The Role of Monolayer/Multilayer Homojunctions for Photoinduced Charge Generation in Metal Chalcogenide Heterostructures

Abstract

Increasing the lifetimes of photoexcited charge carriers in two-dimensional transition metal dichalcogenides (2D-TMDCs) is an important goal for realizing efficient TMDC-based optoelectronic and photoelectrochemical devices. However, the path toward sustaining sufficiently long-lived carriers in high yields is not yet well defined. A promising strategy for overcoming ultrafast decay of the tightly bound excitons inherent to these materials is charge separation using energy level offsets at heterojunction interfaces. For example, in this study we separate charge using a model Type-II heterojunction with single-walled carbon nanotubes as the electron donor and MoS2 as the acceptor. This system can sustain charge past 10 µs (orders of magnitude longer than other reported TMDC-based heterojunctions), but the carrier kinetics vary depending on the TMDC microstructure. In particular, we observe longer-lived charge carriers in monolayer TMDCs containing a small fraction of multilayer islands. This highlights the challenge of optimizing TMDC-based heterostructures while TMDC synthetic methods are still developing, where chemical or structural (i.e. thickness) variations are often evident across neat TMDC substrates. Although obtaining TMDC thickness uniformity across wafer scales is a highly sought-after goal, here we propose that a small fraction of multilayer sites can be beneficial for extending carrier lifetimes. We discuss the photophysical processes that occur at monolayer/multilayer TMDC homojunctions, and we present transient absorption data where we use well-defined carbon nanotube spectral signatures to quantify carrier generation in various monolayer-only or monolayer-rich TMDC heterostructures. Our results provide valuable insight for understanding the role of thickness variation in TMDC dynamic photophysical processes.