Stable and Efficient Sky-blue Organic Light Emitting Diodes Employing a Tetradentate Platinum Complex

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1 Stable and Efficient Sky-blue Organic Light Emitting Diodes Employing a Tetradentate Platinum Complex Guijie Li 1,2,a), Kody Klimes 1, Tyler Fleetham 1, Zhi-Qiang Zhu 1, and Jian Li 1,a) 1 Department of Materials Science and Engineering, Arizona State University, Tempe, AZ 85284, USA 2 College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang , P. R. China a) Electronic mail: Jian.li.1@asu.edu and guijieli@zjut.edu.cn Abstract: A tetradentate Pt(II) complex platinum (II) [10-(9-(4-tert-butylpyridin-2-yl-κN)-9H-carbazol-2-yl-κC 1 )-9,10-dihydro-9,9-dimethyl-3-(1H-pyrazo l-1-yl-κn 2 )acridine-1-yl-κc 1 ] (PtN'1N-tBu) incorporating pyrazolyl-acridine as the lumophore was demonstrated to act as a stable and efficient sky-blue emitter. Phosphorescent OLED employing PtN'1N-tBu without EBL achieved a high EQE of 15.9% and an estimated operational lifetime LT 70 of 635 h. at an initial luminance of 1000 cd/cm 2. The device efficiency could be further improved by adding TrisPCz as EBL, reaching EQE of 17.3% and operational lifetime up to 482 h. at 1000 cd/cm 2. Since Tang and VanSlyke demonstrated a practical organic light-emitting diode (OLED), 1 OLEDs have attracted a great deal of attention in both academic and industrial research for their commercial applications for full-color displays and solid-state lighting. 2-6 To meet commercial requirements for these technologies, both high quantum efficiency and long operational lifetime are critical device performance. Diligent research has enabled highly efficient OLEDs across the visible spectrum employing efficient Ir(III) 7-13 and Pt(II) based phosphors, organic thermally activated delayed fluorescent (TADF) emitters 25-28, or Pd(II) based metal assisted delayed fluorescent (MADF) emitters 29, due to their ability to harvest all the electrogenerated excitons. 30,31 After more than two decades of research efforts, significant progress has been made for the stable and efficient red and green OLEDs which have been utilized in commercial consumer electronics, such as mobile displays and televisions. However, the development of efficient blue OLEDs with long operational lifetimes remains a big challenge and are scarcely reported Essentially, TADF OLEDs for blue emitters face a similar challenge as phosphorescent emitters and their stability is also affected by the same triplet-triplet and triplet-polaron annihilation events that plague phosphorescent OLEDs with longer 1

2 operation lifetimes. This deficiency hinders the development of OLED technologies and their application in cost-effective display and general lighting. Therefore, it is highly urgent to overcome this bottleneck to promote the development of OLED industry. The previous work in the development of stable and efficient blue OLEDs mainly focused on the designs of Ir(III) complexes and the optimization of device structures in the past two decades. In 2014, Forrest's group achieved a significant breakthrough for stable blue OLED employing iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine] (Ir(dmp) 3 ) as the emitter by adopting the strategy of a graded dopant concentration profile for the emissive layer and a two-unit stacked device structure, which demonstrated LT 80, an operational lifetime to 80% initial luminance of 1000 cd/m 2, of 616 ± 10 h. with a peak external quantum efficiency (EQE) of 18.0 ± 0.2%. 33 Moreover, in the past few years, significant progress has been made for Pt(II) based phosphorescent materials that have demonstrated OLED performances comparable or superior to the Ir(III) analogs, indicating that they can be as promising candidates in the development of stable and efficient blue phosphorescent emitters. Recently, our group demonstrated a strategy for efficient blue OLEDs with high operational stability by employing a tetradentate Pt(II) complex with 6-membered chelate rings, PtNON, which exhibited LT 70, an operational lifetime to 70% initial luminance of 1000 cd/m 2, of estimated 624 h. with peak EQE of 10.7% in a general device setting for the design of charge confinement. 34 More importantly, it had been well demonstrated that the optical properties, such as emission spectral shape, of the planar tetradentate Pt(II) complexes could be easily controlled through simple structural modifications, making them more suitable for efficient and practical blue OLED development. In this communication, we developed a stable and efficient tetradentate sky-blue Pt(II) complex designed to contain a 9,9-dimethyl-acridine moiety in the molecular structure, i.e. platinum (II) [10-(9-(4-tert-butylpyridin-2-yl-κN)-9H-carbazol-2-yl-κC 1 )-9,10-dihydro-9,9-dimethyl-3-(1H-pyrazo l-1-yl-κn 2 )acridine-1-yl-κc 1 ] (PtN'1N-tBu). A device employing PtN'1N-tBu achieved an operational lifetime, LT 70, of 67 h. at an initial luminance of 3753 cd/m 2 with a peak EQE of 15.9%. This corresponds to 635 h. at an initial luminance of 1000 cd/m 2 and over h. at 100 cd/m 2. This operational lifetime is among the best performances reported for blue phosphorescent OLEDs. 2

3 Further devices structure modifications yielded even higher EQE of 17.8% for the PtN'1N-tBu device with TrisPCz as electron blocking layer (EBL), while retaining a high LT 70 of 482 h. at an initial luminance of 1000 cd/m 2. In 2013, our group incoporated pyridyl-carbazole to the design of series of deep blue Pt(II) based phosphorescent materials with device EQEs exceeding 25%, demonstrating that the pyridyl-carbazole could be useful as an auxiliary ligand for the tetradentate Pt(II) complexes. 41 Subsequently, several series of efficient deep blue 39,40, green 39 and red 36,37 phosphors were also developed incorperating these moeities. In particular, PtN3N-ptb, which included a tetradentate cyclometalating ligand with 4-phenylpyridyl-carbazole as a lumophore bonded to 4-tert-butylpyridyl-carbazole (Figure 1). The PtN3N-ptb doped OLED achieved an amazing long operational lifetime LT 97, to 97% of initial luminance, of 638 h. with an initial luminance of 1000 cd/m 2, indicating that pyridyl-carbazole could be a stable group for the commercial OLED design. 37 Subsequently, the emission energy of PtN3N-ptb was blue-shifted by raising the energy of the lowest unoccupied molecular orbital (LUMO) through replacing the 4-phenylpyridine group with pyrazole to develop a narrow-band green emitter PtN1N (Figure 1). PtN1N device demonstrated an estimated operational lifetime, LT 70, of 1436 h. at initial luminance of 1000 cd/m 2 with peak EQE of 14.3%. 39 In order to develop a stable and efficient blue emitter, this design is altered by breaking the conjugation of the carbazole ring to form the 9,10-dihydro-9,9-dimethylacridine based emitter, PtN'1N-tBu (illustrated in Figure 1). The 9,10-dihydro-9,9-dimethylacridine moiety is critical to the PtN'1N-tBu, firstly, the two phenyl group can be fastened to form a rigid molecule and increase emission efficiency. Moreover, the two methyl groups can prevent the benzyl carbon from being oxidized facilitating device operational stability. Figure 1. Color tuning strategy for development of sky-blue emitter employing tetradentate Pt(II) complexes. 3

4 The emission spectra of the PtN'1N-tBu and PtN1N in 2-methyl tetrahydrofuran at 77K and in dichloromethane at room temperature are shown in Figure 2a. At 77K, PtN'1N-tBu shows a blue structured emission spectrum with a dominant peak at 476 nm and a small vibronic sideband at 514 nm, yielding Commission Internationale de L'Eclairage (CIE) coordinates of (0.113, 0.301) and an estimated triplet energy of 2.61 ev for PtN'1N-tBu molecule. The dominant emission peak of PtN'1N-tBu blue-shifts by 8 nm compared to that of PtN1N which peaks at 484 nm at 77K, this demonstrates that the triplet energy of PtN'1N-tBu was raised by breaking the conjugation of the carbazole ring. At room temperature, PtN'1N-tBu exhibits a significantly broadened emission spectrum peaking at 486 nm with vibronic features, which is probably due to the increased flexibility from the 9,10-dihydro-9,9-dimethylacridine/carbazole moiety, resulting in distortion in the excited state. Figure 2. The emission spectra and chemical structures (inset) for (a) PtN1N and PtN'1N-tBu and (b) PtN'1N and PtN'1N-tBu in a solution of CH 2 Cl 2 at room temperature and in a solution of 2-methyl-THF at 77 K. In order to investigate the effect of the t-bu group on the photophysical properties of the PtN'1N-tBu molecule, PtN'1N was also synthesized and the emission spectra in 2-methyl tetrahydrofuran at 77K and in dichloromethane at room temperature are illustrated in Figure 2b. At 77K, PtN'1N has a dominant emission peak at 476 nm, whose value is the same as that of PtN'1N-tBu, with a higher sideband. This indicates that both the emission processes of PtN'1N and PtN'1N-tBu originate from the pyrazolyl-acridine moieties at 77K. However, at room temperature, compared to PtN'1N-tBu, PtN'1N exhibits a significantly broader Gaussian type emission spectrum and a large rigidochromic shift peaking at 536 nm. Furthermore, it is worth noting that the tbu group can also increase the thermal stability of the PtN'1N-tBu molecule. The photoluminescent quantum yield (PLQY) of PtN'1N-tBu in a 5% doped poly(methyl methacrylate) (PMMA) film achieves Ф =

5 Figure 3. The device architectures employed in this study. To evaluate the electroluminescent (EL) properties and operational stability of PtN'1N-tBu in the devices setting, OLEDs were fabricated and tested using four different stable device structures (Figure 3), which all adopted 10% PtN'1N-tBu:mCBP (25 nm) as the emitting layer (EML). They had a general structure of ITO/HATCN (10 nm)/npd (40 nm)/ebl/10% PtN'1N-tBu:mCBP (25 nm)/hbl/bpytp (40 nm)/lif (1 nm)/al (100 nm), where the electron blocking layer (EBL) and hole blocking layer (HBL) are arranged as follows: Structure 1: No EBL/EML/BAlq (10 nm) Structure 2: TrisPCz (10 nm)/eml/balq (10 nm) Structure 3: No EBL/EML/mCBT (8 nm) Structure 4: TrisPCz (10 nm)/eml/mcbt (8 nm) HATCN was 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile, NPD is N,N'-diphyenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4''-diamine, mcbp is 3,3-di(9H-carbazole-9-yl)biphenyl, BPyTP is 2,7-di(2,2'-bipyridin-5-yl)triphenylene, BAlq is bis(2-methyl-8-quinolinolato)(biphenyl-4-olato)aluminum, TrisPCz is 9,9,9 -triphenyl-9h,9 H,9 H-3,3 :6 3 -tercarbazole 42 and mcbt is 9,9'-(2,8-dibenzothiophenediyl)bis-9H-carbazole (Figure S11). The EL spectra and current density-voltage (J-V) characteristics of these devices are plotted in Figure 4a,b. All the devices exhibited similar sky-blue emission spectra peaking at 490 nm, which were narrower than the PL spectrum of PtN'1N-tBu in dichloromethane (Figure 4a). 5

6 Figure 4. (a) EL spectra and (b) EQE vs. luminance at constant current of 20 ma/cm 2. The operational stability of the OLEDs (Figure 5) were assessed by accelerated testing at a constant driving current of 20 ma/cm 2 with the initial voltage between V (Figure S13). Device with Structure 1, utilizing BAlq as HBL, demonstrated a peak EQE of 8.2% at a luminance of 1000 cd/m 2, and an operational lifetime, LT 70 of 47 h. with an initial luminance (L 0 ) of 3394 cd/cm 2. This can be translated to a LT 70 of 375 h. at 1000 cd/cm 2 and h. at 100 cd/cm 2 using the formula LT(L 1 )=LT(L 0 )(L 0 /L 1 ) where L 1 is the desired luminance of 1000 or 100 cd/cm 2 (Table 1). Because TrisPCz has a higher LUMO energy level (-1.2 ev) than that of the host material mcbp (-1.6 ev), it helps to confine the electron inside of the emissive layer and results in effective exciton formation. Thus, the device with structure 2 employing TrisPCz as a EBL, improved to a peak EQE of 10.1 % at 1000 cd/m 2, and also higher initial brightness of 4361 cd/cm 2, resulting in increase in estimated operational lifetime LT 70 of 416 h. at 1000 cd/cm 2 (Table 1). Similar results were also reported for PtN1N 39 doped green and PtNON 34 doped blue OLEDs in a related device structure. Furthermore, replacement of the HBL BAlq with a higher bandgap material mcbt, a significantly improved operational lifetime LT 70 of 67 h. with an initial luminance of 3753 cd/cm 2 was achieved for the device with structure 3, resulting in estimated lifetime of 635 h. at 1000 cd/cm 2 and h. at 100 cd/cm 2 with a peak EQE of 15.9%, and still remaining 11.8% at 1000 cd/cm 2 (Table 1). Such long operational lifetime and high efficiency performance demonstrates that the 9,9-dimethyl-acridine moiety can be incorporated in stable phosphorescent Pt(II) complexes designs. Additionally, the device efficiency could be further improved with structure 4 using an EBL of TrisPCz along with the mcbt HBL. A peak EQE of 17.8% was achieved, remaining at 17.3% at 100 6

7 cd/cm 2 and 14.6% at 1000 cd/cm 2 (Table 1), indicating efficient energy transfer from the host material to the dopant molecules and sufficient confinement of the excitons. These high efficiencies are in good agreement with a PLQY of 0.68, but molecular orientation may also play a role in the high efficiencies due to the planar structure of the molecule. Nevertheless, the operational lifetime slightly decreased to 482 h. (Table 1). Figure 5. Relative luminance vs. operation time at constant current of 20 ma/cm 2. Table 1. Device Performances of PtN'1N-tBu EQE (%) lifetime (h) Structure CIE peak 100 cd/m cd/m 2 L 0 (cd/m 2 ) LT 70 at L 0 LT 70 at LT 70 at 1000 cd/m cd/m 2 1 (0.152, 0.473) (0.155, 0.484) (0.155, 0.482) (0.157, 0.491) To further illustrate the importance of molecular design and the development of the device performance, the comparison of PtN'1N-tBu based sky-blue device and our previous reported PtN1N based green device 39 with the same structures and dopant concentration are plotted in Figure 6. In device structure 3 without an EBL, the PtN'1N-tBu doped OLED exhibits higher efficiency but with a larger roll-off at high luminance (Figure 6a). And the operational lifetime of PtN'1N-tBu doped OLED can achieve approximately 45% of the PtN1N doped OLED at initial luminance of 1000 cd/cm 2, which is 635 h. vs 1436 h. Moreover, in device structure 4 adding TrisPCz as EBL, the device efficiencies of both of them are improved significantly, demonstrating the peak EQE of 17.8% and 22.1% for the PtN'1N-tBu and PtN1N doped OLEDs respectively. And their operational lifetime 7

8 are also comparable (Figure 6b), because of the much higher device initial luminance of PtN1N, PtN'1N-tBu doped OLED (482 h.) can still keep 40% of the PtN1N doped OLED (1194 h.) at initial luminance 1000 cd/cm 2. The better device performance of PtN1N can be attributed to the lower triplet energy of the emitter, resulting in improved charge trapping in the emitting layer. However, it expected that the device performance of PtN'1N-tBu can be further improved by the employment of more advanced host, charge transporting, and blocking materials to balance the excitons in the EML, achieving further efficiency enhancement and operational lifetime improvement. Figure 6. (a) EQE vs. luminance and (b) Relative luminance vs. operation time at constant current of 20 ma/cm 2 for the comparison of PtN'1N-tBu and PtN1N. In summary, we demonstrated a stable and efficient sky-blue tetradentate Pt(II) complex PtN'1N-tBu incorporating pyrazolyl-acridine as the lumophore. Phosphorescent OLED employing PtN'1N-tBu as emitter without EBL achieved a high EQE of 15.9% and an estimated operational lifetime LT 70 of 635 h. at an initial luminance of 1000 cd/cm 2. The device efficiency could be further improved by adding TrisPCz as EBL, reaching EQE of 17.3% and operational lifetime up to 482 h. at 1000 cd/cm 2. These high-performance OLEDs are comparable or superior to the best literature reported Pt(II) and Ir(III) based blue OLEDs, confirming that PtN'1N-tBu can act as stable and efficient dopant for device fabrication and 9,9-dimethylacridine can be stable in the Pt(II) based phosphorescent emitters. Therefore, the 9,10-dihydro-9,9-dimethylacridine-based tetradentate Pt(II) emitter design strategy in this communication can pave a way to develop stable and efficient blue phosphorescent emitters for display and solid-state lighting applications. 8

9 SUPPLEMENTARY MATERIAL See supplementary material for the synthesis of PtN'1N and PtN'1N-tBu; 1 NMR spectra of the liangds and materials; optimized molecular structures of PtN1N, PtN'1N and PtN'1N-tBu; DFT calculation of HOMOs and LUMOs for PtN1N, PtN'1N and PtN'1N-tBu; the chemical structures of some organic materials; current density architectures and electrochemical properties of the tetradentate Pt(II) complexes. ACKNOWLEDGEMENTS The authors thank the National Science Foundation (CHE ), Department of Energy (Contract No. EE ), the Universal Display Corporation, National Natural Science Foundation of China (Grant No ) and "Qianjiang Talents Plan" (Grant No. QJD ) for partial support of this work. The authors also thank Dr. Jason Brooks from Universal Display Corporation for the measurement of emission quantum yield of the sample in thin film. References 1. C. W. Tang, and S. A. Vanslyke, Appl. Phys. Lett. 51, 913 (1987). 2. M. A. Baldo, D. F. O Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R. Forrest, Nature 395, 151 (1998). 3. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, and K. Leo, Nature 459, 234 (2009). 4. C. Adachi, M. A. Baldo, S. R. Forrest, and M. E. Thompson, Appl. Phys. Lett. 77, 904 (2000). 5. M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Appl. Phys. Lett. 79, 156 (2001). 6. X. Yang, X. Xu, and G. Zhou, J. Mater. Chem. C, 3, 913 (2015). 7. R. C. Kwong, M. R. Nugent, L. Michalski, T. Ngo, K. Rajan, Y.-J. Tung, M. S. Weaver, T. X. Zhou, M. Hack, M. E. Thompson, S. R. Forrest and J. J. Brown, Appl. Phys. Lett. 81, 162 (2002). 8. E. L. Williams, J. Li, and G. E. Jabbour, Appl. Phys. Lett. 89, (2006). 9. X. Yang, D. C. Müller, D. Neher, and K. Meerholz, Adv. Mater. 18, 948 (2006). 10. T. Qin, J. Ding, L. Wang, M. Baumgarten, G. Zhou, and K. Müllen, J. Am. Chem. Soc. 131, (2009). 11. C.-H. Lin, Y.-Y. Chang, J.-Y. Huang, C.-Y. Lin, Y. Chi, M.-W. Chung, C.-L. Lin, P.-T. Chou, G.-H. Lee, C.-H. Chang, and W.-C. Lin, Angew. Chem. Int. Ed. 50, 3182 (2011). 9

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