Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides

Abstract

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO 2 as a prototypical system, we use ab initio nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds. Polaron migration among Ti atoms is strongly correlated with quenching through an electron−hole (e−h) recombination process. At low temperature, the polaron is localized on a single Ti atom and polaron quenching occurs within several nanoseconds. At increased temperature, as under solar cell operating conditions, thermal phonon excitation stimulates the hopping and delocalization of polarons, which induces fast polaron quenching through the e−h recombination within 200 ps. Our study proves that e−h recombination centers can be formed by photoexcited polarons, which provides new insights to understand the efficiency bottleneck of photocatalysis and photovoltaics in TMOs.

Effects of oxygen vacancies on the photoexcited carrier lifetime in rutile TiO2

“…56 We use the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional with Hubbard U = 4.5 eV for Ti 3d orbitals in all calculations, in accordance with our previous report. 42,57 A 2 Â 2 Â 3 supercell containing 24 TiO 2 units (72 atoms in total) is employed to model the rutile TiO 2 bulk. The tests of the supercell and U value can be found in the ESI.…”

“…The development and application of the ab initio nonadiabatic molecular dynamics (NAMD) method in recent years makes it possible to study the carrier lifetime in solid systems using a first-principles method. [40][41][42][43][44][45][46] Scientists have studied how the defects and impurities affect the photoexcited carrier lifetime in different semiconductors. 9,10,15,16,[47][48][49][50][51][52] Concerning TiO 2 , Long et al have studied how the impurity affects the carrier lifetime in N-doped anatase TiO 2 .…”

“…49 Furthermore, we revealed that photoexcitation can generate small polarons in rutile TiO 2 . 42 The hopping of such small polarons distinctly accelerates the e-h recombination. As discussed above, besides the photoexcitation, the excess charge can also introduce small polarons in rutile TiO 2 , and an O V is a general existing n-type defect in rutile TiO 2 which can provide two excess electrons.…”

Polaron in TiO2 from First‐Principles: A Review

“…Rutile TiO 2 was very recently investigated by Zhang et al. [ 73 ] who focused on the temperature‐dependent excess electrons trapped at 100, 300, and 700 K, using AIMD with Hubbard on‐site potential of 4.5 eV. Their results suggest that the polaron formation time scale does not significantly depend on the temperature.…”

“…According to Zhang et al. [ 73 ] reduction of charge‐carrier lifetime attribute to increasing the overlap of orbitals between valence band maximum (VBM) and polaron due to increase hopping mobility and delocalization at high‐temperature. Consequently, strengthening the nonadiabatic coupling between VBM and polaron accelerates the electron‐hole recombination.…”