Project R-11168

Inge Verboven: Printing of organic light emitting diodes on flexible substrates (Research)

To keep up with the rapidly changing technology and needs of men, lighting today is expected to be light weighted, flexible, highly efficient, non-expensive and fabricated in an environment friendly way. Organic light emitting diodes meet all these requirements with their nanometer thick layers (5-100 nm), their high efficiency, surface emission and low DC turn-on voltage (~ 3 V DC). Furthermore these devices can be applied using inexpensive and roll-to-roll compatible printing techniques such as ultrasonic spray coating. The latter is a versatile and high throughput deposition technique where a liquid is atomized due to ultrasonic vibrations in the nozzle into a spray of evenly sized droplets. This PhD research focusses on the ultrasonic spray coating of an entire OLED stack. For this purpose a state-of the art device is fabricated on ITO patterned glass substrates using spin coating and thermal evaporation and then layer by layer converted into a fully ultrasonically spray coated device. Not all materials are suited to be ultrasonically spray coated so adequate alternative materials are being researched that fulfil the morphological, electrical and optical requirements for a certain layer. Often ITO is deployed as transparent electrode, but it is rather expensive due to low indium reserves and high processing costs and furthermore it has a high rigidity causing crack formation on the surface upon flexing and bending. A low-cost alternative with promising comparable optical and electrical properties can be found in silver nanowires (AgNW) films. Subsequently poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is deposited on top of the nanowires to fill the uncoated areas and to serve as hole transport layer. Consequently the emissive PPV-polymer Super Yellow, optimized by a previous PhD research, is coated. For the electron injection and transport layer mostly low work function materials that facilitate the electron mobility, such as barium (Ba) or calcium (Ca), are applied using expensive and non-continuous vacuum deposition techniques due to their high reactivity. Efficient and stable alternatives can be found in the aliphatic amines, polyethylenimine (PEI) and polyethylenimine(ethoxylated) (PEIE). In direct presence of a metal contact the PEI(E) layers cause a work function shift to improve the electron mobility. Finally the OLED is completed with an aluminum layer. All layers are subjected to a complete morphological and electro-optical characterization. For all manufactured devices current and voltage characteristics and luminous performances are obtained and compared to the reference non-printed OLED device.

01 February 2021 - 30 April 2021