Blue-emitting Phosphors for Solid State Lighting Applications

Context: The invention of artificial lighting, dating from Joseph Wilson Swan and Thomas Edison's seminal contributions to the invention and commercialization of the incandescent light bulb in 1879, is arguably one of the most important inventions of humankind. Artificial lighting permits most human activities to continue past sundown, thus immeasurably increasing worldwide human productivity. Though Edison's device was much brighter than candle lighting, it was inefficient, converting only 0.2% of electricity into light. Since this seminal invention, many other lighting devices have been developed, from the tungsten lamp, to fluorescent tubes to halogen lighting to light-emitting diodes (LEDs) to organic light-emitting diodes (OLEDs). With each further iteration in lighting technology, the quality (pureness of colour), power efficiency and brightness of the light produced by the device have each improved. Light emission also enables information displays, televisions and computer screens.

Producing devices that are energy efficient is of particular importance as, according to the US Department of Energy, it is estimated that 1/3 of commercial electricity use and 10% of household electricity consumption in the United States alone is dedicated towards artificial lighting. Artificial lighting represents a $15 Billion market in the United States alone and almost $91 Billion worldwide, corresponding to 20% of total worldwide energy output. The environmental impact related to this energy consumption is enormous and is estimated to be responsible for 7% of global CO2 emissions. Whereas inorganic LED and organic or polymer OLED lighting is now the state of the art in artificial lighting, their high cost and small active surface area are still barriers to wide adoption. In fact, for large surface area outdoor lighting applications, low-pressure sodium lamps are still the technology of first choice. Within this context, there is an urgent need to find alternative artificial lighting technologies that are of lower production cost, more energy efficient, colour tunable and can be used in environments not currently accessible to current LED and OLED technologies. It is implicit that in a similar manner to OLEDs, such a new lighting technology would have applications in visual displays, telecommunication and sensors.

Organometallic complexes capable of harnessing light and/or electrical current and transforming such energy into useful work are at the heart of many important applications. An application that is of particular interest to my research group is energy-efficient visual displays and flat panel lighting based on either a phosphorescent light-emitting electrochemical cell (LEEC) architecture or an OLED architecture. Currently, most ionic transition metal complex-based (iTMC) LEECs rely on the use of a charged iridium(III) complex as the luminophoric material. These complexes can be readily solution processed. Iridium complexes phosphoresce and thus the maximum photoluminescence quantum efficiency (PLQY) theoretically attainable is unity. The external quantum efficiency (EQE) of a LEEC device has been found to scale proportionately to the solid-state PLQY and as such bright devices are possible. Despite the advantages listed above, LEECs incorporating iTMCs have several weaknesses: (i) low EQE; (ii) limited stability of the device and (iv) colour quality, particularly with reference to blue light emission.

This grant proposal targets the development blue-emitting iridium(III) cationic complexes that will act as a luminophoric material in both LEEC and OLED devices. The two main goals are: 1. to obtain a LEEC that emits brightly in the blue region of the spectrum and that is stable over thousands of hours and that can quickly illuminate upon the application of an external voltage; to produce higher performance deep blue emitting OLEDs.

This research aims to develop bright and stable blue-emitting light-emitting electrochemical cells (LEECs) and organic light-emitting diodes (OLEDs) for solid-state lighting applications. Blue-emitters are a prerequisite to achieving white-light emission in an operational device. 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 the grand challenge that remains is the development of bright and stable blue emitters.

The project relies on the design and development of new ligand or complexation motifs that result in bright blue-emitting complexes. The key feature in each design is the use of "fluorine-free" ligands (no C(aryl)-F bonds that are labile in electroluminescent, EL, devices). These complexes will then be incorporated into LEECs and OLEDs to produce blue-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 £ 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 R&D research in industry from fine chemical companies such as Johnson Matthey, Umicore, Merck, Eastman, Alfa Aesar, BASF and Solvay to lighting device companies such as Cambridge Display Technologies (who are supporting this project), Thorn Lighting, Coughtrie DesignLED, Pufferfish, and OSRAM. These same materials will 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. Moreover, the ligands proposed will themselves be of interest to other synthetic organic chemists and their properties may also be of interest to the greater scientific community. This project combines computational modelling, organic and inorganic syntheses, optoelectronic characterization and materials development under one umbrella.

The PDRA working on this project will thus become highly trained with a desirable and multidisciplinary skill set (capable of conducting and interpreting computations, synthesizing organic and organometallic compounds, measuring optoelectronic properties and evaluating functional lighting devices), making the individual employable and attractive to a wide selection of industries. The researcher will additionally benefit from the opportunity to interact with an interdisciplinary team of researchers, including chemists, physicists and materials scientists. The research experience will be enhanced through visits to the University of Valencia, where the LEECs will be fabricated and tested and to the School of Physics at the University of St Andrews, where OLEDs will be fabricated and tested, and where advanced ultrafast spectroscopy exists.