Mechanisms of Ultrafast Charge Separation in a PTB7/Monolayer MoS2 van der Waals Heterojunction

TL;DR

This study investigates ultrafast charge separation mechanisms at a PTB7/monolayer MoS2 heterojunction, revealing sub-picosecond electron transfer and insights into optimizing polymer-2D semiconductor devices for optoelectronics.

Contribution

It provides a detailed analysis of the time-dependent photophysical processes at PTB7/MoS2 interfaces, highlighting ultrafast charge transfer pathways and their implications for device performance.

Findings

Electron transfer from PTB7 to MoS2 occurs in less than 250 fs.

Charge transfer yield from PTB7 to MoS2 is approximately 58%.

Photogenerated carriers have a lifetime of 3-4 ns limited by MoS2 defects.

Abstract

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.