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

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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.