Directional organic light-emitting diodes using photonic microstructure
Abstract
This thesis describes investigations into the optical and device design of organic
light-emitting diodes (OLEDs) with the aim of exploring the factors controlling the
spatial emission pattern of OLEDs and developing novel OLEDs with directional
emission by applying wavelength-scale photonic microstructure. The development of
directional OLEDs was broken down into two steps: the development of efficient
narrow linewidth OLEDs and the integration of wavelength-scale photonic
microstructures into narrow linewidth OLEDs.
The narrow linewidth OLEDs were developed using europium (Eu) complexes. The
electrical optimisation of solution-processed Eu-based OLEDs using commercially
available materials was investigated. The optimised Eu-based OLEDs gave an external
quantum efficiency of 4.3% at a display brightness of 100 cd/m². To our knowledge,
this is the highest efficiency reported for solution-processed Eu-based OLED devices,
and the efficiency roll-off has been reduced compared with other reported references.
Photonic microstructures were applied to develop directional OLEDs using the efficient
Eu-based OLEDs. Two contrasting strategies were used. One was to embed photonic
microstructures into Eu-based OLEDs, the other was to couple photonic
microstructures externally onto the devices. The microstructured devices developed by
the former strategy boosted the emitted power in desired angles in both s- and
p-polarisations and doubled the fraction of emission in an angle range of 4⁰. The
devices developed by the external coupling strategy achieved even higher directionality
and the out-coupled emission was a confined beam with easy control of beam steering.
Around 90% of the emitted power was confined in an angular range of 20⁰ in the
detection plane. The optical properties can be optimised independently without
compromising the electrical properties of devices, which gives major advantages in
terms of effectiveness and versatility. Optical models were also developed to investigate
the out-coupling mechanism of various trapped modes and develop OLEDs with
stronger directionality.
Type
Thesis, PhD Doctor of Philosophy
Rights
Embargo Date: 2018-10-06
Embargo Reason: Thesis restricted in accordance with University regulations. Electronic copy restricted until 6th October 2018
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