As our societies struggle to meet their ambitions for decreasing greenhouse gas emissions and the Paris Climate agreement, research into alternative solar energy conversion and storage technologies becomes even more relevant. Solar fuel production by photocatalysis is an economically promising route, especially when driven by earth abundant and organic materials. While organics’ bottom-up design possibilities promise tailorable structure-function relationships for enhanced activity, the advancement is often hindered by limiting knowledge of interwoven photo-physical processes and properties that lead to recombination losses.[1]

In this talk, I will explain how time-resolved (transient) spectroscopy techniques in combination with varying environmental conditions can be used to provide insights into the very beginning of the solar energy conversion process chain, focussing on exciton generation and separation, and charge stabilization. This enables us to better understand light-matter interactions, and to tailor them to address bottlenecks associated with exciton recombination.

Our recent study on a series of functionalized polymers with varying porosity has revealed that not a maximization of a BET surface area is key, but rather the active interaction area with the photocatalytic environment, if exciton or charge separation is driven by sacrificial agents.[2]

In context of photocatalysis, interactions with aqueous ions, which are highly relevant for enabling applications in sea water conditions, are typically disregarded. Our study on suspended nanoparticles in presence of different salt shows how ions can impact stabilization of excitons and significantly extend their lifetimes, thereby enabling a new way to address excitons’ commonly fast and rate-limiting recombination.[3]

Lastly, I will introduce Terahertz permittivity measurements as convenient technique to probe the complex permittivity, and with that the dielectric properties of organic semiconductors on ps-time scales. The dielectric response defines exciton binding and is hence relevant for charge carrier photogeneration in all solar energy conversion technologies, but its values are highly frequency dependent, and commonly extracted at timescales orders magnitude off the ps-regime. Our study focussing on carbon nitrides now reveals dielectric screening and transport properties at the early time scales of solar energy conversion process chains. At the same time, it shows that also in this ultrafast regime, the environment and ions can matter, and strongly enhance photophysical parameters.[4]

References:

[1] T. Banerji, F. Podjaski, J. Kröger et al.: Polymer photocatalysts for solar-to-chemical energy conversion. Nat. Rev. Mater. 6, 168–190 (2021).

[2] B. Willner, C. M. Aichiston, F. Podjaski et al.: Correlation between the Molecular Properties of Semiconducting Polymers of Intrinsic Microporosity and Their Photocatalytic Hydrogen Production. J. Am. Chem. Soc. (2024), https://doi.org/10.1021/jacs.4c08549.

[3] F. Podjaski*, S. Gonzalez Carrero, C. Aichiston, P. Khlat, K. Steward, L. Hart, S. Hillman, J.-S. Kim, I. McCulloch, J. R. Durrant. In preparation.

[4] R. Jahangir, F. Podjaski*, P. Alimard, S. A. J. Hillman, S. Davidson, S. Stoica, A. Kafizas, M. Naftaly, J. R. Durrant: Terahertz-permittivity of Carbon Nitrides: Revealing humidity-enhanced dielectric properties on the picosecond timescales relevant for charge carrier photogeneration. Submitted. Preprint: https://www.arxiv.org/abs/2411.06226.