In a recent collaborative study, published in Science Advances, the authors present unconventional polaronic charge transport in hematite—one of the most promising semiconductor materials for solar energy conversion.
Polarons are charged quasiparticles, typically generated through chemical doping or light exposure, that often inhibit charge transport and efficient energy conversion. This inhibition occurs because polarons tend to become immobilized within lattice traps in the material, preventing the generation of a charged current. However, using an advanced technique that probes local electric fields near the hematite surface, experimental work at Charles University in Prague (led by M. Setvin) and the Technical University of Vienna (led by G. Parkinson and U. Diebold), supported by theoretical interpretations at the University of Vienna (C. Franchini), reveals unusual polaron transport behavior in hematite crystals. Contrary to expectations, doping hematite results in enhanced conductivity, suggesting that dopants do not serve as trapping centers for electrons. Computer aided simulations performed by M. Reticcioli and F. Ellinger at the University of Vienna reveal peculiar mechanisms at play that shed light on the experimental measurements. While polaron diffusion is typically dominated by hopping of the localized charge carriers towards nearest neighbor sites, in hematite transport seems to occur preferentially via a transient delocalized charge state. The preference for this mechanism, known as random flight, can be understood by considering the particularly shallow polaronic eigenstates, lying only a few meV below the conduction band of hematite. The analysis of the density of states, clearly show a tendency of the shallow polaronic states towards the transient delocalized states.
These findings identify the random flight as an important type of electron-polaron diffusion in hematite, and the methodology introduced here may open a new way to understanding fundamental charge transport mechanism.
This work was developed within the special research program TACO (Taming Complexity in Materials Modeling), granted by the Austrian Science Fund FWF.
Original article:
Jesus Redondo et al. Real-space investigation of polarons in hematite Fe2O3.Sci. Adv.10,eadp7833(2024). DOI:10.1126/sciadv.adp7833
Image caption:
Electron polaron in Hematite, credits: Michele Reticcioli;
Further links:
TACO: https://sfb-taco.at/consortium/p07/
Group of Cesare Franchini: https://homepage.univie.ac.at/cesare.franchini/
