Improvement in the Ductility of Organic Semiconductor Materials Used in a Flexible Organic Light Emitting Diode

Transcription

1 Journal of JSEM, Vol.14, Special Issue (214) s189-s193 Copyright C 214 JSEM Improvement in the Ductility of Organic Semiconductor Materials Used in a Flexible Organic Light Emitting Diode Toshiro KOBAYASHI 1, Takashi YOKOYAMA 1, Yuichi UTSUMI 2, Hideyuki KANEMATSU 3, Tsuyoshi MASUDA 4 and Motomichi YAMAMOTO 5 1 Department of Electronics & Control Engineering, Tsuyama National College of Technology, Okayama , Japan 2 Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Hyogo , Japan 3 Department of Materials Science & Engineering, Suzuka National College of Technology, Mie , Japan 4 Q-Light Co., Ltd., Iwate , Japan 5 Faculty of Engineering, Hiroshima University, Hiroshima , Japan (Received 28 December 213; received in revised form 23 March 214; accepted 19 April 214) Abstract Tensile strain testing was conducted on standard thin films: Tris(8-hydroxyquinolinato)aluminum (Alq 3 ), Bis [N-(1- naphthyl)-n-pheny] benzidine) (α-npd), and mixed layers of 1/1 Alq 3 /α-npd. Alq 3 films were found to possess the highest ductility and the ductility of α-npd thin films was found to improve by mixing with Alq 3. In order to improve the ductility of the OLED with a conventional structure of glass/indium-zinc-oxide (IZO)/molybdenum oxide (MoO)/α-NPD/ Alq 3 /MgAg, the α-npd layer was replaced by a mixed layer of Alq 3 and α-npd. Key words Organic Semiconductor, Light Emitting Diode, Flexibility, Power-Luminance Efficiency, Alq 3, α-npd, IZO 1. Introduction Because organic semiconductors can be manufactured by printing and have greater flexibility than inorganic semiconductors such as silicon-based semiconductors, research and development of organic semiconductors, including solar cells, has been extensively conducted regardless of their inferior electrical characteristics [1]. In addition, organic light emitting diodes (OLEDs) a type of organic semiconductor have been practically applied in displays and lighting because they are thin, self-luminous, and fast responding [2]. Flexibility can be obtained by applying a polymer sheet or films, such as polyethylene naphthalate (PEN) or polyethylene terephthalate (PET) [3], as the substrate of an organic semiconductor, instead of applying a conventional glass substrate. However, this would require films made of a transparent conductive oxide (TCO) such as indium tin oxide (ITO) or indium zinc oxide (IZO) [4-6]. Recently, it has been reported that a transparent, organic conductor material known as PEDUT:PSS (polyethylene dioxythiophene doped with polystyrene sulfonate) does not show cracking even under a tensile strain of 6%; however, its electric conductivity is relatively low [7]. More recently, it has been reported that multilayer transparent electrodes with a structure of ZnS/Ag/WO 3 show much greater flexibility than ITO and have comparable electric properties [3]. Therefore, improvements in the flexibility of OLEDs can be expected. The establishment of a method of assessing the flexibility of the thin film constituting an organic semiconductor has become an important issue. However, systematic consideration has not been found. Some authors have studied methods for observing the cracking of inorganic TCO and transparent organic light emitting materials [8], and have reported that even organic materials experience cracking at low strain as well as inorganic TCO [9]. In the present work, for a typical OLED device, the crack initiation strain of each layer was evaluated, and a method for improving the ductility in which the relatively less ductile layer is replaced by another more ductile material was proposed. In addition, it was also verified that the improved OLED device had almost same powerluminance efficiency. 2. Experimental 2.1 Tensile test The organic thin film test specimen was Tris(8- hydroxyquinolinato)aluminum (Alq 3 ), Bis[N-(1-naphthyl)- N-pheny]benzidine] (α-npd), and mixed layers of Alq 3 /α- NPD=1/1 of nm thickness deposited on a PEN substrate of 25 nm thickness. The deposition of the α- NPD thin film was conducted using a multi-chamber type vacuum deposition system for organic thin films. The specimens were cut to 4 mm in width, and the occurrence of film cracking under tensile strain was observed using an optical microscope with the specimen clamped to impart tensile strain with the micrometer. The gauge length was 2 mm. Therefore, assuming an error of ±5 μm in the micrometer and mechanical system, an error of.25% was estimated at a strain of 1%, or in other words, a relative error of 2.5% [8]. Moreover, in order to view the samples using a transmission type optical microscope, the components below the test specimens were composed of transparent acrylic and included an opening. 2.2 Observation of cracking Digital photos were taken while specimens were observed by the optical microscope to investigate the relationship between cracking and applied strain. The thin films observed in the present study had a high light transmittance, and the substrate was a transparent plastic sheet; thus, they were not clearly visible under a reflection type optical microscope in which light is irradiated from the objective lens. Therefore, a transmission type optical microscope in which light is introduced from below the transparent specimen to the objective lens was employed (Fig.1). In -s189-

2 T. KOBAYASHI, T. YOKOYAMA, Y. UTSUMI, H. KANEMATSU, T. MASUDA and M. YAMAMOTO addition, because the cracking of the organic thin films was too fine to observe clearly even with the transmission type optical microscope, a laser microscope and a field emission type scanning electron microscope (JOEL, JSM 6F) were used in combination for observing the occurrence and the configuration of the cracking. For the observation of organic films, gold was deposited at a thickness of 3 nm on the specimen to prevent charge up due to the high electrical resistance of the materials. Sample Camera Microscope Rump Micrometer Fig.1 Schematic diagram of the experimental method [8] 2.3 Evaluation of light emission performance The performance of the prepared OLED device was measured using an evaluation apparatus for OLEDs, as shown in Fig.2. It consisted of a multi-channel spectrometer, a luminance meter, and a control system. Luminance was measured with increasing voltage. Luminance efficiency was estimated using the following equation because electric efficiency η e [lm/w] total light flux [lm] divided by the input power [W] is generally used as the luminance efficiency [1]. 3. Results and Discussion 3.1 Thin film cracking under tensile strain Alq 3 The tensile test results for an Alq 3 thin film on a PEN substrate are shown in Fig. 3. per 2 mm length was plotted against the applied strain. It was found that the cracking was initiated at a strain of 4.8% and the number of cracks increased to approximately at 8% strain. Example photographs taken at and 4 magnifications by an optical microscope are shown in Fig. 4. Compared with oxide films like Al 2 O 3 and IZO [9], the number of cracks was greater and the pitch of the cracks was narrower. Alq 3 Fig.3 Relation between strain and the number of cracks per unit area in Alq 3 film on PEN substrate η e [lm/w]=π[sr]l[cd/m 2 ]/P i [W/m 2 ] (1) L [cd/m 2 ] is luminance measured, and P i [W/m 2 ] is electric power density. In addition, the light flux irradiated from a light source with one candela [cd] in a solid angle with one steradian [sr] is defined as one lumen [lm], i.e., 1 [lm] = 1 [cd sr]. The device measured was mm. μm μm (a) IZO 6.4% strain (b) Al 2 O 3 8.% strain Luminance meter OLED device Rotation stage (c) Alq % strain (d) Alq 3 > 8. % strain Controller Multi -channel spectrometer Power source Fig.4 A transmission type optical micrograph of Alq 3 film in comparison with inorganic films Fig.2 Configuration of evaluation equipment for OLED device α-npd The tensile test results for an α-npd thin film are shown in Fig. 5. The tendency of increased cracking with increasing applied strain was same as that of Alq 3. However, the cracking was initiated at a lower strain of approximately -s19-

3 Journal of JSEM, Vol.14, Special Issue (214) 3.5%, and the number of cracks at 1% strain was approximately. Therefore, it is suggested that the Alq 3 thin film is more ductile than the α-npd thin film. The reason is suggested that the original properties of Alq 3 is better than those of α-npd or crystal orientation during vacuum deposition process mainly influenced on the mechanical properties. The morphology of the α-npd film was almost the same as that of the Alq 3 film, as shown in Fig. 6. The formation of the cracks was observed using laser microscopy and field emission type scanning electron microscopy (FE-SEM). Cracking was more readily recognized using laser microscopy at magnification of 4, as shown in Fig. 7. In addition to the vertical lineshaped cracks, granular or short horizontal stripes were observed in the case of a strain more than 8%. From observation at a magnification of 1, by FE-SEM, as shown in Fig. 8, it was determined that the vertical stripes are concave cracks. It was also estimated that the convex shape, which was recognized as short horizontal lines or granular shapes, unlike the vertical protrusions, was an overlapping or bridged shape created by buckling that caused the coating film to peel and resulted from the compressive stress. The compressive stress was produced by the shrinkage in the longitudinal direction due to the tensile strain in the horizontal direction. Fig.7 Images of the specimen shown in Fig.6 observed by a laser microscope at 4 times magnification α-npd (a) times magnification (b) times magnification Fig.8 Images of specimen at more than 8% strain by FE- SEM Fig.5 Relation between strain and the number of cracks per unit area in α-npd film on PEN substrate (a) 4. %Strain (b) 6.5 % strain Fig.6 A transmission type optical micrograph of α-npd film Mixture of Alq3 and α-npd As shown above, the Alq 3 film has higher ductility than the α-npd film, and the ductility of the mixed layer with a weight ratio of Alq 3 /α-npd = 1/1 was measured. In Fig. 9, the tensile test results are shown. The cracking was initiated at a strain of 4.% and the number of cracks increased to approximately - at 8% strain. It is suggested that the ductility of the mixed layer film was between those of the Alq 3 and α-npd films, but was nearer to that of the α-npd film, although the materials were used in an equal weight ratio. Consequently, these results indicate that the threshold strain for crack initiation was improved by using the mixture of Alq 3 and α-npd instead of α-npd alone. -s191-

4 T. KOBAYASHI, T. YOKOYAMA, Y. UTSUMI, H. KANEMATSU, T. MASUDA and M. YAMAMOTO Fig.9 Relation between strain and the number of cracks per unit area in the mixture of Alq 3 and α-npd film on PEN substrate Discussion The average numbers of cracks in the Alq 3, α-npd, Alq 3 /α-npd, and IZO films are shown in Fig. 1 and were estimated using an approximation formula. The IZO film showed a lower number of cracks than the organic films even at a strain of 8%, with narrower crack spacing and approximately 5 cracks per 2 mm. The reason could be that the films may be delaminated or have lateral cracking along the interface at the edge of the cracks near the interface between the substrate and the films due to the higher elastic modulus of IZO and lower adhesion than that of the organic films. On the other hand, the spacing between the cracks in the organic films was narrower, and the cracks, although more in number, were finer. The threshold strains of crack initiation were approximately 3.4%, 4.9%, 4.%, and 4.2% for the α-npd, Alq 3, Alq 3 /α-npd, and IZO films, respectively. Therefore, it is suggested that the threshold strain of crack initiation can be increased by approximately 17% by replacing the α- NPD layer with the mixed layer. In addition, because the thickness of the films used for the tensile tests was thicker than that of the layer in the device, experiments using the thickness of the actual device are desired, although these will not be easy. In future work, changes to improve the flexibility should be examined by tensile or bending testing of the light emitting device Alq 3 /NPD=1 Alq3 α-npd Alq3/α-NPD=1/1 IZO Approximation 3.2 Evaluation of light emission performance Fig. 11 shows the cross-sectional structure of the evaluated basic OLED device. The threshold strain of crack initiation is expected to be increased by replacing the α-npd layer with the mixed layer, which has a greater ductility, as shown in Fig. 1. However, the electrical performance of the device may be decreased by replacing this layer; therefore, α-npd was vacuum deposited at the interface between IZO and the mixed layer. The amount of α-npd was equivalent to 15 nm in thickness; thus, the layer of α- NPD will exist as an island structure which is expected to not cause cracking. An example of the OLED emission is shown in Fig. 12. The device was prepared on a glass substrate and encapsulated by another cover glass, and the emitting area was mm. The characteristics of all structures were measured using four pieces, and representative data were plotted in Fig. 13, which shows the influence of an applied voltage on the power luminance efficiency. The device with the mixed layer had a little lower efficiency than the basic device; however, the device with the mixed layer and the island-structured layer exhibited efficiency almost equivalent to that of the basic device. As a result, it is suggested that the combination of a mechanical approach of eliminating the low ductile layer and an electrical modification to overcome the trade-off is useful for improving the flexibility of an organic semiconductor. Hole transport layer Hole injection layer Transparent conductive layer (Anode) Metal electrode (Cathode) Electro injection, Electron transport and Emission layer Fig11 Cross-sectional structure of the evaluated basic OLED device Fig.12 An example of the OLED emission (the emitting area is mm) Fig.1 Relation between strain the average number of cracks per unit area by using the approximation formula for Alq 3, α-npd, Alq 3 /α-npd, and IZO films -s192-

5 Journal of JSEM, Vol.14, Special Issue (214) Power Luminance Efficiency / [lm/w] Voltage / V Applied Voltage [V] M-1-g Basic Mixed M-3-g +Interfacial M-2-g Mixed Fig.13 Influence of an applied voltage on the power luminance efficiency for the basic OLED device, the device with the mixed layer and the device with a thin interfacial electron-injection layer 4. Conclusion The ductility of materials constituting organic semiconductors was investigated, and the power luminance efficiency of an organic light emitting diode (OLED) layered with the organic semiconductor materials was measured. The following results were obtained. (1) The Alq 3 films had the highest ductility, and the ductility of the α-npd thin films was improved by mixing with Alq 3. (2) To improve the ductility of the OLED with a conventional structure of glass/indium-zinc-oxide (IZO)/molybdenum oxide (MoO)/α-NPD/ Alq 3 /MgAg and to compensate for the decrease in device performance, a method was proposed in which the α- NPD layer was replaced by a mixed layer of Alq 3 and α-npd, and further a thin interfacial electron-injection layer was inserted. (3) The power luminance efficiency of the OLED obtained by the proposed method was equivalent to that of conventional material. In further studies, reciprocal flexibility testing of the OLED obtained by the proposed method is needed. Acknowledgement This work was supported by JSPS KAKENHI Grant Number The guidance and technical advice provided by H. Ogasawara, Y. Yanagi and T. Hirano of Mitsubishi Heavy Industries, Ltd. was greatly appreciated. References [1] Salleo, A., Chabinyc, M.L. and Yang, M.S.; Street, RA.: Polymer thin-film transistors with chemically modified dielectric interfaces, Applied Physics Letters (IEEE) (2), [2] (in Japanese). [3] Hyunsu, C., Changhun, Y., Jae-Woo, P. and Seunghyup, Y.: Highly flexible organic light emitting diodes base on ZnS/Ag/WO 3 multilayer transparent electrodes, Organic Electronics, 1 (9), [4] Kim, W. H. and Kafafi, Z. H.: Flexible organic light emitting devices using conductive polymeric anodes, Soc. Inf. Display. Dig. Tech. Papers XXXⅢ, (2), [5] Lan, Y. F., Peng, W. C., Lo, Y. H. and He, J. L.: Durability under mechanical bending of the indium tin oxide films deposited on polymer substrate by thermionically enhanced sputtering, Organic Electronics (21), [6] Leteir, Y., Medico, L., Demarco, F., Manson, J. A. E., Betz, U., Escola, M. E., Olsson, M. K. and Atamny, F.: Mechanical integrity of transparent conductive oxide films for flexible polymer-based display, Thin Solid Films, 46 (4), [7] Cairns, D. R. and Crawford, G. P.: Electromechanical Properties of Transparent conducting Substrates for Flexible Electronic Displays, Proceedings of The IEEE, 93-8(8), [8] Kobayashi, T., Yokoyama, T., Utsumi, Y., Kanematsu, H. and Masuda, T.: Study on Evaluation Methods for Mechanical Properties of Organic Semiconductor Materials, Journal of Physics: Conference Series, 433 (213), 19. [9] Kobayashi, T., Yokoyama, T., Utsumi, U., Kanematsu, H. and Masuda, T.: Measuring the Ductility of Organic Semiconductor Materials, 19th International Vacuum Congress, Paris, France, September 9-13, (213). [1] Kobayashi, T., Hirai, E., Hirano, T., Satou, K., Ogasawara, H., Yanagi, Y. and Tsumoto, Y.: Development of a Vacuum Deposition Equipment for Organic-Light-Emitting-Diode Panels Used for Lighting, Journal of Japan Society for Design Engineers, 48-1(213), s193-