Resolving Spin Quantum Properties in Organic Light-Emitting Diodes

Abstract

Devices which exploit the quantum properties of materials are widespread, with information processors and sensors showing significant recent progress. Organic materials offer interesting opportunities for quantum technologies owing to their engineerable spin properties, with spintronic operation and magnetic field sensing demonstrated in research grade devices, as well as proven compatibility with large scale fabrication techniques. Yet several important challenges remain as we move toward scaling these proof-of-principle quantum devices to larger integrated logic systems or spatially smaller sensing elements – particularly those associated with the variation of spin properties both within and between devices.

In this thesis, we explore three aspects influencing the homogeneity of spin interactions experienced by excitations in their local molecular environments – spatial, temporal and energetic variations. The resolution of these variations is realised through magneto-optical spin spectroscopy, whereby the modulation of optoelectronic processes in organic light-emitting diodes are imaged under the application of external magnetic fields. Using this technique, we map the spatiotemporal and energetic distributions of important spin quantum properties common to many molecular compounds, the results of which highlight the challenges of miniaturising and integrating these technologies for sensing and logic-based applications.

In addition to characterising the variability of hyperfine interactions across the microscopic molecular landscape, we observe the spatial correlation of this property for lengths up to 7 micrometres in both a polymer and small molecule material, and dynamic at room temperature. The energy dependence of exchange interaction strengths were also resolved in thermally activated delayed fluorescence materials, with variabilities exceeding 50% and which should be accounted for in future design rules of high performance fluorescent molecules.

Our investigations into the variation and correlation of spin interactions in space, time and energy provide important characterisations of the spin properties possessed by molecular materials for use in quantum devices. The miniaturisation, integration and scaling of technologies employing these materials will have to contend with this variation.