Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.235
Publication date: 6th July 2018
Developing energy storage mechanisms is a crucial step to secure a sustainable energy economy. This is particularly important for solar power because there is a mismatch between the periods of peak energy harvesting and peak consumption. Photo-electrocatalytic energy storage has traditionally been dominated by inorganic systems. In particular, transition metal oxides are attractive due to their stability in liquid electrolytes and the remarkable catalytic flexibility of metallic centres. However, inorganic systems often underperform and only poor quantum yields are achieved, even when synthetic care is taken to eliminate defects and impurities. In the dark, oxides like Fe2O3 and BiVO4 are poor conductors due to the localization of charges on specific atomic sites forming small polarons. Under illumination, transport improves but the mechanisms behind it are unknown. Recent x-ray and terahertz studies have hinted at the existence of light-induced small polarons and their link with structural properties. 1-3 However, these measurements lack device sensitivity and it is difficult to know if such states really influence performance. In this talk, I will present ultrafast optical studies with photocurrent detection of photoelectrochemical devices. These measurements are capable of probing light-initiated changes that control actual device activity.4 I will present data demonstrating that, in oxides such as efficient Fe2O3 thin-films,5 charges intrinsically self-trap within femtoseconds of photo-generation hindering transport. Importantly, I will show how in efficient systems, crossover between molecular-type, localised transport and band-like transport is critical and necessary for activity. This behavior is remarkably similar to that observed in molecular crystals and some of the strategies used to improve molecular semiconductors might apply to metal oxides. I will discuss the material implication of these findings and how it might be possible to achieve synthetic control of charge hopping and delocalisation rates in oxides.
References:
1Carneiro L. et al. Nat Mater. 2017, 16, 8, 819
2Biswas S. et al. Nano Lett. 2018,18,1228
3Butler K.T. et al. J.Mat.Chem.A, 2016,4,18516
4Bakulin A. et al. J.Phys.Chem. Lett. 2016, 7, 2, 250
5Steier L. et al. ACS Nano, 2015,9,11775
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