High Efficiency Fluorescent Blue Organic Light-emitting Device with Balanced Carrier Transport

High-Efficiency Fluorescent Blue Organic Light-Emitting

Device with Balanced Carrier Transport

Jiun-Haw Lee,zYu-Hsuan Ho, Tien-Chun Lin, and Chia-Fang Wu

Graduate Institute of Electro-optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan

We demonstrate a blue fluorescent organic light-emitting device having a current efficiency of 19.2 cd/A and an external quantum efficiency of 8.32% at 100 cd/m2_{. While driving at 20 mA/cm}2_{, the device showed a current efficiency of 16.5 cd/A with a CIE} coordinate at 共0.155, 0.239兲 and an operation voltage of 5.14 V. The estimated half-decay lifetime is 15,611 h at an initial luminance of 1000 cd/m2_{. The improved performance is the result of better carrier balance which is achieved by using bis} 共10-hydroxybenzo关h兴quinolinato兲beryllium 共Bebq2兲 as the electron-transport layer.

© 2007 The Electrochemical Society. 关DOI: 10.1149/1.2737659兴 All rights reserved.

Manuscript submitted December 28, 2006; revised manuscript received March 16, 2007. Available electronically May 22, 2007.

Organic light-emitting devices共OLEDs兲 have attracted much at-tention owing to their advantages of low power consumption, high brightness, high contrast, and potentially low cost.1,2Among the three primary stimuli, blue OLED typically exhibits lower current efficiency and shorter operation lifetime than red and green ones which has significantly hampered the development of full-color OLED.3,4Compared to green and red devices, blue OLED has the shortest wavelength and hence the highest photon energy. Therefore, in achieving a blue phosphorescent OLED, very wide bandgap ma-terials for both host and dopant are needed, which is rather difficult in terms of material design and synthesis.5The high cost of heavy metal phosphorescent material is also one of the key obstacles that prevent the widespread of phosphorescent OLED. It is therefore believed that making a blue fluorescent OLED with high efficiency and long operation lifetime is one of the most important tasks for OLED development in the near future.

In our blue fluorescent OLED, we used 9,10-bis共2

⬘

-naphthyl兲 anthracene 共ADN兲 and 4,4

⬘

-bis关2-共4-共N, N-diphenylamino兲phenyl兲 vinyl兴 biphenyl 共DPAVBi兲 as the host and dopant materials for the emitting layer共EML兲, respectively. A device with similar combina-tion of the organic materials reported by Lee et al.6showed a current efficiency of 9.7 cd/A with CIEx,y共0.16, 0.32兲 and operation voltage of 5.7 V at 20 mA/cm2_{. The half-decay lifetime at an initial}

bright-ness of 1000 cd/m2is 46,000 h. Typically, the hole mobility of a hole-transport layer共HTL兲 material is much higher than the electron mobility of an electron-transport layer共ETL兲 material. Such unbal-anced carrier mobility between HTL and ETL materials results in low device efficiency and shortens the operation lifetime.7 A composite-HTL共c-HTL兲 was used to impede the hole transport for achieving charge balance and significantly increased the current ef-ficiency from 10.4 to 16.2 cd/A.8However, the operation voltage is slightly increased due to the lower hole-mobility of the c-HTL. In-stead of decreasing the hole mobility, we used a high electron-mobility ETL, bis共10-hydroxyben-zo关h兴quinolinato兲beryllium 共Bebq2兲, to achieve carrier balance. This material exhibits an

elec-tron mobility of one order magnitude higher than that of the typi-cally used ETL material, tris-共8-hydroxyquinoline兲 aluminum 共Alq3兲.

9

Because the charge is more balanced and the mobility value of the ETL is higher, the current efficiency increases with lower operation voltage, which, in turn, leads to lower power consump-tion.

Experimental

In our blue fluorescent OLEDs, we used N,N

⬘

-diphenyl-N,N

⬘

-bis共1-naphthyl兲-1,1⬘-biphenyl-4,4

⬘

-diamine 共NPB兲 as the HTL material, DPAVBi of different concentrations doped in ADN as the EML, and Bebq₂as the ETL, followed by 1.2 nm thick LiF as the electron-injection layer between the ETL and 100 nm thick

alu-minum cathode. Table I illustrates the OLED structures. Dopant concentrations of devices 1 to 6 were 0, 1, 2, 3, 4, and 8%, respec-tively, for investigating the electrical and optical characteristics of DPAVBi in ADN matrix. Layer thicknesses of HTL, EML, and ETL were 40, 45, and 15 nm, respectively. After thin-film deposition, devices were encapsulated in the N₂glove box with calcium oxide dessicant. Keithley 2400 source meter was used to determine the current-voltage共I-V兲 characteristics of the device, whereas Minolta CS-1000 photometer was used to measure the luminance, the spec-trum, and the CIE coordinate of the device. Operation lifetime was measured while the device was driven by constant dc current with different initial luminances.

Results and Discussion

Figure 1 shows the current density-voltage共J-V兲 characteristics of devices 1 to 6. At the current density of 100 mA/cm2_{, the driving}

voltage changes slightly from 6.2 to 6.6 V. Note that the dopant concentration did not affect the J-V characteristics significantly. Typically, if a dopant acts as a trap in a matrix, the operation voltage will increase with increasing dopant concentration. However, in our DPAVBi/ADN devices, the driving voltage is nearly the same; no obvious trend of voltage-shift is observed with different dopant con-centrations. Hence, we attribute the minor voltage difference to the inevitable experimental errors. Compared to other OLEDs, the op-eration voltages of our devices are quite low.6Such low operation voltage is the result of using high electron-mobility ETL material, Bebq₂.

Figure 2 shows the dependence of the current efficiency共in terms of cd/A兲 and the current density. It is obvious that the current effi-ciency increases rapidly as the dopant concentration increases from 0 to 2% which is due to the enhanced energy transfer from the host to the dopant materials. When the dopant ratio is between 2 and 4%, the current efficiency reaches a plateau with maximum value. The optimized dopant concentration is 3%共device 4兲 with a current ef-ficiency of 16.5 cd/A at 20 mA/cm2_{and 19.2 cd/A at 100 cd/m}2_.

Such high current efficiency is attributed to the improved carrier balance. The improved device performance is due to the

incorpora-z

E-mail: jhlee@cc.ee.ntu.edu.tw

Table I. Layer structures of the OLED devices.

EML

Device HTL Host Dopant共%兲 ETL

CIE coordinates

NPB ADN DPAVBi Bebq2

1 40 nm 45 nm 0 15 nm 共0.178, 0.213兲 2 1 共0.175, 0.220兲 3 2 共0.159, 0.238兲 4 3 共0.154, 0.238兲 5 4 共0.154, 0.242兲 6 8 共0.160, 0.270兲

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tion of high-mobility ETL, Bebq2, which leads to better charge

bal-ance in the blue OLEDs because they are typically hole-limited devices. When the dopant concentration further increases to 8%, the current efficiency drops共in Fig. 3兲 and the emission red-shifts 共in Table I兲, which represents a typical concentration quenching effect.10Note also that as the dopant concentration increases from 0% to 8%, the y-value of the CIE coordinates increases, which may come from the solid-state solvation effect.11

Figure 3 shows the normalized spectra and luminance profile of our optimized blue OLED, Device 4共DPAVBi = 3%兲, at different viewing angles. Typically, the thin-film structure of OLED can be viewed as a Fabry-Perot microcavity.12,13From Fig. 3a, when nor-malizing the peak at 460 nm, we observe the decrease of the peak at 485 nm with increasing the viewing angles because the resonant wavelength of the microcavity is proportional to the value of the cos␪, where ␪ is the angle to the normal of the device that accounts for this blue shift.14,15In addition, a small hump near 525 nm is also observed, which comes from the emission of Bebq2. Because ADN

exhibits ambipolar carrier-transport characteristics, the holes may transport through the EML to recombine with electrons in the ETL.16 This explains why our blue OLED with higher electron-mobility ETL has higher efficiency compared to the conventional blue OLEDs with Alq3 as the ETL.17As shown in Fig. 3b, the

luminance is nearly constant at different viewing angles. It is a typi-cal Lambertian source which was predicted by the ‘‘half-space’’ optical model that accounts for optical interference effects of the metal cathode-reflector in a weak-microcavity OLED.18The exter-nal quantum efficiency of the optimized device is as high as 8.32%, which is close to the physical limit of the fluorescent device reported in Ref. 19.

Figure 4a shows the results of the accelerated operation lifetime tests of Device 4 with different initial luminances. The extrapolated half-decay lifetimes under the initial luminance of 5000, 7500, Figure 1. Current density vs voltage curves of devices 1–6.

Figure 2. Luminance vs current density of devices 1–6.

Figure 3. 共Color online兲 共a兲 Spectra and 共b兲 luminances at different view

angles of device 4.

Figure 4. 共a兲 Accelerated operation lifetime measurement and 共b兲

half-lifetime with different initial luminances of device 4.

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10,000 and 12,500 cd/m2are 109.8, 27.0, 16.6, and 5.6 h, respec-tively. The half-decay lifetimes under different initial luminances were determined by the equation shown below as illustrated in Fig. 4b20

L*_t 1/2

n

= C 关1兴

where n is the acceleration coefficient and t1/2is half-decay lifetime.

The best fitting acceleration coefficient value is 3.088. Hence, the estimated half-decay lifetime of 15611 and 1836 h are achieved at an initial luminance of 1000 and 2000 cd/m2, respectively. This value is quite similar to that obtained in Ref. 6 共2400 h at initial luminance of 1940 cd/m2_{兲 due to similar organic materials used for}

the EML.

Conclusion

In summary, we have shown a high-efficiency and long operation lifetime blue OLED using DPAVBi doped ADN the EML and Bebq₂ as high electron-mobility ETL. The current efficiency and EQE of the optimized device at 100 cd/m2 are 19.2 cd/A and 8.32%, re-spectively. While driving at 20 mA/cm2_{, the device demonstrated a}

current efficiency of 16.5 cd/A with CIE_x,y 共0.15, 0.239兲 and an operation voltage of 5.14 V. The estimated half-decay lifetime is 15,611 h. at an initial luminance of 1000 cd/m2.

Acknowledgments

This work was supported by the National Science Council, ROC, under grant no. NSC 95-2221-E-002-305 and by the Chi-Mei Opto-electronics Corp.

National Taiwan University assisted in meeting the publication costs of this article.

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