The functional layer encompasses ~15 nm thick Zinc Oxide nanoparticles (ZnO NPs) in a polyethylenimine ethoxylated (PEIE) matrix. PEIE acts as a surface modifier on ZnO NPs and also interlinks the individual ZnO NPs resulting in enhanced electron injection and transport. Owing to the deep valence band energy state of the ZnO NPs, the hole mobility is reduced significantly. The optimized light outcoupling in the OLED using the functional layer is achieved with the combination of optical light outcoupling simulation and thickness optimization by spin coating. The outcome of these optimizations is a solution processable high efficient, low-cost OLED.
Lighting today is expected to be light weighted, flexible, highly efficient, non-expensive and fabricated in an environment friendly way. Organic light emitting diodes (OLEDs) meet all of these requirements and can be applied using inexpensive and roll-to-roll compatible printing techniques. This work demonstrates the ultrasonic spray coating (USSC) of polyethylenimine (PEI) and polyethylenimine(ethoxylated) (PEIE) as electron injection/transport layer (EIL/ETL) for OLEDs. This high-end printing technique employs ultrasonic atomization to break down a liquid into a spray of homogeneous small (20 µm) droplets. The PEI(E) layer was optimised using USSC and subjected to a complete morphological and electro-optical characterisation. For all manufactured devices current and voltage characteristics and luminous performances were obtained. This study confirms the versatility of USSC and the suitability of PEI(E) as excellent EIL/ETL for OLEDs and paves the way towards fully printed devices.
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 literature1,3 , 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.
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