Spin Dynamics in Multilayer Organic Photovoltaics and Organic Light-Emitting Diode Films'

My project is a collaboration between Chemistry and Physics, which aims to develop new high-performing organic materials for multilayer organic devices such as organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). We will also, for the first time, explore the use of ferromagnetic resonance spectroscopy (FMR) to characterise the spin states of these devices.

Spin-flip mechanisms are crucial to the design of efficient organic materials for renewable energy applications. The interplay of spin-singlet and spin-triplet excited states mediates the performance of solar-to-electricity energy generation in photovoltaics and electricity-to-light energy conversion in OLEDs. However, there are various factors which are still significantly restricting the efficiency of these devices. For example, the maximum efficiency for a single p-n junction photovoltaic cell is limited to below 33.7% by the Shockley-Queisser limit, whereas the internal quantum efficiency of singlet-state generation in OLEDs is limited to only 25% due to the spin-forbidden radiative decay from the triplet to singlet state.

Promising solutions to the problems mentioned above have been proposed. For example, the unique properties in certain organic semiconductors can be utilised to overcome the limitations of single-junction inorganic photovoltaics via singlet fission. Singlet fission is a process by which a high-energy singlet exciton is converted into two triplet excitons, each carrying about half the energy. Coupling such an organic semiconductor to a low band gap inorganic semiconductor allows fabrication of a two-bandgap OPV in a single junction, which will in principle have a higher efficiency than conventional photovoltaics. On the other hand, the use of fluorescence emitters which exhibit thermally activated delayed fluorescence (TADF) in OLEDs could be considered. By designing molecules with a small energy difference between the S1 and T1 levels, the small energy gap may enable reverse intersystem crossing to occur, where excitons in T1 are converted to S1 in a thermally activated process. Once in the S1 state the excitons will be able to decay back to the S0 ground state via fluorescence. Using the TADF mechanism, internal quantum efficiencies of 100 % can be achieved and it is hoped that TADF will allow the creation of a stable and high efficiency OLEDs.

The first stage of my project will, therefore, be the development of organic materials, focusing on highly conjugated systems such as acene derivatives that are predicted to undergo singlet fission. Once the new organic materials have been synthesised, they will be subjected to FMR spectroscopy to characterise their spin states. FMR probes spin flip and reverse intersystem crossing processes by measuring the precessional damping of magnetisation in an adjacent ferromagnetic thin-film spin source. It will give us direct access to the rates of the fundamental organic spin processes described above. Although FMR has rarely been used on organic materials, this approach has great potential in studying OPVs and OLEDs because it can be applied to probe the entire multilayer system.

We hope that by developing high-performing materials for OPVs, and OLEDs, the broad applicability of these devices means that our technological developments could bring positive environmental impacts to society.

ReNU's enhanced doctoral training programme delivered by three uniquely co-located major UK universities, Northumbria (UNN), Durham (DU) and Newcastle (NU), addresses clear skills needs in small-to-medium scale renewable energy (RE) and sustainable distributed energy (DE). It was co-designed by a range of companies and is supported by a balanced portfolio of 27 industrial partners (e.g. Airbus, Siemens and Shell) of which 12 are small or medium size enterprises (SMEs) (e.g. Enocell, Equiwatt and Power Roll). A further 9 partners include Government, not-for-profit and key network organisations. Together these provide a powerful, direct and integrated pathway to a range of impacts that span whole energy systems.

Industrial partners will interact with ReNU in three main ways: (1) through the Strategic Advisory Board; (2) by providing external input to individual doctoral candidate's projects; and (3) by setting Industrial Challenge Mini-Projects. These interactions will directly benefit companies by enabling them to focus ReNU's training programme on particular needs, allowing transfer of best practice in training and state-of-the-art techniques, solution approaches to R&D challenges and generation of intellectual property. Access to ReNU for new industrial partners that may wish to benefit from ReNU is enabled by the involvement of key networks and organisations such as the North East Automotive Alliance, the Engineering Employer Federation, and Knowledge Transfer Network (Energy).

In addition to industrial partners, ReNU includes Government organisations and not for-profit-organisations. These partners provide pathways to create impact via policy and public engagement. Similarly, significant academic impact will be achieved through collaborations with project partners in Singapore, Canada and China. This impact will result in research excellence disseminated through prestigious academic journals and international conferences to the benefit of the global community working on advanced energy materials.