Lighting out of the printer
The fibres are based on light-emitting electrochemical cells made of organic materials, so-called OLECs (Organic Light-emitting electrochemical cells). Closely related to OLEDs, they however do not incorporate a solid material but a viscous one. This active viscous layer assumes the task of conducting electric current as well as emitting the light. In the liquid phase, this material contains freely mobile ions. As soon as a voltage is applied, they accumulate at the edge. Only then, electric charges can be injected into the light emitting layer, effectively creating a light emitting diode. By combining various materials appropriately, the cells basically can emit light in any desired color.
In a pilot production line in Augsburg, Osram researchers led by Dr. Frank Vollkommerwere able to create functioning lighting cells at a size of 14 x 15 centimeters. They used a plastic foil as substrate with a conductive transparent layer on it. On top, they applied another conductive polymer layer by means of a precision slotted nozzle. After an infrared drying step, the light-emitting layer has been applied by means of the same method. Finally, standard metal electrodes have been vapour deposited. "In contrast to LED and OLED production, this process does not require cleanrooms", Vollkommer said. Only the machine itself needs to be kept clean of particles which can be achieved through constantly supplying clean air.
The research has been conducted within the European research project CELLO. Besides Osram, also Siemens and five research institutes participated in the project. So far the researchers were able to achieve LECs with an efficiency of 17 lumens per watt in the green part of the light spectrum – as a comparison an incandescent light bulb achieves an efficiency of some 10 lm/w. The next goals for the project are now higher efficiency and longer operating life. Another challenge is the homogeneity of the light emission, Vollkommer said.
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