Mechanisms of Ultrafast Charge Separation in a PTB7/Monolayer MoS van der Waals Heterojunction.

Mixed-dimensional van der Waals heterojunctions comprising polymer and two-dimensional (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 Letter reports a systematic exploration of the time-dependent photophysical processes that occur upon photoexcitation of a type-II heterojunction between the polymer PTB7 and monolayer MoS. In particular, photoinduced electron transfer from PTB7 to electronically hot states of MoS occurs in less than 250 fs. This process is followed by a 1-5 ps exciton diffusion-limited electron transfer from PTB7 to MoS and a sub-3 ps photoinduced hole transfer from MoS 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 probably limited by MoS defects.