Material and device properties of highly birefringent nematic glasses and polymer networks for organic electroluminescence

Light-emitting nematic liquid crystals are promising materials for organic light-emitting devices because their orientational anisotropy allows polarized electroluminescence and improved carrier transport. Two classes of nematics, i.e., room-temperature glasses and crosslinked polymer networks are discussed. The latter class has an additional advantage in that photolithography can be used to pixelate a full-color display. We show that the order parameter and birefringence of a new light-emitting nematic liquid crystal with an extended aromatic core both have values greater than 0.9. The performance of green light-emitting devices incorporating liquid crystals of different conjugation lengths is discussed. Efficacies up to 11.1 cd/A at 1160 cd/m2 at an operating voltage of 7 V were obtained. A spatially graded, color organic light-emitting device obtained by overlapping pixels of blue-, green-, and red-emitting liquid crystals were demonstrated. Some regions of the red pixel were only partially photopolymerized in order to obtain different hues in the overlapping region with green. We also show that the photolithographic process has micron-scale resolution. © Copyright 2006 Society for Information Display.Link_to_subscribed_fulltex

Material and device properties of highly birefringent nematic glasses and polymer networks for organic electroluminescence

Light-emitting nematic liquid crystals are promising materials for organic light-emitting devices because their orientational anisotropy allows polarized electroluminescence and improved carrier transport. Two classes of nematics, i.e., room-temperature glasses and crosslinked polymer networks are discussed. The latter class has an additional advantage in that photolithography can be used to pixelate a full-color display. We show that the order parameter and birefringence of a new light-emitting nematic liquid crystal with an extended aromatic core both have values greater than 0.9. The performance of green light-emitting devices incorporating liquid crystals of different conjugation lengths is discussed. Efficacies up to 11.1 cd/A at 1160 cd/m2 at an operating voltage of 7 V were obtained. A spatially graded, color organic light-emitting device obtained by overlapping pixels of blue-, green-, and red-emitting liquid crystals were demonstrated. Some regions of the red pixel were only partially photopolymerized in order to obtain different hues in the overlapping region with green. We also show that the photolithographic process has micron-scale resolution. © Copyright 2006 Society for Information Display.Link_to_subscribed_fulltex

Organic electroluminescence using polymer networks from smectic liquid crystals

We report the synthesis of a red light-emitting and photopolymerizable smectic liquid crystal (reactive mesogen). We investigate the suitability of polymer networks formed from smectic reactive mesogens for use in organic light-emitting diodes (OLEDs). The use of mixtures of smectic reactive mesogens is shown to lower the processing temperature for the fabrication of OLEDs to room temperature. We also report efficient energy transfer from a nematic polymer network host to a smectic light-emitting dopant and polarized emission from a polymer network formed from an aligned smectic reactive mesogen. © 2006 Taylor & Francis.Link_to_subscribed_fulltex

Photoluminescence study of crosslinked reactive mesogens for organic light emitting devices

The spectroscopic properties of photoluminescent nematic liquid crystals were studied. On polymerization these crystals formed polymer networks when ultraviolet light is used. A rigid polymer backbone was formed by crosslinking resulting in an increase of the photoluminescent quantum efficiency. An increase in viscosity is confirmed by the spectral shifts of the vibronic transitions.Link_to_subscribed_fulltex

A Full-Color Electroluminescent Device and Patterned Photoalignment Using Light-Emitting Liquid Crystals

The principle of combining liquid crystals and organic light-emitting diodes (OLED) technology to produce full-color OLEDs with enhanced capabilities was analyzed. The deposition and the photolithographic patterning of polymerizable and light-emitting liquid crystals using standard liquid crystal display (LCD) manufacturing procedures, to fabricate a three-color OLED, was described. A latest class of organic copolymers designed specifically as hole-transporting, photoalignment layers for OLEDs was discussed. The results demonstrate the viability of the polymerizable liquid-crystal approach to give full-color, high-information-content, large-area OLEDs, as well as to generate latest kinds of 3D OLEDs.Link_to_subscribed_fulltex

Light-emitting fluorene photoreactive liquid crystals for organic electroluminescence

Light-emitting liquid crystals for organic light-emitting diodes (OLEDs) require low-temperature liquid crystal phases for room-temperature processing and a range of molecular energies for electron and hole injection, as well as tunable color and color purity for multicolor OLEDs. We report a number of light-emitting polymerizable liquid crystals (reactive mesogens) based on 2,7-disubstituted-9,9-dialkylfluorene, whose energy levels can be tuned for optimized charge injection and light emission. As a consequence of these systematic property/structure investigations small molecule reactive mesogens have been synthezised, which exhibit low melting points, even below room temperature and nematic phases above room temperature as single components. Many of the molecules retain a supercooled nematic phase on cooling to room temperature. Simple binary eutectic mixtures of reactive mesogens with identical aromatic cores form light-emitting nematic phases at room temperature with a high clearing point to generate a high order parameter. The ionization potential of six-ring fluorene reactive mesogens can be tuned between 4.93 and 5.57 eV by chemical modification of the aromatic cores. Similarly the emission spectrum can be tuned from blue to green. A typical performance for an OLED using such liquid crystalline materials as a cross-linked polymer network is described.Link_to_subscribed_fulltex

Linearly polarised organic light-emitting diodes (OLEDs): synthesis and characterisation of a novel hole-transporting photoalignment copolymer

We describe the synthesis and characterisation of a novel hole transporting photoalignment copolymer for linearly polarised emission. The copolymer has a coumarin side-chain which undergoes (2 + 2) cycloaddition on irradiation with ultraviolet light. A fluorene side-chain, whose ionisation potential is well matched to the work-function of indium tin oxide, provides hole transporting properties. Polarised green electroluminescence was obtained by spin coating a novel polymerisable and light-emitting liquid crystal onto the photoaligned copolymer. A polarisation ratio value of 13 : 1 and a polarised irradiance of 200 cd m-2 was obtained at 9 V. Polarised red emission is also described in a guest-host configuration. The spatial patterning of the polarisation direction is also shown. © The Royal Society of Chemistry 2005.Link_to_subscribed_fulltex

Light-emitting Polymerizable Liquid Crystals: Micron Scale Photolithographic Patterning and Green Electroluminescence.

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 ni and gives a luminance of 990 cd m-2 at an operating voltage of 10 V. © 2005 Materials Research Society.Link_to_subscribed_fulltex