Effect of Defects on the Small Polaron Formation and Transport Properties of Hematite from First-Principles Calculations

TL;DR

This study uses first-principles calculations to analyze how defects like oxygen vacancies and dopants affect small polaron formation and transport in hematite, providing insights for improving solar fuel conversion efficiency.

Contribution

It offers a quantitative analysis of defect impacts on polaron formation and mobility in hematite, aiding the design of better photoanode materials.

Findings

Oxygen vacancies and dopants influence polaron formation energies.

Defects alter carrier mobility and concentration.

Insights for designing improved hematite-based photoanodes.

Abstract

Hematite ($\alpha$-Fe$_2$O$_3$) is a promising candidate as photoanode materials for solar-to-fuel conversion due to its favorable band gap for visible light absorption, its stability in an aqueous environment and its relatively low cost in comparison to other prospective materials. However, the small polaron transport nature in $\alpha$-Fe$_2$O$_3$ results in low carrier mobility and conductivity, significantly lowering its efficiency from the theoretical limit. Experimentally, it has been found that the incorporation of oxygen vacancies and other dopants, such as Sn, into the material appreciably enhances its photo-to-current efficiency. Yet, no quantitative explanation has been provided to understand the role of oxygen vacancy or Sn-doping in hematite. We employed density functional theory to probe the small polaron formation in oxygen deficient hematite, N-doped as well as Sn-doped hematite. We computed the charged defect formation energies, the small polaron formation energy and hopping activation energies to understand the effect of defects on carrier concentration and mobility. This work provides us with a fundamental understanding regarding the role of defects on small polaron formation and transport properties in hematite, offering key insights into the design of new dopants to further improve the efficiency of transition metal oxides for solar-to-fuel conversion.