Optimizing the outcoupling efficiency and the radiation pattern of organic light emitting devices by inkjet printing lens arrays films

It is known that organic light emitting diodes (OLEDs) can reach an internal quantum efficiency close to 100 %(1). Outcoupling of the generated photons however is not that efficient resulting in an extraction efficiency of only around 20 %(2). This is mainly due to total internal reflection at the OLED-substrate and substrate-air interfaces. In recent literature(1,3), lenses are proven to be an adequate solution, but lens production techniques are complex, expensive and unsuitable for mass production. The aim of this research is therefore to investigate the development of a cost-effective lens array film by inkjet printing. These inkjet printed lenses are validated by pixelated OLEDs. Firstly, circular patterns of anisole are printed in a regular hexagon on PMMA-foil. Due to the coffee ring effect, reservoirs are formed in this foil which prevent the liquid lenses from merging. Afterwards these lenses, i.e. spherical droplets of NOA74, are deposited into these reservoirs and cured by ultraviolet light. Finally, the lenses are connected to printed pixelated OLEDs. The developed lens array film increases the OLED's outcoupling efficiency by more than 20 % as is also expected from a theoretical study on these light extraction principles. The combination of the above-mentioned route for lens printing with the deposition of patterned OLED pixels, will not only improve the outcoupling to a large extend but will also help to develop OLEDs with a tailored emission pattern. A throughout understanding of the principles behind it will lead to optimized extraction efficiencies for large area printed OLED panels.