Organic Photonics, 14-R9734

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Principal Investigators
Joseph N. Mitchell
Jeffrey L. Boehme
Jonathan S. Schulze
W. Royall Cox

Inclusive Dates:  07/01/07 – Current

Background - Despite the excellent operating characteristics of traditional semiconductor light emitting diodes and photodiodes, a new class of organic-based devices have several advantages including flexible devices, no requirement for packaging in a metal or plastic can, simplicity of fabrication, potential for large sheets of devices, and the ability to customize the electro-optical characteristics by altering the chemistry of the active materials. A typical organic photonic device comprises four thin-film layers: an anode made from a transparent, flexible conductor, a hole transfer polymer layer, an active electroluminescent (or photoconductive) polymer layer, and a cathode layer (usually a metal film). By patterning the top cathode layer into a series of crossing electrodes, arrays may be readily fabricated to produce a display or imaging device. Although vacuum processing is commonly used with these devices, they have the potential for greatly simplified fabrication by processing in a standard lab environment.

Applications for organic photonic devices include use as solar cells for optical power harvesting, illumination applications, touchscreens, proximity sensors, document scanners, biomedical sensors, ultrathin and flexible image sensors, and active camouflage. Most organic photonic materials only emit in the visible spectrum. However, there are many possible applications by extending the sensitivity of these devices into the near-infrared (NIR) portion of the spectrum.

Approach - The objective of this project is to develop novel flexible light emitters and photodetectors using organic thin-film technology that operate in NIR wavelengths (700-2,000 nm) and are fabricated using non-vacuum processing on various substrates. The first task is to develop organic device fabrication techniques that do not use vacuum processing and can produce emitters and detectors on both rigid and flexible substrates. The second task is to develop organic electroluminescent (EL) NIR materials whose wavelength is adjusted by the addition of quantum dots (QD). The third task is to pattern devices to create arrays and to develop a technology demonstrator.

Accomplishments - To date, we have assembled working organic light emitting diodes (OLEDs) using blends EL and QD materials. The devices use a clear cathode of Indium-Tin Oxide (ITO) on both glass and flexible polyethylene terephthalate (PET) substrates, although issues with adhesion have been found with PET-based devices. Various anode options have been tested, but the most successful has been silver epoxy, whereas evaporated aluminum did not. The current/voltage characteristics, emission spectra, and intensities have been measured. Although much of the testing has been their operation as emitters, they can also operate as photovoltaics. We have begun measuring the bias voltage vs. current using a white light source as well as measuring the conversion efficiency of these devices. Finally, some screen-printing tests have been conducted using the silver epoxy as the anode.

This visible light organic photonic emitter is on an ITO-coated PET substrate with poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) for the electroluminescent material and silver epoxy anodes.

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