Project R-9876

Dotted design for OLEDs (Research)

Organic light emitting devices (OLED) are a promising light source for future applications because of their high luminous efficacy. However due to high losses of the out coupling – and the Lambertian radiation pattern – the practical usage of these devices is challenging. For external efficiency and modulation of the radiation pattern a micro lens array can be used. As the state of the art fabrication using lithography is expensive and difficult in mass fabrication a new method by using an inkjet printer (inkjet etching) combined with surface modification is investigated. A typical OLED-stack consists of different materials stacked in a layered layout. A wave-guided loss occurs because difference of refractive index n of the layers. For example, using a glass substrate with nglass = 1.5 differs from air with nair =1.0. Thus, light will only be out coupled at angles larger than the critical angle as lower ones will be totally reflected (equation below). Furthermore, this happens not only in the air/glass intersection but in all layers with different refractive indexes. To reduce the loss due to total reflection at the substrate-air interface spherical lenses have been produced to avoid total reflection at substrate/air interface. For a simpler production using the inkjet printing process macrolenses are used. State of the art processes are photolithography and related processes to create lenses or lens arrays. However, these processes are not suited for mass production and are expensive. Furthermore, the material waste is very high and design-changes are difficult to fulfil. Thus, drop on demand inkjet printing suits the application. In the following report, the creation and enhancement of a printing process for lenses will be described, characterized and optimized. The lenses will be inkjet printed into a reservoir-structure to enhance their contact angle (CA) and create high-CA lens(-arrays), which can be tuned in their geometry. For the reservoir structure, an inkjet etch process using PMMA substrate will be applied. Creation of a lens reservoir using inkjet etching. The substrate used is a thin PMMA foil, which can be etched witch Anisole. Due to the coffee stain effect, the dissolved PMMA will be transported to the pinned edge of the Anisole droplet. As evaporation is faster at the edge (and low Marangoni-effect due to low surface tension of anisole) a laminar flow from the inside to the outside is formed and transports the PMMA. When evaporating, the PMMA solidifies and creates a wall-like structure. Instead of printing a big droplet, a ring-like shape is needed, as the etching of the PMMA will lead to an increased surface roughness. The ring-shaped printing pattern preserves the low roughness on the inside of the reservoir, where the lens will be placed thus unwanted light scattering can be avoided. Critical for the printing process is the pre-treating of the surface to increase the quality and height of the reservoirs dramatically. The PMMA substrate is first plasma treated and then silanized (APTES ((3- Aminopropyl)triethoxysilane)) to enhance the contact angle and reduce the spread of the liquid creating sharp printed lines. To create the lens itself, the material (Norland NOA 89) will be inkjet printed inside the reservoir structure and then cured using UV light. To maximize the lens' contact angle the silanization needs to be redone as described above but with a different silane. FOTCS (Trichloro(1H,1H,2H,2H-perfluorooctyl)silane) at room temperature showed promising results for big contact angles.

01 June 2019 - 30 June 2019