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-voltage-luminance 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 RL = ∫λ2 λ1 V(λ)S(λ)R(λ)dλ/∫λ2 λ1 V(λ)S(λ)R(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 illuminations. The pixel contrast ratio (PCR) was calculated as PCR = (Lon + LRL)/(Loff + LRL) , where Lon and Loff are the brightness of a pixel at on-and 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.

 

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.

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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 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 electron-injection 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.

 

Fig. 2. Calculated contrast ratios of OLEDs under different ambient illumination.

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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.

 

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.

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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.