Simon R. Farrar

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.

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.

Photoluminescence study of crosslinked reactive mesogens for organic light emitting devices

We study the spectroscopic properties of luminescent liquid crystals which show a glassy nematic phase at room temperature and then form polymer networks by polymerization using ultraviolet light. The reactive mesogens possess fluorene-based aromatic cores with either diene or acrylate photoreactive groups at the end of aliphatic spacers. The photoluminescence quantum efficiency is enhanced when a rigid polymer backbone is formed by crosslinking of the diene endgroups. Spectral shifts of the vibronic transitions confirm an increase in the viscosity of the matrix following photopolymerization. Continuous and time-resolved photoluminescence measurements show that the quantum efficiency is limited by exciton diffusion to traps. Either the diffusion constant or the density of traps is reduced by photopolymerization.

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.

Electroluminescent Liquid Crystals

Abstract A new class of liquid crystals for use as transport layers and/or emission layers in organic light-emitting diodes has been prepared. Depending on the nature and number of the terminal substituents enatiotropic nematic, smectic and columnar phases are observed. Chain branching gives rise to materials with a low melting point. Several of these liquid crystals electroluminesce in the visible region. The charge transport properties of some of these materials have been evaluated.

Pulsed laser deposition of small molecules for organic electroluminescence

A XeCl excimer laser at 308 nm is used for the pulsed laser deposition (PLD) of a range of novel electroluminescent molecules. An optimum fluence dependent on the absorption length is found as a compromise between deposition rate and photodecomposition. There is minimal degradation of the aromatic luminescent core of the molecule when the fluence is optimised. Electroluminescent devices are prepared and show similar properties to those incorporating spin-cast films. Photodecomposition is observed for films deposited using higher fluences. The decomposition mechanism is ablative and some photoproducts are oxidised at high fluences. Thin films of reactive mesogens, containing end-groups which can undergo free radical polymerisation, are also prepared by PLD. These are not polymerised on the substrate by the deposition process. Subsequent irradiation with ultraviolet light results in an insoluble cross-linked network.

Electroluminescent Nematic Polymer Networks with Polarised Emission

Abstract Electroluminescence with a polarisation ratio of 12:1 from a uniformly aligned nematic network is reported. Diene photo-active end-groups were used, which polymerise by a selective cyclisation reaction. Surface alignment was achieved for the first time using a doped polymer photoalignment layer, oriented by exposure to polarised UV light. Threshold voltages between 2 V and 8 V were found and a brightness of 90 cd m−2 was obtained. Both the PL and EL intensities were higher after photochemical crosslinking.

Pulsed-laser deposition for organic electroluminescent device applications

An anthracene-based compound has been synthesized and used in the preparation of organic light-emitting diode devices by pulsed-laser deposition. Spectrally resolved electroluminescence has been observed and combined with current–voltage measurements in an investigation of the electro-optical dependence on laser fluence and device conditioning history. The device performance has been assessed and the charge-transfer process characterized. A space-charge-limited conduction regime with an exponential distribution of traps is proposed for the elevated electric fields sufficient to produce electroluminescence. The suitability of the pulsed-laser deposition technique is evaluated for this application. Evidence for molecular degradation associated with deviation from optimum deposition parameters is provided by Fourier transform infrared spectroscopy and scanning electron microscopy observations, comparing pulsed-laser deposition with established spin-coating and physical-vapor deposition techniques.