A novel, fully automated, fabrication and characterization apparatus for polymer light-emitting diodes (PLEDs) was developed. This high throughput apparatus allows the fabrication of 49 devices with a controlled variation of essential parameters like material, material composition, blend concentration, layer thickness, and annealing temperature. Up to now, due to a lack of elaborate design tools, extensive experimental effort is required in order to optimize novel materials, material combinations and device structures for polymer based LEDs. Our novel apparatus provides an extensive dataset which can be used for device optimization and a profound device modeling offering a deeper theoretical understanding of underlying device physics in PLEDs.
With increasing demand for flat panel displays, which usually incorporate indium tin oxide (ITO) thin films, the price of
indium will rise dramatically in the future. For simple and cheap applications (such as LogoLED™, see
www.logoled.com) alternative anode materials have to be used. We will show that polymer-only anodes and wires are
sufficient to fabricate patterned polymer light-emitting devices (PLEDs), such as seven segmented displays. As another
approach to replace ITO we will present results from aluminum / PEDOT anodes devices with better stability and
bottom & top emission.
A polymer thin-film optical touch and proximity sensor is presented. The sensor is based on the monolithic integration of
polymer light emitting diodes, logos or displays, and polymer photodiodes on a common substrate. The main interest in this
new form of optical sensor lies in its potentially cost-effective manufacture on thin and flexible substrates. Potential
applications of such systems range from simple information displays with integrated touch-screen to biochemical sensors.
We present a complete characterization of Organic Light Emitting Diode (OLED) structures performed in an ultra-pure Ultra High Vacuum (UHV) environment and under controlled influence of oxygen and atmospheric gases. We fabricated and characterized standard NPB/Alq3 devices with an Indium Tin Oxide (ITO) anode and a magnesium cathode in an UHV system. With this system we are able to study the injection properties of very clean, controllable interfaces in the absence of any impurity gas. We found that the threshold voltage for OLED operation always increased after exposure to any atmospheric gas, an indication of deteriorated injection properties. However, the luminescence efficiency can become higher after exposure to impurity gases. Without contact to air the OLED do not degrade with appearance of so called 'black spots.' To investigate the intrinsic stability of the OLEDs in ultra-high vacuum we performed a realtime observation on the surface of a 35 nm thin magnesium cathode with Ultraviolet Photoelectron Spectroscopy (UPS). We found that even with a 35 nm thin magnesium-cathode, the underlying organic layer never appeared at the surface also after hours of operation in the ultra-pure conditions. The only sign of deterioration at the cathode is a slow oxidation of the magnesium surface. Thus, OLEDs with semitransparent cathodes are stable if the are operated under ultra-high vacuum conditions.
Alq3-based organic light emitting diodes (OLEDs) were prepared by molecular beam deposition in Ultra-High Vacuum (UHV) and their electrical properties were studied by impedance spectroscopy before they were exposed to any impurity gases. To characterize the fundamental injection processes, we studied the simplest OLED structure, consisting of a single layer of Alq3 between an ITO anode and a Mg cathode. In UHV, and below the threshold voltage for luminescent, instabilities in the frequency dependence of the resistivity, as well as in the current vs. voltage characteristics, are observed, which could be related to inhomogeneous contacting at the electrodes. When the same experiment is performed in air, both kinds of instabilities disappear, which demonstrates that exposure to atmospheric gas plays a role in stabilizing the contact and injection properties in the devices. Additionally, the impedance spectrum in air below the threshold voltage for luminescent shows an additional feature that hints to two different regions in the Alq3 layer, which we assign to the effect of Mg atoms which diffused into the Alq3 layer during deposition.
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