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Heterojunctions Between Zinc Oxide Nanostructures and Organic Semiconductor Heterojunctions between zinc oxide nanostructures and organic semiconductor Amal Wadeasa Norrköping 2011 Heterojunctions between zinc oxide nanostructures and organic semiconductor Amal Wadeasa Linköping Studies in Science and Technology. Dissertations, No. 1405 Copyright, 2011, Amal Wadeasa, unless otherwise noted ISBN: 978-91-7393-046-8 ISSN: 0345-7524 Abstract Lighting is a big business, lighting consumes considerable amount of the electricity. These facts motivate for the search of new illumination technologies that are efficient. Semiconductor light emitting diodes (LEDs) have huge potential to replace the traditional primary incandescent lighting sources. They are two basic types of semiconductor LEDs being explored: inorganic and organic semiconductor light emitting diodes. While electroluminescence from p-n junctions was discovered more than a century ago, it is only from the 1960s that their development has accelerated as indicated by an exponential increase of their efficiency and light output, with a doubling occurring about every 36 months, in a similar way to Moore's law in electronics. These advances are generally attributed to the parallel de velopment o f semiconductor te chnologies, optics a nd m aterial science. Organic light emitting diodes (OLEDs) have rapidly matured during the last 30 years driven by the possibility to create large area light-emitting diodes and displays. Another driving force to specifically use semiconducting polymers is the possibility to build the OLED on conventional flexible substrates via low-cost manufacturing techniques such as printing techniques, which open the way for large area productions. This thesis deals with the demonstration and investigation of heterojunction LEDs ba sed o n p-organic semiconductor and n-ZnO nanostructures. The ZnO- organic heterojunctions are fabricated using low cost and simple solution process without the need for sophisticated vacuum equipments. Both ZnO-nanostructures and the organic materials were grown on variety of substrates (i.e. silicon, glass and plastic substrates) using low temperature methods. The growth mechanism of the ZnO nanostructures has been systematically investigated with major focus in ZnO nanorods/nanowires. Different organic semiconductor materials and device configurations are explored starting with single polymer emissive layer ending up with separate emissive and blocking layers, or even blends. Interestingly, the photoluminescence and electroluminescence spectra of the hybrid LEDs provided a broad emission band covering entirely the visible spectrum [∼400-∼800nm]. The hybrid light emitting diode has a white emission attributed to ZnO intrinsic defects and impurities in combination with the electroluminescence from the conjugated polymers. The ZnO nanostructures in contact with a high workfunction electrode constitute an air stable electron injecting contact for the organic semiconductor. Hence, we have shown that a white light emission can be achieved in a ZnO-organic hybrid light emitting diode using cheap and low temperature growth techniques for both organic and inorganic materials. Acknowledgments I would like to extend my heartfelt gratitude and acknowledge the help of the following people, for making this thesis a reality. I would like to express my sincere gratitude to: Xavier Crispin, my supervisor, for his support, understanding and patience during the past two years. Without his guidance as a great supervisor, this work would not have been possible, and I certainly wouldn’t be here. Magnus Berggren, for giving me the opportunity to work and study in Organic Electronics group. Sophie, for all practical help and making everything related to administration so easy for me. The entire Organic Electronics group both past and present group members, for their help, support and creating such warm working environment. Especially, I would like to thank Olga, for being great friend and for her support in the times that I needed it most during the last two years. My special gratitude goes to my previous group members and supervisors for their support at that time. I thank Parisa and Fengi, for many years of true friendship, and for believing in me and encouraging me in this winding road. I am also greatly indebted to companionship of a small circle of close friends outside of Linköping University whom have continuously supported me throughout these years. Without the love and support of my family, this would have been a very hard journey. I thank my dear father and mother, Mohammed and Thoraia, for teaching me good values such as hard work and appreciation for the gift of life. I strive everyday to make you proud! Tahra, my sister and second mother, for endless love and strength throughout the years which really helped me in sailing through everything. Ghada, Abdelrahman, Mohammed and Wadeisa, my siblings, without your love, wisdom and care I would not be the person that I am today. My love for all of you is eternal. My sincere thanks go to Margit, Göran, Andreas, Maria and Jonas, for always being there for me, and their kindness and the many memorable moments and all the unforgettable times we have spent together. Last, but not least, I would like to dedicate this work to my wonderful husband, Tobias, for giving me an unwavering love and support. I dare not even imagine how it would have been without you. List of included papers Paper 1: The demonstration of a hybrid n-ZnO nanorod/p-polymer heterojunction light emitting diodes on glass substrates. A.Wadeasa, S. L. Beegum, S. Raja, O. Nur, and M. Willander Applied Physics A: Materials Science and Processing, 2009, 95, 807-812 Paper 2: The effect of the interlayer design on the electroluminescence and electrical properties of n-ZnO nanorod/p-type blended polymer hybrid light emitting diodes A. Wadeasa, O. Nur and M. Willander Nanotechnology, 2009, 20, 065710-065715 Paper 3: Solution processed ZnO nanowires/polyfluorene heterojunctions for large area lightening A. Wadeasa, G. Tzamalis, P. Sehati, O. Nur, M. Fahlman, M. Willander, M. Berggren and X. Crispin Chemical Physics Letters, 2010, 490, 200–204 Paper4: ZnO-Polymer hybrid electron only rectifiers A.Wadeasa, M.Berggren, and X. Crispin Submitted Related work not included in this thesis Light emission from different ZnO junctions and nanostructures M. Willander, Yu. E. Lozovik, A. Wadeasa, O. Nur, A. G. Semenov and N. S. Vonorova Physica Status Solidi A, 2009, 206, 853–859 Photonic Devices in Some Low dimensional Systems M. Willander, A. Wadeasa, L. L. Yang, Q. X. Zhao and O. Nur ECS Transactions, 2009, 16, 17-30 Zinc oxide nanowires: controlled low temperature growth and some electrochemical and optical nano devices M. Willander, L. L. Yang, A. Wadeasa, U. S. Ali, H. M. Asif, X. Q. Zhao and O. Nur Journal of Materials Chemistry, 2009, 19, 1006–1018 Light-emitting diodes based on n-ZnO nano-wires and p-type organic semiconductors M. Willander, A. Wadeasa, P. Klason, L. Yang, S. Lubana Beegum, S. Raja, X. Q. Zhao and O. Nur Proceedings of SPIE - The International Society for Optical Engineering, 2008, 6895, 68950O Table of Contents 1. General introduction ................................................................................ 3 1.2. Organic light emitting diodes ................................................................ 5 1.3. Inorganic light emitting diodes ............................................................. 6 1.4. External quantum efficiency .................................................................. 8 2. Thesis goal ................................................................................................... 10 3. Organic semiconductors ....................................................................... 11 3.2. Introduction ...............................................................................................11 3.3. Bonds in molecules ..................................................................................14 3.4. Conjugated polymers ..............................................................................18 3.5. Charge transport in organic semiconductors................................20 3.6. Optical properties of organic semiconductors materials .........23 4. Zinc oxide ..................................................................................................... 27 4.2. Introduction ...............................................................................................27 4.3. Structure ......................................................................................................30 4.4. Defects and optical properties ............................................................35 4.5. Growth techniques ..................................................................................39 4.6. Aqueous chemical growth ....................................................................40 5. Manufacturing and characterization .............................................. 42 5.2. Device Fabrication ...................................................................................42 5.3. Substrate ......................................................................................................42 5.4. Metal contacts ............................................................................................42 5.5. Organic semiconductor layers ............................................................42 5.6. ZnO nanostructures .................................................................................42 5.7. Insulator layer
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