Highly Directional Emission and Beam Steering from Organic LightEmitting Diodes with a Substrate Diffractive Optical Element

In planar OLED structures, a signifi cant fraction of the internal light emission is trapped in guided modes. This trapped light is a major limitation on the external quantum effi ciency, and a number of studies have integrated photonic microstructures into OLEDs to improve the device effi ciency by extracting trapped light such as the substrate modes, [ 3 ] the waveguide modes [ 4 ] and the surface plasmon polariton modes [ 5 ] into the escape cone. Here we investigated an interesting related problem that has received much less attention, namely how photonic microstructures could be used to achieve high directionality of emission. [ 6 ] We have recently reported a solution-processable microstructured OLED with enhanced directionality of emission by embedding a photonic crystal into the organic layers using a simple nano-replication technique. [ 7 ] This embedded photonic microstructure gives a compact footprint, but requires a delicate balance between the optimisation of electrical and optical properties. In this communication we develop photonic microstructured substrates for OLEDs which give enhanced directionality while not interfering with the optimised electrical design of the OLEDs. A nanoimprinted diffractive optical element (DOE) is integrated in the substrate of the OLED adjacent to the emitting pixel, which extracts the directional substrate mode without any Lambertian background. We steer the direction of the emitted beam both by varying the period of the DOE and by bending a fl exible DOE substrate. A broadband OLED with a DOE embedded in a waveguide was previously used by Ramuz et al. [ 8 ]