Periodic Reporting for period 2 - SCORS (Spin Control in Radical Semiconductors)
Reporting period: 2023-04-01 to 2024-09-30
Organic semiconductors have been developed over the past few decades to provide a high level of performance in opto-electronic applications, and in particular, as light-emitting diodes as used in OLED displays. To achieve this success, some fundamental challenges inherent to organic semiconductors have been addressed. One centrak issue is that Coulomb interactions between electronic charges are strong, so that photo-excited states are generally bound electron-hole pairs, termed excitons. This brings an associated large spin-exchange energy which is large, resulting in optically accessible spin singlet excitons and dark, lower energy spin triplet excitons. These triplet excitons generally limit performance of OLEDs and also organic solar cells.
The starting point for SCORS is the recent realisation that organic semiconductors which have a net electron spin, so are radicals, can show efficient light absorption and emission. Organic radical molecules have not generally been regarded to be very luminescent, however, in 2018 we demonstrated efficient OLEDs based on radical (spin ½)-based organic semiconductor (ROSCs) molecules for emission. The key design feature is that these materials operate entirely within the spin-doublet manifold, avoiding non-emissive spin configurations based on spin-triplets. This opens a completely new domain for the operation and design of organic semiconductors materials and devices, one that is radically different from what has been possible till now. It is these opportunities that are being explored in the SCORS project. The broad set of challenges starts with the optimisation of luminescence efficiencies and colours through chemical design and synthesis and their use in efficient OLEDs. There are opportunities to explore novel spin-dependent energy transfer pathways between the doublet excitons in radical semiconductor and singlet or triplet excitons in host materials, and this can lead to new concepts for spin-optical control and novel quantum objects.
The starting point for SCORS is the recent realisation that organic semiconductors which have a net electron spin, so are radicals, can show efficient light absorption and emission. Organic radical molecules have not generally been regarded to be very luminescent, however, in 2018 we demonstrated efficient OLEDs based on radical (spin ½)-based organic semiconductor (ROSCs) molecules for emission. The key design feature is that these materials operate entirely within the spin-doublet manifold, avoiding non-emissive spin configurations based on spin-triplets. This opens a completely new domain for the operation and design of organic semiconductors materials and devices, one that is radically different from what has been possible till now. It is these opportunities that are being explored in the SCORS project. The broad set of challenges starts with the optimisation of luminescence efficiencies and colours through chemical design and synthesis and their use in efficient OLEDs. There are opportunities to explore novel spin-dependent energy transfer pathways between the doublet excitons in radical semiconductor and singlet or triplet excitons in host materials, and this can lead to new concepts for spin-optical control and novel quantum objects.
Substantial advances have been made in radical semiconductor design and synthesis, working with chemistry groups of Prof Feng Li at Jilin University and Prof Hugo Bronstein in Cambridge. This has supported significant improvements on OLED performance, particularly in the operation of OLEDs at higher current densities. There has been considerable success in device structures where there can be efficient energy transfer between doublet excitons and both singlet and triplet excitons in host organic semiconductors. Energy transfer between doublet and triplet excitons is in principle spin-allowed and we have shown very efficient energy transfer from ‘dark’ triplets to ‘bright’ luminescent doublet excitons, and have used this to improve OLED performance in TADF OLED systems, where we achieve fast emission from the doublet excitons.
One area of the project has advanced with unexpected success. We have explored molecular systems where the radical semiconductor is covalently attached to a molecular unit that supports a spin triplet exciton that has the same energy as the doublet exciton on the radical. Photoexcitation of the radical exciton leads to a rapid and efficient generation of an excitation that sits on both the radical group and the triplet-supporting group, and remarkably, intersystem crosses to form a high spin quartet state. Extensive electron spin resonance experiments show that this process is reversible, back to the spin double state, and we are able to get efficient luminescence back from the spin quartet state. This optical write – optical read of the electron spin system is very rare (the NV centre in diamond is the best known of these), and our realisation of such a molecular system represents a big advance. There are many opportunities to make extended spin systems, and we have shown that when a second radical group is attached to the other side of the triplet-supporting group, we can photogenerate a spin quintet state that couples the two radical spins. On relaxation to the ground state, these two spins remain in a long lived coherent state. This work was published in August 2023 (Nature, 10.1038/s41586-023-06222-1).
One area of the project has advanced with unexpected success. We have explored molecular systems where the radical semiconductor is covalently attached to a molecular unit that supports a spin triplet exciton that has the same energy as the doublet exciton on the radical. Photoexcitation of the radical exciton leads to a rapid and efficient generation of an excitation that sits on both the radical group and the triplet-supporting group, and remarkably, intersystem crosses to form a high spin quartet state. Extensive electron spin resonance experiments show that this process is reversible, back to the spin double state, and we are able to get efficient luminescence back from the spin quartet state. This optical write – optical read of the electron spin system is very rare (the NV centre in diamond is the best known of these), and our realisation of such a molecular system represents a big advance. There are many opportunities to make extended spin systems, and we have shown that when a second radical group is attached to the other side of the triplet-supporting group, we can photogenerate a spin quintet state that couples the two radical spins. On relaxation to the ground state, these two spins remain in a long lived coherent state. This work was published in August 2023 (Nature, 10.1038/s41586-023-06222-1).
We are directing efforts to develop these new spin-optical high spin radical systems, using newly designed molecular systems. We have promising new results with diradical and polyradical structures, some of which show promise for quantum information.