High performance, light absorbing semiconductors are of intense interest for the solar-driven generation of sustainable fuels in photocatalytic and photoelectrochemical systems. Whilst metal oxides are the most-studied photoactive materials for the evolution of hydrogen from water and methanol from carbon dioxide and water, organic semiconductors can advantageously be synthesised from abundant sources; offer control over microstructure, light harvesting properties and redox potentials; and have the potential to be used in technologically simple devices. Despite substantial increases in the activities of organic photo-driven systems over the last five years, few studies[1] have managed to deconvolute the many factors (structural, optical, and electronic) that define the performance of these materials.

In this talk, I will primarily use transient absorption spectroscopy to demonstrate that generating charges which can live for microseconds is critical to the performance of a wide variety of different carbon nitride and linear conjugated polymer photocatalyst and photoelectrode systems [2-5]. However, generating large yields of these long-lived charges is a serious challenge in organic materials as excitons and charges typically recombine on the picosecond-nanosecond timescale. I will present several design strategies which can be employed to retard these processes, with particular focus on the timescales at which sacrificial agents and water influence the photophysics of the polymer system.

I will first use a series of novel sulfone-containing linear conjugated polymer photocatalysts to demonstrate that excitonic hole transfer from the polymer to the sacrificial donor can occur on the picosecond timescale, strongly competing with exciton recombination [2]. Crucially, the ability of the best-performing polymer to generate long-lived electrons is dependent on the thermodynamic driving force for hole transfer to the sacrificial donor.

Alternatively, excitons can be separated using an organic heterojunction. However, whilst a PM6:Y6 photoanode can efficiently separate excitons on the picosecond timescale with no applied bias, long-lived active charges are only observed when spatially separated by an applied bias [3]. We similarly find that low charge mobility in carbon nitride photoanodes limits their ability to physically separate charges, and can be improved by introducing a conductive network into the electrode [4]. Building on these observations, we demonstrate that the lifetime and yield of PM6 hole polarons can be improved in the PM6:Y6 photoanode by adding a PM6 overlayer on top of the bulk heterojunction. The PM6 overlayer aids the spatial separation of charges, retarding bimolecular recombination [3]. Finally we show that the inclusion of oligo(ethylene glycol) side chains to a polymer photocatalyst also extends the lifetime and yield of the active electrons relative to its alkylated analogue [5]. In this case, the hydrophilicity of the side chains causes substantial swelling, with the resulting spatial separation again reducing recombination.

Overall, I aim to show that a wide range of photo(electro)chemical organic materials and device structures can be improved through several common design themes.