5946
Langmuir 1998, 14, 5946-5950
Degradation of Organic Electroluminescent Devices. Evidence for the Occurrence of Spherulitic Crystallization in the Hole Transport Layer P. F. Smith,* P. Gerroir, S. Xie, A. M. Hor, and Z. Popovic Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1
M. L. Hair Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8 Received August 20, 1997. In Final Form: March 18, 1998 Electroluminescent devices show major promise for the next generation of flat panel displays. These devices consist of a hole transport material in conjunction with an electron transport material, sandwiched between electrodes of different work function.This paper provides the first evidence of the crucial role of spherulitic crystallization of the hole transport material in the degradation of organic electroluminescent devices. The evolution and growth of nonemissive dark spots are studied using a combination of atomic force microscopy, electron microscopy, and energy-dispersive X-ray analysis. Spherulites are imaged and shown to nucleate from a 1 µm defect on the indium tin oxide anode. These spherulites cause the device to delaminate, and this results in a decrease in the luminescence around the defect and, finally, the failure of the electroluminescent device.
Introduction Since the first report of electroluminescence from organic materials,1 there has been considerable interest in the development of organic electroluminescent devices (OELDs) for use in flat panel displays. The general principle of this technique is that a thin layer of electroluminescent material is sandwiched between two other thin semiconductor layers that transport hole or electrons selectively. When a field is applied, the transported holes and electrons are neutralized in the middle layer and light is emitted. Tri(8-hydroxyquinoline)aluminum (Alq3) is a material of choice for the actual emitting layer, and use of this material simplifies the device as it can function both as the emitter and the electron-transporting film. The light produced using this method can be extremely bright, but a major problem is the lifetime of the device. To be of practical use, the device must have a lifetime of longer than 10 000 h while producing the intensity of a computer screen (100-200 cd m-2). It has been shown that the lifetime of the device is very sensitive to atmospheric contaminants. The lifetime of the devices in the laboratory is increased if they are kept in a dry, nitrogen atmosphere. When OELDs are operated under atmospheric conditions, the electroluminescent intensity decreases dramatically within the first hour.2 Other workers have observed that this decrease in luminescence has been accompanied by the evolution of dark spots, but until recently this principle mechanism of the degradation and failure has not been examined in detail.3,4 Encapsulation methods have been examined * To whom correspondence should be addressed. (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Smith, P. F.; Hair, M. L.; Xie, S.; Popovic, Z.; Hor, A. M. Unpublished results, Xerox Research Centre. (3) Tokailin, H.; Matsuura, M.; Higashi, H.; Hosokawa, C.; Kusumoto, T. Proc. SPIE 1993, 1910, 39.
with the aim of curtailing the occurrence of dark spots.5-7 The elucidation of the mechanism of the degradation process and the prevention of the “dark spots” are extremely important if electroluminescent devices are ever to replace conventional display devices. This paper examines the degradation of an electroluminescent device both in air and under dry nitrogen. We report on the use of atomic force microscopy (AFM), time lapse photography, scanning electron microscopy (SEM), and energy-dispersive X-ray analysis (EDXA) to study the process. Experimental Section The design of a typical electroluminescent device is shown in Figure 1a. In this study the device was viewed from the indium tin oxide (ITO) side as shown in Figure 1b. The structure shown is that used in most laboratories. The device anode consists of a glass plate that has been sputter coated with ITO and was obtained from Donnelly Applied Films, Ltd. Resistance was 1.5 kΩ cm-2 over a 5 cm2 area. A strip of ITO (50 mm × 6 mm) was prepared by appropriately masking the ITO surface and then etching using a Zn/HCl solution. This strip was cleaned using Decon fluid, MILLI-Q distilled water (18 MΩ cm at 25 °C), and UV/ozone. A hole-transporting organic material, N,N′-diphenylN,N-bis(3-methylphenyl)1,1′-biphenyl-4,4′-diamine (TPD), was vacuum deposited from a molybdenum boat at a pressure of less than 1 × 10-6 Torr, to a thickness of 50 nm. A 50 nm thick layer of tris(8-hydroxyquinoline)aluminum (Alq3) was then deposited on top of the TPD, and the device was completed by depositing a Mg/Ag alloy to a thickness of 150-200 nm. The alloy was (4) Burrows, P. E.; Bulovic, V.; Forrest, S. R.; Sapochak, L. S.; McCarty, D. M.; Thompson, M. E. Appl. Phys. Lett. 1994, 65 (23), 2922. (5) McElvain, J.; Antoniadis, H.; Hueschen, M.R.; Miller, J. N.; Roitman, D. M.; Sheats, J. R.; Moon, R. L. J. Appl. Phys. 1996, 80 (10), 6002. (6) Littman, J. E.; Scozzafa, M. U.S. Patent No. 5,059,861. (7) Scozzafava, M.; Tang, C. W.; Littman, J. E. U.S. Patent No. 5,073,446, 1991.
S0743-7463(97)00940-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/27/1998
Degradation of Organic Electroluminescent Devices
Langmuir, Vol. 14, No. 20, 1998 5947 as the stylus damages the layer of deposited material. This effect has been noted previously, when examining thin film magnetic heads.8,9 To investigate the degradation, a device was run at a constant driving current at room temperature and the evolution of the nonemissive dark spots examined.
Results and Discussion
Figure 1. (a) Structure of an electroluminescent device. (b) Viewing position of the electroluminescent device. formed by coevaporation of Mg and Ag through a suitable mask in order to produce an array of electrodes along the ITO strip. The formation of dark spots was followed using a Mitutoyo microscope (magnification ×1800), which was attached to a JVC time lapse video recorder and a Sony video printer, UP5100. Atomic force microscopy images of the topmost film were obtained using a Topometrix TMX 1010 Explorer, equipped with a 130 µm or a 2 µm scanner. The samples were scanned in contact mode, using a Si3N4 cantilever that had a force constant of 0.3 N m-1. The devices were also analyzed using a scanning electron microscope. A thin film of carbon was evaporated onto the OELD using an Edwards coating system, model E306A. Energydispersive X-ray analysis was applied to the film using a Kevex Delta Plus analyzer on a JEOL SEM, model 6300F, equipped with a field emission gun (FEG). The EDXA spectra were all acquired with the SEM operating at 10 kV. Upon completion of EDXA, the sample was sputtered with a thin film (80 Å) of chromium using a Plasma Sciences, Inc., sputtering system, model CrC-100. Electron micrographs of secondary electron images were then recorded on the SEM using a gun potential of 1.0 kV. The emitting characteristics of such devices are dependent on the absolute thickness of the thin film layers. These should be known with some accuracy, and the measurement of this is nontrivial since this thickness is deduced from the thickness monitor of the vacuum deposition system. To obtain reliable thickness values, the vacuum deposition system was calibrated using the atomic force microscope operating with a 2 µm scanner. (Deposited layers for the OELD device are on the order of 50 nm, and hence the thickness calibration required the use of the 2 µm scanner in order to obtain an accurate measurement in the z direction.) The average thickness of the deposited layer was obtained by examining line profiles of scanned areas of 20-25 µm. The thickness vaules measured by AFM were then plotted against the various nominal thickness values from the quartz crystal monitor. It is noted that the use of a Tallystep profilometer for the thickness calibration is not feasible for these organic films
In the first series of experiments we detected the evolution of dark spots by utilizing the emitted light from the devices, which is imaged by an optical microscope. Initially a small current is run through the device to remove any shorts. This is necessary to ensure that the device emits light. Figure 2a shows the surface of the Mg/Ag electrode taken through the ITO reflecting back from the Mg/Ag cathode (configuration shown in Figure 1b) after the initial “breaking in” treatment. An edge view of the device is shown in Figure 1a. Defects on the electrode (that were not present before shorting) appear as dark spots of about 1 µm in diameter. Parts b and c of Figure 2 are taken with the device luminescing under normal atmospheric conditions. The bright areas correspond to high amounts of luminescence; the dark areas correspond to nonemitting areas in the device. The area shown in parts b and c of Figure 2 is identical to the area imaged in Figure 2a. Comparison of the center of the dark spots in Figure 2b and Figure 2c with those in Figure 2a shows that the nonemissive areas correspond directly to the dark defects produced by the introductory “shorting” of the device. Although the actual defect at the center of the dark spot is only 1 µm in diameter, the nonemissive area extends to a distance approximately 10-20 µm from the center. This is most clearly seen in Figure 2b, where the original nucleus of the dark spot is obvious. In this image, the area around the original nucleus is still luminescing slightly as evidenced by the gray area around the black center spot. After 360 min of operation, the area around each defect has stopped luminescing. The nonemissive area has spread further, in some cases with the dark spots eventually merging. As the device ages, it is also noticeable that the edge of the device degrades rapidly. The dark area around the edge extends to almost 10 µm from the edge of the device. The growth of the dark spots and the degradation from the edge that produce the decrease in light intensity are accompanied by an increase in the operating voltage of the device (Figure 3). The decrease in working area and the consequent increase in resistance of the device are not unexpected and relate to the practical observation that a continual increase in voltage is needed to maintain a constant driving current. Devices operated under a dry, nitrogen atmosphere degrade in exactly the same manner, although the time taken for the extension of the nonemitting areas and hence the time taken for the device to die are considerably longer. After the device is operated either in dry nitrogen or under atmospheric conditions, holes appear in the Mg/Ag cathode surface. These holes can be clearly imaged using scanning electron microscopy (Figure 4). The device shows delamination around the hole, and three distinct layers are revealed. When analyzed using EDXA, these layers are shown to be ITO (layer E), Alq3 (layer C) and Mg/Ag (layer A). The central layer (E) exhibits a pattern of branched structures that radiate from a central nucleus. It seems that the layers originally present in the OELD device have delaminated cleanly indicating a lack of (8) Smallen, M.; Lee, J. J. K. Trans. ASME, J. Tribol. 1993, 115, 382. (9) Veisfeld, N. J. Microsc. Soc. Am. 1995, 1 (4), 163.
5948 Langmuir, Vol. 14, No. 20, 1998
Smith et al.
Figure 2. (a) Ag/Mg electrode after initial “shorting”. (b) EL device luminescing (30 min). (c) EL device luminescing (360 min).
adhesion between the layers. Analysis of the circular, branched structures was undertaken by positioning the cantilever tip of the AFM in the central hole and scanning over 5-10 µm areas. Spherulitic crystal structures with heights of approximately 50 nm (the thickness of the deposited hole transport layer) and diameters in the range 100-200 nm are clearly seen (Figure 5). The crystal-
lization appears to nucleate from a circular 1 µm defect and extends over all of the exposed ITO. It is likely that crystallization has been nucleated by the heat produced when a short circuit develops in the device since it appears that the crystallization nucleates from the original defect caused by the shorting. The crystallization continues across the device, extending between the anode and
Degradation of Organic Electroluminescent Devices
Langmuir, Vol. 14, No. 20, 1998 5949
Figure 3. Variation of voltage and light intensity during device operation.
Figure 4. Electron micrograph of a typical defect in an EL device (a) and enlargement of defect (b). The areas A, B, C, D, and E are the areas subjected to EDXA analysis as described in the text: A, contains Mg and Ag (i.e., the electrode); B, also contains Mg and Ag but no Al; C, the emitting layer contains only Al (i.e., no Mg or Ag); D, the fibrils contain only Al; E, the transparent electrode shows only In and Sn (no Al).
cathode. The crystalline spherulites are almost certainly the cause of the reduced luminescence around the hole in
Figure 5. Atomic force micrographs of dendritic crystallization on the ITO surface.
5950 Langmuir, Vol. 14, No. 20, 1998
the device, the gray areas seen in Figure 2b. Such crystallization would disrupt and reduce the contact between the layers allowing oxygen and water vapor to permeate between the spherulites and so penetrate between the layers in the device. Spherulitic crystallization will therefore explain the degradation of the devices even under an inert, dry atmosphere. If the degradation was due solely to water or oxygen, then in a controlled environment there should be no growth in the nonemissive areas surrounding the initial defect. A very recent paper by Fujihira et al.10 suggests on spectroscopic grounds that the dark spots result from the diffusion of Alq3 into the TPD layer. We point out that such diffusion should be observed when analyzing for Al in the series of EDXA experiments. However, the EDXA experiments on the delaminate hole (Figure 4) show no evidence of Al except in the Alq3 layer. The surface (A) and the lip of the hole (B) are Mg/Ag, the layer C and the (10) Fujihira, M.; Do, L.-M.; Koike, A.; Han, E.-M. Appl. Phys. Lett. 1996, 68 (13), 1787.
Smith et al.
crystalline structures D contain Al, and analysis of the layer E shows only ITO. Note: pure organic molecules cannot be detected using EDXA, although it is obvious from the AFM images that TPD is present on this surface. The absence of Al in the residual TPD layer on the ITO (layer E) suggests that the crystallization of the TPD plays a major role in the device passification. Conclusions These results provide the first recorded evidence that the degradation of an electroluminescent device can be attributed in part to the crystallization of the hole transport layer, which in turn provides avenues for the diffusion of substances such as water or oxygen into the hole or electron transport layers. The crystallization appears to be nucleated from a defect present in the anode, which is indicated by the radial growth of dendritic structures from a 1 µm structure. LA9709406
