  The dynamics of photo-excited charge carriers, particularly their transport
and interactions with defects and interfaces, play an essential role in
determining the performance of a wide range of solar and optoelectronic
devices. A thorough understanding of these processes requires tracking the
motion of photocarriers in both space and time simultaneously with extremely
high resolutions, which poses a significant challenge for previously developed
techniques, mostly based on ultrafast optical spectroscopy. Scanning ultrafast
electron microscopy (SUEM) is a recently developed photon-pump-electron-probe
technique that combines the spatial resolution of scanning electron microscopes
(SEM) and the temporal resolution of femtosecond ultrafast lasers. Despite many
recent excellent reviews for the ultrafast electron microscopy, we dedicate
this article specifically to SUEM, where we review the working principle and
contrast mechanisms of SUEM in the secondary-electron-detection mode from a
users' perspective and discuss the applications of SUEM to directly image
photocarrier dynamics in various materials. Furthermore, we propose future
theoretical and experimental directions for better understanding and fully
utilizing the SUEM measurements to obtain detailed information about the
dynamics of photocarriers. To conclude, we envision the potential of expanding
SUEM into a versatile platform for probing photophysical processes beyond
photocarrier dynamics.

Materials Science