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.
