Spectroscopic Study of Triplet Exciton Dynamics at the Hybrid Organic-Inorganic Interface
The control and utilisation of spin-triplet excitons in organic semiconductors is highly sought after for the next generation of Optoelectronic applications. Of particular interest is the utilisation of these photoexcited states in the singlet-fission photon-multiplier and triplet-triplet annihilation upconversion processes. Singlet fission is an exciton multiplication process in organic molecules, in which a photogenerated spin-singlet exciton is rapidly and efficiently converted into two spin-triplet excitons. Conversely, triplet-triplet annihilation is essentially the same reaction operating in reverse. These processes offer two spectral management mechanisms to break the Shockley–Queisser limit by overcoming the thermalisation and absorption losses inherent to all single-junction photovoltaics. Such spectral management technologies have been predicted to increase the maximum possible efficiency of Si-based cells from 32 % to greater than 40 %, breaking the Shockley–Queisser limit. Harnessing these processes would be facilitated if the energy of the triplet exciton could be efficiently interchanged with photons. However, utilising triplet excitons like this poses a significant challenge, as transitions between the ground state and triplet excited states typically have negligible mediation by photons. Transitions such as these are spin forbidden, and have a correspondingly weak oscillator strength. In this thesis, we investigate a promising method to overcome this impasse, using inorganic quantum dots (QDs) to efficiently convert between triplet excitons and photons. We develop a variety of novel hybrid organic-inorganic systems that perform singlet-fission photon-multiplication and triplet-triplet annihilation upconversion. We find that in order to achieve efficient triplet transfer at the hybrid organic-inorganic interface, it is critical to engineer the surface of the QD with a triplet transfer ligand. The triplet transfer ligand facilitates transfer by acting as an intermediate excited state during the transfer process, encouraging the formation of an optimal solid-state morphology, and providing a weak adsorption site for rapid transfer to occur at. Among the many highlights, we develop a solid-state singlet-fission photon-multiplier with an exciton multiplication efficiency of ~190%, showing significant promise for real-world application. Additionally, advanced spectroscopic techniques and mathematic modelling are applied to gather an in-depth understanding of the impact of a variety of photophysical processes on operation under realistic working conditions. These results establish a variety of highly tuneable platforms to understand the triplet transfer process at the organic semiconductor and inorganic QD interface, providing clear design rules for new materials that perform spectral management.
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