Liquid crystal OLEDs

In recent years there have been major advances in liquid crystal displays (LCDs), with very large area TV screens being widely available. Despite their commercial success, alternative technologies are still being sought since LCDs are very complex, have slow switching times and relatively poor contrast. Organic light emitting devices (OLEDs) are actively pursued as they have many intrinsic advantages to LCDs, e.g high contrast and fast switching times. To date most OLEDs use small light-emitting molecules deposited by thermal evaporation although there remains a strong interest in polymer OLEDs.

The Organophotonics group has pioneered a unique liquid crystal approach to OLEDs based on nematic liquid crystals, which can be polymerised by irradiation with ultraviolet light to form insoluble thin films. Hence multilayer devices can be made by solution processing and patterned by photolithography. The nematic molecules can be uniaxially aligned to give polarised electroluminescence.  We design and synthesise new materials and test them in OLEDs. We investigate the photophysical and semiconducting properties of the materials to gain a good understanding of the correlation between their properties and OLED performance.

The figure shows how a red, green and blue pixellated LC-OLED can be fabricated using photolithography. A thin film of red-emitting film is deposited by solution processing, irradiated by UV light through a mask to define an insoluble pixel and the remaining material removed by washing in chloroform. Pixels of the green and blue light-emitting materials are then sequentially formed in different regions by a similar process.