  The outstanding performance of organic-inorganic metal trihalide solar cells
benefits from the exceptional photo-physical properties of both electrons and
holes in the material. Here, we directly probe the free-carrier dynamics in
Cs-doped FAPbI3 thin films by spatiotemporal photoconductivity imaging. Using
charge transport layers to selectively quench one type of carriers, we show
that the two relaxation times on the order of 1 microsecond and 10 microseconds
correspond to the lifetimes of electrons and holes in FACsPbI3, respectively.
Strikingly, the diffusion mapping indicates that the difference in
electron/hole lifetimes is largely compensated by their disparate mobility.
Consequently, the long diffusion lengths (3 ~ 5 micrometers) of both carriers
are comparable to each other, a feature closely related to the unique charge
trapping and de-trapping processes in hybrid trihalide perovskites. Our results
unveil the origin of superior diffusion dynamics in this material, crucially
important for solar-cell applications.
