Self-trapped exciton dynamics in hematite nanoforms

What is it about?

Hematite is a potential material for a wide range of applications from optoelectronics to light harvesting. Despite its exciting features, its performance is restricted by the short minority charge diffusion distance, poor oxidizing kinetics and most importantly the short life time due to the presence of trap states. Hence, understanding and controlling the underlying recombination processes in the hematite-based systems is crucial for both device performance and fundamental physics. In this work, a non-degenerate pump probe transmission spectroscopy (Pump-3.15 eV and Probe 1.57 eV) is used to explore the self-trapped exciton (STE) dynamics in hematite nanoforms at various pump fluences. The kinetics of STE formation and annihilation were studied using coupled rate equation based kinetic model for the first time. According to our study, the free excitons and STEs interact nonlinearly to annihilate one another in a way similar to the trap-assisted bimolecular Auger recombination. Another notable observation from our studies is the strong carrier concentration dependence of the kinetics of STE formation and the exciton decay.

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Photo by Vinayak Varma on Unsplash

Photo by Vinayak Varma on Unsplash

Why is it important?

Hematite is a potential material for a wide range of applications from optoelectronics to light harvesting. Despite its exciting features, its performance is restricted by the short minority charge diffusion distance, poor oxidizing kinetics and most importantly the short life time due to the presence of trap states. Hence, understanding and controlling the underlying recombination processes in the hematite-based systems is crucial for both device performance and fundamental physics. In the recent Extreme UV(XUV) pumped transient white light probe, and optically pumped THz time resolved experiments, the presence of self-trapped excitons (STEs) and polarons are confirmed to play a major role in the photo-excited hematite although previous transient absorption data were speculated in terms of free carriers and free excitons. Thus, there was an urgent need to re-look at the photo-excited hematite through nondegenerate pump-probe measurements as that would enable to look at exclusively the photoexcited species at the conduction band from where free excitons and self-trapped excitons can be tracked.