TADF Emitters for OLEDs

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Lighting accounts for almost 20% of world electricity demand. Widescale adoption of energy efficient lighting would have a profound positive effect on energy consumption and the environment. In addition, in the developing world improved lighting would increase economic output. Organic semiconductors are attractive materials for lighting applications because of the scope to tune their properties (such as colour of light emission) and deposit them by simple processes from solution. This means they have the potential for large area light source to be made inexpensively, and the possibility of such light sources being flexible or conformable. The light emission process involves injecting opposite charges which combine to form excited states called singlets and triplets. In most materials triplets do not emit light, but for high efficiency the triplets need to be harvested to contribute to light emission. The present state-of-the-art lighting devices (organic light-emitting diodes, OLEDs) employ the use of iridium and other metal complex phosphors. The rarity of these metals and their associated cost precludes their use in inexpensive lighting devices. In this project we propose to explore thermally activated delayed fluorescence (TADF) emitters which offer a tantalizing solution as these materials are based on small molecule organic compounds i.e. are made from very abundant materials. TADF emitters rely on a molecular design where the singlet and triplet energy levels are close enough together that conversion of triplets to light-emitting singlets is efficient, thereby leading to highly efficient light-emitting devices. The inexpensive starting materials and relatively low-cost synthesis coupled with comparable optoelectronic properties to the exepnsive metal complexes makes this next-generation of emitters very promising.In our proposal we address grand challenges in the development of TADF emitters: (1) the development of a bright and stable deep blue emitter; (2) the improvement of high efficiency red emitters based on organic compounds; (3) the fabrication of low cost EL devices. For (1) and (2) we will design and evaluate materials that address these challenges, guided by detailed photophysical and optoelectronic measurements to relate the properties of the materials to their structure. Most work on TADF materials to date has focused on evaporated materials. Instead we will develop materials that can be processed from solution to address challenge (3) of simple fabrication to enable the full benefit of these low cost but efficient emitters to be realised.