The Impacts of Dopants on the Small Polaron Carrier Mobility and Conductivity in Hematite – The Role of Disorder

Congratulations to Mingpeng Chen, Andrew C. Grieder, Tyler J. Smart and Prof. Yuan Ping in collaboration with Kiley Mayford, Samuel McNair, Anica Pinongcos, Samuel Eisenberg, Prof. Frank Bridges and Prof. Yat Li for their work for their work on “The Impacts of Dopants on the Small Polaron Carrier Mobility and Conductivity in Hematite – The Role of Disorder”, published in Nanoscale!  Link to the published article.

Abstract: Hematite (α-Fe2O3) is a promising transition metal oxide for various energy conversion and storage applications due to advantages of low cost, high abundance, and good chemical stability. However, its low carrier mobility and electrical conductivity have hindered wide application of hematite-based devices. Fundamentally, this is mainly caused by the formation of small polarons, which conduct through thermally activated hopping. Atomic doping is one of the most promising approaches to improve the electrical conductivity in hematite. However, its impact on carrier mobility and electrical conductivity of hematite at the atomic level remains to be illusive. In this work, through a kinetic Monte-Carlo sampling approach for diffusion coefficient combined with carrier concentrations computed at the charge neutrality condition, we obtained electrical conductivity of doped hematite. We took contributions from individual Fe-O layers, given that the in-plane carrier transport dominates. We then studied how different dopants impact carrier mobility in hematite using Sn, Ti, and Nb as prototypical examples. We found that the carrier mobility change is closely correlated with the local distortion of Fe-Fe pairs, i.e., the more stretched Fe-Fe pairs are compared to the pristine systems, the lower carrier mobility will be. Therefore, elements which limit distortion of Fe-Fe pair distances from pristine are more desired for higher carrier mobility in hematite. The calculated local structure and pair distribution functions of doped systems have remarkable agreement with experimental EXAFS measurements on hematite nanowires, which further validates our first-principles predictions. Our work revealed how dopants impact carrier mobility and electrical conductivity of hematite, and provided practical guidelines to experimentalists on the choice of dopants for optimal electrical conductivity of hematite and performance of hematite-based devices.

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