Micro-Heterogeneous Annihilation Dynamics of Self-Trapped Excitons in Hematite Single Crystals

Abstract

The Auger recombination in bulk semiconductors can quickly depopulate the charge carriers in a nonradiative way, which, fortunately, only has a detrimental impact on optoelectronic device performance under the condition of high carrier density because the restriction arising from concurrent momentum and energy conservation limits the Auger rate. Here, we surprisingly observed enhanced Auger recombination in an α-Fe2O3 single crystal, a wide bandgap semiconductor with low carrier mobility. The Auger process was ascribed to the Coulombically coupled self-trapped excitons (STEs), and the relaxation of momentum conservation due to the strong spatial localization of these STEs should account for the enhancement. The STE-density dependent kinetics suggested that the strong polaronic effect could cause a micro-heterogeneous distribution of STEs in a high-quality bulk single crystal, which also gave rise to the micro-heterogeneous annihilation dynamics, and a stochastic recombination model was developed and successfully described the STE annihilation dynamics.

Barrierless Self-Trapping of Photocarriers in Co3O4

“…The fast decay of the PIA may reflect the decrease in the absorption coefficient due to hot carrier cooling, ,, which however is ruled out because such a phenomenon is also observed by pumping the sample at the absorption-edge so that hot carriers are absent (Figure S3). As demonstrated previously, the free carriers have a greater absorption coefficient than the localized carriers, especially in the NIR and IR regions, ,, and thus, we attribute the rapid PIA decline to the free-carrier localization. The residual PIA signals at long delays then correspond to the optical transitions from localized states to higher energy states. , Further evidence will be presented later to validate this attribution.…”

“…The self-trapping of free carriers has detrimental impacts on the optoelectronic applications because this process suppresses the carrier mobility, ,,− enhances the nonradiative recombination, − and reduces chemical potential. , While in TMOs the small polarons and the associated insufficient characteristics are well-accepted, the self-trapping mechanism of photocarriers is still not well-understood. Here, we investigated the self-trapping of photocarriers in Co 3 O 4 epitaxial monocrystalline thin films by using transient absorption (TA) and time-resolved terahertz (TR-THz) spectroscopies.…”

Recombination of Polaronic Electron–Hole Pairs in Hematite Determined by Nuclear Quantum Tunneling

“…Transition metal oxide semiconductors are widely utilized in photovoltaic applications, such as solar cells and photoelectrochemical cells, and the lifetime of photocarriers in the oxides is the key factor to the energy conversion efficiency. To gain insight into the photocarrier lifetime, the carrier recombination mechanisms are the subject of considerable experimental and theoretical study. − Compared with high-mobility semiconductors, the formation of small polarons is ubiquitous in metal oxides because of the ionic bonding character, − which not only reduces the carrier mobility − but also substantially complicates the carrier recombination mechanisms. , For instance, the photocarrier recombination in hematite ( -Fe 2 O 3 ) is dominated by an intrinsic nonradiative channel under low excitation intensity, which is independent of the defect concentration . Under high excitation intensity, the Auger process in -Fe 2 O 3 leads to an annihilation fashion distinct from that in high-mobility semiconductors .…”

“…To gain insight into the photocarrier lifetime, the carrier recombination mechanisms are the subject of considerable experimental and theoretical study. − Compared with high-mobility semiconductors, the formation of small polarons is ubiquitous in metal oxides because of the ionic bonding character, − which not only reduces the carrier mobility − but also substantially complicates the carrier recombination mechanisms. , For instance, the photocarrier recombination in hematite ( -Fe 2 O 3 ) is dominated by an intrinsic nonradiative channel under low excitation intensity, which is independent of the defect concentration . Under high excitation intensity, the Auger process in -Fe 2 O 3 leads to an annihilation fashion distinct from that in high-mobility semiconductors . Thus, the quantitative description of carrier recombination in metal oxides is a problem of both great fundamental and technical importance, but the strong polaronic effect may render the theoretic model mostly developed for high-mobility semiconductors insufficient.…”

“…As -Fe 2 O 3 is not only an important material for solar-driven water splitting but also an ideal platform for the small polaron studies, ,,− herein, we focused on the recombination mechanism in -Fe 2 O 3 under weak excitation. Because of the strongly localized nature, polaronic electron–hole pairs, the predominant photogenerated species, were treated as pseudocomplexes in a solid medium, and thus, we were able to employ a mathematical equation developed previously for the F-center transition to quantitively describe the experimentally measured nonradiative recombination rate in -Fe 2 O 3 .…”