Organic Electronics Yousof Mortazavi VLSI Course Presentation December 2004

2 References L. Ficke,M. Cahay, “The bright future of organic LEDs”, IEEE Potentials, Jan J. N. Bardsley, “International OLED technology roadmap”, IEEE J. Selected Topics in Quantum Electronics, Vol. 10, No. 1, Feb 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 T. Shimoda, T. Kawase, “All-polymer thin film transistor fabricated by high- resolution ink-jet printing”, In Proceedings IEEE International Solid-State Circuits Conference, S. Forrest, P. Burrows, M. Thompson, “The dawn of organic electronics”, IEEE Spectrum, Aug G. Schmid, et al., “Organic electronics: perspectives towards applications”, ISSCC K. Nomoto, et al., “A bottom-contact organic-thin-film-transistor for flexible display application”, ISSCC M. G. Kane, “Organic electronics: what is it good for?”, ISSCC D. Gundlach, et al., “High-mobility, low voltage organic thin film transistors”, IEDM 1999.

3 Outline Motivations OLED Fundamentals OTFTs Advantages of Organic Electronics Applications OLEDs for Color Displays Challenges

4 Motivations Microelectronics vs. “Macroelectronics”: –Microelectronics: try to make smaller transistors to reduce cost and boost performance –Macroelectronics: reduce costs in order build ever larger devices, with acceptable performance Thin Film Transistors: –Active layer is silicon (a-Si) deposited on glass. –For high mobilities, a-Si can be crystallized (p- Si) by laser-pulses at high temperatures. –Can’t easily use flexible substrates, such as plastics Organic Thin Film Transistors –Organic semiconductors were discovered in –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 Cornell U.) Cost/areaCost/function Bulk Si ICs $10K/ft µcents/ transistor a-Si TFTs on glass $150/ft 2 1 mcents/ transistor Printed Organic TFTs $30/ft µcents/ transistor [Kane (ISSC’04)]

5 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 ITO-Covered Substrate HTL ETL Metal Cathode Transparent Anode

6 OTFT (OFET) Typical OTFT: –Bottom gate, inverted staggered structure –Pentacene (C 22 H 14 ) active –Gate dielectric SiO 2 PMMA PVP OTFTs operation: –accumulation –depletion Mobilities as high as 1 cm 2 /Vs has been obtained with Ion/Ioff ratio of Very low fabrication temperature (<60°C) allows use of cheap plastics. Conventional MOSFET equations are used to model OTFTs however, mobility is voltage dependent. Pentacene: Formula: C 22 H 14 Metling Point: 300°C Optical Bandgap: 2.8 eV SAM dielectric to reduce gate thickness to 2.5 nm [Schmid et al.] W/L = 240 µm/44 µm T gate= 1700 Å.

7 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

8 Applications Flexible Displays –PM-OLED –AM-OLED –Wearable Displays Sensor Arrays –Artificial Skin –Gas Sensors RF ID Tags –Inductors –Capacitors X-ray imaging panels Solid-State Lighting

9 OLEDs for Color Displays [Forrest, et al.]

10 Challenges Choice of electrodes Encapsulation Reliability and yield Lifetime Brightness control with feedback Particle migration control with AC driver A. Giraldo, et al.

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