Contrast and efficiency enhancement in organic light ... - OSA Publishing

Contrast and efficiency enhancement in organic light-emitting devices utilizing high absorption and high charge mobility organic layers W. F. Xie, Y. Zhao, C. N. Li, and S. Y. Liu National Laboratory of Integrated Optoelectronics, Jilin University, Changchun 130012, China [email protected]

Abstract: Organic light-emitting devices utilizing copper phthalocyanine (CuPc) and C60 as the hole- and electron-injection layers were reported. Compared with a conventional device without CuPc and C60 layers, the improvement of the contrast is more than 100% under 140-lx ambient lighting at a brightness of 300 cd/m2. A maximum current efficiency of 3.93 cd/A, which is higher than 3.62 cd/A for a conventional device, was obtained at 9 V. The device has a maximum luminance of 17170 cd/m2 at 15 V. The high contrast and high efficiency of the device are attributed to the high absorption and high charge mobility of CuPc and C60 films.

©2006 Optical Society of America OCIS codes: (160.4890) Organic materials; (230.3670) Light-emitting diodes.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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Received 30 March 2006; revised 11 July 2006; accepted 17 July 2006 21 August 2006 / Vol. 14, No. 17 / OPTICS EXPRESS 7954

1. Introduction Great success has been achieved in organic light-emitting devices (OLEDs) in recent years. Different colors, high brightness, and high efficiency in OLEDs with different device structures and materials have been reported [1-5]. For many applications, however, high contrast and great visibility in the illumination condition are more important. Extensive effort has been exerted recently on enhancement of the contrast ratio of OLEDs. For example, Krasnov et al. [6] used a contrast-enhancing stack to increase the contrast ratio of the devices; Lee et al. [7] reported the contrast enhancement in OLEDs using CuPc as an electron transport layer; Hung et al. [8] used an organic, absorptive layer of oxygen-deficient zinc oxide and an Al cathode; Xie et al. [9] constructed a multilayer of Alq3/Sm/Alq3; Wu et al. [10] used a hybrid cathode; Lau et al. [11] used a Sm:Ag alloy cathode to improve the contrast of the OLEDs; and Wong et al. found a nonreflective black cathode by doping the oxygen-deficient silicon monoxide (SiO) into aluminum (Al) [12]. The contrast of OLEDs has been improved using different methods; however, the efficiency of the various devices lessened owing to the decreasing of the reflected light from cathodes. In this paper, a high-efficiency and high-contrast OLED utilizing copper phthalocyanine (CuPc) and C60 as hole and electron injection layers was reported. Both the contrast and efficiency of the device with CuPc and C60 layers are improved compared with the device without the layers. 2. Experimental details (Indium tin oxide) ITO-coated glass substrates were cleaned with acetone and ethanol by using an ultrasonic bath, rinsed with deionized water, and then dried in an oven. Organic layers and cathodes were sequentially deposited on the ITO-glass substrates without breaking the vacuum. Two kinds of devices were prepared: Device A: ITO/CuPc (25 nm)/NPB (25 nm)/Alq3 (20nm)/LiF (0.3 nm)/Al (0.6 nm)/C60 (30 nm)/Mg:Ag (100 nm) Device B: ITO/NPB (50nm)/Alq3 (50nm)/Mg:Ag (100nm) In the devices, CuPc, N,N’-bis-(1-naphthl)-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB), tris(8-quinolinolato) aluminum (Alq3), and C60 were used as a hole-injection layer (HIL), a hole-transporting layer (HTL), an emitting layer, and an electron-injection layer, respectively. The ultrathin bilayer of LiF/Al was used to enhance the electron injection from C60 to Alq3. All the films were deposited at pressures below 4×10-6 Torr. Deposition rates were measured with a quartz oscillating-thickness monitor. The characteristics of the current-voltageluminance were measured with a programmable Keithley Model 2400 power supply and a Photo-research PR650 spectrometer in room-temperature air. 3. Results and discussion First, optical constants of the organic materials were measured with variable angle spectroscopic ellipsometry (VASE); then the reflection spectra of the devices were calculated by using a transfer matrix method. Figure 1(a) shows the optical reflection of the devices with different organic layers. It can be seen that Device B has the highest reflection among all of the devices. The device with CuPc has low reflection at 600–780 nm, and the device with C60 has low reflection at 380–500 nm. Device A with both CuPc and C60 has the lowest reflection, with an average reflection of about 44%. We attribute it to the high absorption of C60 at 380– 500 nm and CuPc at 600–700 nm as shown in Fig. 1(b). The luminous reflectance, or RL, of a device is defined as λ2

λ2

λ1

λ1

RL = ∫ V (λ ) S (λ ) R(λ )dλ / ∫ V (λ ) S (λ )dλ [9], where λ1 and λ2 are 380 nm and 780 nm, respectively; V(λ) is the standard photonic curve; R(λ) is the optical reflection of the device; and S(λ) is the spectral power distribution of the light source D65. The calculated luminous reflection of Device A is 41%, which is about half that of Device B, which is 87%. Figure 2 shows the calculated contrast ratios of the OLEDs under different ambient #69538 - $15.00 USD (C) 2006 OSA


illuminations.

The

pixel

contrast

ratio

(PCR)

was

calculated

as

PCR = ( Lon + LR L ) /( Loff + LR L ) , where Lon and Loff are the brightness of a pixel at onand off-state, respectively; L is the ambient illumination; and RL is the luminous reflectance. In the calculation, the brightness of the device at on- and off-state is set to be 300 and 0 cd/m2, respectively. The contrast ratio of Device A is calculated to be 17.4:1 under an ambient illumination of 140 lx, which is about twice that of Device B. Also, the improvement of the contrast is 58% under 1000 lx strong ambient lighting. The high contrast of the device is attributed to the low optical reflection of the device. 120 110

D evice A D evice B ITO /C uPc(25nm )/NP B(25nm)/Alq 3 (50nm )/M g:Ag

(a)

100

ITO /N PB(50nm)/A lq 3 (20nm )/C 60 (30nm)/M g:Ag

Reflection (%)

90 80 70 60 50 40

am bient light reflection

30 ITO glass organic layer cathode

20 10 400

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W avelength (nm )

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kAlq

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60

0.8

2.2 0.6 2.0 0.4

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Extinction coefficient 'k'

Refractive index 'n'

nAlq

1.4 0.0

1.2 400

450

500

550

600

650

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750

Wavelength (nm)

Fig. 1. (a) Calculated optical reflection spectra of the devices with different organic layers, and (b) optical constants of the organic materials used in this work.

Figure 3(a) shows the characteristics of current density, operating voltage, and brightness (J-V-B) of the devices. It can be observed that the current density and luminance of Device A are higher than those of Device B. For example, at a voltage of 10 V, the current density and luminance of the Device A are 101.2 mA/cm2 and 3975 cd/m2, while those of Device B are #69538 - $15.00 USD (C) 2006 OSA


60.72 mA/cm2 and 1994 cd/m2, respectively. The improvement of the luminance and current density of Device A can be explained as follows. First, the presence of CuPc can improve the hole-injection efficiency from the ITO anode [13]. Second, Mg-doping occurs during the deposition of the Mg:Ag top electrode onto the C60 film [14], and the electron mobility of the Mg-doped C60 film is much higher than that of C60 and Alq3. Finally, the presence of the ultrathin bilayer of LiF/Al [15] can enhance the electron injection from C60 to Alq3. To confirm the effect of the LiF/Al ultrathin bilayer on the electron injection from C60 to Alq3, the device without the LiF/Al bilayer was fabricated. We found that the maximum brightness of the device without the LiF/Al bilayer was less than 100 cd/m2, while the maximum brightness of Device A was 17170 cd/m2. It indicated that the ultrathin bilayer of LiF/Al effectively enhanced the electron injection from C60 to Alq3. Figure 3(b) shows the energy level diagram of the device. It clearly can be seen that the C60 layer formed a high electroninjection barrier with the Alq3 layer without the LiF/Al ultrathin bilayer, resulting in increased electron-hole recombination in the nonemissive C60 layer, and thus a substantial reduction in luminance and efficiency. 50

Contrast ratio

40

Device A Device B

30

20

10

0 0

500

1000

1500

2000

Ambient illuminance (lx) Fig. 2. Calculated contrast ratios of OLEDs under different ambient illumination.

Figure 4 shows the current density–current efficiency and current density–power efficiency characteristics of the devices. The maximum current efficiency of Devices A and B are 3.93 and 3.62 cd/A, respectively. We also can see that the maximum power efficiency of Device A (2.99 lm/W) is much higher than that of Device B (1.75 lm/W). The high efficiency can be attributed to the high hole/electron mobility of CuPc/C60 and the balance of the charge injection. It was found that the efficiency of Device A is more sensitive to the current density; we attribute it to the electron-injection barrier between C60 and Alq3 layers, although the presence of the ultrathin bilayer of LiF/Al can decrease the barrier. We think that the charges injected from both electrodes can transport effectively to the Alq3 layer, and the charges are more balanced at a low-current density. However, with the increasing of the current density, electrons injected to the C60 layer cannot transport to the Alq3 layer completely and more electron-hole recombination will occur in the C60 layer. As a result, the efficiency of Device A decreases more quickly than that of Device B at the high current density, and the detailed mechanism of this phenomenon is in progress.

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600

10

4

10

3

10

2

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1

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(a) 2

400 300 Device A Device B

200 100

Brightness (cd/m2)

Current Density (mA/cm )

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0 2

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Voltage (V)

(b)

2.5

LiF/Al

3.0

Mg:Ag 4.5 C60

Alq3

CuPc

ITO

NPB

3.5

5.2

5.5

5.7

6.2

Fig. 3. (a) Characteristics of current density, operating voltage, and brightness (J-V-B) of Device A and B; and (b) energy level diagram of Device A.

3.5 4.0 3.0

3.6

Device A

2.5

3.4 3.2

2.0 3.0 Device B

2.8

1.5

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Power Efficiency (lm/W)

Current Efficiency (Cd/A)

3.8

2.2 0

100

200

300

400

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0.5

2

Current Density (mA/cm )

Fig. 4. Characteristics of current density, current efficiency, and power efficiency of Devices A and B.

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4. Conclusion In summary, high-efficiency and high-contrast OLEDs using CuPc and C60 as hole and electron injection layers have been reported. Both the contrast and the efficiency of the device are improved. The maximum power efficiency of the device is much higher than that of the conventional device without CuPc and C60 layers. Also, improvement of the contrast of the device is more than 100% under 140 lx ambient lighting at a brightness of 300 cd/m2. The high contrast and high efficiency can be attributed to the high absorption and high hole/electron mobility of the CuPc/C60 layer. Acknowledgments This work was supported by the National Natural Science Foundation of China under grant 60376028 and National “973” project of China under grant 2003cb314703.

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