High efficient pin orange organic light emitting diode fabrication with novel Al cathode using DC magnetron sputtering

Tae Hyun Gil, Sebastian Franke, Christian May, Jörg Amelung, Hubert Lakner, Karl Leo

Fraunhofer-Institut für Photonische Microsysteme, Maria-Reiche Str. 2, 01106 Dresden, Germany

ABSTRACT

In this study a high efficient p-i-n type orange organic light emitting diode (OLED) is presented. It is based on doped charge transport layers to realize low operating voltage and emitting layer which consists of alpha-NPD(4,4-bis [N-(1-naphtyl)-N-phenylamino]biphenyl) and Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) as a host and a phosphorescence dye dopant respectively. Organic layers are vacuum-sublimed on ITO-coated glass substrates in vertical inline deposition tool, and aluminium is deposited directly on organic layer by DC magnetron sputtering to form a cathode. Since sputter deposition of top electrode is known to damage organic layers and degrade OLED performance, various sputter process parameters are selected and applied for cathode formations, and the OLEDs are characterized by means of I-V-L measurements. The OLED characteristics are evaluated with the plasma factors based on sputter process parameters in order to explain the damage sources from sputtering process. The characteristics of OLEDs that cathodes are deposited by sputtering and evaporation are compared. The fabricated OLED which has the lowest damage level exhibits almost comparable result to the OLED that the cathode is deposited by evaporation. The OLED shows good performances of driving voltage of 4.25 V and luminous efficacy of 7.77 lm/W and current efficiency of 10.68 cd/A at 1000cd/m2. Keywords: organic light emitting diode, DC magnetron sputtering, p-i-n OLED

1. INTRODUCTION

Since Tang and VanSlyke [1] have released a stacked electroluminescence device based on small molecule organic materials in 1987, organic light emitting diode (OLED) has been attracting a tremendous attention from researchers and industries during two decades. The dominant advantages of OLED are able to be summarized as the realization of full colours (red, green, blue) by means of various organic materials and the relatively simpler fabrication process than other commercially existing display device, e.g. TFT-LCD. Because of their potential for next generation flat panel displays and solid state lighting applications, countless efforts were performed to improve device efficiencies and commercialize OLED to markets. A great breakthrough in efficiency was achieved by the combination of charge-transport host material and doping of phosphorescent material which emits intense phosphorescence from triplet states at room temperature. All of the three major colours could be realized using phosphorescent material with external quantum efficiencies of over 5% [2, 3, 4]. However, the factors that determine the OLED performance is not confined only to the luminescence efficiency of emission layer, but also the charge transport in emission layer and the carrier injection from electrodes. Commonly proposed OLED structures comprised charge transport and injection layers intimated with electrodes, but low carrier mobility of ordinary organic material [5] limited vigorous charge transport because the carriers in organic material are moved by filling traps under high electric field. P. E. Burrows et al reported that the major carrier transport mechanism in organic semiconductor is trap-limited current [6]. Hence, such a limitation gives disadvantages to device operation, e.g. high operation voltage. The carrier injection properties from electrode into organic material have been also important issue. J. Blochwitz et al [7] has proved that the intrinsic layer was totally depleted when a metallurgical contact was formed on organic semiconductor. In this case, the carrier injection property is dependant on thermionic emission, and the carrier injection into organic is inverse exponential function to the energy difference between molecular orbital of organic semiconductor and workfunction of metal [8]. This has been also a critical issue to choose an appropriate cathode material in OLED. However, those two issues were able to be solved by intentional doping in organic material. Blochwitz et al has not only explained the phenomenon of metal-intrinsic organic material contact, but proposed the doped charge transport layer in the report [7]. Moreover, they found out that a significant band bending occured at the

Organic Optoelectronics and Photonics III, edited by Paul L. Heremans, Michele Muccini, Eric A. Meulenkamp, Proc. of SPIE Vol. 6999, 69991C, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.780430

Proc. of SPIE Vol. 6999 69991C-12008 SPIE Digital Library -- Subscriber Archive Copy

metal-organic junction as well as smaller depletion region when a doped organic material is contacted with metal. This means that tunnelling can become a dominant injection mechanism and the doped organic transport layer can provide a huge variety of choices in selecting a contact material. Besides of it, the doped transport layer enables a realization of thick film due to their high carrier density and conductivity. As described above, the doping process brings many solutions and advantages to OLED fabrication simultaneously, and it can be a cutting edge technique undoubtedly in OLED fields.

In order to mass-produce the OLED, development of process equipment as well as of high functional organic material must be performed. A lot of types of equipment were proposed to deposit organic material on large area substrate. Two types of equipment were introduced mainly in the markets, one is In-line type, and the other is Cluster type. Both types of equipment have their own advantages and disadvantages, but the In-line type is thought to prevail against Cluster type in the aspect of productivity. In general, In-line equipment can be embodied as a vertically standing type, and it consists of several process chambers which has linear type evaporation sources in it. Because the vaporizing temperatures of metals are mostly much higher than organic materials, the heat radiation from the source can deteriorate organic material properties. Therefore, alternative metal deposition technique is required to form a cathode, and magnetron sputtering can be one of the candidates to deposit thin films of good quality in vacuum.

Since the magnetron sputtering technique is versatile and eco-friendly method, there were many trials to deposit top cathode using magnetron sputtering [9-12]. If transparent conducting oxide is deposited on organic layers, top emission OLED can be realized which provides more aperture ratio. When organic materials are, however, exposed to plasma, they are damaged easily by plasma factors from sputtering, e.g. high energetic particle bombardment, because they have relatively weaker bonding structures than inorganic materials. Nevertheless, the exact damage sources were not clarified yet sufficiently.

To investigate on it, aluminium (Al) is deposited using direct current (DC) magnetron sputter gun, and several sputter process parameters are selected to identify degradation of OLED. Orange OLEDs are fabricated in vertical In-line equipment, and performance of orange OLED comprising carrier transport layers (Novaled AG) and emission layer consisted of α-NPD (4,4′-bis [N-(1-naphtyl)-N-phenyl-amino] biphenyl) as a host and Ir(MDQ)2(acac) (Iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate)) as a phosphorescent dye dopant is presented. Because the Ir(MDQ)2(acac) was reported as highly efficient in hole transporting host material [13], it can be applied to white OLED fabrication as a orange-red emission layer or area lighting source.

In order to compare the OLED having sputtered aluminium cathode with undamaged OLED, we fabricated orange OLED whereby ytterbium (Yb) is evaporated as a cathode. Both two OLEDs are characterized by Current-Voltage-Luminance (I-V-L) measurements, and the results are discussed for possible damage mechanisms.

2. EXPERIMENTAL DETAILS

2.1 Vertical In-line deposition equipment The equipment used in this study is VES400 (Applied Materials, GmbH & Co. KG), world first vertical In-line OLED

fabrication machine [14]. In-line type deposition tool has been being used in many semiconductor and display mass-production lines due to its high throughput. The implementation of the vertical design has two main advantages. The one is less particle contamination, the other is that large area glass and mask can avoid problems of bending toward bottom.

The installed equipment VES400 in Fraunhofer IPMS was designed for substrate size up to 300×400 mm, and it consisted of several functional chambers as demonstrated in Fig. 1 [15]. The substrate-installed carrier is loaded to the module 1(Glove box), and deposition processes are performed through the process chambers (module 5 ~ 10). A set of three linear type evaporation sources is installed to deposit organic materials in each process chamber and they enable doping by means of co-evaporation. One middle-lying source is used for host material deposition, and the other two surrounding sources are for dopant material depositions. After organic layer deposition, the carrier is transferred to metal evaporation chamber to deposit metal cathode. VES 400 was equipped with a sputter chamber which can deposit metals, and an aluminium target which has area of 87×488 mm2 was installed for this study. The fabricated OLEDs are encapsulated with getter-attached glasses in nitrogen ambient glove box, and they are taken out of glove box for measurement.

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300

400

Fig. 1 Diagram of VES 400 In-line equipment

2.2 Substrate preparation and loading The substrates used in this study have four different sizes of active area as shown in fig. 2. Each 4-quadrant size is

defined by the aperture of passivation, and ITO film of 90 nm is deposited under the passivation. Each diode is named as D1, 2, 3, and D4 in association with increasing size, and the sizes are also indicated in fig. 2. By means of different diode size, homogeneity of OLED performance can be compared in terms of current density versus luminance.

19.909 mm2 79.645 mm2

4.977 mm2 1.245 mm2

Fig. 2 4-quadrant substrate and active area sizes for OLED fabrication

Fig. 3 Substrate loading in the process carrier

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CH3/oCH

The substrates are cleaned by isopropanol and DI water, and the substrates are heated at over 120°C for 1 hour in dry nitrogen ambient in order to minimize outgassing. The substrates are installed in the process carrier as illustrated in fig. 3. The carrier can cover active region of 300×400mm2, and maximum 13 × 4-quadrant substrates can be installed within active region, thus, a distribution of OLED characteristics can be obtained.

2.3 Organic layers and Yb film deposition The orange light emitting OLED utilizing doped transport layers is fabricated in the In-line equipment. The proposed

OLED stack in this study are ITO/NHT5:NDP2/α-NPD/α-NPD:Ir(MDQ)2(acac)/NET5/NET5:NDN1/Yb and Al as illustrated in fig. 4. Fig. 4 (a) is the OLED with evaporated Yb cathode, and fig. 4 (b) is with sputtered Al cathode. For a realization of low voltage operation, p- and n-doped transport materials (Novaled AG) are applied on each anode and cathode side, respectively. The doped transport layers are co-evaporated from host and dopant evaporation sources, and the doping concentration of each transport layer is controlled to yield a conductivity of ~10-5 S/cm.

(a) (b) (c)

Fig. 4 Organic stacks of fabricated OLED, (a) with evaporated Yb cathode, (b) sputtered Al cathode, (c) molecular structure of α-NPD upper and Ir(MDQ)2(acac) lower.

Because the doped transport layer has high conductivity and carrier density compared with intrinsic organic materials, influence of layer thickness becomes weaker on carrier transport. Therefore, a thick transport layer can be achieved to reduce the interlayer defects from particle contamination. The emission layer is also co-evaporated in organic deposition chamber. The molecular structures of these materials are illustrated in fig. 4 (c), and the doping ratio of Ir(MDQ)2(acac) is optimized for highest luminescence. After deposition of doped electron transport layer, Yb cathode was deposited in metal evaporation chamber at over 600oC in order to use it as a damage-free reference OLED.

3. RESULTS AND DISCUSSIONS

3.1 Characteristics of OLED having evaporated Yb cathode and sputtered Al cathode In order to investigate on the phenomena of OLEDs that the cathodes are deposited by evaporation and sputtering, a

Yb film is deposited on the proposed organic stack, and an arbitrary process parameter, DC power of 300 W, Ar flow rate of 300 sccm, is selected to deposit Al film of 100 nm by sputtering on the same organic stack. The OLEDs having evaporated Yb [19] and sputtered Al cathode are fabricated successfully, and they are characterized by means of I-V-L measurements at applied voltage from 5 to -5 V. The I-V-L results are shown in fig. 5 (a), and the luminance spectrums are compared in fig. 6.

Glass

ITO

NHT5:NDP2

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α-NPD:Ir(MDQ)2(acac)

NET5

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α-NPD:Ir(MDQ)2(acac)

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Fig. 5 I-V-L results of (a) Yb evaporated OLED and (b) Al sputtered OLED

The both OLEDs exhibit almost identical operation voltages of 3.27 V at 100 cd/m2, and 4.23 and 4.13 V at 1000

cd/m2 for Yb and Al cathode, respectively. The spectra of the OLEDs are graphed in fig. 6 and are also identical exhibiting peak values at λ=600 nm. In spite of different workfunctions of two metal cathodes, Yb=2.6 eV and Al=4.2 eV, the two OLEDs show almost same performance. It is obvious from the I-V-L results that the carrier injection properties from the both cathodes are comparable by the introduction of doped transport layer, and it proved that the injection is not dependant on thermionic emission but the tunnelling effect. In case of the Yb evaporated OLEDs, the Jrev values at negative voltage regime exhibit low Jrev values of around ~10-4 mA/cm2 at -5 V while the Al sputtered OLEDs exhibit higher values of Jrev. This indicates that the organic layers are deteriorated during the sputter process.

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Fig. 6 EL spectra of OLEDs

Because the component of parallel ohmic resistance of organic layers contributes to the current in OLED at negative

voltage and low positive voltage [6], the Jrev can represent either that the original organic materials are decomposed and form more conductive species, or a lot of traps are formed in the organic stack. Such high leakage current path can induce a joule heating during OLED operation, and it can result in shorter lifetime or sudden death problem of OLED from rapid thermal degradation of organic molecules by high leakage current path. Accordingly, the Jrev can be used as an index to identify defects inside of organic layers, and the Jrev of reference OLEDs will be compared with them of OLEDs having sputtered Al cathode since here.

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3.2 Al deposition process parameters and deposition rates In order to deposit Al film on organic layers by sputtering, five process parameters are selected, and the corresponding

deposition rates are listed in table 1. Sputter powers are varied from 100 to 500 W at fixed Ar flow rate of 500 sccm in process number from 1 to 3, and Ar flow rates are varied from 100 to 500 sccm in process number from 3 to 5 leaving the sputter power of 500 W which is power density of 1.18 W/cm2. Because the In-line type equipment uses a linear motion for process, thickness of film is controlled by carrier moving speed and oscillations in front of sputter cathode. The carrier moving speed in this study is fixed at 0.48 m/min, and the oscillations are determined to deposit Al film of 100 nm in all processes.

Based on the sputter process parameters, the current-voltage relations of sputter cathode are measured from DC power supply. As seen in fig. 7, the both DC voltage and current increase as higher power is applied to the sputter cathode in process 1 to 3. The DC voltage and current increases nonlinearly as the power increases to 500W, but the deposition rate increases at almost proportional rate of the DC power. It is thought that ions of sufficient amount contribute to result in the deposition rate despite of low DC voltage because the DC current at DC power 100 W is not exactly one third with respect to that of DC power 300W. The DC current and voltage in process 3 to 5, however, show a slight parabolic feature which has peak value at process 4. The deposition rate decrease at higher Ar flow rate, and higher collision probability of sputtered Al molecules with neutral Ar gases is expected as the reason for decreased deposition rate at the almost same DC voltage and current.

Table .1 Sputter process parameters and corresponding results

No DC power (W)

Ar (sccm)

Pressure (hPa)

Speed (m/min)

Deposition rate (nm/speed.oscillation)

DC voltage (V)

DC current(A)

1 100 500 5E-3 0.48 2.48 253.8 0.39 2 300 500 5E-3 0.48 8.58 292.7 1.02 3 500 500 5E-3 0.48 14.75 341.6 1.46 4 500 300 3E-3 0.48 18.7 341.6 1.46 5 500 100 1E-3 0.48 24.86 346.1 1.44

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Fig. 7 Current-Voltage relations of sputter cathode as process parameters

3.3 Characteristics of OLEDs as various sputter process parameters OLEDs are fabricated with Al cathodes which are sputtered on organic layers as various sputter process parameters.

For these experiments, 5 substrates are installed in the substrate carrier from 1 to 5 on every process in order to observe a distribution of OLED degradations. The fabricated OLEDs are characterized by I-V-L measurements, and the values of reverse current density Jrev at -1 V are extracted for comparisons because some seriously deteriorated OLEDs reach values of 100mA/cm2 even at -1 V which is the limit value of I-V-L measure instrument. All of the extracted Jrev values are indicated as histograms in fig. 8, and Jrev values of OLED having evaporated Yb cathode are also indicated. Some of the OLEDs had failures during fabrication, and they are not appeared in the graph.

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(a) (b) (c)

Fig. 8 Histograms of reverse current densities from (a) D2, (b) D3, (c) D4

3.3.1 Evaluation of OLEDs as variation of DC powers (Process 1, 2, 3) Al films are deposited on organic layers as the process parameters 1, 2, 3. As shown in fig. 7 the both DC voltage and

current increase evidently as the power increase at a fixed process pressure. The increase of DC current can be explained that electrons obtain higher energy from higher power applied to sputter cathode and they contribute to ionize more neutral gases leading to higher ion current on sputter cathode. The sputter voltage also increases in this sputter cathode because the DC voltage is determined by cathode load, e.g. cathode area and magnetic field intensity. The high energetic particles sputtered from the target surface are important factor in sputter process. Because sputtering mechanism is based on momentum transfer from ion to target atom, the sputtered target atoms can hold energy of which they obtain from ions. On the surface of target, however, not all the ions contribute to the momentum transfer, but considerable number of ions can penetrate into the target surface through scattering. Those ions lose their charge during scattering but still have high kinetic energy. The neutralized gas atoms can be recoiled toward the opposite side of target. Accordingly, both the sputtered target atoms and the recoiled neutral gases move toward the substrates with high energy, and they can bombard the surface of substrate. L. S. Liao et al have studied the degradation of OLED caused by bombardment of energetic particles [16]. The C-C bonding energy varies from 100 to 500 kJ/mol, and the C-H bonding energy varies from 350 to 550 kJ/mol which are mostly consisting of organic molecule. These bond energies are lower than the energies of particles from sputter process, and such energetic particles are capable to destroy organic molecules or create displacement defects within organic layer. Hence, more serious damage can occur at higher sputter voltage and current. The results of I-V-L measurement from process 1 to 3 exhibit corresponding features to other researches except for process 1. If the OLED degradation follows the bombardment theory, process 1and 3 must show the lowest and highest reverse current density respectively. The process 3 exhibits higher Jrev values than process 2 as expected, but the lowest Jrev values are observed in process 2, and the Jrev values in the process 1 are higher than the process 2 despite the sputter voltage and current are lowered. This phenomenon is unexplainable with the theory of high energetic particle bombardment presently, and additional investigation is necessary. Thin film is grown at the beginning of film deposition through nucleation on substrate forming islands. Once Al islands begin to cover the surface of organic layer, those Al islands can protect organic layers against particle bombardment, but the exposed organic molecules are attacked until Al islands cover the whole surface of organic layer. S. Valkealahti has studied the penetration depth of energetic Ar atom into Al film and has reported that the major penetration depth is around 20 nm at Ar incident energy of 400 eV [17]. At this point of view, it is possible to yield an explanation that the deterioration of organic layers from energetic particles can occur probably at initial stage of Al film formation, and fast deposition rate and low sputter voltage are therefore required to reduce damage by particle bombardment.

The OLEDs fabricated with sputter process parameter 2 show good performances. The electrical and luminous characteristics of a representative OLED are shown in fig. 9, and it exhibits very low Jrev values. Not all of the OLEDs in process 2 show good results, but the possibility of sputter deposition on OLED is expectable. The OLED efficiencies are indicated in fig. 9 (b), and current efficiency is placed in the inner corner. The driving voltage was 4.25 V and the luminous efficacy was 7.77 lm/W and current efficiency was 10.68 cd/A at 1000cd/m2.

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Al films are deposited on organic layers as the process parameters 3, 4, 5 which have different process pressures. The

movement of particles in vacuum is determined by mean free path of molecules. The mean free path λ is the average distance that a molecule travels before it collides with another molecule. The λ is given by equation (1),

22 PdkTπ

λ = (1)

d is the diameter of the molecule and P is the pressure. In case of Ar, the mean free path at 0.001 hPa is approximately

60 mm. The mean free path of particles in vacuum process is a strong function of gas pressure, so that particles in a process reactor collide more frequently as process pressure increase loosing their energies. Because the distance between the target and the substrate is set as 84 mm in current sputter system, the energy loss of the particles departed from sputter cathode can be quite low especially in process 5. From the Al deposition results of the process 3 to 5, the deposition rates show that they do not follow the cathode current and voltage relations. As the Ar flow rate increases, the deposition rate decreases gradually, and this is an evident clue to explain collision process. If the energetic particles from sputtering collide with neutral gases, they can lose their kinetic energies as well as their moving direction is changed which can result in lower deposition rate on substrate.

I-V-L results of process 3 to 5 follow the collision process very well. As shown in fig. 8, the overall tendency of the Jrev values increase as the process pressure decreases although those of D3 OLEDs show relatively lower values. This phenomenon is possible to be explained that the energy of particles is reduced effectively by the collision process, and the organic layers are thus less damaged. However, the problem of uneven distribution of the Jrev is still unexplainable with only bombardment and collision effects.

3.4 Al deposition at higher Ar pressure and OLED fabrication As studied above, the OLEDs show lower Jrev values at higher process pressure. As a trial to reduce the OLED

degradation, additional process parameters are selected and they are listed in table 2. In order to process at higher pressure, Ar gas of maximum 900 sccm is injected into the sputter chamber, and the results of Al deposition tests are also indicated in the table 2.

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Table .2 Sputter process parameters and corresponding results at higher Ar flow rate

No DC power (W)

Ar (sccm)

Pressure (hPa)

Speed (m/min)

Deposition rate (nm/speed.oscillation)

DC voltage (V)

DC current(A)

6 300 700 6.5E-3 0,48 6.67 290 1.02 7 500 700 6.5E-3 0,48 12.35 319.3 1.57 8 500 900 7.6E-3 0,48 10.31 305.8 1.62

Based on the sputter process parameters above, the current-voltage relations of sputter cathode are measured from DC

power supply and they are compared with them of lower Ar flow rate at same DC power. Fig. 10 (a) is a graph of which process 2 and 6 are compared, and fig. 10 (b) is of which process 3, 7, and 8 are compared. As seen in fig. 10, the DC voltage and current show only small differences at process pressure over 5E-3 hPa. Saturation of the DC voltage and current is more outstanding at lower DC power due to lower ionization efficiency. The DC voltages and currents slightly decrease and increase respectively in process 3, 7 and 8, but the deposition rate is lower at higher Ar flow rate as explained above with collision process.

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OLEDs are fabricated with the sputter process parameters in table 2. For these experiments, 5 substrates are also

installed in the substrate carrier from 1 to 5 on every process in order to compare directly with process 2 and 3. The fabricated OLEDs are characterized by I-V-L measurements, and their Jrev values at -1 V are extracted for comparisons. All of the extracted Jrev values are indicated as histograms in fig. 11.

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Fig. 11 Histograms of reverse current densities from (a) D2, (b) D3, (c) D4

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3.4.1 Evaluation of OLEDs as Al deposition of higher Ar pressure The Jrev values of Processes 2 and 6 are firstly taken into account. Although the particle energies in process 6 are

estimated to be lower, the OLEDs exhibit almost similar values of Jrev. Decrease of Jrev is clearly observed from the previous experiment of processes 3 to 5 but not at higher process pressure over 5E-3 hPa. In case of process 3, 7 and 8, similar Jrev values are also shown in fig. 11. If the tendency of Jrev in process 3 to 5 is considered, the values of Jrev are expected to be equivalent as process 2, but the real results are not supported only by collision process anymore.

An OLED is investigated on its surface using scanning electron microscopy (SEM) in order to recognize degradation. The two different surfaces of Al film on organic layers were taken and are shown in fig 12. The fig. 12 (a) shows a less deteriorated Al surface. Many hillocks are distributed on the surface and many black spots are also shown. The material contrast in SEM measurement means different material component, and the black spots are estimated to be organic materials in the image. Such deteriorations are difficult to be explained only by bombardment. As discussed already, the particles can hardly destroy organic molecules directly but keep bombarding the Al surface if Al film of several tenth of nanometer is deposited. Organic materials are supposed to be deteriorated continuously during the sputter process, and the decomposed organic materials are presumed to extrude out through the Al films. The deterioration of the Al film surface is occurred more seriously on other surface as shown in fig. 12 (b). The reason for uneven distribution of deterioration on the diode surface is probably due to nonuniformity of plasma, and non-uniformity of Jrev distribution in one carrier may be related to this phenomenon. E. Atanassova. [18] reported damages in thin oxide film caused by ultraviolet (UV) irradiation in magnetized plasma. The ionized atoms in a glow discharge produce broad spectra and high energy levels of radiation when they are recombined with electrons or impinge on electrode surface with high energy. Because the density of metastable states of magnetized plasma is so high and the radiation occurs in vacuum, the photons have energies corresponding to vacuum UV (VUV). Normal organic molecules have strong UV absorption and bandgaps of few eV. If organic molecule is irradiated by VUV, VUV can lead to photochemical reaction as well as the bonds of organic molecules can be broken. E. Atanassova has also studied on VUV penetration in the report, and the perfect VUV blocking was achieved by Al film of over 300nm. Since all the thicknesses of Al film are held at 100 nm, VUV can penetrate through Al film continuously during deposition. In planar magnetron sputtering system the substrate always faces on the target and is exposed to VUV during deposition. Therefore, the VUV besides of energetic particles is thought as possible damage source which results in degradation of OLED.

(a) (b)

Fig. 12 SEM images of OLED deterioration

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4. CONCLUSION

Orange OLEDs are fabricated with evaporated Yb cathode and sputtered Al cathode. The OLEDs are characterized by I-V-L measurements and they show almost identical performance despite of different cathode materials. The OLED fabricated with sputtered Al cathode shows higher reverse current density. In order to investigate on damage sources OLEDs are fabricated as various sputter process parameters, and their I-V-L characteristics are compared. In case of sputter process 1 to 3, the minimum Jrev values are found at the lower DC power, and it can be explained with the particle bombardment energy depends on sputter voltage. Decrease of the Jrev values appears as the process pressure increases. They are explained in terms of reduced particle energy by collision process with neutral atoms. The Jrev values at over Ar flow of 500 sccm, however, show almost similar results. From the SEM measurement, damages appear on the surface of Al film, and VUV is introduced as another possible damage source for the degradation. Nonuniform distributions of damages are also observed in SEM images, and uneven Jrev values are probably due to nonuniformity of magnetized plasma. The OLEDs that Al films are deposited by process 2 show the lowest Jrev values, and show also good performances. Driving voltage of 4.25 V and luminous efficacy of 7.77 lm/W and current efficiency of 10.68 cd/A at 1000 cd/m2 are achieved. Acknowledgement

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