Alternative transparent electrodes for organic light emitting diodes [Elektronische Ressource] / vorgelegt von Yuto Tomita

Yuto

Description

Institut für Angewandte Physik Fachrichtung Physik Fakultät Mathematik und Naturwissenschaften Technische Universität Dresden Alternative transparent electrodes for organic light emitting diodes Dissertation zur Erlangung des Doktorgrades der Naturwissenschaften (Doctor rerum naturalium) vorgelegt von Yuto Tomita geboren am 20.03.1980 in Kita-hiroshima, Hokkaido, Japan Dresden 2008 i Eingereicht am 1. Gutachter: Prof. Dr. Karl Leo 2. Guta. Hubert Lakner 3. Gutachter: Dr. Dietrich Bertram Verteidigt am iiPublications K. Schulze, B. Männig, Y. Tomita, C. May, J. Hüpkes, E. Brier, E. Reinold, P. Bäuerle, and K. Leo, “Organic solar cells on indium tin oxide and aluminium doped zinc oxide anodes”, Appl. Phys. Lett. 91 073521 (2007) K. Schulze, B. Männig, M. Pfeiffer, K. Leo, Y. Tomita, C. May, E. Brier, E. Reinold, and P. Bäuerle, “Comparison of different anode materials in efficient small molecule organic solar cells”, Proceedings of 71. Jahrestagung der Deutschen Physikalischen Gesellschaft und DPG Frühjahrstagung des Arbeitskreises Festkörperphysik, DPG Frühjahrstagung des Arbeitskreises Festkörperphysik (Regensburg, March 26 – 30, 2007) C. May, Y. Tomita, M. Törker, M. Eritt, F. Loeffler, J. Amelung, and K. Leo, “In-Line deposition of organic light-emitting devices for large area applications“, Thin Solid Films, Article in press, doi:10.1016/j.tsf.2007.

Sujets

• Physik

Informations

• Publié par: technische_universitat_dresden
• Publié le: 01 janvier 2008
• Nombre de lectures: 35
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Institut für Angewandte Physik

Fachrichtung Physik

Fakultät Mathematik und Naturwissenschaften

Technische Universität Dresden

Alternative transparent electrodes for organic

light emitting diodes

Dissertation

zur Erlangung des

Doktorgrades der Naturwissenschaften

(Doctor rerum naturalium)

vorgelegt von

Yuto Tomita

geboren am 20.03.1980 in Kita-hiroshima, Hokkaido, Japan

Dresden 2008

i

Eingereicht am

1. Gutachter: Prof. Dr. Karl Leo

2. Gutachter: Prof. Dr. Hubert Lakner

3. Gutachter: Dr. Dietrich Bertram

Verteidigt am

ii

Publications

K. Schulze, B. Männig, Y. Tomita, C. May, J. Hüpkes, E. Brier, E. Reinold, P. Bäuerle, and K. Leo, “Organic solar cells on indium tin oxide and aluminium doped zinc oxide anodes”,Appl.

Phys. Lett.91073521 (2007) K. Schulze, B. Männig, M. Pfeiffer, K. Leo, Y. Tomita, C. May, E. Brier, E. Reinold, and P. Bäuerle, “Comparison of different anode materials in efficient small molecule organic solar cells”,Proceedings of 71. Jahrestagung der Deutschen Physikalischen Gesellschaft und DPG Frühjahrstagung des Arbeitskreises Festkörperphysik, DPG Frühjahrstagung des Arbeitskreises Festkörperphysik (Regensburg, March 26 – 30, 2007) C. May, Y. Tomita, M. Törker, M. Eritt, F. Loeffler, J. Amelung, and K. Leo, “In-Line deposition of organic light-emitting devices for large area applications“,Thin Solid Films, Article in press, doi:10.1016/j.tsf.2007.06.014 Y. Tomita, C. May, M. Toerker, J. Amelung, M. Eritt, F. Loeffler, C. Luber, K. Leo, K. Walzer, K.

Fehse, and Q. Huang, “Highly efficient p-i-n-type organic light emitting diodes on ZnO:Al

substrates”,Appl. Phys. Lett.91, 063510 (2007)

Y. Tomita, C. May, M. Törker, J. Amelung, M. Eritt, F. Löffler, C. Luber, K. Walzer, K. Fehse, Q. Huang, and K. Leo, “PIN type OLEDs for lighting applications on ITO and ZAO”, Proc. EOS conference on Trends in Optoelectronics, 36, World of Photonics Congress 2007 (Munich, June 17-19, 2007) Y. Tomita, C. May, M. Törker, J. Amelung, M. Eritt, F. Löffler, C. Luber, K. Leo, K. Walzer, K. Fehse, and Q. Huang, “Large area p-i-n type OLEDs for lighting”,SID Symp. Digest Tech. Papers,39, 1030 (2007) J. Amelung, M. Toerker, Y. Tomita, D. Kreye, C. Grillberger, U. Vogel, A. Elgner, M. Eritt, Ch. May, U. Todt, C. Luber, R. Hermann, Ch. Zschippang, and K. Leo, “Integration of high-efficiency PIN organic light-emitting devices in lighting and optoelectronic applications”, Proc. SPIE,6486, 64860C (2007)

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C. May, J. Amelung, M. Eritt, O. Hild, F. Löffler, M. Törker, Y. Tomita, and K. Leo, “Verfahren für die Großflächenbeschichtung mit Halbleitermaterialien”,Proceedings of Herstellung organischer Halbleiterbauelemente mittels großflächiger Vakuumbeschichtungs-verfahren, Workshop Verfahren für die Großflächenbeschichtung mit Halbleitermaterialien (Wörlitz, October 18, 2006) Y. Tomita, C. May, M. Törker, J. Amelung, K. Leo, K. Walzer, K. Fehse, and Q. Huang, “High

efficient p-i-n OLED on ZnO:Al (ZAO)“,Proceedings of Organic Electronics Conference & Exihibition 2006, P020410 (Frankfurt am Main, September, 2006) C. May, Y. Tomita, M. Törker, M. Eritt, F. Löffler, J. Amelung, and K. Leo, “Inline deposition of organic light-emitting devices for large area applications”,Proceedings of 6th Int. Conf. On Coatings on Glass and Plastics, p 381 (Dresden, June, 2006) M. Törker, M. Eritt, Ch. May, J. Amelung, C. Luber, R. Hermann, Ch. Zschippang, Y. Tomita, and K. Leo, “In-line deposition of high-efficiency p-i-n organic light-emitting devices”,SID Symp. Digest Tech. Papers,37, 1471 (2006) J. Amelung, M. Toerker, C. Luber, M. Eritt, Y. Tomita, H. Cholewa, R. Hermann, F. Loeffler, C. May, U. Vogel, G. Bunk, A. Heinig, W. Jeroch, H.-J. Holland, K. Leo, “Second generation OLED devices and systems: inline evaporation, highly efficient OLED devices, and novel driver/controller ASICs”,Proc. SPIE,5961, 47 (2005)

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Contents

1. Introduction 1.1.Preface 1.2.Aims and objectives 1.2. Bibliography 2. Literature review 2.1.Literature review of OLEDs for lighting with alternative substrates 2.1.1.Historical background of OLEDs 2.1.1.1.Electrical luminescence from single crystals 2.1.1.2.Architecture of OLEDs: Development of structure 2.1.1.3.Commercial applications of OLEDs 2.1.2.Materials for OLED 2.1.2.1.Hole transporting materials 2.1.2.2.Electron transporting materials 2.1.2.3.Emitter materials 2.1.3.White OLEDs 2.2.Literature review of Aluminium doped zinc oxide (ZnO:Al)

2.2.1.Introduction – transparent conductive oxides

2.2.2.Aluminium doped zinc oxides 2.2.3.Deposition techniques of ZnO:Al 2.2.4.Doping effect 2.2.5.Effect of deposition parameters 2.2.5.1. Deposition temperature effects 2.2.5.2. Reactive gas effects 2.2.5.3. Film thickness effects 2.2.5.4. Deposition rate 2.2.6.Application of ZnO:Al for OLED devices 2.3.review of poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) Literature 2.3.1.Introduction 2.3.2.Properties of PEDOT:PSS 2.3.3.Conductivity of PEDOT:PSS 2.3.4.Applications of PEDOT:PSS for OLEDs as a hole injection layer 2.3.5.Applications of PEDOT:PSS for OLEDs as a transparent anode

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2.4. Bibliography 3. Experimental procedures 3.1.Fabrication and characterisation of ZnO:Al 3.1.1.Deposition system: VES400 Inline system 3.1.2.Magnetron sputtering system 3.1.3.Characterisation 3.1.3.1.Four point probe 3.1.3.2.Hall measurement 3.1.3.3.Atomic force microscopy 3.1.3.4.Scanning electron microscopy 3.1.3.5.Profilometer 3.1.3.6.Optical spectroscopy 3.1.3.7.Spectroscopic ellipsometry 3.1.3.8.X-ray diffraction

3.1.4.Substrate preparation 3.1.4.1.ZnO:Al test substrate for OLED 3.1.4.2.ZnO:Al large size substrate for OLED 3.1.4.3.ZnO:Al substrate for OLED used in TU Dresden IAPP 3.2.of PEDOT substrate Preparation 3.2.1.Metal grid 3.2.2.Preparation of test substrates with PEDOT anode 2 3.2.3.substrate with PEDOT anodePreparation of 50 x 50 mm 2 3.2.4.Preparation of 100 x 100 mm substrate with PEDOT anode 3.2.5.Deposition of PEDOT 3.2.6.Fabrication of OLEDs on PEDOT substrates 3.3. Preparation of OLEDs 3.3.1.Deposition systems 3.3.1.1.In-Line system VES400 3.3.1.2.Cluster system SUNICEL plus 200 3.3.1.3.Evaporator tool at TU Dresden IAPP 3.3.2.Materials for OLEDs 3.3.2.1.p-type hole transporting layers (p-HTLs) 3.3.2.2.Electron blocking layers (EBLs) 3.3.2.3.Emission layers (EMLs) 3.3.2.4.Hole blocking layers (HBLs)

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3.3.2.5.n-type electron transporting layers (n-ETLs) 3.3.2.6.Cathode materials 3.3.3.Characterisation of OLEDs 3.3.4.Integration sphere 3.3.5.Process flow of OLED fabrication 3.4. Bibliography 4. Aluminium doped zinc oxide (ZnO:Al) layers 4.1. Optimisation of ZnO:Al films 4.1.1.Effect of gas flow (oxygen partial pressure) 4.1.2.Effect of temperature 4.1.2.1.Structural properties 4.1.2.2.Electrical properties 4.1.3.Effect of film thickness 4.1.4.Parameters of the optimised ZnO:Al film 4.2. Spectroscopic ellipsometry 4.2.1.Modelling 4.2.2.Optical constants 4.2.3.Thickness homogeneity 4.3. Work function investigation 4.3.1.UPS measurement 4.3.2.m-i-p type Schottky structure 4.4.in properties after structuring process Change 4.5. Other properties 4.6. Conclusion 4.7. Bibliography 5.Organic light emitting diodes on aluminium doped zinc oxide anode 5.1.Introduction 5.2. Green p-i-n OLEDs 5.2.1.device structure 5.2.2.Optical simulation 5.2.3.Results for green p-i-n OLEDs 5.2.4.Doping effect 5.3.EML green p-i-n OLEDs Double 5.4.p-i-n OLEDs Red

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5.4.1.Device characteristics 5.4.2.Stability 5.5.p-i-n OLEDs White 5.5.1.Small size white OLEDs 5.5.2.Influence of conductivity of p-HTL 5.5.3.Up-scaled white p-i-n OLEDs 5.5.4.Luminance homogeneity 5.6. Conclusion 5.7. Bibliography 6.Organic light emitting diodes on PEDOT anode 6.1. Introduction 6.2. Characteristics of PEDOT thin films 6.3. Small size OLEDs using PEDOT substrates 6.3.1.Green p-i-n OELDs 6.3.2.Influence of exposure of n-ETL in nitrogen atmosphere 6.3.3.White p-i-n OLEDs 2 6.4. White OLEDs on 50 x 50 mm PEDOT substrates 2 6.5. White OLEDs on 100 x 100 mm PEDOT substrates 2 6.6.PEDOT substrateslayout for 100 x 100 mm  New 6.7. Conclusion 6.8. Bibliography

7. Summary and outlook 7.1. Summary 7.2. Outlook Appendix A – Definitions of efficiencies

Appendix B – Definitions of colourimetry List of symbols List of Abbreviations Acknowledgments

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Chapter 1 – Introduction

Chapter 1 Introduction 1.1. Preface Since the revolutionary invention of light bulbs by Thomas Edison in 1880s, electric lighting has been established as a standard technology. Today, the lighting market is dominated by fluorescent lamps and incandescent light bulbs because of their long-term stability and low manufacturing costs. The luminance efficiency of incandescent light bulbs is about 13 to 20 lm/W i.e., more than 90 % of the energy is wasted as heat. Fluorescent lamps have a higher

efficiency of 50 to 100 lm/W, however, they require inert gas and mercury in a vacuum glass tube. Further derivative applications of traditional lighting, such as high-intensity discharge lamps and halogen lamps, have been heavily investigated. However, since these technologies are relatively mature and new scientific breakthroughs are not expected, new and environmentally friendly technologies are desirable. Solid state lighting is a new environmentally friendly light source with potentially high efficiency. So far, light emitting diodes (LEDs) and organic LEDs (OLEDs) have been presented as candidates for SSL. SSL is based on semiconductor properties: a photon created by carrier recombination of holes and electrons in p-n junction device. A novel crystal growth technique for gallium nitride materials has enabled the fabrication of LEDs with wavelength range from purple to UV. White lighting sources have then been realised by mixing LEDs different colours or with down conversion materials using lower energy phosphors. Recently, white light LEDs are commercialised as flash lights, lighting, and back lights for liquid crystal displays (LCDs) with various display sizes, ranging from mobile phone screens (a few inches) to personal computer monitors (more than 20 inches). The luminance efficiency of white light LED devices has already exceeded incandescent light bulbs. External power conversion efficiencies exceeding 50% have been reported for red LEDs. However, LEDs are point sources and their total luminous flux is relatively small, therefore, a large number of diodes are needed for practical lighting applications. This raises serious problems in heat dissipation. Moreover, cost efficiency and colour rendering index (CRI) remain challenging. Another promising lighting source which has advanced within the past two decades and has a high expectation for further improvements is the organic light emitting diode (OLED).

1

Chapter 1 – Introduction

Recent advances of OLEDs in device architecture, light-out coupling, and materials have ensured high efficiency, exceeding that of incandescent light bulbs. In contrast to conventional point source LEDs, OLEDs distribute light throughout the surface area and are not restricted by their size. This brings the possibility of having high luminance flux without glare. One remarkable advantage of OLEDs is the ability of colour tuning due to the presence of numerous emitting materials in the visible range. The mixture of these elements enables a wide colour range. Achieving ideal white light in Commission Internationale de l'éclairage (CIE) coordinates of (0.33, 0.33) or a CRI near 100 is already within the reach for OLEDs.

An example of lighting by LEDs. Passenger cabin in the regional train (Deutsche Bahn) in Dresden, Germany.

OLED lifetime is a critical issue: Early OLED degraded in a few hundred hours. Currently, white light OLED devices have reached more than 20000 hours lifetime at practical operation brightness, which is longer than fluorescent lamps [1]. Moreover, monochromatic OLED devices have been demonstrated with ultra long lifetime of more than 10 million hours [2]. Therefore, OLEDs are expected to reach sufficient stability in the near future. The remaining challenge for OLEDs is their cost. New OLED technologies provide cost effective manufacturing methods which reduce the organic material consumption during the deposition. Similar arguments could be presented for transparent electrode materials because indium tin oxide (ITO), a widely used material as a transparent electrode as transparent electrode materials for OLEDs, is less than optimal due to its high element price. Previously, various oxide materials have been investigated. Zinc oxide is one of the

2

Chapter 1 – Introduction

candidates for a replacement of ITO, especially when doped with aluminium impurities. It showed high potential as an alternative material [3]. Aluminium doped zinc oxide (ZnO:Al) which is composed of abundant materials can be obtained at low price. Apart from the oxide materials, highly conductive polymers such as polyethylenedioxythiophene (PEDOT) [4] and carbon nanotubes [5] are also considered as alternative electrode materials. Their application in the fabrication of OLED devices is a relatively new field. This study addresses some of the tasks in this area, such as OLEDs manufacturing with low cost alternative substrates.

Demonstration of lighting by OLED. Image obtained from www.ipms.fraunhofer.de

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