  Mixed-dimensional van der Waals heterojunctions comprising polymer and
twodimensional (2D) semiconductors have many characteristics of an ideal charge
separation interface for optoelectronic and photonic applications. However, the
photoelectron dynamics at polymer- 2D semiconductor heterojunction interfaces
are currently not sufficiently understood to guide the optimization of devices
for these applications. This manuscript is a report of a systematic exploration
of the time-dependent photophysical processes that occur upon photoexcitation
of a type-II heterojunction between the polymer PTB7 and monolayer MoS2. In
particular, photoinduced electron transfer from PTB7 to electronically hot
states of MoS2 occurs in less than 250 fs. This process is followed by a slower
(1-5 ps) exciton diffusion-limited electron transfer from PTB7 to MoS2 with a
yield of 58%, and a sub-3-ps photoinduced hole transfer from MoS2 to PTB7. The
equilibrium between excitons and polaron pairs in PTB7 determines the charge
separation yield, whereas the 3-4 ns lifetime of photogenerated carriers is
limited by MoS2 defects. Overall, this work elucidates the mechanisms of
ultrafast charge carrier dynamics at PTB7-MoS2 interfaces, which will inform
ongoing efforts to exploit polymer-2D semiconductor heterojunctions for
photovoltaic and photodetector applications.
