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Ideal n-type layers are highly desired for high performance organic light emitting diodes (OLEDs). For the first time, we studied the combination of a low-temperature-evaporable n-dopant KBH4 and a high mobility electron transport material 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene (DPyPA). The excellent transporting property of the DPyPA: KBH4 layer allows the fine tuning of the OLED performance by varying the thickness of the n-doped layer in a wide range (from 10 nm to 50 nm, 100 nm, 150 nm and 200 nm). The device with the optimized n-type layer thickness of 150 nm shows the best performance with a high current efficiency of 27.60 cd/A at the brightness 10,000 cd/m2, which is about 40% higher than the device with a 10 nm n-type layer (19.95 cd/A at 10,000 cd/m2). The high performance is attributed to the optimization of optical path and the decrease of the loss in the organic layer/cathode interface due to the thick n-doped layer.
©2011 Optical Society of America
1. Introduction
In recent years, OLEDs have been attracting more and more interests in lighting and display areas [1]. Although the current efficiency has been developed to a high level, the low-voltage operation is still a requirement for achieving low power consumption [2]. Electrical doping method has been proved to be effective for decreasing the operating voltage because of the increased conductivity [2,3]. Additionally, electrical doping method has other advantages: firstly, n-doped layer can be fabricated to a wide thickness range, which is beneficial for the optimization of optical path in order to achieve good color saturation; secondly, when the n-doped layer is thick, it is advantageous to enhance the device efficiency and lifetime because the thicker layer between the light emitting layer and cathode can reduce the loss in the organic layer/cathode interface and more light is out-coupled from the surface plasma mode to air [4].
Generally, highly conductive n-type layer requires a low-work-function dopant and a high mobility electron transport host, though both of which are difficult to achieve [5]. Low-work-function metals such as lithium [5,6] and cesium [7] are very sensitive to oxygen and moisture and are difficult to handle with, and thus their derivatives are extensively studied as alternatives [3,8,9]. However, as the evaporation temperatures of organic materials are generally low, it is difficult to find n-dopants with similar temperatures for co-deposition, due to the high evaporation temperature of most of the alkaline metal derivatives [10]. Fortunately, we’ve found that KBH4 is a thermally decomposable precursor to potassium and the evaporation temperature is as low as 500K at 1.0 × 10−3 Pa [11], making it possible to be evaporated in the low-temperature organic chamber. Recently, Yook et al. demonstrated that a low-temperature-evaporable inorganic n-dopant of cesium azide can very effectively enhance the device performance of the OLEDs as well as simplify the vacuum deposition process and material handling [10]. As the evaporation temperature of KBH4 is even lower than CsN3, it is expected to be an excellent inorganic n-type dopant. On the other hand, we’ve designed and synthesized a stable electron transport material 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene (DPyPA), which was found to possess an extremely high electron mobility (around 10− 3cm2V− 1s− 1) [12]. It is believed that the combination of DPyPA and KBH4 may lead to an ideal n-type layer.
In this work, we studied p-i-n OLEDs by employing DPyPA as the eletron transport host and KBH4 as the n-dopant for the first time. It is found that DPyPA: KBH4 is effective in reducing the driving voltage. Moreover, when the thickness of n-doped layer was varied from 10 nm to 200 nm, the charge injection and transport ability remained almost unchanged. The results further showed that OLEDs with a thick n-doped layer (150 nm) exhibited the highest efficiency. Meanwhile, the electroluminescent (EL) spectra and the color saturation of the device with the optimized thickness were improved as well. The above results indicate that the DPyPA: KBH4 n-type layer is a promising candidate for high performance OLED displays and lightings.
2. Experimental
In this letter, two types of devices were fabricated. The first type are green-emitting devices with a rather thin n-type layer, and the device structure is: glass / ITO / m-MTDATA: 4% F4-TCNQ (100 nm) / NPB (20 nm) / ADN: 3% GD-9 (40 nm) / DPyPA (5 nm) / DPyPA: x % KBH4 (10 nm) (x = 10, 20, 30)/ Ag (150 nm), where m-MTDATA is 4,4’,4”-tris(3-methylphenylphenylamino) triphenylamine, F4-TCNQ is tetrafluorotetracyano-quinodimethane, NPB is 4,4'-N,N'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl, ADN is 9,10-di(naphthalen-2-yl)anthracene, GD-9 is 2,6-di-tert-butyl-N 9,N 9,N 10,N 10-tetrap-tolylanthracene-9,10-diamine. NPB and DPyPA are hole and electron transport layer, respectively. ADN was doped with GD-9 as the green emitting layer. It is worth noting that KBH4 is stable in glove box and dry air and can be stably deposited through thermal deposition at low temperatures similar to organic materials. It was used as the electron injection layer in our previous work and co-deposition of DPyPA and KBH4 was carried out in this work. A control device with a conventional n-type layer of 10nm tris(8-hydroxyquinolinato) aluminum (Alq3): 3% Li was also fabricated for comparison. Aiming at bottom emission OLEDs, we used indium tin oxide (ITO) as the transparent anode with a sheet resistance of approximately 10 Ω/□ (emitting area is 3 × 3 mm2), and 150 nm-Ag as the metal cathode.
The second type of devices had the same structure with Device I except the thickness of the n-doped layer. We studied four different devices with the thickness of 50 nm (Device II-1), 100 nm (Device II-2), 150 nm (Device II-3) and 200nm (Device II-4), respectively. Figure 1 depicts the structures of these devices. All devices were fabricated by vapor phase deposition method, and the electroluminescent characteristics of devices were measured with a Keithley model 2602 source-measure unit and a calibrated silicon photodiode. Spectra and Commission International de L’Eclairage (CIE) coordinates were measured with a PR655 spectrophotometer. The measurement of the device performance was carried out at room temperature.
Fig. 1 Structures of the two types of devices (Devices I are varied with different concentration in the n-doped layer and Device II with different thickness of n-doped layer)
3. Results and discussion
We firstly optimized the doping concentration of KBH4 in DPyPA based on Device I-1, I-2 and I-3, corresponding to 10%, 20% and 30%, respectively. The brightness-voltage-current density (B-V-J) characteristics of the three devices and the Alq3: Li control device are shown in Fig. 2(a) . It is clear that Devices I(1-3) with a DPyPA: KBH4 layer all show lower driving voltage than the control device with Alq3: Li. As suggested by Chan et al. [13], the performance of the n-type layers depends on their electrical conductivities which can be improved by using an electron-transporting host with higher electron mobility. Therefore, better performance of the DPyPA: KBH4 n-type layer can be largely attributed to the high electron mobility of DPyPA. From these curves, we can see that the brightnesses of Devices I(1-3) at the same voltage are similar but there are some differences in the current density. As the KBH4 concentration increases from 10% to 30%, the current density shifts to high values. And the highest current density is obtained in Device I-3. However, the current efficiency and power efficiency versus brightness curves (CE-B-PE) in Fig. 2(b) do not show the same trend as the J-V curves. The efficiency of Device I-3 is lower than that of Device I-1 and I-2, which is attributed to the imbalance of injected carriers and the improved migration of K at higher doping concentrations. Thus, 10% and 20% are both feasible in the latter experiments. By consideration of the over all performance, we choose 20% as the optimal concentration in the fabrication of Device II. The current efficiency of the Alq3: Li device is lower than Device I 1 and 2 but a little bit higher than Device I-3. On the other hand, the power efficiency of the Alq3: Li device is much lower than Devices I(1-3) due to its high driving voltage.
Fig. 2 Characteristics of Devices I (1-3) with different concentration of KBH4 and the control device with a Alq3: Li layer. a) is the J-V-B characteristics (current density and brightness versus voltage), b) is the CE-B-PE characteristics (current efficiency and power efficiency versus brightness)
Device II were fabricated with different thickness of n-doped layer, corresponding to Device II-1(50 nm), Device II-2(100 nm), Device II-3(150 nm), Device II-4(200 nm), respectively. In this segment, the goal is to optimize the optical path for good color saturation as well as long lifetime and high efficiency, because a thicker layer between the light emitting layer and cathode would reduce the loss in the organic layer/cathode interface and more light is out-coupled from the surface plasma mode.
Figure 3 shows the J-V-B and CE-B-PE characteristics of Devices II (1-4) and Device I-2 is chosen for comparison. The J-V curves in Fig. 3(a) is similar, which means that increasing the thickness of the n-doped layer would not bring large changes of the injection and transport ability. The B-V curves exhibit that Device II-1(50 nm) and II-3(150 nm) have similar driving voltage at the same brightness, which is consistent with Device I-2(10 nm) and lower than other devices. The remarkable superiority of Device II-3 appears on the CE-B-PE curves. The current efficiency is 27.60 cd/A at a high brightness of 10,000 cd/m2, approximately 40% higher than 19.95 cd/A of Device I-2. The behavior of other devices are 24.23 cd/A (II-1), 7.34 cd/A (II-2), 11.30 cd/A (II-4), all lower than 27.60 cd/A.
Fig. 3 Characteristics of Devices II (1-4) with different thickness of n-doped layer compared to Device I-2. a) is the J-V-B characteristics (current density and brightness versus voltage), b) is the CE-B-PE characteristics (current efficiency and power efficiency versus brightness)
The varied thickness of n-doped layer induces the change of the optical path, and further changes the emission color and purity. The electroluminescent (EL) spectra and CIE coordinates of these devices are shown in Fig. 4 . From Fig. 4, we can see that the peak wavelength of the EL spectra red-shifts from 516 nm (Device II-2) to 524 nm (Device II-3) and 568 nm (Device II-4) with the increase of the thickness. Device II-2 shows the narrowest spectra and the best color purity among these devices, however, the efficiency is too low (7.34 cd/A). Device II-1 and II-3 exhibits similar spectra and the latter has higher current efficiency (27.60 cd/A). Thus, the optimal thickness of n-doped layer in these devices is 150 nm (Device II-3). The CIE of Device II-1, II-2, II-3 and II-4 and Device I-2 are (0.2851, 0.6507), (0.1554, 0.6824), (0.2789, 0.6715), (0.4548, 0.4992) and (0.2727, 0.6511), respectively. The value of CIEy is improved from 0.6511(Device I-2) to 0.6715 of Device II-3. Device II-3 also shows stable CIE coordinates at all driving voltages, as shown in Table 1 . From 3 V to 6 V, the brightness of the device increases from 46 cd/m2 to 33762 cd/m2, while the variations in CIE coordinates are merely (−0.0083, + 0.0154). It is evident that there is almost no shift of the recombination zone in Device II-3.
Table 1. CIE coordinates of Device II-3 at driving various voltages.
The simulated CIE coordinates using an optical simulation software SETFOS at different thicknesses are compared with the measured data, as shown in Fig. 5 . Here, we can see that the measured and simulated data agree well with each other.
The angular dependence of electroluminescence characteristics is also an important parameter in application. Figure 6(a) shows the change of the EL spectra and intensity at viewing angles of 0°, 30°, 45°, 60° for Device I-2 and Device II-3, respectively. Device II-3 presents enhanced forward luminance than Device I-2 at 0°, and it decreases sharply with the augment of the angle. We can see that EL spectra of Device II-3 at small angles are narrower than those of Device I-2. In addition, Device II-3 does not exhibit obvious color shift with viewing angle. Figure 6(b) compares the angular distributions of the measured EL intensity (normalized to the 0° intensity) for the two devices. By simultaneously achieving high efficiency, high color purity and low color shift with viewing angle, our OLEDs with a thick DPyPA: KBH4 n-type layer are ideal for display and lighting applications.
Fig. 6 a) EL spectra at viewing angle of 0°, 30°, 45°, 60° for Device I-2 and Device II-3, respectively. b) Polar plots of measured EL intensity (normalized to the 00 intensity) for Device I-2 (circle) and Device II-3(triangle).
4. Conclusions
In summary, we have investigated a series of OLEDs with a DPyPA: KBH4 n-type layer. Due to the low evaporation temperature of KBH4, the n-doping process can be easily carried out in the organic chamber. Studies on OLEDs with thick n-doped layer (50 nm, 100 nm, 150 nm and 200 nm) revealed that Device II-3 with 150 nm exhibited the highest efficiency and excellent color saturation. Meanwhile, Device II-3 presented only a small color shift with viewing angle. The results indicate that the method through accommodating the thickness of n-doped layer is useful for the improvement of the performance of OLEDs. Furthermore, the combination of DPyPA and KBH4 are ideal for future applications.
Acknowledgments
This work was supported by the National Key Basic Research and Development Programme of China under Grant Nos. 2009CB930602 and Grant No. 2009CB623604 and the National Natural Science Foundation of China (Grant No. 51173096).
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