Scientists from Philips Research and the Eindhoven University of Technology (TUe) have developed a novel method for accurately measuring the active layer in organic light-emitting diode (OLED) lighting.
This is an important step forward for OLED device optimization and efficiency that is determined by the position of the very narrow zone in which the light is generated.
In the past five years OLEDs have emerged as a promising option for energy-efficient solid-state-lighting. OLEDs can cover large areas; they are extremely thin and can be made on substrates of virtually any shape. This high level of flexibility in terms of design and application makes them highly appealing to lighting designers, manufacturers and consumers.
Unlike incandescent bulbs, which generate light by passing electricity through a wire, or fluorescent lamps, which pass current through a gas, OLED lighting works by passing electricity through one or more incredibly thin layers of organic semiconductors. These layers are sandwiched between two electrodes. The whole "sandwich" is deposited onto a supporting sheet of glass or other transparent material. When current is applied to the electrodes it flows through the organic film and the film emits light. Using different materials in the films makes it possible for the OLEDs to emit differently colored light, and using a combination of materials pleasant white light is emitted.
The novel method has been developed by PhD students Siebe van Mensfoort and Marco Carvelli from the Eindhoven University of Technology working for the Dutch Nano Technology program NanoNed and the Dutch Polymer Institute as well as researchers from Philips Research (Aachen and Eindhoven). "There are many factors that influence the efficiency, or light out-coupling efficiency. Not all the light reaches our eyes, as some of it is trapped and absorbed in the OLED, particularly when it is formed close to the electrodes. In the most efficient OLEDs the light comes nicely from the center. Simply put, we have provided the microscope through which we can see whether we are able to perfectly realize that goal," Marco Carvelli explains.
Initially blue OLEDs were looked at because of the relatively simple single-layer structure. "You need to know the light emission profile with a high degree of certitude to calculate the light out-coupling efficiency, which is the most important efficiency loss factor in state-of-the-art OLEDs," Reinder Coehoorn, Research Fellow, indicates. "We are very proud of the fact that Nature Photonics has picked up the paper and are looking forward to expanding the analysis to multilayer devices, for which a more structured profile is expected."
"This is a great achievement and it demonstrates the good progress we are making in understanding OLEDs," says Dietrich Bertram, manager of the OLED Lighting business center at Philips Lighting. "The insights from this and other research activities are important contributions to further improving the performance of our Lumiblade OLED lighting devices and to secure our leading position in OLED lighting," he continues.
The findings were published online in Nature Photonics on Sunday 14 March.
Reference: Philips Research
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