Liquid crystals for light emitting diodes

Liedtke, Alicia

Physical sciences
May 2009

Thesis or dissertation

© 2009 Alicia Liedtke. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

In this work a series of new semiconducting liquid crystals (LCs), which are applicable for organic light emitting diodes (OLEDs), were investigated. Semiempirical calculations were carried out on monomers and anti-cofacial dimers built from our molecules, representing molecules in solution and thin film respectively. Compared to the monomer a doubling of the oscillator strength in the dimer was found for longitudinal offsets larger than 20 A. Smaller shifts showed a forbidden absorption transition from ground to the lowest excited state. Assuming that the absorption transition is equivalent to the emissive transition, this might explain the reduced optical quantum efficiency observed for all of our materials in the solid state.

OLEDs made from blends of three different blue/green emitters with a red component showed white light emission with voltage independent CIE coordinates close to the ideal white. With polarised microscopy nematic phases frozen in a glassy state at room temperature were observed for all blends. Thus the blends were homogeneous and no phase separation occurred. This is important for homogeneous white emission and the alignment of the LCs due to a rubbed alignment layer below. Polarised white electroluminescence with an average polarisation ratio of 8:1 was shown from an OLED made with a blend deposited onto an alignment layer. Polarised background light for LC displays is desirable as this minimises the losses at the polarisers in the display and thus increases its brightness or lowers the power consumption. The low efficiency of the red emitter however limited the OLED performance.

Surface relief gratings (SRGs) with periods of a few hundred nm and a maximum depth of 66 nm and periods in the nm-range with a depth of 140 nm were spontaneously induced on our films. They were formed through molecular mass transport from the dark to bright regions during crosslinking by irradiation with a sinusoidal light pattern created by a phase mask. The anisotropic properties of LCs are shown to enhance transport. SRGs were formed at room temperature and an elevated sample temperature of 65deg. They are suitable feedback structures for optically pumped organic lasers and can also be employed to enhance the outcoupling of OLEDs.

Department of Physical Sciences, The University of Hull
O'Neill, Mary
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