Realization of an efficient top emission organic light-emitting device with novel electrodes

Thin Solid Films 467 (2004) 201 – 208 www.elsevier.com/locate/tsf

Realization of an efficient top emission organic light-emitting device with novel electrodes C.J. Lee, R.B. Pode *, D.G. Moon, J.I. Han Information Display Research Center, Korea Electronics Technology Institute JinWi MaSan, Pyung Taek-Si, KyungGi-Do 451-865, South Korea Received 8 December 2003; received in revised form 19 March 2004; accepted 6 April 2004 Available online 18 May 2004

Abstract Top emission organic light-emitting devices (TEOLEDs) have generated a considerable interest owing to their use in the development of active matrix displays. Requirements for the next-generation displays, which are high-quality video images, lightweight and thin, and low power consumption can be realized in TEOLEDs. The Ni/4,4-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl(a-NPD)/Alq3/2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP)/Ca/Ag top emitting device has been fabricated. The reflective Ni anode and Ca/Ag semitransparent double layer cathode have been characterized. Suitable explanation is proposed for the high transmittance of Ca/Ag structure. D 2004 Elsevier B.V. All rights reserved. PACS: 85.60 Jb Keywords: Top emission organic light-emitting devices (TEOLEDs); Ni anode; Bilayer structure; Ca/Ag semitransparent cathode; Transmittance; Cavity design

1. Introduction In the digital age, access to information and communication to everyone is a focal theme. An electronic display is the principle channel for interactive communication from a computer to a person. Information display devices with high performance are increasingly important due to remarkable advancements in communication technology [1,2]. The development of new display devices has been accelerated over the past decade. Recently, there has been a considerable progress in the area of organic displays which are known to fulfill all requirements for wide-spread consumer adoption [3,4]. An organic light-emitting device (OLED) is a heterostructure device consisting of an organic emitting layer (ETL) sandwiched between anode, generally transparent indium tin oxide (ITO), and a low work function cathode which is calcium, magnesium, or lithium fluoride –aluminum. The hole transport layer (HTL) is introduced between the anode and ETL to improve the hole injection efficiency. Electroluminescence (EL) in organic materials was first reported in 1965 in anthracene single crystals [5]. It remained as an academic interest for next 2 decades due * Corresponding author. Tel.: +82-31-6104-235; fax: +82-31-6104-126. E-mail address: [email protected] (R.B. Pode). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.04.016

to the difficulty of growing a large-size single crystal and requirement of very high voltage (about 1000 V) to produce luminescence. The first efficient multilayer organic device has been demonstrated by Tang and VanSlyke in 1987 [6]. This significant breakthrough in OLED technology has laid the foundation for the realization of full-color, flat panel displays [7,8]. Earlier investigations were focused on the bottom emission structure in which the light is emitted through a transparent bottom electrode and the glass substrate (Fig. 1a). Major disadvantage of a bottom emitting structure is that the emission aperture shares the substrate with the device electronics, potentially limiting the pixel aperture. This can be avoided by using top emission organic light-emitting devices (TEOLEDs), where (a) light escapes from the device through the transparent cathode and encapsulation, (b) permits larger pixel apertures, and (c) all electronics circuitry could be placed at the bottom as shown in Fig. 1b [9,10]. Essential requirements for the next-generation displays are reproduction of high-quality video images, high brightness and contrast, improved color variation and resolution, low weight and thickness, and reduced cost and low power consumption. Active-matrix pixel driving scheme, top-emitting device structure, and solid state encapsulation are some technological options available in realizing these features.

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cleaned glass substrate by using the radiofrequency (RF) magnetron sputtering. The reflective and opaque Ni anodes were patterned by the photolithography process and wet etching. The following devices were fabricated: (1) Ni/a-NPD (35 nm)/Alq3 (50 nm)/BCP (5 nm)/Ca (10 nm)/Ag (10 nm), and (2) Ni/a-NPD (50 nm)/Alq3 (35 nm)/BCP (10 nm)/Ca (10 nm)/Ag (10 nm), where 4,4-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (aNPD), Alq3 (tris-(8-hydroxyquinoline)-aluminum) and 2,9dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) were used as HTL, ETL, and buffer layers, respectively. All organic and metal layers were deposited by using vacuum thermal evaporation technique in a base pressure of 10 6 to 10 7 Torr. Ni anodes were pretreated by oxygen plasma sputtering at 100 W for 3 min before depositing organic layers. Different double layer structures, such as Mg (10 nm)/Ag (10 nm), Ca (10 nm)/Al (10 nm), and Ca (10 nm)/Ag (10 nm), were fabricated for our investigation on cleaned glass substrates (7
7 cm size). The Ca layer was deposited in a background pressure c 2.78
10 6 Torr with a deposition ˚ /s. The Ag protective metal layer was then rate c 2 – 3 A deposited on the top of the Ca layer in a background pressure c 2.12
10 6 Torr with a same deposition rate as that of Ca metal. The substrate was at room temperature (RT) during the deposition process. Other bilayer cathodes were fabricated in a similar way. Optical transmittance and reflectance were measured using a Hitachi U3410 spectrophotometer with a normal incidence of monochromatic light at the sample surface side. Electrical properties of fabricated cathodes were estimated by using Four Probe Method.

3. Results and discussion Fig. 1. Structure of the bottom (a) and top (b) emission OLEDs.

TEOLEDs displays provide the solution to satisfy all these requirements which needs a highly reflective metal anode and the semitransparent cathode. In this report, studies on reflective Ni anode, semitransparent Ca/Ag cathode, fabrication of TEOLEDs and, finally, conclusions, have been presented. It is proposed that TEOLEDs with reflective Ni anode and semitransparent Ca/Ag cathode satisfy requirements for the development of nextgeneration displays.

2. Experimental details Samsung Corning glass substrates (7
7 cm size) were thoroughly cleaned with detergent and acetone before the deposition of Ni. Nickel (t = 200 nm) was deposited on a

3.1. Nickel anode Optoelectronic devices require transparent electrical contacts to allow charge carriers to be injected into or extracted from the active emitting material while also allowing light to enter or exit the device. In OLEDs, operating voltage and luminescence efficiency of the devices are strongly dependent on the effective charge injection from electrodes and charge transport in the organic materials [11]. In general, to achieve the lowest possible operating voltage it is necessary (i) to have ohmic interfaces between the organic layers and the charge injecting contacts and (ii) to maximize the drift mobility of both types of carriers. The barrier to hole injection is taken as the energy difference between the work function of anode, Um, and the ionization potential, Ic (the highest occupied molecular orbital state, HOMO) of the HTL. There is no Fermi barrier if Um = Ic. Obviously, the best electrodes for injecting holes will be high work

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function metals like Pt, Au, Ag, and Al [11]. Table 1 shows the work functions of various metal anodes [12]. In bottom emitting OLEDs, metal oxides, such as ITO, aluminum or indium doped zinc oxide, tin oxide, magnesium –indium oxide, fluorine tin oxide, nickel –tungsten oxide, and cadmium –tin oxide, have been used as transmissive anodes [13,14]. ITO is the most widely used transparent conducting oxide in optoelectronic devices. Commercial ITO films have a sheet resistance of less than 10 V/sq with a visible transmission of more than 90%. The role of ITO in all of these devices is of a transparent conducting electrode addressing each pixel, or larger zone, on the display screen. While display types can differ significantly, the role of ITO remains essentially the same. Recently, other transparent metal oxides with superior properties have been proposed as alternative anode materials to ITO [15]. One of the serious obstacles with the OLEDs is that they degrade rapidly during operation if proper care is not taken during the fabrication process. Premature failure remains problematic in view of commercial applications. Several studies reported in the recent literature have concluded that the degradation of OLEDs is due to presence of moisture and injection of oxygen into active organic layer [16 – 18]. When oxygen diffuses into the organic materials, it leads to the moisture induced degradation of ETL. One likely source of migrating oxygen is the metal oxide used to prepare the transparent contact. This problem is very severe in the bottom as well as both sides emitting devices. A number of solutions have been proposed to reduce or eliminate the oxygen induced degradation associated with metal oxide transparent contacts, including a plasma surface treatment to passivate the surface of ITO [19], annealing of the ITO film at elevated temperature [20], and the incorporation of oxygen diffusion barriers between the transparent contact and the organic film [21 – 23]. In case of the TEOLED, this problem can be overcome by using metal electrodes instead of metal oxides. The Ni metal therefore has been used as a reflective and opaque anode in the fabrication of TEOLED. The oxygen plasma treatment affects the root-meansquare (rms) surface roughness of Ni anode as shown in Fig. 2. Oxygen plasma power was fixed at 100 W. The surface roughness without oxygen plasma treatment was ˚ and slightly increased after treatment for found to be 16.1 A 1 min. When oxygen plasma treatment is carried out for 3 ˚ and lowest min, the sample has the best roughness of 14.1 A deviation. Furthermore, the surface roughness is increased again when the sample is exposed to plasma for more than 3 Table 1 The work function of metal anodes used in OLEDs

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Fig. 2. Surface roughness of the Ni anode as a function of time of plasma treatment.

min. Explanation has been proposed for the improvement of electrical properties of Ni after O2 plasma treatment, as follows. During O2 plasma treatment, oxygen reacts with Ni at the anode surface forming a nickel oxide (NiO). NiO is a well known p-type transparent conducting oxide [24] which allows holes to be injected from the valence band of the contact into the HOMO level of the organic material [25,26]. Localized interface dipole region is created in NiO, and the Fermi level is pinned above the top of the HOMO state of the HTL at the interface. It has a high work function compared to Ni which is more favorable for the hole injection into organic layers [27,28]. A thin layer of NiO (1– 2 nm) is sufficient to increase the work function of the anode at the interface [29]. As NiO is not as transparent as ITO, thickness of NiO and substrate temperature during deposition are very important in the bottom emission OLED. Therefore, in order to increase the work function of anode in TEOLEDs, a thin layer of NiO (1– 2 nm) which satisfies both the requirements of high conductance and transmittance has been used. Moreover, an easy processibility and good electrical properties of Ni are favorable for anode use in the TEOLED. Other metals have disadvantages; Cu is vulnerable to corrosion and has a low reliability; Pt has a poor etching properties at low temperature and requires adhesion layer; Au is more expensive than Ni with a similar work function, easy processibility and cheap. Therefore, Ni has been proposed as a suitable reflective anode (reflectivity ~60% at k = 515 nm) [30] for the top emission configuration and used in fabricated TEOLEDs. 3.2. Ca/Ag semitransparent cathode

Metals

Work function (eV)

Ref.

Au Al Cu Ni Pt

5.4 4.2 4.65 5.2 5.7

[12] [12] [12] [12] [12]

The electron injection barrier is generally taken as the difference between the metal work function and the electron affinity of the ETL. The most suitable cathodes are metals having low work function ranging from 2.63 to 4.70 eV [31]. These include Al, Mg, Ca, Ag, Mg:Ag, and LiF:Al.

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The work function of these cathode metals are shown in Table 2 [31,32]. Low work function, high transmittance, and high electrical conductivity are the basic requirements of the transparent cathode in TEOLEDs. Rigorous efforts have been made by several researchers to develop highly conducting and visibly transparent cathode electrodes for TEOLEDs in the last few years [33 –37]. As a consequence, metal free top emission cathode, CuPc capped with a film of sputtered ITO and transparent cathode BCP/Li capped with ITO [33], and semitransparent metal cathode Mg:Ag/ITO [34] have been proposed to achieve desired results. CuPc and BCP layers were used to prevent damage to underlying organic layers by high energetic ions emitted during the ITO sputtering process. TEOLEDs comprising reflective anodes (Ni and Ag/ITO) and transparent cathodes (Mg:Ag /ITO and Ca/ITO) have also been fabricated and studied [30]. Some transparent conducting oxides, such as indium zinc oxide, ZnO:Al, cadmium stannate, zinc stannate, etc., can also be used as transparent cathodes in TEOLEDs [37]. The sputtering process has been employed for deposition of transparent cathodes over organic layers. However, this method is not suitable as it causes a serious damage at the metal/organic interface, limiting the device performance. The vacuum thermal evaporation process for the deposition of transparent cathodes in TEOLEDs is believed to be damage free without causing any permanent defect to underlying organic layers. To our knowledge, no work has been reported so far on the damage-free deposition of transparent metal cathode for TEOLEDs. Al and Ag have high reflectivity in the visible region (92% and 96% at k = 500 nm, respectively) and are effectively used in bottom emitting structure but cannot be used as top electrodes. As far as Mg and Ca are concerned, being strongly sensitive to ambient, they have to be covered by another protective layer. The nature of Mg/Ag double-layer transmission curve is similar to Ag, whereas the Ca/Al double layer structure is hazy and measurement of transmittance is difficult in this structure. On the other hand, the transmittance is surprisingly improved in Ca/Ag double layer structure. Results of measurement are displayed in Fig. 3. The strong absorption at 480 nm in Ag monolayer has been significantly suppressed in Ca/Ag double layer structure. A transmission over 70% and average reflectance of 12 –14% through out the visible spectral range have been measured in this structure. The possibility of formation of intermetallic com-

Fig. 3. Transmittance spectra as a function of wavelength for various cathodes.

pounds/alloys at the interface is ruled out as bilayer system was not annealed. Measurements were made on a bilayer system of Ca and Ag films at RT. Transmittance curves of Ca/Ag double-layer structures with different thicknesses on glass substrates are shown in Fig. 4. Results show that the transmittance is strongly dependent on the thicknesses of individual layers in the structure and also on the total thickness of the stack. The geometry of the whole structure is influencing the transmittance behavior. From these results, it appears that the optimum uniform transmittance has been measured in Ca (10 nm)/Ag (10 nm) structure. Another important aspect of the thin cathode is its ability to inject electrons into organic layers with a low resistance. The electrical resistivity of thin metal films increases with decreasing thickness, which is attributed to increased surface scattering as the film thickness becomes smaller than the intrinsic electron mean-free path [38]. Surprisingly, very low and uniform sheet resistance is noticed in Ca/Ag

Table 2 The work function of metal cathodes used in OLEDs Metals

Work function (eV)

Ref.

Ca Al Mg Ag

2.9 – 3.00 4.2 3.7 4.26

[31,32] [31,32] [31,32] [31,32]

Fig. 4. Transmittance spectra of Ca/Ag double-layer structure cathode as a function of individual layer thickness.

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double-layer structures which are tabulated in Table 3. It varies with the thicknesses of individual layers as well as with the total thickness of the stack. The low sheet resistance of Ca/Ag cathode may be explained as follows. A thin evaporated layer of Ag on glass would also have a much higher sheet resistance because it would nucleate in small isolated islands. Instead, the Ca wets the glass surface and allows the Ag layer to be continuous. Therefore, the Ca film contributes to making the electrode more conductive. A better tradeoff between the transmittance and sheet resistance has been achieved in a Ca (10 nm)/Ag (10 nm) cathode structure. Fig. 5 shows the photograph displaying the degree of transparencies of Ag (10 nm) and Ca (10 nm)/Ag (10 nm). The business card of KETI is placed beneath partially covering both Ca/Ag double and Ag single layer structures. Careful observation of this photograph shows a high visibility of letters through the double-layer structure as compared to Ag monolayer. At the first instant, this indicates that not only Ca (10 nm) layer is transparent to visible light, but it also improves the transparency of the Ag (10 nm) layer. We suspect that the Ca/Ag interface some how modifies the transmittance of Ag monolayer which may be explained as follows. The transparent Ca/Ag cathode should have (i) a low resistance to injection of charge carriers at the metal/organic interface, and (ii) a high transparency in the visible range. The electron injection efficiency is primarily due to Ca metal having a low work function c 2.9 to 3 eV, while the Ag protective layer improves the lateral electrical conductance of the cathode as reported by Hung et al. [39] in the LiF:Al/Ag cathode. A clean Ca film can be formed by evaporating the Ca in a base pressure c 10 10 Torr [40], wherein Ca/Ag structure forms an efficient electron-injecting contact with the organic interface and the device is expected to have a low turn-on voltage. However, this model of Ca/Ag cathode is failed to explain the observed high transparency. On the other hand, Broms et al. [41] have shown that the Ca film is completely oxidized during the deposition in a back ground pressure V 3.75
10 6 Torr leading to device failure. Assuming the formation of CaO/ Ag structure, the observed high transparency can be suitably explained, but the electron injection efficiency is likely to be deteriorated due to complete oxidation of Ca. Earlier, Gu et Table 3 Sheet resistance variation with the thickness in the Ca/Ag double-layer cathode Ca (nm)

Ag (nm)

Total thickness (nm)

Sheet resistance (V/sq)

2.5 2.5 5 10 15 10 20

5 10 15 10 15 20 10

7.5 12.5 20 20 30 30 30

– – 19.7 12 11 9.6 7.1

205

Fig. 5. Photograph of Ag (10 nm) and Ca (10 nm)/Ag (10 nm) double layers depicting the degree of transparency.

al. [42] reported the increase in barrier to electron injection at the Mg:Ag/ITO interface due to oxidation of Mg:Ag during ITO deposition. Therefore, CaO/Ag model of cathode structure is also inadequate to explain the observed results. Either a clean Ca/Ag cathode deposited in very high vacuum conditions or cathode deposited in a poor back ground pressure is not desirable for transparent contacts in optoelectronic devices. Results of Andersson et al. [43,44] showed that the partially oxidized calcium film is produced during the deposition of Ca in a background pressure ˚ /s. c 10 8 to 10 6 mbar with a deposition rate c 4 A Thus, the partially oxidized Ca film is believed to be formed during the deposition of Ca in a background pressure of 2.78
10 6 Torr. The surface of the partially oxidized Ca film may be further oxidized to form Ca – O due to various contaminants like O2, moisture, CO, Ca-hydroxyl (OH), and Ca-carbonates groups present in the vacuum chamber. The contamination layer (2 – 3 nm), thus formed, will then protect the deeper layers of the Ca film against any modification [45]. Furthermore, there may be some adsorption of Ag on the surface of partially oxidized Ca film. Thus, it appears that the described cathode has a partially oxidized Ca/Ag structure instead Ca/Ag. The partially oxidized Ca layer contents metallic as well as oxidized components of Ca [43]. The formation of partially oxidized Ca/thin CaO (refractive index of CaO, n = 1.830) significantly improves the transparency of the whole structure. Recent results of Song et al. showed that the oxidation of Ni (8 nm) – Mg (8 nm) (solid solution)/Au contacts for optoelectronic devices by annealing at 550 jC for 1 min in air improves the transmittance to 79% as compared to 40% of unoxidized Ni (8 nm) – Mg (8 nm)/Au [46]. The addition of Mg to Ni significantly improves the optical transparency of oxidized contacts [47]. The possibility of complete oxidation of Ca and diffusion of Ag into CaO in fabricated Ca/Ag cathode is ruled out as the oxidation conditions in performed experiments are totally different. We argue that these evidences favor the hypothesis of the improvement of optical transparency of Ca/Ag structure due to partial oxidation of Ca. The unoxidized component of Ca contributes towards the electron injection into ETL; the oxidized component provides the transparency; and the surface adsorption of Ag facilitates the conduction of charge carriers at the partially oxidized Ca/Ag interface (or at the CaO/Ag interface). Thus,

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The incident light is represented by Winci ðx; tÞ ¼ ð2=LÞ1=2 sinðmpx=LÞexpðiEm t=hÞ: For single-mode cavity, the above equation reduces to Winci ðx; tÞ ¼ ð2=LÞ1=2 sinðpx=LÞexpðiEt=hÞ: The reflected wave is represented by Wrefl ðx; tÞ ¼ ð2=LÞ1=2 sinðpx=L þ dÞexpðiEt=hÞ

Fig. 6. Current density and brightness variation as a function of voltage in the Ni/a-NPD(35 nm)/Alq3(50 nm)/BCP(5 nm)/Ca(10 nm)/Ag(10 nm) TEOLED. The inset shows the EL efficiency as a function of voltage.

the partially oxidized Ca/Ag cathode structure fully satisfies all the requirements of the cathode for TEOLEDs. 3.3. Fabrication of TEOLED The Fabry –Perot resonator has been employed to design the cavity for the TEOLED. The vacuum electric field and electric field density of states in a resonator are responsible for the emission characteristics. A Fabry –Perot cavity is used to control the vacuum electric field, whereas electric field density of states at a dipole position can be modified through end mirrors reflectivity and cavity length. A cavity modifies both the electric field and the density of states of the field at its location. Placing the dipole emitter at a maximum of the field (antinode position) will dramatically enhance radiation, while at a node of the field will result in the suppression of radiation. High reflectivity mirrors and small cavities are required to achieve a strong radiative emission as electric field density of states is directly proportional to the reflectivity of mirrors and reciprocal of the length of cavity [48]. An OLED, thin organic film sandwiched between two electrodes, is principally a one-dimensional device. This onedimensional photonic confinement is employed to modify the optical properties of the device. A prior knowledge of antinode (or constructive interference region where the energy transfer is maximum) of the cavity could be beneficial to tailor the device by positioning the emitting layer at the antinode site. The standing wave of the emitted and reflected lights in the optical cavity has been constructed by assuming the exciton dipole as a particle in one-dimensional box. We use cavity of length L < k/2n, where n is the refractive index of the organic medium (n = 1.7) and k is the emission wavelength. The total length of the cavity is taken to be 90 nm. The emitted light has been reflected at the Ni surface and then transmitted out at the semitransparent cathode. Because the thickness of the cavity is very small ( < 100 nm) compared with the emission wavelength (resonating wavelength k = 524 nm for Alq3 is taken for quantitative estimation) [49], single mode cavity (m = 1) is considered here.

where d is the phase shift between incident and reflected waves. For Ni, d = 0.74p and intensity of reflection coefficient = 0.448. Normally, the optical thickness of the emitting layer is of the order of k/4. However, thick Alq3 film increases the operating voltages, so in practice, the best power efficiency is obtained with the Alq3 optical thickness of around f (2/3)k/4 (optical thickness is the product of actual thickness and the refractive index of the material). To avoid the nonradiative energy loss, the exciton dipole should be positioned at a suitable distance from the cathode. By considering all these facts, the actual thickness of the emitting layer is taken as 50 nm. The HTL thickness is taken as 35 nm. The details of the cavity design are given in Ref. [50]. The fabricated TEOLED is: Ni/a-NPD (35 nm)/ Alq3 (50 nm)/BCP (5 nm)/Ca (10 nm)/Ag (10 nm) (L = 90 nm). The organic buffer layer BCP (5 nm), which has a HOMO state at 6.4 eV and LUMO state (lowest unoccupied molecular orbital state) at 2.9 eV, has been introduced between the cathode and ETL to prevent the diffusion of Ca and destroying the Alq3 molecular structure by attacking the phenoxyl oxygen. Fig. 6 shows the current density and brightness variation with the biasing voltage. The turn on voltage is found to be 7.5 V. The inset of the figure shows the EL efficiency as a function of voltage. The EL curve is shown in Fig. 7. No microcavity effects which result in Fabry –Perot interference

Fig. 7. Electroluminescence curve of the Ni/a-NPD(35 nm)/Alq3(50 nm)/ BCP(5 nm)/Ca(10 nm)/Ag(10 nm) TEOLED.

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Fig. 8. Normalized emission spectra of the TEOLED: Ni/a-NPD(50 nm)/ Alq3(35 nm)/BCP(10 nm)/Ca(10 nm)/Ag(10 nm) with different biasing voltage: (a) 7 V, (b) 21 V (strip 1), and (c) 21 V (strip 2).

fringes, are observed in Ni/a-NPD (35 nm)/Alq3 (50 nm)/ BCP (5 nm)/Ca (10 nm)/Ag (10 nm) TEOLED structure. Whereas for other configuration, Ni/a-NPD (50 nm)/Alq3 (35 nm)/BCP (10 nm)/Ca (10 nm)/Ag (10 nm) (L = 95 nm), interference fringes have been noticed as displayed in Fig. 8. Intensities of fringes are strongly dependent on the biasing voltage. Some microcavity effects are expected to be present in TEOLEDs due to reflective nature of both electrodes. Thicknesses of all layers between electrodes have to be tightly controlled to achieve the clean EL spectrum. Moreover, the thickness of the organic stack is quite important to the spectral distribution of the EL peak. No interference fringes have been observed by reducing the cavity length to 90 nm and tailoring the internal layers thicknesses. Furthermore, the cathode structure also significantly affects the behavior of EL spectrum. Thus, by intelligently designing the microcavity, the interference fringes can be avoided in the top emitting structure with a reflective anode and the semitransparent cathode.

anode for the TEOLED. The semitransparent cathode plays an important role in TEOLEDs. Small residual contact reflection causes serious problem in the device performance. The Ca (10 nm)/Ag (10 nm) double-layer structure has a high transmittance over 70% and low sheet resistance about 12 V/sq. The enhancement of transparency of the Ca/Ag cathode structure is attributed to the partial oxidation of Ca during the deposition. Therefore, it has been selected as the cathode. In TEOLED with semitransparent cathode and reflective anode, some microcavity effects are bound to occur due to reflective nature of both electrodes. In such cases, the thicknesses of various layers between electrodes and the thickness of the organic stack have to be tightly controlled to achieve the clean EL spectrum. The microcavity is designed by constructing the standing wave of emitted and reflected lights, assuming the exciton dipole as a particle in one-dimensional box. No interference fringes have been noticed in the EL spectrum of designed TEOLED whereas fringes are noticed in other TEOLED, fabricated without optimizing thicknesses of various layers in the cavity.

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4. Conclusions OLEDs have emerged as the leading next-generation information displays. Top emitting configuration provides the solution to satisfy the requirements of these displays. The TEOLED, Ni/a-NPD (35 nm)/Alq3 (50 nm)/BCP (5 nm)/Ca (10 nm)/Ag (10 nm), is fabricated using a highly reflective and opaque Ni anode and the Ca/Ag semitransparent cathode. Due to the favorable properties of Ni, it has been selected as an anode. It can be deposited by using the RF magnetron sputtering on a glass substrate and has a good adhesion. The sputtered Ni has a low surface roughness, easy processibility, and good electrical properties compared to Au, Cu, and Pt. O2 plasma treatment of Ni produced NiO, which has a high work function and promotes good hole injection properties. Therefore, Ni has been proposed as an

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