ir(CˆC:)3 complexes as the overwhelming products. These mer-Ir(CˆC:)3 phosphors are air stable and have been fully characterized by NMR spectroscopy, mass spectrometry and X-ray crystallography (Figures S1S5). The UV-visible absorption spectra of these five complexes (Figure 1) were recorded in tetrahydrofuran (THF). Similar to the related reports, the absorptions at the high-energy region (< 300 nm) are assigned to the π to π* transition of NHC ligand with
and 3 dramatically increase to 97.5%, 85% and 99%, respectively (Table S2). The ET values of 15 are estimated to be 2.96, 2.79, 2.77, 2.76 and 2.94 eV, respectively, from the emission maxima of their PL spectra at 77 K. Similar to other Ir(CˆC:)3 complexes, their HOMO and LUMO levels are higher than that of bis[2-(4,6-difluorophenyl)pyridinatoC2,N](picolinato)iridium(III) (FIrpic) derivative (Table S2 and Figure S9). Additionally, the decomposition temperatures (Tdec) of all the phosphors are higher than 300 oC (Figure S10), confirming the good stabilities of these phosphors and therefore they are suitable for thermal evaporation and vacuum deposition processes in device fabrication. At first, two different configurations of blue PHOLEDs devices were designed to confine the charge recombination on phosphor. The configurations of the two devices are:
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higher ET levels of 5 and TSPO1 (5: 2.94 eV; TSPO1: 3.36 eV) than that of 4 (2.76 eV) leads to well-confined triplet excitons inside EML, which enables efficient radiative emission conversion. As a result, by employing 5 and 4 as EBL, the deep (device A4) and sky blue (device B4) PHOLEDs afford maximum EQE of 15.2% and 9.6% with CIE coordinates of (0.145, 0.186) and (0.152, 0.277) (Figures 3a and 3c), respectively, and exhibit high brightness of 9,908 and 34,836 cd m2 with low driving voltage (Von) of 3.3 and 3.6 V (Figure 3b and Table 1).
a) A4
B4
b)
Figure 2. The energy diagrams of blue (device A4, B4 and C4) light-emitting devices based on phosphor 4 and the chemical structures of device materials used. Here, the devices made from structure A or B by using phosphor n were donated by device An or Bn (n = phosphor 1, 2, 3, 4 or 5).
A: ITO/HAT-CN (20 nm)/TAPC (40 nm)/5 (5 nm)/emitter doped in TSPO1 (10 wt%, 20 nm)/TSPO1 (25 nm)/LiF (1 nm)/Al (100 nm) and B: ITO/HAT-CN (20 nm)/TAPC (40 nm)/emitter (5 nm)/emitter doped in TSPO1 (10 wt%, 20 nm)/TSPO1 (25 nm)/LiF (1 nm)/Al (100 nm). Dipyrazino[2,3f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN), LiF, 4,4'-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC), TSPO1 and ITO/Al were used as hole injection layer (HIL), electron injection layer (EIL), HTL, ETL and electrodes, respectively. All the devices display deep or sky blue emissions. By taking emitter 4 as an example: Firstly, the energy barriers from TAPC (HOMO = 5.5 eV) to holeconductive EBL 5 (HOMO = 5.20 eV) and 4 (HOMO = 5.05 eV) (Figure S12) are merely 0.30 and 0.45 eV, which ensure hole transportation to the HOMO of EBL and phosphor smoothly (Figure 2). Secondly, a large amount of electrons would aggregate on the LUMO of host due to the strong electron transport ability of TSPO1. The recombination can be successfully achieved on phosphor because the energy barrier for electron hopping from TSPO1 (LUMO = 2.5 eV) to 4 (LUMO = 2.12 eV) is barely 0.38 eV (Figure 2). Thirdly, the
c)
Figure 3. a) EL spectra (inset: images of devices A4 and B4), b) current density-voltage-luminance (J-V-L) curves and c) EQE curves of devices A and B.
Due to the much shallower LUMO level of the emitter than that of TSPO1, the charge recombination prefers taking place between the LUMO of TSPO1 and the HOMO of emitter, which results in exciplex formation.27,28 Two different devices: I: ITO/HAT-CN (20 nm)/TAPC (40 nm)/5 (5 nm)/4 (20 nm)/TSPO1 (25 nm)/LiF (1 nm)/Al (100 nm) and II: ITO/HAT-CN (20 nm)/TAPC (40 nm)/5 (5 nm)/TSPO1 (25
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nm)/LiF (1 nm)/Al (100 nm) were fabricated to confirm such exciplex formation. The comparison between the spectra of device I and B4 (Figure 4a) reveals that the peak at 480 nm is assigned to the emission of exciplex from device I, which is further confirmed by the energy difference between LUMO of TSPO1 and HOMO of 4 (2.55 eV, corresponding to the wavelength at 485 nm) (Figure 4b, left). On the other hand, from the PL spectrum of 10 wt% of 4 in TSPO1 film (Figure 4a), the peak at 450 nm in devices A4 and B4 are confirmed to be originated from the emission of exciton in 4, which are formed through the direct recombination of the electrons and holes on the phosphor. When 5 was used as EBL, the relative emission intensity of exciton in 4 to the exciplex formed between TSPO1 and 4 in device A4 increased (0.74 for A4 and 0.54 for B4) (Figure 4a). This is probably due to the stronger holeconductivity of 5 than 4 (Figure S12), which allows more holes to be transferred to the HOMO of 4 that can recombine with the electrons at the LUMO of 4 in EML. Therefore, both of the exciton in 4 and the exciplex formed between TSPO1 and 4 contribute to the deep blue emission of A4, while sky blue emission is obtained in the more exciplex-dominant B4.
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In the recent literature report, an efficient deep-blue PHOLED made from bis-tridentate Ir(III) metal phosphor shows a peak EQE of 20.7% with CIE coordinates of (0.15, 0.17),31 which inspires us to use DPEPO as the host material. DPEPO has a much shallower LUMO level (around 2.0 eV) than that of our phosphors, and therefore effective electron transfer to dopant from host can occur, which leads to a much more balanced charge recombination inside EML. By changing the host TSPO1 to DPEPO, the EQE of blue PHOLED (device C4 in Figure 2) is enhanced, affording a peak EQE of 19.0% with CIE coordinates of (0.148, 0.192) (Figure 5). We have also applied other host materials with high triplet energy levels or shallow energy levels of LUMOs, such as 1,4bis(triphenylsilyl)benzene (UGH2, ET = 3.23 eV)22 and 2,6bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridine (2,6tBumCPy, 6, as a new host, LUMO = 2.18 eV, see details in SI). Unfortunately, the efficiency of blue phosphor 4 based PHOLED of the former case decreases (Figure S13), while the latter one results in tuning the emission colors of these devices to the low-energy region and white light was generated (Device W, in Figures S16 and S17 and Table S3).
a)
b)
Figure 5. The EQE curve and EL spectrum of device C4.
3. CONCLUSION
Figure 4. The origins of deep and sky blue emissions: a) EL spectra of devices I, II, A4, B4 and the PL spectrum of 10 wt% of 4 in TSPO1 film and b) schematic diagrams of exciplex formation between TSPO1 and 4 or 5.
By comparing with the results of those devices made from 2 and 2', the substituents on the imidazole ring almost make no difference to the performance of the devices. It is noted that J1/2 value for A4 and B4 are 83 and 215 mA cm2, respectively, corresponding to the L1/2 values of 7,489 and 29,417 cd m2. It is also noted that the EQE roll-offs of these two devices are 4.6% and 3.1%, respectively, when the luminance was increased to 2,000 cd m2, which are much smaller than most of the reported blue OLEDs.29,30 Other devices also exhibit high J1/2 over 60 mA cm2, indicating a relatively small efficiency roll-off (Table 1).
In conclusion, we have designed five carbene-based Ir(III) phosphors and their single-doped deep blue and sky blue OLEDs were fabricated. The maximum EQEs for deep and sky blue OLEDs made from the blue-emissive phosphor 4 are 19.0% and 9.6%, respectively, with restricted efficiency rolloff. Meanwhile, the tunable emission colors of the devices are based on the exciplex and electromer formation.
ASSOCIATED CONTENT Supporting Information. The synthesis, characteristics, electrochemical and thermal properties of phosphors 15 and host 6; the data of electroluminescence performance.
AUTHOR INFORMATION Corresponding Authors *Email:
[email protected] (W.-Y.W.). *Email:
[email protected] (C.-L.H.).
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*Email:
[email protected] (S.C.).
ACKNOWLEDGMENT W.-Y. Wong thanks the Hong Kong Research Grants Council (HKBU 12304715), Areas of Excellence Scheme of the University Grants Committee HKSAR (AoE/P-03/08) and the Hong Kong Polytechnic University (1-ZE1C) for financial support. C.-L. Ho thanks the Hong Kong Research Grants Council (HKBU 12317216), National Natural Science Foundation of China (21504074), Hong Kong Baptist University (FRG2/15-16/074) and the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20160531193836532) for their financial support. We also thank the Inter-institutional Colloborative Reseach Scheme from Hong Kong Baptist University (RCICRS/14-15/02) for financial support.
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Kuei, C.-Y.; Chi, Y.; Lee, G.-H. Bis-Tridentate Ir(III) Metal Phosphors for Efficient Deep-Blue Organic Light-Emitting Diodes. Adv. Mater. 2017, DOI: 10.1002/adma.201702464.
Dibenzothiophene-Based Phosphine Oxide Host and ElectronTransporting Materials for Efficient Blue Thermally Activated Delayed Fluorescence Diodes through Compatibility Optimization. Chem. Mater. 2015, 27, 51315140. (31) Kuo, H.-H.; Chen, Y.-T.; Devereux, L. R.; Wu, C.-C.; Fox, M. A.;
Table 1. The performance data of devices A, B and C. Dopant
EBL
Host
CE (cd A-1)
EQE (%)
EQE Roll-Off (%)
L (cd m-2)
Vonc) (V)
J1/2 (mA cm-2)
CIE
A2
2
5
TSPO1
10.0a), 6.5b)
7.6a), 4.9b)
35.5
3,467
4.6
64
0.153, 0.187
A3
3
5
TSPO1
14.6a), 12.9b)
10.8a), 9.6b)
11.1
5,225
3.9
63
0.146, 0.185
Device
a)
b)
a)
b)
A4
4
5
TSPO1
20.5 , 19.6
15.2 , 14.5
4.6
9,908
3.3
83
0.145, 0.186
B2
2
2
TSPO1
17.9a), 15.4b)
7.8a), 6.8b)
12.8
14,705
3.9
70
0.190, 0.356
17.2
18,692
3.3
110
0.158, 0.272
3.1
34,836
3.6
215
0.152, 0.277
7.4
8,934
3.0
77
0.148, 0.192
a)
b)
a)
b)
B3
3
3
TSPO1
20.7 , 17.4
11.6 , 9.6
B4
4
4
TSPO1
17.5a), 17.1b)
9.6a), 9.3b)
C4 a)
4
5 b)
DPEPO
a)
b)
27.1 , 25.1
a)
b)
19.0 , 17.6
-2 c)
-2
maximum efficiency, efficiency recorded at the luminance of 2,000 cd m ; voltage at the brightness of 1 cd m .
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