Delocalized Impurity Phonon Induced Electron–Hole Recombination in Doped Semiconductors

Abstract

Semiconductor doping is often proposed as an effective route to improving the solar energy conversion efficiency by engineering the band gap; however, it may also introduce electron-hole (e-h) recombination centers, where the determining element for e-h recombination is still unclear. Taking doped TiO as a prototype system and by using time domain ab initio nonadiabatic molecular dynamics, we find that the localization of impurity-phonon modes (IPMs) is the key parameter to determine the e-h recombination time scale. Noncompensated charge doping introduces delocalized impurity-phonon modes that induce ultrafast e-h recombination within several picoseconds. However, the recombination can be largely suppressed using charge-compensated light-mass dopants due to the localization of their IPMs. For different doping systems, the e-h recombination time is shown to depend exponentially on the IPM localization. We propose that the observation that delocalized IPMs can induce fast e-h recombination is broadly applicable and can be used in the design and synthesis of functional semiconductors with optimal dopant control.

Ab initio nonadiabatic molecular dynamics investigations on the excited carriers in condensed matter systems

“…In this case the NAMD simulation in several ps can barely obtain the accurate e–h recombination timescale. It can be estimated to be around 0.5 ns by fitting the energy decay curve in the first 4 ps using an exponential decay function . Such a recombination timescale in nanosecond magnitude is in agreement with previous studies qualitatively .…”

“…Besides the charge transfer dynamics at different interfaces, ab initio NAMD simulations can also be used to investigate the photogenerated e–h recombination dynamics. Zhang et al chose a wide band gap semiconductor TiO 2 , which is one of the most intensively studied photocatalyst, as a prototypical system to study how the cation and anion doping affect the e–h recombination rate . Different cation doping in wide band gap semiconductors is proposed to be an effective route to reduce the band gap, improve the photon energy absorption and enhance the solar energy conversion efficiency .…”

“…However, it may also introduce e–h recombination centers which will affect the lifetimes of the photogenerated electrons and holes. Based on NAMD simulations, Zhang et al proposed that the localization of impurity‐phonon modes (IPMs) is the key factor for the e–h recombination rate …”

“…(g–i) The averaged NAC elements in undoped, CrN‐ and VN‐doped TiO 2 at 300 K. The inset in (b) shows the spatial distribution of the excess charge induced by CrN codoping, in which the Ti, O, Cr and N atoms are marked by large light blue, small red, large deep blue and small purple balls, respectively. (Reprinted with permission from Reference . Copyright 2018 American Chemical Society)…”

“…A typical TiO 2 bulk phonon mode in CrN‐doped TiO 2 is marked by a purple arrow. (Reprinted with permission from Reference . Copyright 2018 American Chemical Society)…”