Light-emitting Polymerizable Liquid Crystals: Micron Scale Photolithographic Patterning and Green Electroluminescence
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Light-emitting polymerizable liquid crystals: Micron scale photolithographic patterning and green electroluminescence. Matthew P. Aldred1, Adam E. A. Contoret2, Peter E. Devine2, Simon R. Farrar, Robert Hudson, Stephen M. Kelly1, Gene C. Koch2, Mary O’Neill, Wing C. Tsoi, Kai L. Woon and Panos Vlachos2. Department of Physics, University of Hull, Cottingham Rd., Hull, HU6 7RX, United Kingdom. 1 Department of Chemistry, University of Hull, Cottingham Rd., Hull, HU6 7RX, United Kingdom. ZLX Techno Ltd., 3 Trinity Lane, Beverley, HU17 0DY, United Kingdom. ABSTRACT We report the synthesis of photoreactive and light-emitting liquid crystals which can be patterned photolithographically to form polymer networks for use in organic light-emitting diodes. The resolution capability of the photopatterning is investigated using a phase mask of period one micron to spatially modulate the irradiance of ultraviolet light incident to the monomer thin film. A surface relief grating of the same period and depth 85 nm is formed on exposure and washing. An OLED incorporating a novel green light-emitting liquid-crystal has a luminous efficiency of 4.3 lm W-1 at 100 cd m-2 and gives a luminance of 990 cd m-2 at an operating voltage of 10 V. INTRODUCTION We have pioneered a liquid crystal approach to organic electroluminescence, whereby thin, uniform films of light-emitting and charge-transporting liquid crystals are formed as insoluble polymer networks by photopolymerization using ultraviolet light [1-3]. The liquid crystals, socalled reactive mesogens, contain a polymerizable endgroup attached via a flexible spacer to each end of a light-emitting aromatic core. Polymerization and crosslinking occur by photoinduced generation of free radicals on irradiation with ultraviolet light. Electroluminescent polymer networks formed from reactive mesogens have many advantages compared to other approaches to organic electroluminescence. They are monodisperse after standard purification procedures. The reactive mesogens can be uniformly aligned to obtain polarized emission. Recently we have developed a new hole-transporting polymer which also acts as a photoalignment layer for these materials [4]. The polymer develops an anisotropy on irradiation with polarized ultraviolet light. Polarized electroluminescence was obtained with a polarisation ratio of 13:1 and the polarization direction was spatially patterned simply by changing the polarization direction of the ultraviolet beam incident to the photoalignment layer [4]. Rod-like liquid crystals show orientational order resulting in shorter intermolecular distances and faster carrier transport by hopping. Thin films of the reactive mesogens are made insoluble by photopolymerization so that multilayer devices are easily made using solution processing. Another advantage is that the films are photopatternable by standard photolithography. Recently we fabricated a red, green and blue pixellated organic light-emitting device (OLED) on a single substrate using standard photolithography [4]. Efficacie
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