From Event: SPIE Organic Photonics + Electronics, 2018
Current OLED displays rely on a circularly polarised (CP) filter to enhance contrast by trapping ambient light inside the display. However, this means that 50% of the randomly polarised light emitted from each OLED pixel never leaves the screen, halving display efficiency and operational lifetime. One simple route to fabricate CP-emitting OLEDs is to use electroluminescent (EL) polymer – small molecule blends. Our approach is to pair a chiral small molecule with a non-chiral device optimised polymer, which allows for CP-dependent applications while retaining much of the performance properties of the original polymer. Previously circularly polarised polymer emission has been achieved via thick cholesteric stacks of liquid crystalline polymers, where linearly polarised light becomes circularly polarised. Here we show that it is possible to control whether cholesteric packing or chiral dipole dominates emission using film thickness; remarkably this allows us to change the handedness of the CP EL emission in the same materials system. We compare how the chemical structure of the non-chiral polymer and post-deposition processing impacts the chiroptical response of the resulting device, in an effort to provide a set of design rules for future high performance CP-OLEDs. We demonstrate a liquid-crystalline light emitting polymer with a record high induced absorption dissymmetry factor, which additionally shows no change in device characteristics (no trapping, etc) in the blends, as well as strong CP-PL and EL emission.
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