Exciton Dissociation, Charge Transfer, and Exciton Trapping at the MoS2/Organic Semiconductor Interface

Hybrid inorganic–organic semiconducting devices consisting of monolayer transition metal dichalcogenides (TMDs) represent a new frontier in advanced optoelectronics due to their high radiative efficiencies and capacity to form flexible p–n junctions with inherent device tunability. However, understanding how excitons and charges behave at the interface between TMDs and organic systems, a key requirement to advance the field, remains underexplored. Herein, a heterostructure consisting of a highly conjugated organic system, 9-(2-naphthyl)-10-[4-(1-naphthyl)phenyl]anthracene (ANNP), and monolayer molybdenum disulfide (MoS2) on quartz is elucidated via transient absorption and photoluminescence spectroscopies. Upon direct excitation of MoS2 at 532 nm, hole transfer to ANNP of ∼5 ps and a charge separation time constant of ∼2.4 ns are observed. When the sample is excited at 400 nm (where both ANNP and MoS2 absorb), a self-trapped exciton within ANNP is formed. The emission of the self-trapped exciton is long-lived compared to the exciton lifetime of ANNP, decaying within 20 ns. The trapping of the ANNP exciton is caused by structural deformities of the ANNP crystal lattice when grown on MoS2, which are removed by annealing the film. These observations highlight how exciton dissociation and charge transfer dominate at the interface of ANNP and MoS2 whereas the exciton dynamics within ANNP are prone to the formation of trap states brought about by crystal defects within the film. These insights will aid in future developments of TMD-containing optoelectronics.