0.7% Roll-off for Solution-Processed Blue Phosphorescent OLEDs

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0.7% Roll-off for Solution-processed Blue Phosphorescent OLEDs with a Novel Electron Transport Material Liming Xie, Jinyong Zhuang, Xiaolian Chen, Zhong-Zhi Xie, Ruifeng He, Li Chen, Wenlou Wang, Dongyu Zhang, Wenming Su, Jian-Xin Tang, Xiaolin Yan, and Zheng Cui ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00882 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 21, 2017

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0.7% Roll-off for Solution-processed Blue Phosphorescent OLEDs with a Novel Electron Transport Material Liming Xiea,b, Jinyong Zhuang*,a, Xiaolian Chena, Zhongzhi Xiec, Ruifeng Hed, Li Chenb, Wenlou Wangb, Dongyu Zhanga, Wenming Su*,a, Jianxin Tangc, Xiaolin Yane and Zheng Cuia a

Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese

Academy

of

Sciences,

Suzhou,

P.

R.

China.

Email:

[email protected];

[email protected]. b

Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, P.

R. China. c

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, China. d

Guangzhou ChinaRay Optoelectronic Materials Co. Ltd, Guangzhou, P.R. China.

e

TCL Corporate Research, Nanshan District, Shenzhen, P.R. China.

ABSTRACT

A

novel

cross-linkable

electron

transport

material

1,3,5-tris(5-(4-vinylphenyl)pyridin-3-yl)benzene (TV-TmPY) for solution-processing as well

as

a

small

molecular

1,3,5-tris(5-phenylpyridin-3-yl)benzene

(TmPY)for

vacuum-deposition were designed and synthesized for OLEDs. TV-TmPY and TmPY with the identical core structure are fully characterized to systematically investigate the

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impact of solution-processing and vacuum-deposition on the performance of phosphorescent OLEDs. Over 90% EQE (external quantum efficiency) was achieved for the solution-processed TV-TmPY based device compared to that of the vacuum-deposited TmPY at the luminance of 1000 cd m-2. An EQE deviation of 0.7% was observed ranging from 100 cd m-2 to 1000 cd m-2 with TV-TmPY, which is the smallest value to date for the solution-processed OLEDs. And over 12% EQEs were achieved for the trilayered solution-processed green and blue phosphorescent OLEDs. KEYWORDS:

electron

transport

material,

roll-off,

solution-process,

blue

phosphorescent OLEDs, high efficiency

Organic light emitting diodes (OLEDs) have been successfully applied in full-color display and solid-state lighting since the pioneer work of Tang and VanSlyke.1 In the last few years, high-quality displays based on active matrix (AM) OLEDs have been widely used in smart phones, tablet PCs, laptops, digital cameras and large-sized TVs. In comparison with the vacuum-deposited technology, solution-processing for OLEDs has gained great interest as it potentially leads to low cost mass production for large-sized OLED display and lighting panels.2-7 Multi-layer solution processing in OLEDs fabrication often results in intermixing and corroding of previously deposited layers, which leading to poor device performance.8 Using the cross-linkable material has been proved

to

be

a

feasible

way

to

fabricate

the

multi-layered

devices

by

solution-processing.5, 9-12 Cross-linkable materials will have excellent solvent resistance after cross-linking thus can well solve the intermixing and corroding problems at the interfaces. To date, oxide TFTs are highly recommended for AMOLEDs backplane due to their high carrier mobility, high transparency, good uniformity, low cost and low temperature processing. There have been reports that inverted OLEDs (IOLEDs) are more compatible than conventional OLEDs with oxide TFTs, in addition to the advantages such as high air-stability, long-lifetime, large aperture ratio and little image


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sticking phenomenon.13-15 High electron mobility and sufficient triplet energy (ET) are the crucial factors for highly efficient OLEDs.16-17 Besides, suitable HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) levels are also very important as these energy levels directly relate to the hole blocking and electron injection abilities in OLEDs. As a strong electron-withdrawing moiety, pyridine group is beneficial to improving the electron mobility, which is widely employed for the construction of electron transport materials (ETMs) for OLEDs.18-21 In this work, we designed and synthesized a pyridine-based thermally cross-linkable electron transport material, 1,3,5-tris(5-(4-vinylphenyl)pyridin-3-yl)benzene (TV-TmPY) for solution-processed OLEDs. In order to evaluate the impact of solution-processing and vacuum-deposition

on

the

device

performance,

a

small

molecule

TmPY

(1,3,5-tris(5-phenylpyridin-3-yl)benzene) with the structure identical to the core of TV-TmPY was also synthesized. The thermally cross-linkable vinyl groups are attached to the core structure, which render TV-TmPY with excellent solvent resistance after thermal curing without any initiators. Besides, the introduction of cross-linkable unit also improves the solubility of TV-TmPY, which facilitates the solution-processing. High triplet energy of 2.87 eV, deep HOMO level of -6.49 eV and low LUMO level of -2.82 eV were obtained for the cross-linked TV-TmPY. Maximum external quantum efficiency (EQE) of 20.5% was obtained for the TmPY based blue phosphorescent OLEDs, which is the highest EQE value to date for the inverted blue OLEDs. In addition, the TV-TmPY-based device has the smallest EQE deviation value of 0.7% to date compared with the vacuum-deposited one of 24.5% ranging from the luminance of 100 cd m-2 to 1000 cd m-2. High EQEs of 15.4% and 13.9% at the luminance of 1000 cd m-2 were achieved for the TmPY (vacuum-deposited) and TV-TmPY (spin-coated) based blue phosphorescent OLEDs, respectively. Our results indicates that over 90% EQE can be achieved for the solution-processed device in comparison with the vacuum-deposited one. And over 12% EQEs were achieved for the trilayered solution-processed green and blue


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phosphorescent OLEDs.

Scheme 1. Schematic illustration of the synthesis of TV-TmPY and TmPY.

The synthetic routes to TV-TmPY and TmPY are shown Scheme 1. Both the TV-TmPY and TmPY were obtained by a two-step Suzuki coupling reaction. The compounds were purified through chromatography with the yield of 82.6% and 80.0% for TV-TmPY and TmPY, respectively. And the chemical structures of the compounds were confirmed by proton nuclear magnetic resonance (1H NMR), carbon 13 NMR (13C NMR) and high resolution mass spectrometry. The detailed synthetic procedures and material characterization are in Supporting Information. The thermal stabilities of TV-TmPY and TmPY were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. TV-TmPY showed the decomposition temperature (Td, corresponding to 5% weight loss) of 398 °C. Both TV-TmPY and TmPY showed very similar DSC cures, the two compounds have the same glass transition temperature (Tg) as high as 148 °C due to the identical and rigid core structure. In addition, the broad exothermic peak at 230 °C corresponds to the


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thermal polymerization of TV-TmPY (see Supporting Information). No obvious exothermic or endothermic signal was observed in the second scan of TV-TmPY, which indicates the excellent thermal stability of TV-TmPY.

Figure 1 (a) UV-Vis absorption spectra of the curved TV-TmPY films before and after rinsing with 1,2-dichloroethane, THF, toluene and chlorobenzene. (b) AFM topographic images of TV-TmPY

films and vacuum-deposited TmPY. The solvent resistance of the thermally cured TV-TmPY films was measured using UV-Vis spectroscopy on the quartz substrates. As shown in Figure 1 (a), the absorption spectra of the curved films before and after rinsing with commonly used solvents


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(1,2-dichloroethane, THF, toluene and chlorobenzene) are almost identical. These results well prove the excellent solvent resistance of TV-TmPY after cross-linking. The morphology of spin-coated TV-TmPY was investigated. As shown in Figure 1 (b), the root-mean-square (RMS) surface roughness of the film before cross-linking is 0.61 nm, the value slightly increased to 0.87 nm after cross-linking. After rinsed with chlorobenzene, the surface morphology showed slight change with the roughness of 0.95 nm. The excellent morphology of the homogeneous films suggests the good manufacturability and potential utility of TV-TmPY in solution-processed devices. The morphology of the vacuum-deposited TmPY was also investigated with the roughness as low as 0.43 nm. The HOMO levels of TV-TmPY and TmPY were measured with ultraviolet photoelectron spectroscopy (UPS), and the LUMO levels were calculated using the optical energy gap from the absorption onset. The HOMO levels of TV-TmPY and TmPY are -6.49 eV and -6.43 eV, respectively, which imply the excellent hole blocking ability of the compounds. Both the cross-linked TV-TmPY and TmPY exhibited very similar LUMO levels, which indicate that the cross-linkable unit does not significantly alter the electronic structure of the TmPY backbone. The molecular simulation was carried out by the B3LYP/6-31G(d) basis set. And the spatial distributions of the calculated HOMO and LUMO energy levels are shown in Supporting Information. The dihedral angle of the ground state molecular geometry between the benzene ring in the core and the adjacent pyridine was 38.63° for TV-TmPY and 38.51° for TmPY, and the dihedral angle between the pyridine and the peripheral benzene ring is 37.35° and 38.67°, respectively. With these small and close dihedral angles, strong π-π interactions between the molecules can be expected, which in turn may improve the electron transport ability.19 For phosphorescent OLEDs, it is very important that the triplet excited state of electron transport material should be higher than that of the emitter in order to confine the generated triplet excitons in the emitting layer (EML).18 The triplet energies of TV-TmPY


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and TmPY are calculated based on the highest-energy vibronic sub-band of phosphorescence spectra at 77 K, which were determined to be 2.87 eV and 2.74 eV, respectively. The high triplet energies of the compounds ensure their application for blue phosphorescent OLEDs, which will confine the excitons effectively in the emitting layer.

Table 1 Thermal and photophysical properties of TV-TmPY and TmPY [a] [a] [b] [c] TCL Td λabs Eg[d] λPL[c] HOMO[e] LUMO ET Compound Tg [°C] [°C] [°C] [nm] [eV] [nm] [eV] [eV] [eV]

TV-TmPY 148

TmPY

230

398

263

-3.72

431

-6.49

-2.82 2.87

-

235

259

-3.81

448

-6.43

-2.62 2.74

148

[a] Obtained from DSC measurement. [b] Obtained from TGA measurement. [c] Measured in thin films by spin-coating from 1,2-dichloroethane solution. [d] Calculated from the edge of the UV-vis absorption (Eg=1240/λ). [e] Calculated from 21.22-Ek from UPS.

Figure 2 Schematic energy-level diagrams of the solution-processed devices.

In order to evaluate the electron transport materials in OLEDs, we fabricated the devices

with

the

TV-TmPY (20 nm)/EML

following (30

structure:

ITO/ZnO (35 nm)/TmPY

nm)/1,1-bis[(di-4-tolylami-no)phenyl]cyclohexane

(TAPC) (30nm)/1,4,5,8,9,11-hexaazatriphenylene (HAT-CN) (10 nm)/MoO3

or

(10 nm)/Al.

The

hexacarbonitrile emitting


layer

consists

of

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2,6-bis(3-(carbazol-9-yl)phenyl)pyridine

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(26DCzPPy)

as

the

fac-tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-phenyl-1H-imidazole]

host

and

iridium(III)

(Ir(dbi)3, 1:0.1 wt ratio) as the dopant.22-23 Meanwhile, the control device without the electron transport layer was fabricated for comparison. The schematic energy-level diagrams of the solution-processed devices is shown in Figure 2. For systematically investigating the impact of solution-processing and vacuum-deposition technologies on the device performance, the EML was deposited by vacuum-deposition since TmPY has weak solvent resistance. After the coating of ZnO, the TV-TmPY was spin-coated onto ZnO. The samples were baked at 80 °C for 20 min to remove the residual solvent, then they were heated at 230 °C for 60 min for cross-linking before transferred to the vacuum chamber. Unlike TV-TmPY based device, TmPY was vacuum-deposited and followed the other functional layers without exposure in air. Maximum external quantum efficiency (EQE) of 20.5% was obtained for the TmPY based blue phosphorescent OLEDs, which is the highest EQE value to date for the inverted blue OLEDs. The excellent device performance indicates that TmPY is a very efficient electron transport material, thus decent performance could be expected for the TV-TmPY based devices. Further, the devices using TV-TmPY as the electron transport material were fabricated by spin-coating. The thickness of the electron transport layer was optimized, which is 10 nm for

TmPY

and

20

efficiency-luminance-power

nm

for

TV-TmPY,

efficiency

(CE-L-PE),

respectively. EQE-luminance

electroluminescent (EL) spectra are shown in Figure 3.


The

current

curves

and

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Figure 3 CE-L-PE (a) and EQE-luminance (b) curves of the solution-processed bilayered devices. Inset: EL spectra.

As shown in Figure 3, the TmPY based device exhibited a high EQE of 20.4% at the luminance of 100 cd m-2, and 15.4% for 1000 cd m-2. As for the solution-processed TV-TmPY, the EQE is 13.8% and 13.9% corresponding to the luminance of 100 cd m-2 and 1000 cd m-2, respectively. The performance of the TV-TmPY and TmPY based devices were both better than the control device, which was fabricated without the electron transport material. The working voltage of the TV-TmPY based device at the luminance of 100 cd m-2 and 1000 cd m-2 is 8.7 and 11.1 V, respectively. As for the TmPY based device, the working voltage is a little higher, which is 8.7 and 11.1 V, respectively. The maximum luminance of the TV-TmPY based device was 8779 cd m-2, which is higher than that of TmPY based device (6425 cd m-2) with an increment of 36.6%. The lower working voltage and higher maximum luminance of the TV-TmPY based device implies that TV-TmPY may have better electron transport ability than TmPY. Thus, more balanced charge carrier can be expected in the solution-processed TV-TmPY based device. In comparison with the roll-off of 24.5% for the vacuum-deposited device from 100 cd m-2 to 1000 cd m-2, the device based on the solution-processed TV-TmPY showed an EQE deviation as low as 0.7%. To the best our knowledge, this is the smallest roll-off value for the solution-processed OLEDs up to now. The lifetimes of the devices based on solution-processed TV-TmPY and vacuum-deposited TmPY were tested without


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encapsulation with high humidity of 75% in air. And the performances of TV-TmPY and TmPY based devices were both better than the control one (Table 2).

Table 2 EL data of the solution-processed bilayered phosphorescent OLEDs 100 cd m-2

1000 cd m-2

T50 Maximum Maximum Maximum

Device

(L0=300 Voltage CE

PE

EQE

Voltage CE

PE

EQE

CE

PE

EQE cd/m2)

TV-TmPY

8.7

29.4 10.5

13.8

11.1

29.5 8.4

13.9

30.0

11.1

14.1

21.2

TmPY

8.9

42.4 14.8

20.4

11.3

31.6 8.6

15.4

42.5

18.5

20.5

28.7

Control

9.0

10.6 3.69

4.8

10.8

15.0 4.3

6.8

15.0

4.5

6.8

15.8

Voltage: V; CE: cd A-1; PE: lm W-1 ; EQE : % ; T50: min

Encouraged by these results, solution-processed trilayered devices for both green and blue phosphorescent OLEDs were also fabricated. The trilayered devices have the same

structure

as

the

bilayered

one

except

for

the

emitting

layers:

CR-ZTG-006:Ir(mppy)3 (1:0.1 wt ratio) for green and 26DCzPPy:Ir(dbi)3 (1:0.1 wt ratio) for blue phosphorescent OLEDs by spin-coating. CR-ZTG-006 was received from Guangzhou ChinaRay Optoelectronic Materials Co. Ltd as host material for the green phosphorescent OLEDs. The maximum EQE of the trilayered solution-processed blue device was 12.8%, which is comparable with the bilayered solution-processed one of 14.1%. The maximum EQE of the green solution-processed OLEDs was 12.1%, and the maximum current efficiency is 40.3 cd A-1. Both the performance of the green and blue OLEDs by trilayered solution-processing are among the best in literature.15,

24-29

The

detailed figures are demonstrated in Supporting information and device performances are summarized in Table S1. In conclusion, we have designed and synthesized a novel cross-linkable electron


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transport material TV-TmPY for solution-processing as well as a small molecular TmPY for vacuum-deposition. TV-TmPY and TmPY with the identical core structure are systematically investigated in phosphorescent OLEDs. Our results indicate that over 90% EQE can be achieved for the solution-processed TV-TmPY in comparison with the vacuum-deposited TmPY at the luminance of 1000 cd m-2. High EQEs of 13.8% (100 cd m-2) and 13.9% (1000 cd m-2) were obtained for the solution-processed blue phosphorescent OLEDs based on TV-TmPY, respectively. An EQE deviation of 0.7% was observed from the luminance of 100 cd m-2 to 1000 cd m-2 based on TV-TmPY, which is the smallest value to date for the solution-processed blue phosphorescent OLEDs.

ASSOCIATED CONTENT Supporting Information Experimental procedures, synthesis, characterization, absorption spectra, PL spectra, phosphorescent spectra, TGA, DSC, UPS data, computational studies, AFM images, lifetime test, EL perforamance of the bilayered and trilayered solution-processed devices.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Program on Key Basic Research Project (973 Program, grant number 2015CB351901), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDA09020201), the National Natural Science Foundation of China (NSFC) (21402233), the Natural Science Foundation of


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Jiangsu Province (BK2012631 & BK20140387), Project on the Integration of Industry, Education and Research of Jiangsu Province (BY2014066), Research Fund for the Nano-tech Program of Suzhou City (ZXG201418), The authors also thank Youth Innovation Promotion Association CAS (No.2013206) for financial support. The authors also thank Guangzhou ChinaRay Optoelectronic Materials Co. Ltd for offering the host material CR-ZTG-006.

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413-422. 12. Aizawa, N.; Pu, Y.-J.; Sasabe, H.; Kido, J., Thermally Cross-linkable Host Materials for Enabling Solution-processed Multilayer Stacks in Organic Light-emitting Devices. Org. Electron. 2013, 14, 1614-1620. 13. Hsing-Hung Hsieh, T.-T. T., Chin-Yu Chang, Hsiao-Han Wang, Jung-Yen Huang,; Shih-Feng Hsu, Y.-C. W., Tze-Chien Tsai, Ching-Sang Chuang, Lee-Hsun Chang,; Lin, a. Y.-H., A 2.4-in. AMOLED with IGZO TFTs and Inverted OLED Devices. SID Symposium Digest of Technical Papers 2010, 11.2, 140-143. 14. Lee, J.-H.; Wang, P.-S.; Park, H.-D.; Wu, C.-I.; Kim, J.-J., A High Performance Inverted Organic Light Emitting Diode Using an Electron Transporting Material with Low Energy Barrier for Electron Injection. Org. Electron. 2011, 12 , 1763-1767. 15. Chen, J.; Shi, C.; Fu, Q.; Zhao, F.; Hu, Y.; Feng, Y.; Ma, D., Solution-processable Small Molecules as Efficient Universal Bipolar Host for Blue, Green and Red Phosphorescent Inverted OLEDs. J. Mater. Chem. 2012, 22, 5164-5170. 16. Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A., Electron Transport Materials for Organic Light-Emitting Diodes. Chem. Mater. 2004, 16, 4556-4573. 17. Hughes, G.; Bryce, M. R., Electron-transporting materials for organic electroluminescent and electrophosphorescent devices. J. Mater. Chem. 2005, 15, 94-107. 18. Su, S.-J.; Chiba, T.; Takeda, T.; Kido, J., Pyridine-Containing Triphenylbenzene Derivatives with High Electron Mobility for Highly Efficient Phosphorescent OLEDs. Adv. Mater. 2008, 20, 2125-2130. 19. Sasabe, H.; Tanaka, D.; Yokoyama, D.; Chiba, T.; Pu, Y.-J.; Nakayama, K.-i.; Yokoyama, M.; Kido, J., Influence of Substituted Pyridine Rings on Physical Properties and Electron Mobilities of 2-Methylpyrimidine Skeleton-Based Electron Transporters. Adv. Funct. Mater. 2011, 21, 336-342. 20. Ye, H.; Chen, D.; Liu, M.; Su, S.-J.; Wang, Y.-F.; Lo, C.-C.; Lien, A.; Kido, J., Pyridine-Containing Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll-Off. Adv. Funct. Mater. 2014, 24, 3268-3275. 21. Shih, C.-H.; Rajamalli, P.; Wu, C.-A.; Hsieh, W.-T.; Cheng, C.-H., A Universal Electron-Transporting/Exciton-Blocking Material for Blue, Green, and Red Phosphorescent Organic Light-Emitting Diodes (OLEDs). ACS Appl. Mater. Interfaces 2015, 7, 10466-10474. 22. Su, S.-J.; Sasabe, H.; Takeda, T.; Kido, J., Pyridine-Containing Bipolar Host Materials for Highly Efficient Blue Phosphorescent OLEDs. Chem. Mater. 2008, 20, 1691-1693. 23. Zhuang, J.; Li, W.; Su, W.; Liu, Y.; Shen, Q.; Liao, L.; Zhou, M., Highly Efficient Phosphorescent Organic Light-emitting Diodes Using a Homoleptic Iridium(III) Complex as a Sky-blue Dopant. Org. Electron. 2013, 14, 2596-2601. 24. Lee, H.; Park, I.; Kwak, J.; Yoon, D. Y.; Lee, C., Improvement of Electron Injection in Inverted Bottom-emission Blue Phosphorescent Organic Light Emitting Diodes Using Zinc Oxide Nanoparticles. Appl. Phys. Lett. 2010, 96, 153306 (1-3). 25. Thomschke, M.; Hofmann, S.; Olthof, S.; Anderson, M.; Kleemann, H.; Schober, M.; Lüssem, B.; Leo, K., Improvement of Voltage and Charge Balance in Inverted Top-emitting Organic


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Electroluminescent Diodes Comprising Doped Transport Layers by Thermal Annealing. Appl. Phys. Lett. 2011, 98, 083304 (1-3). 26. Zhang, K.; Zhong, C.; Liu, S.; Liang, A.-h.; Dong, S.; Huang, F., High Efficiency Solution Processed Inverted White Organic Light Emitting Diodes with a Cross-linkable Amino-functionalized Polyfluorene as a Cathode Interlayer. J. Mater. Chem. C 2014, 2, 3270-3277. 27. Fan, C.; Lei, Y.; Liu, Z.; Wang, R.; Lei, Y.; Li, G.; Xiong, Z.; Yang, X., High-Efficiency Phosphorescent Hybrid Organic–Inorganic Light-Emitting Diodes Using a Solution-Processed Small-Molecule Emissive Layer. ACS Appl. Mater. Interfaces 2015, 7, 20769-20778. 28. Chiba, T.; Pu, Y.-J.; Kido, J., Solution-Processed White Phosphorescent Tandem Organic Light-Emitting Devices. Adv. Mater. 2015, 27, 4681-4687. 29. Wei, C.; Zhuang, J.; Chen, Y.; Zhang, D.; Su, W.; Cui, Z., Highly Air-Stable Electron-Transport Material for Ink-Jet-Printed OLEDs. Chem. Eur. J. 2016, 22, 16576-16585.


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ACS Photonics

Scheme 1. Schematic illustration of the synthesis of TV-TmPY and TmPY. 219x135mm (300 x 300 DPI)


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Figure 1 (a) UV-Vis absorption spectra of the curved TV-TmPY films before and after rinsing with 1,2dichloroethane, THF, toluene and chlorobenzene. (b) AFM topographic images of TV-TmPY films and vacuum-deposited TmPY. 109x143mm (300 x 300 DPI)


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ACS Photonics

Figure 2 Schematic energy-level diagrams of the solution-processed devices. 82x55mm (300 x 300 DPI)


ACS Photonics

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Figure 3 CE-L-PE (a) and EQE-luminance (b) curves of the solution-processed bilayered devices. Inset: EL spectra. 156x56mm (300 x 300 DPI)


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0.7% Roll-off for Solution-Processed Blue Phosphorescent OLEDs

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