Why Silicon Doping Accelerates Electron Polaron Diffusion in Hematite

It is common that dopants enhance the conductivity of hematite Fe2O3, a popular photoanode, but the origin of the enhancement remains unclear. We establish the detailed mechanism by performing ab initio molecular dynamics simulations on electron polarons (EPs) in Fe2O3, obtained by an excess electron (e@EP) and a substitutional Si doping (Si@EP). For the first time, we observe EP hopping in both pristine and doped Fe2O3. We find that the neighboring Fe-Fe distance is the main driving force for the EP hopping, which occurs by the adiabatic charge transfer mechanism. The EP transport is determined by interplay of probabilities to reach a favorable configuration and to hop in that configuration. The hopping barrier decreases as the Fe-Fe distance decreases; however, hops can take place at larger Fe-Fe distances, because such configurations are easier to achieve. Importantly, we demonstrate that the Si dopant speeds up the EP transfer process by increasing the EP mobility. The origin can be ascribed to the following three factors: longer Fe-O bonds, smaller activation energies, and creation of low energy metastable EP states, in Si@EP Fe2O3 compared to e@EP Fe2O3. Interestingly, the EP hopping is random in e@EP Fe2O3, but quasi-random in Si@EP Fe2O3 with specific pathways promoting efficient EP transfer. These new findings greatly enrich the understanding of charge transport in Fe2O3 photoanodes.