Organic Photonics, 14-R9734

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

Inclusive Dates:  07/01/07 – 06/30/09

Background - Despite the excellent characteristics of traditional semiconductor light emitting diodes and photodiodes, a new class of organic-based devices has advantages that include flexible substrates, no requirement for rigid can packaging, simplicity of fabrication, potential for large sheets of devices, and the ability to customize the electro-optical characteristics. A typical organic photonic device comprises several thin-film layers including a transparent anode and an active electroluminescent (EL) or photoconductive (PC) polymer layer. By patterning the cathode (usually a low work-function metal film) and EL layers into a series of crossing electrodes, arrays may be readily fabricated to produce a display or imaging device. Although vacuum processing is commonly used these devices have the potential for simplified fabrication in a standard lab environment.

Applications for organic photonic devices include use as solar cells for optical power harvesting, illumination, 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 was to develop novel flexible light emitters and detectors, using organic thin-films, and fabricate them using non-vacuum processing on various substrates. The first task was to develop fabrication techniques that do not use vacuum processing and that produce emitters and detectors on both rigid and flexible substrates. The second task was to modify the active organic materials to develop devices with NIR optical properties. The third task was to pattern devices to create arrays.

Accomplishments - We developed techniques for fabricating organic photonic devices with non-vacuum benchtop fabrication and produced a range of emitters and detectors. Devices were produced to operate in the visible spectrum using EL materials, and methods were developed to evaluate their optical emission and photovoltaic properties. To extend device operation into the infrared portion of the spectrum, blends of quantum dots and NIR laser dyes were used with the EL materials and their photosensitivity measured. It was found that a particular brand of silver epoxy provided the required low work-function cathode material to work with the high work-function indium-tin oxide (ITO) anodes. Devices were fabricated on both glass and polymer substrates, although the polymer substrates were found to have adhesion issues.

Device shorting and short lifetimes were found to be challenges. Shorting appeared to be caused by agglomeration of materials in the active layer, especially in the quantum dot solutions. Fabrication in a clean dry-nitrogen box may reduce this issue. Epoxy sealing of the devices was shown to be effective in greatly increasing the device lifetime. Arrays of devices were successfully fabricated using SwRI's laser micromachining tool to pattern the elements and using direct-write printing to pattern the upper cathode layer. Up to 16-element arrays were successfully operated using an external control circuit.

Voltage is applied to the left two elements of this array of organic photonic emitters that was patterned by laser micromachining.

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