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Extremely High-Efficiency Multi-Photon Emission Blue Phosphorescent

OLEDs with External Quantum Efficiency Exceeding 40%

Hisahiro Sasabe*a,b

, Kazuhiro Minamoto b

, Yong-Jin Pua,b

, Masakatsu Hirasawaa,b

, Junji Kido*a,b

aDepartment of Organic Device Engineering, Yamagata University, Yonezawa, Yamagata, 992-8510

Japan; bResearch Center for Organic Electronics (ROEL), Yamagata University, Yonezawa,Yamagata, 992-8510 Japan

ABSTRACT

We developed high-efficiency multi-photon emission (MPE) blue phosphorescent OLEDs with external quantum

efficiency exceeding 40% at 100 cd m –2

. In these MPE devices, we used a blue phosphorescent emitter, FIrpic and

pyridine-containing electron-transporters, 3,5,3’’,5’’-tetra-3-pyridyl-[1,1’;3’,1’]terphenyl (B3PyPB) and bis-4,6-(3,5-dipyridylphenyl)-2-methylpyrimidine derivatives (B3PyMPM, B4PyMPM). We also used a well-known electron-

transporter, 2,9-dimethyl-4,7-diphenylphenanthroline (BCP) for comparison. We used a combination of

TAPC/MoO3/Al/Liq layers as the charge-generation layer unit. An optimized MPE device showed an extremely high

current efficiency of over 90 cd A –1 and a high power efficiency of over 40 lm W –1 at 100 cd m –2 without any

outcoupling enhancement.

Keywords: phosphorescent OLED, electron-transport material, tandem device, charge-generation layer

1. INTRODUCTION

A tandem OLED, so-called multiphoton emission (MPE) OLED is recognized as a key technology for use in general

lighting, because we can obtain high-brightness at low current density leading to an OLED with high efficiency and long

life.[1,2] MPE device consists of multiple emissive units connected with charge generation layer (CGL). In this device,

each CGL generates electrons and holes upon voltage application. Injected holes and electrons recombine in each

emissive layers (EML). Therefore, extremely high external quantum efficiency (η ext) can be obtained. In this study, we

developed high-efficiency MPE blue phosphorescent OLEDs with EQE exceeding 40% at 100 cd m –2

.

2.

RESULTS AND DISCUSSIONPrior to the fabrication of MPE device, we evaluated FIrpic-based conventional OLEDs using a pyridine-containing

electron-transporting layer (ETL), such as 3,5,3’’,5’’-tetra-3-pyridyl-[1,1’;3’,1’]terphenyl (B3PyPB), bis-4,6-(3,5-

dipyridylphenyl)-2-methylpyrimidine derivatives (B3PyMPM, B4PyMPM)[3,4] and a well-known electron-transporter,

2,9-dimethyl-4,7-diphenylphenanthroline (BCP) for comparison (Figure 1).

N N

H3C CH3

BCP

N

N N

N

N N

N

N N

N

N NN

N N

N

CH3 CH3

B3PyPB B3PyMPM B4PyMPM

Figure 1. Chemical structures of materials

We fabricated a blue phosphorescent OLED with a structure of [ITO (130 nm) / TAPC (60 nm) / TCTA: 7wt% FIrpic

(5 nm) / TCTA: FIrpic 20wt% (5 nm) / ETL (20 nm) / ETL: 25 wt% Liq (35 nm) / Liq (1 nm) / Al (100 nm)].

* [email protected]; phone +81 238 26-3924; fax +81 238 26-3412; yamagata-univ.

Invited Paper

Organic Light Emitting Materials and Devices XVI, edited by Franky So, Chihaya Adachi,Proc. of SPIE Vol. 8476, 847604 · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.927997

Proc. of SPIE Vol. 8476 847604-1

wnloaded From: http://proceedings.spiedigitallibrary.org/ on 09/24/2013 Terms of Use: http://spiedl.org/terms



EL spectra showed an emission only from FIrpic with no emission from neighboring materials (inset in Figure 2(b)).

The current density–voltage – luminance ( J–V–L) and the power efficiency–luminance ( PE–L) characteristics are shown

in Figure 2(a) and Figure 2(b), respectively. A reference OLED using BCP gave a very poor η ext of ~5.6% and an η p of

~10 lm W –1 probably because of two factors: (a) much lower E T (2.63 eV) than that of FIrpic (2.77 eV) and (b) lower

carrier balance. Among pyridine-containing ETLs, B3PyPB showed superior performances in FIrpic-based OLED

despite the high-lying E a of 2.62 eV. B3PyPB-based OLED showed much reduced driving voltage of 2.87 V at 100 cd

m

–2

, and gave an η p,100 of 56.5 lm W

–1

(51.6 cd A

–1

, η ext,100 21.8%) at 100 cd m

–2

and 43.2 lm W

–1

(46.3 cd A

–1

, η ext,100 19.6%) at 1000 cd m

–2, respectively. These are among the best performances in FIrpic-based OLEDs. Whereas

B3PyMPM- and B4PyMPM-based OLEDs showed lower η ext of ~15% and η p,100 of ~30 lm W –1

indicating carrier

imbalance in EML.

Figure 2. (a) J–V–L characteristics and (b) PE–L characteristics of blue OLEDs using B3PyPB (circle), B3PyMPM

(square) and B4PyMPM (triangle), respectively. Inset: EL spectra of the devices.

B3PyPB (20 nm)

TAPC (60 nm)

ITO

TCTA: 7 FIrpic (5 nm)


B3PyPB: 25 wt% Liq (35 nm)

Liq (1 nm)

Al

1 unit device

B3PyPB (55 nm)

TAPC (60 nm)

ITO



Liq (1 nm)

Al (1 nm)

B3PyPB (55 nm)

TAPC (60 nm)

MoO3 (5 nm)



Liq (1 nm)

Al

MPE I

TAPC (60 nm)

ITO



Liq (1 nm)

Al (1 nm)

B3PyPB (20 nm)

TAPC (60 nm)

MoO3 (5 nm)



Liq (1 nm)

Al

B3PyPB: 25wt% Liq (35 nm)

B3PyPB (20 nm)

B3PyPB: 25wt% Liq (35 nm)

MPE II

Figure 3. Device structure of 1 unit device, MPE I and MPE II.

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102

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104

0 1 2 3 4 5 6 7

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d e n s i t y ( m A c m

2 )

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2 )

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(a)

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1 10 100 1000 10000

P o w e r e f f i c i e n c y ( l m W – 1 )

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(b)





Next, we fabricated two types of MPE devices using Liq. Device structures are shown in Figure 3. MPE I is using Liq,

and MPE II is using Liq-doped B3PyPB layer. As a reference, we also showed 1 unit device. J–V and QE–L

characteristics are shown in Figure 4(a) and Figure 4(b), respectively. By using Liq as an EIL, performances in MPE

device were clearly improved. Especially, MPE II using Liq-doped B3PyPB gave superior performances.[5] MPE II

showed much reduced driving voltage of 6.93 V at 100 cd m –2, which is 2.4 times higher than that in 1 unit device. An

η p,100 of 40.8 lm W –1 (90.0 cd A –1, η ext,100 41.1%) at 100 cd m –2 and 28.4 lm W –1 (77.6 cd A –1, η ext,100 35.4%) at 1000 cd

m

–2

were also obtained. These are highest performances in a blue phosphorescent OLEDs so far.

Figure 4. (a) J–V characteristics and (b) QE–L characteristics of blue OLED using conventional structure (circle), Liq

(triangle) and Liq-doped B3PyPB (square) respectively.

3. CONCLUSION

By using FIrpic and pyridine-containing electron-transporter B3PyPB, we successfully developed extremely high-

efficiency MPE blue phosphorescent OLEDs with an η ext,100 exceeding 40%. To maximize a FIrpic-based OLED

performance, we pointed out the importance of high-lying E a level as well as high E T for an ETL due to the deep I p of

FIrpic. These findings will provide a guideline to design a next-generation ETL for a high-performance blue phosphorescent OLED. In these MPE OLEDs, we introduced a combination of TAPC/MoO3/Al/Liq layers as a CGL

unit. For CGL unit, Liq plays a critical role to reduce a driving voltage of a MPE device. An optimized MPE OLED

showed an extremely high η c,100 of over 90 cd A –1

and a high η p,100 of over 40 lm W –1

without any outcoupling

enhancement. These performances are the highest in a blue phosphorescent OLED so far. We believe that this MPE

concept will provide a key solution for a high-performance white OLED for general lighting.

REFERENCES

[1]

Kido, J., Matsumoto, T., Nakada, T., Endo, J., Mori, K., Kawamura, N. and Yokoi, A., "High Efficiency Organic

EL Devices having Charge Generation Layers," SID 03 Digest 964-995 (2003).

[2]

Matsumoto, T., Nakada, T., Endo, J., Mori, K., Kawamura, N., Yokoi A. and Kido, J., "Multiphoton Organic EL

Device having Charge Generation Layers," SID 03 Digest 979-981 (2003).[3]

Sasabe, H., Gonmori, E., Chiba, T., Li, Y-J., Tanaka, D., Su, S-J., Takeda, T., Pu, Y-J., Nakayama, K. and Kido, J.,

"Wide-Energy-Gap Electron-Transport Materials Containing 3,5-Dipyridylphenyl Moieties for an Ultra High

Efficiency Blue Organic Light-Emitting Device," Chem. Mater. 20, 5951-5953 (2008).

[4]

Sasabe, H. and Kido, J., "Multifunctional Materials in High-Performance OLEDs: Challenges for Solid-State

Lighting," Chem. Mater. 23, 621-630 (2011).

[5]

Endo, J., Matsumoto, T. and Kido, J., "Organic Electroluminescent Devices Having Metal Complexes as Cathode

Interface Layer," Jpn. J. Appl. Phys. 41, L800-L803 (2002).

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