Influence of the Energy Level Alignment on Charge Transfer and Recombination at the Monolayer-MoS2/Organic Hybrid Interface

Abstract

Monolayer (ML) transition-metal dichalcogenides (TMDCs) exhibit numerous unique optoelectronic features. This motivates recent efforts to combine TMDCs with organic semiconductors to form heterostructures with tailorable properties that feature the advantages of both materials. Here, we study the photoinduced charge transfer across hybrid interfaces of ML-MoS2 and a series of organic semiconductors─often used as hole transport materials─where we systematically tune the offsets of the frontier energy levels. Steady-state photoluminescence and ultrafast transient absorption spectroscopy reveal that a larger energy level offset causes a lower efficiency of photoinduced charge transfer but also a longer lifetime of the charge separated state. Both observations are explained in the framework of Marcus’ theory of electron transfer. In fact, our observations question direct electron–hole recombination across the hybrid interface as the main decay pathway for photogenerated carriers in the considered systems. Instead, back transfer of holes to ML-MoS2 is suggested as the key decay channel. Adding a 1 nm LiF interlayer causes a significant slowdown of interfacial carrier recombination while not suppressing free carrier formation. This strategy serves as a guideline for optimizing further hybrid systems toward high-performance ML-TMDC/organic-based optoelectronic devices.