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

Abstract

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.

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…Therefore, an X-ray absorption spectrum should be able to detect both localized charges (typically as small polarons at regular sites or at defects) and delocalized ones, as recently demonstrated in the case of perovskites. 274 It seems however that in the case of TiO2 (Figure 18), the signal is dominated by charge localization, which we found to occur on an ultrafast time scale of < 200 fs (Figure 18d). 272 Recently, time-resolved photoemission has been implemented to probe the interfacial dynamics in transition metal oxides.…”

Perspective: Opportunities for ultrafast science at SwissFEL

“…In a very recent work, Hill and co-workers reported the first fs magnetic RIXS after photo-doping the Mott insulator Sr 2 IrO 4 , and directly determined its magnetization dynamics 41 . Although concerning solid materials, such studies are also of interest to photovoltaics and photocatalysis when it comes to transition metal oxides 125,156 and perovskites, 157 as the bulk properties of these materials need to be nailed down, which means studying single crystals as was recently demonstrated in the case of TiO 2 158 …”

Bottleneck-Free Hot Hole Cooling in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption

“…This difference has been observed experimentally in CH 3 NH 3 PbBr 3 , in which transient hard x-ray spectroscopy showed that photoexcited electrons are delocalized between several Pb atoms but holes were likely localized on a single Br. 39 A range of computational work supports this difference in delocalization, predicting the electron polaron to be larger than the hole polaron. 35,37 The difference in size can be connected to the onset of the hot phonon bottleneck, which has been proposed to stem from spatial overlap of polarons above a critical carrier density.…”