Exciton Dynamics in MoS₂-Pentacene and WSe₂-Pentacene Heterojunctions

We measured the exciton dynamics in van der Waals heterojunctions of transition metal dichalcogenides (TMDCs) and organic semiconductors (OSs). TMDCs and OSs are semiconducting materials with rich and highly diverse optical and electronic properties. Their heterostructures, exhibiting van der Waals bonding at their interfaces, can be utilized in the field of optoelectronics and photovoltaics. Two types of heterojunctions, MoS₂-pentacene and WSe₂-pentacene, were prepared by layer transfer of 20 nm pentacene thin films as well as MoS₂ and WSe₂ monolayer crystals onto Au surfaces. The samples were studied by means of transient absorption spectroscopy in the reflectance mode. We found that A-exciton decay by hole transfer from MoS₂ to pentacene occurs with a characteristic time of 21 ± 3 ps. This is slow compared to previously reported hole transfer times of 6.7 ps in MoS₂-pentacene junctions formed by vapor deposition of pentacene molecules onto MoS₂ on SiO₂. The B-exciton decay in WSe₂ shows faster hole transfer rates for WSe₂-pentacene heterojunctions, with a characteristic time of 7 ± 1 ps. The A-exciton in WSe₂ also decays faster due to the presence of a pentacene overlayer; however, fitting the decay traces did not allow for the unambiguous assignment of the associated decay time. Our work provides important insights into excitonic dynamics in the growing field of TMDC-OS heterojunctions.