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 (NCs) 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 are 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. These results can explain the rather modest charge carrier mobilities in
these materials.Comment: 19 pages, 3 figure