Renewable energies technology is to play a massive part in the future of energy generation especially with the global concern of climate change. Increasing the efficiency of organic light-emitting diodes (OLEDs) is one focus in striving for a greener future. More than 20% of the world's energy consumption is due to lighting and displays and developing more efficient materials for OLED applications will help reduce this usage and save energy. A key goal in the development of OLEDs is to produce stable systems that have 100% internal quantum efficiency (IQE) i.e. every charge injected into the device creates a photon. The original OLEDs were limited to 25% IQE due to the quantum mechanical nature of the charges but recent developments allow us to push beyond this limit. Thermally activated delayed fluorescence (TADF) is one of those developments and will be a main focus on our journey to achieve 100% IQE.
The principal focus of my project is optimising organic compounds in the blue region. This is particularly critical as high energy blue emission is a difficult region to obtain stable TADF compounds. Commercial blue OLEDs are often based on emitters that undergo triplet-triplet annihilation; a different phenomenon that is limited to maximum 62.5% IQE. Finding a stable, blue TADF emitter of 100% IQE will have significant efficiency gains on the current state-of-the-art.
Approach:N-alkylation of donor - acceptor organic compounds: Working with collaborators at the University of Glasgow I will characterize groups of quinoline and pyridine salts with carbazole moieties and differing counter ions in terms of their photoluminescent properties. The Etherington group has begun to study the effect of N-alkylation and quaternisation with these compounds and my project will aim to develop and probe the excited state processes that occur from this. I will analyse how N-alkylation tunes the compounds such that TADF can be activated. The study therefore provides a systematic way to achieve the stable blue light-emitting materials desired by commercial companies.
N-alkylation of natural products: Working with Dr Jonathan Knowles in Applied Sciences at Northumbria University I will probe how N-alkylation of organic compounds, for natural product like compounds such as quinine. This will allow us to understand how the synthetic procedure affects the more fundamental processes such as charge transfer and conjugation that lead to the macroscopic phenomena such as TADF.
Methods:These compounds will be measured both in solution and solid form, across a series of host environments and the preparation method of film samples will also be investigated by comparing the drop cast and spin coating methods. I will obtain their photoluminescence quantum yield (PLQY), which defines a measure of the efficiency of photon emission as a ratio of photons emitted and photons absorbed and is a crucial property of these compounds if they are to be of commercial use. A detailed understanding of PLQY, steady-state and time-resolved photoluminescence is key for these systems and will provide both fundamental knowledge and also the criteria that are of interest to commercial OLED companies.
More efficient displays mean reduced energy consumption
The world is now heavily reliant on portable electronics and mobile phones. Increasing the efficiency of their displays is a crucial way to reduce energy consumption in these devices. Understanding the physical processes that can improve their efficiency puts this work at the core of EPSRC's remit. This project will allow me to use my chemical background and explore the physical properties of materials for OLEDs, making this a truly interdisciplinary project across the physical sciences. It will combine a fundamental and applied approach to organic light-emitting materials and will give me the opportunity to train on and compile data from a wide range of equipment at the universities of Northumbria and Durham.