Anal. Chem. 2001, 73, 2245-2253
Analysis of Dark Spots Growing in Organic EL Devices by Time-of-Flight Secondary Ion Mass Spectrometry Atsushi Murase,* Masahiko Ishii, Shizuo Tokito, and Yasunori Taga
Toyota Central R&D Laboratories, Inc., Nagakute, Aichi, 480-1192, Japan
Chemical structural analysis of tape-stripped surfaces at dark spots growing in organic electroluminescent (EL) devices during exposure to the atmosphere was done by time-of-flight secondary ion mass spectrometry (TOFSIMS). The EL devices consist of indium-tin-oxide, triphenylamine-tetramer, tris(8-hydroxyquinoline)aluminum (Alq3), and a Mg-Ag cathode deposited in order under vacuum on a glass substrate. It was found that the interface between the Alq3 layer and the Mg-Ag cathode was exposed as a result of tape-stripping, where a large number of dark spots were observed on both sides. Secondary ion images of O-, Mg+, and Alq2+ were observed from the dark spots on the cathode side. On the other hand, Mg+ and O- images with a nucleus in the center were observed from the Alq3 side. It is concluded from the results that the constituent element Mg of the cathode was oxidized at the interface adjacent to the Alq3 layer during exposure to the atmosphere, forming a dark spot with a nucleus in the center. Finally, it was confirmed that the TOF-SIMS analysis of the tape-stripped surface is useful for the analysis of the mechanism of dark spot formation. Organic electroluminescent (EL) devices are expected to be used in various fields because of their excellent luminescence properties, if reliability for long-life operation is guaranteed.1 A brightness drop and a growth of dark spots are well-known to be the main problems of degradation in practical use of EL devices, and their mechanisms have been investigated in recent years by various approaches.2-9 As a result of depth analysis with dynamicSIMS and Monte Carlo simulation,2-4 the brightness drop occurring gradually with operation is thought to be caused by (1) Carver, G. E.; Velasco, V. J. Synth. Met. 1997, 91, 117. (2) Matsumura, M.; Jinde, Y. Synth. Met. 1997, 91, 197. (3) Nguyen, T. P.; Spiesser, M.; Garnier, A.; de Kok, M.; Tran, V. H. Mater. Sci. Eng. 1999, B60, 76. (4) Brese, N. E.; Rohrer, C. L.; Rohrer, G. S. Solid State Ionics 1999, 123, 19. (5) Aziz, H.; Popovic, Z.; Xie, S.; Hor, A.-M.; Hu, N.-X.; Tripp, C.; Xu, G. Appl. Phys. Lett. 1998, 72, 756. (6) Do, L.-M.; Choi, K.-H.; Lee, H.-M.; Hwang, D.-H.; Jung, S.-D.; Shim, H.-K.; Zyung, T. Synth. Met. 1997, 91, 121. (7) Kawaharada, M.; Ooishi, M.; Saito, T.; Hasegawa, E. Synth. Met. 1997, 91, 113. (8) Smith, P. F.; Gerroir, P.; Xie, S.; Hor, A. M.; Popovic, Z.; Hair, M. L. Langmuir 1998, 14, 5946. (9) Aziz, H.; Popovic, Z.; Tripp, C. P.; Hu, N.-X.; Hor, A.-M.; Xu, G. Appl. Phys. Lett. 1998, 72, 2642. 10.1021/ac001087+ CCC: $20.00 Published on Web 04/18/2001
© 2001 American Chemical Society
electrochemical reaction at the interface of each layer. The growth of dark spots has been presumed phenomenologically to originate by delamination at the interface of each layer due to oxidation by moisture.1,5 Recently, several investigations of the growing mechanism of dark spots by analytical methods have been performed.6-9 By observations with scanning electron microscopy (SEM) and atomic force microscopy (AFM), the morphological changes in the cathode surface have been confirmed at the position of the dark spots.6 Cross-sectional observation using a combination of a focused ion beam (FIB) and transmission electron microscopy (TEM) revealed the existence of a nucleus in the center of a dark spot.7 The formation of dark spots was found to be attributed in part to the crystallization of the hole transport layer by SEM and energy-dispersive X-ray analysis (EDXA).8 By reflection infrared microspectroscopy of the organic layers after removing the cathode, delamination between the emitting layer and the hole transport layer was detected.9 From these studies, however, no standard view on the growing mechanism of dark spots has been obtained, because different types of devices were tested under different conditions by each researcher, and the growing mechanism will be affected by the type of device and the condition of exposure. The presumption that oxidation or delamination at the interface of the layers causes a dark spot has not been confirmed by data on chemical changes at the position of the dark spots, because a limited number of suitable analytical methods for a small area of organic thin films under 100 nm in thickness is available. To characterize the chemical structure at the position of the dark spots, an analytical method for detecting chemical changes at the interface of organic layers is required. Generally, for the purpose of characterization of interfaces of organic thin films, the following three methods have been used. The first method is depth profiling by sputtering from the surface. The second method is mapping or line analysis of the position of the interfaces of the cross section. The third method is surface analysis of a mechanically delaminated interface. The first method enables us to obtain the depth distribution of elements by using dynamic-SIMS or Auger electron spectroscopy (AES), but no information on organic chemical structures can be obtained. The second method requires high spatial resolution of nanometer order to identify several monolayers at interfaces; therefore, it is limited to morphological analysis such as by TEM or AFM. The third method enables us to utilize various kinds of surface analysis techniques such as XPS or TOF-SIMS which are suitable for the analysis of organic Analytical Chemistry, Vol. 73, No. 10, May 15, 2001 2245
Figure 1. Schematic illustration of the structure of the studied EL device.
materials. Especially for the evaluation of dark spots of several 10-100 µm in diameter, TOF-SIMS is expected to be useful because of its ability for molecular imaging at high lateral resolution.10 Therefore, this third method is thought to be the only way to evaluate chemical changes at the dark spots in organic EL devices, if the position of the mechanically stripped surface is specified and if some chemical changes corresponding to dark spots occur at the surface. In this paper, we report the results of the investigation on the evaluation of chemical changes at dark spots by TOF-SIMS analysis of mechanically stripped surfaces of organic EL devices. EXPERIMENTAL SECTION Preparation of Organic EL Devices. The structure of the studied EL device is shown in Figure 1. The EL device had a typical two-organic layer structure of indium-tin-oxide (ITO, ∼160 nm) as a transparent anode, triphenylamine-tetramer (TPTE, ∼70 nm) as a hole transporting layer, tris(8-hydroxyquinoline)aluminum (Alq3, ∼70 nm) as an emitting layer, and a MgAg cathode (∼150 nm). The chemical structures of TPTE and Alq3 are shown in Figure 2. The total thickness of the device is ∼400 nm. The device was covered with a MgF2 protector of ∼300 nm in thickness. All materials are commercially available reagents. TPTE and Alq3 were refined by vapor deposition before using in the study. Occurrence of Dark Spots and Their Evaluation. For the purpose of the occurrence of dark spots, the surface of ITO was exposed to the atmosphere for 2 days before vapor deposition of the organic layers, cathode, and MgF2 protector. By exposing the whole device to air, a large number of dark spots of approximately 50-100 µm in diameter were grown in it. Optical microscopy images of the dark spots grown in the device are shown in Figure 3. The occurrence and growth of the dark spots were observed by a video microscope system. Chemical structural analysis of the dark spots was performed by TOF-SIMS analysis of the tapestripped surfaces. (10) Benninghoven, A. Surf. Sci. 1994, 299/300, 246.
2246 Analytical Chemistry, Vol. 73, No. 10, May 15, 2001
Figure 2. Chemical structures of triphenylamine-tetramer (TPTE) and tris(8-hydroxyquinoline)aluminum (Alq3).
Figure 3. Optical microscopy image of dark spots grown in the device.
Tape-Stripping of EL Devices. The cathode side of the devices was peeled off with adhesion tape from the anode side as shown in Figure 4. By TOF-SIMS analyses of both sides of the stripped surfaces, high mass resolution secondary ion mass spectra and high lateral resolution secondary ion images were obtained. TOF-SIMS Analysis. TOF-SIMS measurements were performed with a Physical Electronics TFS-2100 (TRIFT2) instrument controlled by OS/2 Cadence software. High mass resolution spectra of M/∆M > 3000 at m/z 27 (Al+) or 25 (C2H-) were acquired using a bunched 69Ga+ ion pulse at an impact energy of 15 keV, an ion current of 600 pA for 1 pulse, a pulse width of 14 ns (700 ps after bunching), and a pulse frequency of 10 kHz. High lateral resolution images of 0.2 µm and low mass resolution spectra of M/∆M > 300 at m/z 27 (Al+) or 25 (C2H-) were acquired using an unbunched 69Ga+ ion pulse at an impact energy of 25 keV, an ion current of 600 pA for 1 pulse, a pulse width of 30 ns, and a pulse frequency of 10 kHz. This mass resolution was sufficient
Figure 5. Positive ion spectra of tape-stripped surfaces of an undegraded device. Upper, anode side; lower, cathode side. The measured area is 50 × 50 µm.
Figure 4. Schematic illustration of tape-stripping of a device.
for the identification of molecular ions of TPTE and Alq3. Total ion doses in these measurement were approximately
