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

Accelerating Small Electron Polaron Dissociation and Hole Transfer at Solid-Liquid Interface for Enhanced Heterogeneous Photoreaction

In a photocatalysis process, quick charge recombination induced by small electron polarons in a photocatalyst and sluggish kinetics of hole transfer at the solid-liquid interface have greatly limited photocatalytic efficiency. Herein, we demonstrate hydrated transition metal ions as mediators that can simultaneously accelerate small electron polaron dissociation (via metal ion reduction) and hole transfer (through high-valence metal production) at the solid-liquid interface for improved photocatalytic pollutant degradation. Fe3+, by virtue of its excellent redox ability as a homogeneous mediator, enables the BiVO4 photocatalyst to achieve drastically increased photocatalytic degradation performance, up to 684 times that without Fe3+. The enhanced performance results from Fe(IV) species production (via Fe3+ oxidation) induced by dissociation of small electron polarons (via Fe3+ reduction), featuring an extremely low kinetic barrier (5.4 kJ mol-1) for oxygen atom transfer thanks to the donor-acceptor orbital interaction between Fe(IV) and organic pollutants. This work constructs a high-efficiency artificial photosynthetic system through synergistically eliminating electron localization and breaking hole transfer limitation at the solid-liquid interface for constructing high-efficiency artificial photosynthetic systems.

Correlation between Dynamics of Polaronic Photocarriers and Photoelectrochemical Performance in Mo-Doped Bismuth Vanadate

Bismuth vanadate (BiVO4) has received intense research interest due to its outstanding performance for solar water splitting, and doping it with molybdenum (Mo) ions can effectively boost photoelectrochemical performance. In this material, highly localized polarons play a key role in the photoconversion process. Herein, we uncovered the influence of Mo dopants on the dynamics of polaronic transient species using transient absorption spectroscopy. We find that the preexisting electron small polarons stemming from the thermal ionization of dopants provide additional centers to capture itinerant holes, which significantly decrease the hole lifetime. However, the introduction of dopants increases the lifetime of self-trapped excitons that arise from the binding of electron polarons and holes. The dependence of the photoelectrochemical performance of BiVO4 photoelectrodes on doping levels can be well explained by combining the dopant effects on the lifetimes of delocalized and self-trapped transient species. Our findings provide guidance for rational optimization of dopant concentration to maximize the PEC efficiency.

Broadband Emission Origin in Metal Halide Perovskites: Are Self-Trapped Excitons or Ions?

It has always been a goal to realize high efficiency and broadband emission in single-component materials. The appearance of metal halide perovskites makes it possible. Their soft lattice characteristics and significant electron-phonon coupling synergistically generate self-trapped excitons (STEs), contributing to a broadband emission with a large Stokes shift. Meanwhile, their structural/compositional diversity provides suitable active sites and coordination environments for doping of ns2 ions, allowing 3 Pn ( n =0,1,2) →1 S0 transitions toward broadband emission. The ns2 ions emission is phenomenologically similar to that of STE emission, hindering in-depth understanding of their emission origin, and leading to failure to meet the design requirements for practical applications. In this scenario, herein, the fundamentals and development of such two emission mechanisms are summarized to establish a clear and comprehensive understanding of the broadband emission phenomenon, which may pave the way to an ideal customization of broadband-emission metal halide perovskites.

Self-trapped excitons in soft semiconductors

Self-trapped excitons (STEs) have attracted tremendous attention due to their intriguing properties and potential optoelectronic applications. STEs are formed from the lattice distortion induced by the strong electron (exciton)-phonon coupling in soft semiconductors upon photoexcitation, which features in broadband photoluminescence (PL) emission spectra with a large Stokes shift. Recently, significant progress has been achieved in this field but many remain challenges that need to be solved, including the understanding of the underlying physical mechanism, tuning of the performance, and device applications. Along these lines, for the first time, systematic experimental characterizations and advanced theoretical calculations are presented in this review to shed light on the physical mechanism. The possibility of tuning the STEs through multiple degrees of freedom is also presented, along with an overview of the STE-based emerged applications and future research perspectives.

Bimolecular Self-Trapped Exciton Formation in Bismuth Vanadate

Bismuth vanadate (BiVO₄) is a promising photoanode material for solar-driven water splitting, and knowledge of the photocarrier dynamics in BiVO₄ could offer guidance to propel the development of the photoanode performance. Herein, we uncovered the nature of various photogenerated transient species in BiVO₄ and extracted their respective dynamics. We found spectral and dynamic evidence that the electrons in the conduction band collapsed into severely localized small electron polarons on a subpicosecond time scale, while the holes in the valence band remained delocalized and accounted for the photoconductivity. In the following tens to hundreds of picoseconds, the electron polaron captured the hole to form a self-trapped exciton via a bimolecular reaction mechanism, and in consequence, the hole was immobilized. Our finding suggests that exciton dissociation strategies should be taken into account in the design of the BiVO₄-based water-splitting applications in order to enhance charge transport and suppress charge recombination.

Barrierless Self-Trapping of Photocarriers in Co3O4

The self-trapping of a free carrier in transition-metal oxides can lead to a small polaron, which is responsible for the inadequate performance of the oxide-based optoelectronic applications. Thus, fundamental understanding of the self-trapping mechanism is of key importance for improving the performance of these applications. Herein, the self-trapping in Co₃O₄ epitaxial monocrystalline films is investigated primarily by transient absorption spectroscopy. The spectral evolution corresponding to the ultrafast transition from free carriers to small polarons is identified, which allows us to extract the self-trapping kinetics. The relationship between the self-trapping rate and temperature suggests a lack of thermal activation energy. A barrierless self-trapping mechanism derived from the small polaron framework is then proposed, which can successfully describe the observation that self-trapping rate decreases linearly with increasing temperature. Given that small polarons are ubiquitous in transition-metal oxides, this self-trapping mechanism is potentially a general phenomenon in these materials.

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

Here, we investigate the intrinsic nonradiative recombination mechanism in hematite single crystals that decides the photocarrier lifetime under solar illumination. On the basis of the small polaron theory, we propose that the photogenerated electron-hole pair along with its induced lattice deformation in hematite could be treated as a pseudocoordination-complex (PCC) dispersed in a solid medium. We demonstrate that the nonradiative recombination rate at different temperatures determined from the transient absorption spectroscopy can be excellently described by the nonradiative transition theory developed previously for parallels of the PCC model. Our finding suggests that at room temperature the nonradiative recombination in hematite substantially depends on the probability of quantum tunneling of the nuclear configuration.