Printing of flexible light emitting devices: A review on different technologies and devices, printing technologies and state-of-the-art applications and future prospects
Inge Verboven, Wim Deferme
Abstract
The primary purposes of lighting remain visibility and safety, but our quality of life can be improved by creating a complete visual environment that includes needs like health and communication. Therefore, it is indispensable to go past rigid, planar lighting towards flexible lighting. This also implies the use of inexpensive application techniques qualified for continuous manufacturing like printing techniques. Suited to fulfil these lighting needs are the alternating current powder electroluminescent (ACPEL) device, the organic light emitting diode (OLED) and the light emitting electrochemical cell (LEC) thanks to thin flexible and printable layers. ACPEL devices can be screen printed establishing economic, simple manufacturing. Their high AC-driving voltage makes them more fitted for non-wearable applications. Well-matched for wearable lighting are OLEDs thanks to low DC-driving voltages. The high sensitivity to oxygen and water of the OLED materials however necessitates a high barrier encapsulation. Unlike OLEDs, LECs realize a balanced charge injection using mobile ions in their active layer broadening the choice in electrodes, but the improvement in lifetime is limited because of incomplete knowledge of the working mechanisms. This review gives an overview of these devices and their working principles, materials, deposition techniques, applications, state of the art and an outlook.