ABSTRACT 1. INTRODUCTION

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1 An Effective Intermediate Al/Au Electrode for Stacked Color-Tunable Organic Light Emitting Devices Tianhang Zheng, and Wallace C.H. Choy* Department of Electrical and Electronic Engineering, the University of Hong Kong, Pokfulam Road, Hong Kong. ABSTRACT Bright and efficient stacked color-tunable organic light emitting devices (OLEDs) using intermediate Al/Au electrode have been reported. The effects of the thickness of Al and Au layer on the luminance characteristics have been comprehensively studied. After the optimization, After the optimization, the bottom-emission single-unit OLED of 4,4',4''-Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine /N,N -diphenyl-n,n -bis(1-naphthyl)-(1,1 -biphenyl)- 4,4 -diamine /tris(8-hydroxyquinoline) aluminum has a maximum luminance efficiency (η L ) of 3.37 by using Al/Au as the cathode and 2.92 cd/a by using Al/Au as the anode. By introducing the optimized intermediate Al/Au electrode into the stacked color-tunable OLEDs, red unit with maximum η L of 4.73 cd/a and blue unit with maximum η L of 3.96 cd/a have been obtained. The color can be tuned efficiently along a linear route from pure red with the Commission Internationale de l'eclairage (CIE) coordinates of (0.662, 0.330) to sky blue with the CIE coordinates of (0.155, 0.340). This scheme can be a potential candidate for achieving high brightness and efficient stacked color-tunable OLEDs. Keywords: Organic light emitting devices; Al/Au electrode; Color-tuning; Electroluminescence 1. INTRODUCTION Organic light emitting devices (OLEDs) are of great significance due to their potential applications in full-color displays and as solid state lighting sources [1, 2]. Recently, stacked OLEDs have been studied intensively not only because they are a scheme of achieving high efficiency[3, 4], but also they are an alternative architecture to construct a multi- or fullcolor displays or lighting sources in only one single device[5]. The color, grey scale, and brightness can be tuned independently by controlling external bias instead of by mixing the color from several horizontal OLED pixels. The stacked color-tunable OLEDs can contribute to improving the pixel resolution, increasing the fill factor, potential highefficiency, and continuously color tuning [5-7]. Two- and three-color tuning results in stacked color-tunable OLEDs have been previously reported[7-10]. In these devices, one of the key factors that influences the performance of the whole device is the semitransparent intermediate electrode. The electrode needs to function as simultaneous hole and electron injection for the bottom and top OLEDs respectively. Meanwhile, the electrode should be transparent enough to transmit visible light effectively. Various strategies have been proposed to meet such targets. In 1996, Forrest et al first reported a stacked color-tunable OLEDs using Mg/Ag/ITO (indium-tin-oxide) intermediate electrode, and subsequently, they used same structure to realize three color tunable OLEDs. Good color purity and video level s brightness have been demonstrated [5, 11]. However, ITO is generally deposited by using magnetron sputtering and thus increases the complexity of processing. It also needs to adjust material compositions to get a good electrical conduction of ITO thin film. Another group they used Ca/Ag as the anode-cathode layer to demonstrate efficient connection[10]. But the stability of device may also be a problem because Ca is easy to be oxidized like other Alkali metals[12], which results in a short lifetime. The lower work function of Ag also increases barrier energy for hole injection into organic materials. Alternatively, several groups proposed to use Al/Au bilayer structures which can be easily fabricated by thermal evaporation as individual intermediate electrode. Al/Au structures can potentially provide good carrier injection and optical properties along with offering device stability for top-emission and stacked OLEDs[13, 14] potentially. It is because Al/LiF is a good cathode with good electron injection [15] whereas Au is a good anode material for its high work function (5.0eV). In addition, thin Al/Au film is semi-transparent while it keeps high conduction and good contact properties with organic materials. By employing Al/Au as bottom anode, a luminance efficiency (η L ) about 3 cd/a and maximum brightness of 8000 cd/m 2 has been reported for top-emission OLEDs with a structure of glass /Al /Au /4,4',4''- Tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-mtdata)/ N,N -diphenyl-n,n -bis(1-naphthyl)-(1,1 - Organic Optoelectronics and Photonics III, edited by Paul L. Heremans, Michele Muccini, Eric A. Meulenkamp, Proc. of SPIE Vol. 6999, 69992R, (2008) X/08/$18 doi: / Proc. of SPIE Vol R-1

2 HZ - o-o D) 59 H 3 :jz H 0 o ) o o -n Ci) - Z 0 I. I, -Q - Z -OOJ CD -=' 1 T 3 biphenyl)-4,4 -diamine (NPB) / tris(8-hydroxyquinoline) aluminum (Alq 3 ) /LiF/ Al /Ag [14]. In stacked OLEDs, Al/Au bilayer also performs excellent as intermediate the cathode-anode structure of the top and bottom units respectively for high efficiency device [16]. However, there are still few studies to use Al/Au as independent intermediate electrode in stacked OLEDs. A detailed investigation is needed by optimizing the structure of the intermediate Al/Au electrode systematically to achieve efficient OLEDs. In this work, we demonstrate efficient stacked color-tunable OLEDs by employing Al/Au bilayer structures as independent intermediate electrode. The dependence of the electrical and optical properties of single OLEDs on the thickness of Al and Au layer has been studied for improving the emission characteristics. For the single m- MTDATA/NPB/Alq 3 OLEDs, the luminance efficiency reaches a maximum η L of 3.37 by using Al/Au as cathode and 2.92 cd/a by using Al/Au as anode. By introducing the optimized Al/Au electrode into stacked OLEDs, top red unit with maximum η L of 4.73 cd/a and bottom blue device with maximum ηl of 3.96 cd/a have been obtained. The color could be tuned along a linear route from pure red to sky blue efficiently. This provides a scheme for achieving high brightness and efficient stacked color-tunable OLEDs. 2. EXPERIMENT Two types of devices denoted by single and stacked OLEDs have been fabricated. All thin films were prepared by vacuum thermal evaporation at the base pressure of ~10-6 Torr. In single device, firstly, using Al/Au (10 nm) electrode as cathode, Device A with structure of ITO/ m-mtdata(20 nm)/npb(25 nm)/alq 3 (50 nm)/lif(1 nm)/al(x nm)/au(10 nm), as shown in Figure 1 (a), was fabricated onto a ITO glass substrate with a sheet resistance of ~15Ω/ to optimize the Al thickness by characterizing its electrical and optical properties (Prior to organic layer growth, ITO glass were cleaned and then treated by UV-ozone ambient for 15 minutes). Then, Device B with structure of Al(15 nm)/au(y)/m- MTDATA(20 nm)/npb(25 nm)/alq 3 (50 nm)/lif(1 nm)/al(100 nm), as shown in Figure 1(b), was fabricated onto a bare glass and investigated for optimizing the Au thickness. For comparison, a normal device m-mtdata(20 nm) /NPB(25 nm) /Alq 3 (50 nm) /LiF(1 nm) /Al(100 nm) using ITO as the anode was also fabricated. In these three devices, all organic materials were grown with the evaporation rate of ~1.0 Å/s. In stacked device (Device C), it has the structure with the optimized individual intermediate electrode as follows: ITO/ m-mtdata: F4TCNQ(30nm, 3wt.%)/ NPB(30nm)/ MADN:DSA-Ph(30nm), 3wt.%/ Alq 3 (15nm)/ LiF(1nm)/ Al(15nm)/ Au(8nm)/NPB(40nm)/ NPB:Ir(piq) 3 (30nm, 10wt.%)/ Bphen(20nm)/ Alq 3 (30nm)/ LiF(1nm)/ Al(100nm) Here, F4TCNQ, MADN, DSA-Ph, Ir(piq) 3 and Bphen are 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane, 2- methyl-9,10-di(2-naphthyl)-anthracene, p-bis(p-n,n-diphenyl-aminostyryl)-benzene, tris(1-phenylisoquinoline) iridium (III), and 4,7-diphenyl-1,10-phenanthroline, respectively. All devices have an area of 3.57cm 2 defined by using a shadow mask, and their configurations along with the chemical structures of two dyes are shown in Figure 1. N Ir(piq)3 (d) DSA-Ph Figure 1. Schematic structure of OLEDs and the molecule structures of red and blue emitters. (a) OLEDs with Al/Au cathode; (b) OLEDs with Al/Au anode; (c) stacked color-tunable OLEDs with intermediate Al/Au electrode; (d) molecule structures of Ir(piq) 3 and DSA-Ph. Proc. of SPIE Vol R-2

3 The current density-voltage-luminescence (J-V-L) characteristics of OLEDs were measured simultaneously using Keithley 2400 programmable source meter and photometer IL1400A. The electroluminescence (EL) spectra were recorded by an Oriel spectrometer with Cornerstone 260i. The transmission spectra were obtained using a system that employs a continuous output of Xe lamp as the excitation source and Ocean Optics HR4000CR spectrometer. All measurements were carried out under ambient atmosphere. It should be noted that all OLEDs we studied are bottom emission OLEDs. 3. RESULTS AND DISCUSSIONS In Device A, the dependence of luminance efficiency (η L ) of single OLEDs on Al thickness is studied, as shown in figure 2. When Al thickness is lower than 10nm, the device shows worst carrier injection and it breaks down easily at high bias. The highest tolerated current density is only about 70 ma/cm 2 and there is nearly no emitted light. However the peak is improved greatly by increasing Al thickness. The η L takes off quickly from zero to 2.51 cd/a, 3.10 cd/a, 3.27 cd/a, and 3.34 cd/a at a current density of J=100 ma/cm 2 when Al thickness is 15 nm, 20 nm, 23 nm and 27 nm, respectively. The value is comparable with the typical result of NPB/Alq 3 OLEDs using LiF/Al as cathode[16]. The maximum luminance of these devices also follows the trend of η L, as shown in the inset of figure 2, and it is more than 18,000 cd/m 2 for above four OLEDs. Our results show that there should be a critical value of Al thickness existing (between 10 nm and 15 nm) for achieving a good electron injection as the luminance and η L increase slowly further after a sharp increment between 10nm and 15 nm I 1-0- I I I I Current Density(mNcm2) Figure 2. Luminance efficiency of OLEDs with Al/Au cathode as functions of current density. Inset: the dependence of maximum luminance and luminance efficiency on Al thickness. As we all known, a qualified intermediate electrode should not only provide excellent injection and connection, but also it must have good transmission to guarantee visible light to pass through easily[10, 17]. Figure 3 shows the transmission spectra of Al/Au (10 nm) thin films in the visible range with the Al thickness being varied from 5 nm to 30 nm. All spectra are dependent on Al thickness, and the transmission decreases gradually with the increase of Al thickness, particularly, in the range from 500 to 700 nm. Therefore, it should be careful to determine its thickness for stacked OLEDs. By simultaneously consideration of η L, Al/Au thin films with 15 nm Al is appropriate to be used as the intermediate electrode because its average transmission is over 65% while the corresponding OLEDs still have a high efficiency (2.51 cd/a at a current density of 100 ma/cm 2 ) and a low driving voltage (5.3V at a luminescence of 100 cd/m 2 ). Thus, in the following studies, the thickness of Al is fixed at 15nm. Proc. of SPIE Vol R-3

4 1.0 I I 0.9 O \ o U.0....S? 0.5 E 04 Al 3Onm/Au lonm F 0.3 Al 2Onm/Au lonm 02 All5nm/AulOnm V Al lonm/au lonm 0.1.Al Snm/Au lonm 0.0- I I Wavelength(nm) Figure 3. Transmission spectra of Al/Au thin film with different Al thickness. The effects of Au thickness on η L and current density-voltage characteristics are studied in Device B. As shown in figure 4(a), the ability of hole injection from Al/Au electrode is enhanced gradually with the increasing of Au thickness. The open voltage is reduced from 15V to 7V while Au thickness is increased from zero to 14 nm, and also the maximum current density is improved from less than 300 ma/cm 2 to 700 ma/cm 2 due to the high work function of Au[14, 18]. Although the driving voltage is larger than that of normal OLEDs using ITO as anode, its open voltage (5.5V) and maximum current density (850 ma/cm 2 ) approaches the performance of the normal device. The luminance is also strong enough (around 13,000 cd/m 2 ) when Au thickness is more than 8 nm, as shown in the inset of figure 4 (a). However, there is no obvious change in the current density of OLEDs with 5 nm, 8 nm and 11 nm Au thin films which may be ascribed to the morphology of Au film. Early studies have proved that there is an internal phase transition of the dynamic and growth modes of the vapor-deposited one metal film on another metal film in the process of deposition, which is influenced by the conditions such as lattice mismatch, the nature of cohesion and its thickness[19-21]. We also know that the quality of each thin film significantly affects the characteristics of the OLEDs [22-24], Consequently, the abnormal changes of Au morphology induced by some internal phase transitions can lead to irregular features of current densityvoltage curves in certain region, as shown in the cases of Au=5, 8, 11 nm in Figure 4 (a). Voltage(V) (a) Efficiency(cd/A) N) N)... A Ạ A Ạ AuOnm Au2nm A Au5nm Au 8nm Au 1mm Au l4nm Normal o Current Density(mA/cm2) Figure 4. EL characteristics of OLEDs with Al/Au anode. (a) Current density vs. voltage curves. Inset is luminance vs. voltage for the OLEDs with Al (15)/Au (8nm) anode. (b) Luminance efficiency vs. current density. Inset: the dependence of luminance efficiency on Au thickness. Proc. of SPIE Vol R-4

5 Figure 4(b) shows the results of η L of Device B with the various Au thicknesses. At first, η L increases to 2.92 cd/a when Au is 8 nm, and then it reduces to 1.55 cd/a for the OLEDs with 14 nm (see inset in Figure 4(b)). These values are recorded at a current density of J=100 ma/cm 2. The reason for this phenomenon can be explained below. The non-linear changes of η L with injection should result from the status of the balance between the injected electrons and holes. We know that the external EL efficiency (η EL ) depends on the combination of several factors[25, 26], as expressed by equation (1). η = η η = η γ η η EL out int out e/ h exciton PL where η out is the light out-coupling efficiency, γ e/h is the ratio of electrons to holes injected from cathode and anode, η exciton is the fraction of radiative excitons to total excitons which is related to the used emission materials, and η PL is the intrinsic quantum efficiency of photoluminescence (PL). In Device B, the only active factor that affects η EL is γ e/h. The carriers injected from cathode and anode side is more balanced[27] firstly due to the increasing of hole injection with Au thickness, and a highest η EL is obtained when Au layer is 8 nm. With thicker Au layer, even though the injection is still improved, the η EL of the device would make a drop due to the unbalance between electrons and holes. This drop is not likely related to the transparency of Al/Au electrode because the transparency is not sensitive to Au thickness in such small range about 6 nm (from 8 nm to 14 nm). However, the normal device has a higher carrier injection than that of Al/Au based OLEDs, and it is also highly efficient. It may be the reason of the variation of carrier and exciton distribution inside the device and then induces the change of recombination zone to form another new balance besides the cause of transmission difference between ITO and Al/Au electrode. In addition, when the transparency of Al/Au electrode is just above 65% on average for OLEDs with Al(15nm)/Au(10nm), in theory, the η L should have a value of 4.46 cd/a ( ), far larger than that of normal ITO device. So, there is an enhancement that is from the result of microcavity effect due to semi-transparency properties of Al/Au electrode. This effect has been used extensively to improve the η L for OLEDs[28, 29]. From the EL spectra shown in figure 5, it also demonstrates the existing of this effect. Figure 5 compares the difference of EL spectra for Device A and B. Device A has a normal spectrum with peak at the wavelength of 545 nm, and the EL spectrum is independent of the Al thickness. However, Device B exhibits some differences comparing with Device A. Its peak not only shifts toward blue side around 15 nm and shows slight dependence on the thickness of Au layer, but its full width of half maximum (FWHM) becomes narrow apparently, which is the result of microcavity effect in Device B. Thus, the microcavity effect can be used to tune the color purity for OLEDs besides enhancing its η L [30]. In the following stacked color-tunable OLEDs, due to the microcavity effect, the color of top red OLEDs is more saturated comparing with that of single bottom-emission OLEDs, and the Commission Internationale de l'eclairage (CIE) coordinates just locate on the red point of the current National Television System Committee (NTSC) standard color triangle. (1) U) u 0.6 N 0.4 E Figure 5. EL spectra of Device A and Device B. Proc. of SPIE Vol R-5

6 With the optimization of the hole and electron injection as anode and cathode and the simultaneous consideration of the transmission properties, an efficient intermediate Al(15nm)/Au(8nm) electrode has been used for studying stacked color tunable OLEDs. For Device C to realize the red-blue stacked color-tunable OLEDs, in which the emitter is phosphorescent Ir(piq) 3 for red color and fluorescent DSA-Ph for blue color, respectively. The maximum η L of the top unit (red emission) and bottom unit (blue emission) of the stacked color-tunable OLEDs are 4.73 cd/a and 3.96 cd/a at J=12.44 ma/cm 2 and ma/cm 2, respectively. For comparison, the η L of single blue OLEDs using Al/Au as cathode is also studied. As expected, there is a little difference for these two devices, which demonstrates the effectiveness of Al/Au electrode in stacked structures. The maximum luminance for red and blue units is 6900 cd/m 2 and 12,500 cd/m 2, respectively, as shown in Figure single blue device using Al/Au electrode 400 red device in stacked OLEDs blue device in stacked OLEDs. c.' E o I )UU I 1000E I 250 i )nn a) 150 I 100 E =. _j ( T I I Voltage(V) Figure 6. Current density and luminance vs. voltage characteristics of stacked color-tunable OLEDs. Figure 7 shows the EL spectra of individual red and blue units along with the tuning spectra in stacked color-tunable OLEDs. In figure 7(a), it is obvious that the individual EL spectra for red and blue are independent on the applied voltage. And the spectra of blue OLEDs in stacked structure is nearly same with that in single OLEDs, but that of top red OLEDs is different with that of its normal case due to the existing of microcavity effect. Its center locates at 620 nm and has a redshift around 9 nm comparing with the normal spectrum. The color becomes more saturated after the influence by microcavity effect and just situates at the red point of NTSC standard color triangle. By changing the biases to each unit of the stacked OLEDs at the same time, the spectrum can be changed back and forth to make the device emit different color from pure red to sky blue efficiently, as shown in Figure 7 (b) and (c). 0.2 (1).!::i 0.0 blue 8V biuelov red 8V red IOV red normal blue normal Z Wavelength(nm) Proc. of SPIE Vol R-6

7 2.0x106 8.Oxl Oxl U) - 4.0x x105o.o =. U) 1.6x x105 4 (h'ifl ' I I Wave Iength(n m) Wavelength(nm) Figure 7. EL spectra of stacked color-tunable OLEDs. (a) EL spectra of single red and blue OLEDs; (b) the EL spectra of blue OLEDs biased at 14V and red OLEDs biased at 8V; (c) the EL spectra of blue OLEDs biased at 11V and red OLEDs biased at 9V. The corresponding CIE coordinates for the stacked color-tunable OLEDs are plotted in figure 8. The top red OLEDs gives a saturated red emission that has CIE coordinates of (0.662, 0.330). The bottom blue OLEDs has CIE coordinates of (0.155, 0.340) that corresponds to sky blue color. The CIE coordinates of these two OLEDs are fairly stable over a range of applied voltages. Consequently, following a linear route, we can tune two units individually to make the stacked device emit light from pure red to sky blue and also include white light efficiently. When the applied voltage is 13V and 9V on blue and red OLEDs, the device releases white light with the CIE coordinates of (0.336, 0.341), and its luminance surpasses 10,000 cd/m 2. Figure 8. The CIE coordinates of stacked color-tunable OLEDs at various applied voltages along with that of single red and blue OLEDs. The triangle is the National Television System Committee standard color triangle. B is blue unit, and R is red unit. Proc. of SPIE Vol R-7

8 4. CONCLUSIONS In summary, efficient stacked color-tunable OLEDs using Al/Au thin films as individual intermediate electrode have been demonstrated. In single OLEDs, the thickness of Al and Au layer has been optimized in light of the value of luminance efficiency (η L ). The m-mtdata/npb/alq 3 OLEDs using Al/Au as cathode and anode have a maximum η L of 3.37 and 2.92 cd/a, respectively. High brightness greater than 12,000 cd/m 2 was achieved in both devices. In stacked OLEDs, red unit with maximum η L of 4.73 cd/a and blue unit with η L of 3.96 cd/a have been obtained using the optimized intermediate Al/Au electrode. The color could be tuned along a linear route from pure red with the CIE coordinates of (0.662, 0.330) to sky blue with the CIE coordinates of (0.155, 0.340) including white color emission with the CIE coordinates of (0.336, 0.341) in certain condition. These results provide a promising scheme for achieving high brightness and efficient stacked color-tunable OLEDs for signage and display applications. ACKNOWLEDGMENTS We acknowledge the support of the grant (# ) from the Research Grant Council of the HK Special Administrative Region, China. We thank e-ray Optoelectronics for supplying part of the organic materials at special prices. We would like to thank C.J. Liang for his valuable discussions. REFERENCES [1] Service, R. F., "Organic LEDs look forward to a bright, white future," Science 310, (2005). [2] Wang, S., Liu, Y.Q., Huang, X.B., Xu, S.L., Gong, J.R., Chen, X.H., Yi, L., Xu, Y., Yu, G., Wan, L.J., Bai, C.L., Zhu, D.B., "Organic light-emitting diodes with improved hole-electron balance by using molecular layers of phthalocyanine to modify the anode surface, " Appl. Phys.A: Mater. Sci. Process. 78, (2004). [3] Chang, C.C., Chen, J.F., Hwang, S.W., and Chen, C. H., "Highly efficient white organic electroluminescent devices based on tandem architecture," Appl. Phys. Lett. 87, (2005). [4] Kanno, H., Holmes, R. J., Sun, Y., Cohen, S. K., Forrest, S. R., "White Stacked Electrophosphorescent Organic Light-Emitting Devices Employing MoO 3 as a Charge-Generation Layer," Adv. Mater. 18, (2006). [5] Burrows, P. E., Forrest, S. R., Sibley, S. P., and Thompson, M. E., "Color-tunable organic light-emitting devices," Appl. Phys. Lett. 69, (1996). [6] Kalinowski, J., Di Marco, P., Cocchi, M., Fattori, V., Camaioni, N., and Duff, J., "Voltage-tunable-color multilayer organic light emitting diode," Appl. Phys. Lett. 68, (1996). [7] Shen, Z., Burrows, P. E., Bulovi, V., Forrest, S. R., and Thompson, M. E., "Three-Color, Tunable, Organic Light- Emitting Devices," Science 276, (1997). [8] Gu, G., Khalfin, V., and Forrest, S. R., "High-efficiency, low-drive-voltage, semitransparent stacked organic lightemitting device," Appl. Phys. Lett. 73, 2399 (1998). [9] Chen, C.W., Lu, Y.J., Wu, C.C., E. Wu, H.E., Chu, C.W., and Yang, Y., "Effective connecting architecture for tandem organic light-emitting devices," Appl. Phys. Lett. 87, (2005). [10] Sun, J. X., Zhu, X. L., Peng, H. J., Wong, M., and Kwok, H. S., "Bright and efficient white stacked organic lightemitting diodes," Org. Elect. 8, (2007). [11] Gu, G., Parthasarathy, G., Burrows, P. E., Tian, P., Hill, I. G., Kahn, A., and Forrest, S. R., "Transparent stacked organic light emitting devices. I. Design principles and transparent compound electrodes," J. Appl. Phys. 86, (1999). [12] Pode, R. B., Lee, C. J., Moon, D. G., and Han, J. I., "Transparent conducting metal electrode for top emission organic light-emitting devices: Ca Ag double layer," Appl. Phys. Lett. 84, (2004). [13] Wu, E. H. E., Liem, H.M., Chen, C. W., Li, G., Xu, Q.F., and Yang,Y., "An Efficient Method for Fabrication of a Semi-transparent Multilayer Stacked Metal Cathode for Polymeric Light-Emitting Diodes," Proceedings of the International Symposium on Super-Functionality Organic Devices IPAP Conf. Series 6, (2005). [14] Lin, S. J., Ueng, H. Y. and Juang, F. S., "Effects of Thickness of Organic and Multilayer Anode on Luminance Efficiency in Top-Emission Organic Light Emitting Diodes," Jpn. J. Appl. Phys. 45, (2006). [15] Sun, J. X., Zhu, X. L., Peng, H. J., Wong, M., and Kwok, H. S., "Effective intermediate layers for highly efficient stacked organic light-emitting devices," Appl. Phys. Lett. 87, (2005). Proc. of SPIE Vol R-8

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