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Photoexcited Electrons in Hematite Trap Themselves by Polaron Formation

Effect of polaron formation across hematite’s absorption. Electrons localize at orange colored Fe atoms, red colored O atoms are repelled by the localized charge. At higher photon energies more energy goes into the lattice, and it becomes harder for charges to remain trapped, leading to higher charge mobility, and thus higher photoconversion efficiency.

Scientific Achievement
Elucidated the role of electron self-trapping in the photoexcited state of hematite.

Significance and Impact
Understanding the fate of photoexcited electrons helps explain why efficiencies have remained low in Fe2O3 photoelectrochemical cells and helps set the aim for future research.

Research Details

  • Once excited, electrons can lose excess energy by heating the lattice through excitations known as phonons. The electrons can couple to excited phonons forming a quasiparticle known as a polaron.
  • In the case of hematite, the polaron formation is strong enough to trap electrons on the photoexcited Fe atom, decreasing the photoexcited electron's mobility.
  • Accordingly, polaron formation is measured highest in the spectral regions where hematite’s photoconversion efficiency is lowest.

L. M. Carneiro, S.K. Cushing, C. Liu, Y. Su, P. Yang, A.P. Alivisatos, S.R. Leone, Nature Materials (2017) DOI:10.1038/nmat4936