Self-Trapped Excitons in Bismuth Vanadate: A Play of Counterparts

Licentiatavhandling, 2026

Climate change and the persistent dominance of fossil fuels in global energy systems highlight the urgent need for sustainable alternatives. Hydrogen, produced via photoelectrochemical (PEC) water splitting, offers a green energy carrier. However, the efficiency of the PEC process is generally limited by charge carrier dynamics in photoanode materials. Bismuth vanadate (BiVO₄), a potential material for PEC water splitting, is particularly prone to charge localization in the form of small polarons and self-trapped excitons (STEs), which can potentially affect its performance. While the formation of polarons in BiVO₄ has been widely studied, the precise nature, stability, and behaviour of STEs, especially under operating conditions, remain poorly understood. In this thesis, these gaps are addressed using advanced computational methods. First, time-dependent density functional theory (TD-DFT) with hybrid functionals is employed to investigate the localization, stability, and optical properties of STEs in BiVO₄. The results reveal two distinct localized exciton configurations: one separated, more mobile state, and one compact, more rigid state, with comparable energies. Second, hybrid density functional theory (DFT) combined with the nudged elastic band (NEB) method is used to quantify activation barriers for STE hopping, dissociation, and transformation. This analysis reveals distinct kinetic behaviours for the two STE types, where the separated, more mobile state exhibits much lower barriers than the compact, more rigid state. Additionally, an alternative charge-trapping mechanism involving O–O hole dimers is explored, providing insights into a multipolaron binding pathway with much slower kinetics compared to STEs.