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North Carolina Agricultural and Technical State University




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Environmentally Stable Flexible Displays


    The North Carolina A&T State University (NCA&T), in collaboration with the RTI International, established a Center of Excellence for Battlefield Capability Enhancements (CENTER) which focuses on enabling technologies for environmentally stable flexible displays.  The technology focus of the Center will be to develop a unique, environmentally stable, hybrid wide-band-gap organic/inorganic semiconductor device and to demonstrate the use of the structure in high performance, robust luminescent devices for flat panel and flexible displays for the Objective Force.  


    The information-intensive Objective Force requires mobile electronic devices for soldier and vehicle-based communications and weapons systems.  To date, all commercial display technologies are based on glass substrates and require significant ruggedization, adding weight and cost to electronic components.  The promise of organic light-emitting diode (OLED) based displays on flexible substrates is highly impaired by the environmental sensitivity of the organic materials and reactive organic/cathode interfaces.  Novel hybrid device structures that will effectively eliminate the shortcomings of OLEDs, provide good power efficiency in an emissive display, and be compatible with flexible substrates.  The primary stability issues in OLEDs are related to the electron transport layer (ETL) / cathode interface.  On the other hand, numerous organic hole transport materials are available, many of which are stable under ambient conditions.  While the performance of organic materials is superior for hole transport (p-type), inorganic semiconductors tend to offer better electrical properties as electron-transport (n-type) layers.  In addition, devices based on inorganic wide-bandgap semiconductors such as light-emitting diodes (LEDs) suffer minimal degradation under ambient conditions.  Unfortunately, defects in inorganic materials are effective non-radiative recombination centers, and therefore high quality epitaxial films are required for opto-electronic devices.  This makes them incompatible with large area processing.


    The technology will be a hybrid light-emitting device (HLED) comprises of a disordered wide-bandgap inorganic semiconductor acting as a high resistivity electron transport layer, combined with organic semiconductors acting as a high resistivity hole transport and emission layer (HTL and ETL, respectively).  The hybrid device takes advantage of the environmental stability of all layers, making a tremendous advance over OLED technology by eliminating the highly reactive ETL/cathode layers.  The HLED also eliminates the luminescence quenching of defects in the inorganic layer by confining recombination in the organic film.  Moreover, the current-driven diode based display will be able to leverage OLED and LCD infrastructure in rapidly transitioning from passively- to actively- addressed arrays.


    The joint effort of these two teams build the OLED expertise at MCNC and the MBE deposition and characterization expertise of NCA&TSU, to explore a unique integration of organic and inorganic components in HLED device with three goals in mind: (a) to expand the choices of the electron injection component, (b) to use the high mobility properties exhibited by the III-V semiconductor to further enhance the LED efficiency and (c) to  further advance the understanding of the transport properties in the hybrid device, thus pushing further the envelope in the flat panel technology.


    This research surrounds the planned focus of the unique combination of III-V semiconductor system for cathode material with appropriate organic semiconductor for anode. The use of either polycrystalline or amorphous III-V semiconductor is expected to yield electron mobility, though considerably degraded from the corresponding epitaxial layers, but still higher than that is currently achievable in n-type organic semiconductors.  This work may lead to novel structures that have not been hitherto envisaged.


©2010  MBE Lab., North Carolina A&T State University, Greensboro, NC 27411