HIGHLY STABLE AMORPHOUS SILICON THIN FILM TRANSISTORS AND INTEGRATION APPROACHES FOR RELIABLE ORGANIC LIGHT EMITTING DIODE DISPLAYS ON CLEAR PLASTIC Bahman Hekmatshoar A DISSERTATION PRESENTED TO THE FACULTY OF PRINCETON UNIVERSITY IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY RECOMMENDED FOR ACCEPTANCE BY THE DEPARTMENT OF ELECTRICAL ENGINEERING ADVISOR: JAMES C. STURM SEPTEMBER 2010© Copyright by Bahman Hekmatshoar, 2010. Al Rights Reserved.iiiHydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are currently in widespread production for integration with liquid crystals as driver devices. Liquid crystal displays are driven in AC with very low duty cycles and therefore fairly insensitive to the TFT threshold voltage rise which is well-known in a-Si:H devices. Organic light-emitting diodes (OLEDs) are a future technology choice for flexible displays with several advantages over liquid crystals. In contrast to liquid crystal displays, however, OLEDs are driven in DC and thus far more demanding in terms of the TFT stability requirements. Therefore the conventional thinking has been that a-Si:H TFTs are too unstable for driving OLEDs and the more expensive poly-Si or alternative TFT technologies are required. This thesis defies the conventional thinking by demonstrating that the knowledge of the degradation mechanisms in a-Si:H TFTs may be used to enhance the drive current half-life of a-Si:H TFTs from lower than a month to over 1000 years by modifying the growth conditions of the channel and the gate dielectric. Such high lifetimes suggest that the improved a-Si:H TFTs may qualify for driving OLEDs in commercial products. Taking advantage of industry-standard growth techniques, the improved a-Si:H TFTs offer a low barrier for industry insertion, in stark contrast with alternative technologies which require new infrastructure development. Further support for the practical advantages of a-Si:H TFTs for driving OLEDs is provided by a universal lifetime comparison framework proposed in this work, showing that the lifetime of the improved a-Si:H TFTs is well above those of other TFT technologies reported in the literature. Manufacturing of electronic devices on flexible plastic substrates is highly desirable for reducing the weight of the finished products as well as increasing their ruggedness. In addition, the flexibility of the substrate allows manufacturing bendable, foldable or rollable electronic systems which is not possible with conventional rigid substrates. The most reliable TFTs require a temperature higher than that possible with existing clear flexible plastic substrates. Successful integration of a-Si:H TFTs with OLEDs on new high temperature flexible clear plastic substrates, capable of being processed at 300oC, is presented in this thesis. Controlling the mechanical stress and adhesion of the layers is found to be critical at high process temperatures to avoid cracking and delamination on clear plastic, and TFTs with a lifetime of 100 years on clear Abstractivplastic have been achieved. In addition, a new “inverted” integration technique is demonstrated both on glass and clear plastic to allow the programming of standard bottom-emission OLEDs with a-Si:H TFTs independent of the OLED characteristics which may change over time and vary from device to device in manufacturing. This technique also enhances the pixel drive current by nearly an order of magnitude for the same programming voltage. Finally, an approach for the design of reliable pixels is presented. Based on the individual TFT and OLED device stability, a guideline to the overall circuit configuration that will provide the most stable light emission is provided. AbstractvFirst of all, I would like to thank my advisor, Prof. Jim Sturm, for his constant guidance, critical advice and invaluable support which was essential for the all the achievements presented in this thesis. His approach to advising graduate students is not just to guide them to learn the right technical issues, but also, in his words, “to develop the right approach and the right confidence in their abilities to be independent researchers”. Not to mention other reasons, and to say the least, I can hardly imagine a better advisor. I would also like to thank Prof. Sigurd Wagner for the numerous discussions and the invaluable advice and help that I received from him throughout my studies at Princeton University, for which I will be always grateful. Also, I greatly appreciate the time spent by Prof. Sigurd Wagner and Prof. Andrew Houck for reading this thesis and their valuable comments. I would also like to thank Prof. Stephan Chou, Prof. Craig Arnold and Prof. Gerard Wysocki for taking time to serve on my FPO committee. I would like to thank the present and former members of Prof. Sturm’s and Prof. Wagner’s labs particularly those involved in the large-area projects for their help and cooperation: Ke Long, Troy Graves-Abe, Hongzheng Jin, I-Chun Cheng, Jian-Zhang Chen, Alex Kattamis, Kuni Cherenack, Prashant Mandlik, Yifei Huang, Lin Han, Ting Liu and Noah Jafferis. I am also grateful to the other lab members for their help along the way: Rebecca Peterson, David Inglis, John Davis, Kun Yao, Keith Chung, Weiwei Zheng, Sushobhan Avasthi, Jiun-yun Li, Oliver Graudejus, Jane Leisure, Wenzhe Cao and many others. Special thanks to Yifei Huang, Lin Han, Ting Liu and Bhadri Lalgudi for their help with the lingering repair and maintenance work on the PECVD tool. I would also like to thank the staff members of PRISM cleanroom, Helena Gleskova, Joe Palmer, Pat Watson and Mike Gaevski for their great efforts to keep the cleanroom up and running. I also appreciate all the help from PRISM and ELE staff, Sheila Gunning, Carolyn Arnesen, Sarah Braude, Roelie Abdi and many others. Finally, I want to thank my family for their love, encouragement and support throughout all the years that I have been away from home. AcknowledgementAbstract ……………………………………………………………………………… iii Acknowledgements ………………………………………………………………….. v Chapter 1 Motivation and Organization of this Thesis ……………………..….. 1 1.1 Large-Area and Flexible Electronics and Displays ……………………. 1 1.2 Flexible Active Matrix OLED Displays ……………………………….. 2 1.3