Optimising molecular systems for optoelectronic applications requires a deep understanding of the exciton dynamics in such systems. Pentacene is an archetypal organic semiconductor, with multiple device applications depending on molecular aggregation and orientation within the structure. Isolated pentacene molecules within crystals are used for the room-temperature MASER (a device analogous to the laser for microwave amplification by stimulated emission of radiation) but limitations remain due to relaxation processes and heating effects. Aggregated systems undergo singlet fission, the mechanism whereby two triplet excitons are generated from a single photon absorption. This process is of interest to build efficient triplet-harvesting layers for organic photovoltaic applications as it overcomes the thermodynamic limitations of these devices.

This project designs a thin film system that varies the concentration of pentacene to investigate the exciton properties and dynamics as a function of microstructure and molecular configuration. Electron paramagnetic resonance spectroscopy is used to identify the spin species in the pentacene films and the spectral signatures of the free and coupled triplets for different configurations of dimers are identified with distinct kinetic parameters. To explain these differing dynamics, DFT calculations are carried out alongside temperature-dependent measurements to obtain the activation energies of the photoexcited processes. To obtain a complete overview of the dynamics of this molecular system, ultra-fast transient absorption spectroscopy was also employed to study the early-time processes. The intermediate states involved in singlet fission are observed and are tentatively assigned according to the molecular configuration of the pentacene pairs.

This project provides a methodology of film design and characterisation that can be extended to other compatible host-dopant systems used in optoelectronic and spintronic devices. Consequently, not only does this work extend our understanding of the photoexcited mechanisms in pentacene, it also provides a template for investigations of similar molecular systems.