  Optically dark excitons determine a wide range of properties of photoexcited
semiconductors yet are hard to access via conventional spectroscopies. Here, we
develop a time-resolved ultrafast photocurrent technique (trPC) to probe the
formation dynamics of optically dark excitons. The nonlinear nature of the trPC
makes it particularly sensitive to the formation of excitons occurring at the
femtosecond timescale after the excitation. As proof of principle, we extract
the interlayer exciton formation time 0.4~ps at 160 $\mu$J/cm$^2$ fluence in a
MoS$_2$/MoSe$_2$ heterostructure and show that this time decreases with
fluence. In addition, our approach provides access to the dynamics of carriers
and their interlayer transport. Overall, our work establishes trPC as a
technique to study dark excitons in various systems that are hard to probe by
other approaches.
