2D transition metal dichalcogenides are an attractive family of materials in the field of electronics and optoelectronics. They are excellent candidates for sensitive photodetectors, light harvesting devices (photovoltaics, photocatalysts or photoelectrodes), lasers or non-linear optical devices. All these applications rely on light-matter interactions, which processes will ultimately decide the efficiency of the derived devices. Understanding carrier dynamics and the interaction of various exotic quasi-particles (excitons, trions) in these materials is of paramount importance to design better performing devices. Pump-probe transient absorption/reflection spectroscopy (TAS/TRS) is a powerful technique to study photophysical processes involved in charge carrier generation and recombination on the ultrafast timescale. In MoS₂ photoelectrodes after light excitation different types of excitons are generated, and their decay can be monitored with these techniques. Coupling electrochemical techniques with these ultrafast methods, allows to probe the decay of the excited state in these materials under working conditions. In this manner the effect of trap state filling [1] or the effect of charge extraction [2] on the charge carrier dynamics of photoelectrochemical systems can be revealed. In my presentation I will show ultrafast spectroelectrochemical measurements on ITO/MoS₂ photoelectrodes and reveal how the decay of excitons are influenced by the applied electrochemical bias. By comparing results from TAS/TRS spectroscopy the separation of carrier dynamics on the surface and bulk of these systems can be performed. These measurements reveal that the dissociation of excitons occurs at the ITO/MoS₂ interface, resulting in a long living exciton population on the surface of these samples. Charging/discharging studies carried out in these systems reveal that the trap states involved in the dissociation of excitons in these systems can be permanently filled. The electrochemical filling of these trap states allows the tuning of the excited state lifetime of these systems, which can aid the better design of photoelectrochemical devices based on MoS₂. References [1] ACS Energy Lett. 2019, 4, 3, 702–708 [2] J. Am. Chem. Soc. 2018, 140, 1, 86–89