Global economies are transitioning towards net-zero emissions, but technological leaps are still needed to accelerate decarbonisation within the energy sector and beyond. Here, several novel material systems are studied to uncover physical properties which will dictate their suitability for use in state-of-the-art light-harvesting structures such as thin-film photovoltaics and photoelectrochemical fuel cells. Such materials offer promising avenues to cheap and efficient sustainable energy solutions.

Metal halide perovskites excel in the pursuit of highly efficient thin film photovoltaics and light emitters. Substitution of the monovalent cations has advanced luminescence yields and device efficiencies. In this thesis, the change in photocarrier recombination behaviour caused by cation alloying is explored. Time-resolved optical spectroscopy and microscopy are used to reveal local charge accumulation in mixed cation perovskites, creating p- and n-type photodoped regions, unearthing a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes.

Operational stability of perovskite solar cells remains a barrier to their commercialisation, yet a fundamental understanding of degradation processes, including the specific sites at which failure mechanisms occur, is lacking. Here, multimodal microscopy techniques are utilised to show that nanoscale defect clusters, which are associated with phase impurities, are sites at which material degradation seeds. The trapping of charge carriers at sites associated with phase impurities, itself reducing performance, catalyses redox reactions that compromise device longevity. Importantly, this reveals that both performance losses and intrinsic degradation can be mitigated by eliminating these defective clusters.

Carbon nanodots are an emergent material whose ease of fabrication and water solubility make them exciting candidates for photocatalytic processes. However, a full understanding of their excited charge carrier dynamics and interaction with common electron donors/acceptors is not yet established. This work identifies charge transfer processes in hybrid photocatalytic systems with carbon nanodot absorbers and builds bottom-up mechanistic insight.