TADF Emitters for OLEDs

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.

The research stemming from this project will lead to bright and stable blue- and red-emitting electroluminescent devices for solid-state lighting applications. Light-emitting electrochemical cells (LEECs) hold great potential as an alternative technology to organic light emitting diodes (OLEDs) as they possess simpler architectures, are easier and cheaper to fabricate, can operate using AC current, can function in air without the need for encapsulation, are thinner and most importantly, use less electricity. OLED technology is more mature and robust, with devices exhibiting better stability profiles than LEECs. For both technologies, bright and stable deep blue emitters are still in short supply while green and red emitters used in typical devices are based on iridium(III) complexes where the iridium is both rare and expensive.

The project targets the design and development of new TADF emitters. These compounds will then be incorporated into OLEDs and LEECs to produce both blue and red emission in the EL device. The potential environmental and economic impacts of this work are huge. An inexpensive, energy efficient and long-lasting white-light device, especially one that could be fabricated to create large surface area lighting for public and outdoor lighting, would reduce in a dramatic way UK energy consumption and respond to current UK energy policy. Moreover, such directional lighting devices would help to reduce overall light pollution that is caused from omnidirectional lighting currently being used throughout the UK. Dynamic lighting technologies used in visual displays such as mobile phones and televisions would also benefit from cheap, thin and flexible emitters. The lighting and display industries each have multibillion pound markets within the EU. It is thus evident that introducing a new technology or improving the current state-of-the art will have the potential for tremendous impact upon these markets.

In the short to medium term, the optoelectronic functional materials developed in the course of this project will also impact fine chemical companies in the UK such as Johnson Matthey, Merck and Eastman, and in Europe Umicore, Alfa Aesar, BASF and Solvay to lighting device companies in the UK such as Cambridge Display Technology (who are supporting this project), Thorn Lighting, Coughtrie, DesignLED, Pufferfish, and in Europe Cynora and OSRAM. These same materials have the potential to also impact research in the field of solar fuels (generating hydrogen and oxygen from water), organic synthesis using photoredox catalysts, in the area of luminescent probe development for health sciences and for environmental analyte detection.

This project combines computational modelling, organic synthesis, optoelectronic characterization and materials development under one umbrella. The two postdoctoral researchers working on this project will thus become highly trained with a desirable and multidisciplinary skill set (capable of conducting and interpreting computations, synthesizing organic compounds, measuring optoelectronic properties and evaluating functional lighting devices), making the individual employable and attractive to a wide selection of industries. The researchers will additionally benefit from the opportunity to interact with an interdisciplinary team of researchers, including chemists, physicists and materials scientists.