  Efficient charge separation at the interfaces between the perovskite and with
the carrier transport layers is crucial for perovskite solar cells to achieve
high power conversion efficiency. We systematically investigate the hole
injection dynamics from MAPbI$_3$ perovskite to three typical hole transport
materials (HTMs) PEDOT:PSS, PTAA and NiO$_x$ by means of pump-probe
transmission measurements. We photoexcite only near the MAPbI$_3$/HTM interface
or near the back surface, and measure the differential transient transmission
between the two excitation configurations to extract the carrier dynamics
directly related to the hole injection. The differential transmission signals
directly monitor the hole injections to PTAA and PEDOT:PSS being complete
within 1 and 2 ps, respectively, and that to NiO$_x$ exhibiting an additional
slow process of 40 ps time scale. The obtained injection dynamics are discussed
in comparison with the device performance of the solar cells containing the
same MAPbI$_3$/HTM interfaces.
