Time-resolved Element-selective Probing of Charge Carriers in Solar Materials.

We review our recent results on the implementation of picosecond (ps) X-ray absorption spectroscopy to probe the electronic and geometric structure of centres formed by photoexcitation of solar materials such as TiO2 polymorphs and inorganic Cs-based perovskites. The results show electron localization at Ti defects in TiO2 anatase and rutile and small hole polaron formation in the valence band of CsPbBr3, all within 80 ps. This method is promising for the study of the ultrafast time scales of such processes, especially with the advent of the Swiss X-ray Free Electron Laser (SwissFEL).

Transient mid-IR study of electron dynamics in TiO2 conduction band.

The dynamics of TiO2 conduction band electrons were followed with a novel broadband synchrotron-based transient mid-IR spectroscopy setup. The lifetime of conduction band electrons was found to be dependent on the injection method used. Direct band gap excitation results in a lifetime of 2.5 ns, whereas indirect excitation at 532 nm via Ru-N719 dye followed by injection from the dye into TiO2 results in a lifetime of 5.9 ns.

Revealing hole trapping in zinc oxide nanoparticles by time-resolved X-ray spectroscopy.

Nanostructures of transition metal oxides, such as zinc oxide, have attracted considerable interest for solar-energy conversion and photocatalysis. Both applications are sensitive to the transport and trapping of photoexcited charge carriers. The probing of electron trapping has recently become possible using time-resolved element-sensitive methods, such as X-ray spectroscopy. However, valence-band-trapped holes have so far escaped observation. Herein we use X-ray absorption spectroscopy combined with a dispersive X-ray emission spectrometer to probe the charge carrier relaxation and trapping processes in zinc oxide nanoparticles after above band-gap photoexcitation. Our results, supported by simulations, demonstrate that within 80 ps, photoexcited holes are trapped at singly charged oxygen vacancies, which causes an outward displacement by ~15% of the four surrounding zinc atoms away from the doubly charged vacancy. This identification of the hole traps provides insight for future developments of transition metal oxide-based nanodevices.

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy.

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Hunting for the elusive shallow traps in TiO2 anatase.

Understanding electron mobility on TiO2 is crucial because of its applications in photocatalysis and solar cells. This work shows that shallow traps believed to be involved in electron migration in TiO2 conduction band are formed upon band gap excitation, i.e., are not pre-existing states. The shallow traps in TiO2 results from large polarons and are not restricted to surface.