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Organic Electronics Yousof Mortazavi VLSI Course Presentation December 2004 References • • • • • • • • • L. Ficke,M. Cahay, “The bright future of organic LEDs”, IEEE Potentials, Jan. 2004. J. N. Bardsley, “International OLED technology roadmap”, IEEE J. Selected Topics in Quantum Electronics, Vol. 10, No. 1, Feb. 2004. T. Y. Winarski, “Patenting bright ideas; the current state of patented technology in the field of organic light emitting diodes”, IEEE Circuits and Devices Magazine, Apr. 2004. T. Shimoda, T. Kawase, “All-polymer thin film transistor fabricated by highresolution ink-jet printing”, In Proceedings IEEE International Solid-State Circuits Conference, 2004. S. Forrest, P. Burrows, M. Thompson, “The dawn of organic electronics”, IEEE Spectrum, Aug. 2000. G. Schmid, et al., “Organic electronics: perspectives towards applications”, ISSCC 2004. K. Nomoto, et al., “A bottom-contact organic-thin-film-transistor for flexible display application”, ISSCC 2004. M. G. Kane, “Organic electronics: what is it good for?”, ISSCC 2004. D. Gundlach, et al., “High-mobility, low voltage organic thin film transistors”, IEDM 1999. 2 Outline • • • • • • • Motivations OLED Fundamentals OTFTs Advantages of Organic Electronics Applications OLEDs for Color Displays Challenges 3 Motivations • Microelectronics vs. “Macroelectronics”: – – • Cost/area Cost/function Bulk Si ICs $10K/ft2 100 µcents/ transistor a-Si TFTs on glass $150/ft2 1 mcents/ transistor Printed Organic TFTs $30/ft2 200 µcents/ transistor Thin Film Transistors: – – – • Microelectronics: try to make smaller transistors to reduce cost and boost performance Macroelectronics: reduce costs in order build ever larger devices, with acceptable performance Active layer is silicon (a-Si) deposited on glass . For high mobilities, a-Si can be crystallized (pSi) by laser-pulses at high temperatures. Can’t easily use flexible substrates, such as plastics Organic Thin Film Transistors – – – – Organic semiconductors were discovered in 1987. Organic compounds are a natural match for plastic substrates. Use of polymers allows large-areas to be coated and patterned without conventional photolithography (e.g. spin-coaters and ink-jet printers). Organic TFTs may be made large or small (30 nm @ Cornell U.) [Kane (ISSC’04)] 4 OLED Fundamentals • In 1987, Tang, et al. published “Organic electroluminescent diodes”. • Currently more than 500 U.S. Patents have been issued on organic electronics. • Challenges: – Choice of anode for ohmic contact (for low voltage devices) – Diffusion of In, O into HTL HIL interface between ITO and HTL – Protection from oxygen and water encapsulation Cathode Metal ETL HTL ITO-Covered Substrate Transparent Anode 5 OTFT (OFET) • Typical OTFT: – Bottom gate, inverted staggered structure – Pentacene (C22H14) active – Gate dielectric • SiO2 • PMMA • PVP • Pentacene: Formula: C22H14 Metling Point: 300°C Optical Bandgap: 2.8 eV OTFTs operation: – accumulation – depletion • • • Mobilities as high as 1 cm2/Vs has been obtained with Ion/Ioff ratio of 108. Very low fabrication temperature (<60°C) allows use of cheap plastics. Conventional MOSFET equations are used to model OTFTs however, mobility is voltage dependent. W/L = 240 µm/44 µm Tgate= 1700 Å. SAM dielectric to reduce gate thickness to 2.5 nm 6 [Schmid et al.] Advantages of Organic Electronics • Thin, lightweight, flexible displays • Low voltage, low power, emissive source • High brightness • Broad color gamut • Wide viewing angle (~180º) • Good contrast • High resolution (<5 µm pixel size) • Fast switching (1-10 µs) • Low bill of materials and fabrication cost [Bardsley, 2004] Dupont Thermal Multilayer Transistor Process 7 Applications • Flexible Displays – – – • Sensor Arrays – – • PM-OLED AM-OLED Wearable Displays Artificial Skin Gas Sensors RF ID Tags – – Inductors Capacitors • X-ray imaging panels • Solid-State Lighting 8 OLEDs for Color Displays [Forrest, et al.] 9 Challenges • • • • • Choice of electrodes Encapsulation Reliability and yield Lifetime Brightness control with feedback • Particle migration control with AC driver A. Giraldo, et al. 10 Thank You