  Organometal halide perovskite has emerged as a promising material for solar
cells and optoelectronics. Although the long diffusion length of
photo-generated carriers is believed to be a critical factor responsible for
the material's high efficiency in solar cells, a direct study of carrier
transport over long distances in organometal halide perovskites is still
lacking. We fabricated highly oriented crystalline CH$_3$NH$_3$PbI$_3$
(MAPbI$_3$) thin film lateral transport devices with long channel length (~ 120
$\mu$m). By performing spatially scanned photocurrent imaging measurements with
local illumination, we directly show that the perovskite films prepared here
have very long transport lengths for photo-generated carriers, with a minority
carrier (electron) diffusion length on the order of 10 $\mu$m. Our approach of
applying scanning photocurrent microscopy to organometal halide perovskites may
be further used to elucidate the carrier transport processes and vastly
different carrier diffusion lengths (~ 100 nm to 100 $\mu$m) in different types
of organometal halide perovskites.
