Organic-inorganic metal-halide perovskites have attracted great attention in recent years due to their remarkable semiconductor properties. While great advances have been made towards understanding dynamics under steady-state conditions, the importance of non-equilibrium phenomena and their effect on device performances remains elusive.
To provide experimental access to the unexplored regime of spatiotemporal dynamics occurring on ultrafast timescales, we combined the extreme temporal resolution provided by ultrafast spectroscopy with the nm-level localisation capabilities of optical microscopes. We focused our investigation on the spatial carrier dynamics of well characterised methylammonium lead iodide system (MAPI3-xClx).
Intriguingly, the consecutive fs-TAM images reveal a pronounced spatial expansion of the carrier distribution within a few tens of fs. The mean-squared-displacement (MSD) profile of the non-equilibrium carriers grows non-linearly, in contrast to the linear behaviour expected for normal diffusion. Further, the MSD profile is well described by a power law fit, signifying that the non-equilibrium carriers in perovskite thin films propagate in a ballistic manner during the initial 20 fs. In addition, the linear fit reveals the ballistic transport length of 153 nm. T ballistic transport velocity of 7.5 ×106 ms-1 is in approximate agreement with the group velocity of electrons within the conduction band as modelled by density functional theory. This implies that photogenerated carriers propagate as coherent wavepackets and have a non-interacting nature at the earliest times following photon absorption. Our results suggest that at least ~25% of carriers generated in a typical perovskite PV device reach the charge collection layers ballistically