The performance of semiconductor optoelectronic devices depends not only on the intrinsic material properties of the active material but also on the applied electrochemical bias, as charge injection, extraction, and interfacial charge transfer processes strongly shape carrier dynamics under realistic operating conditions. Performing transient absorption spectroscopy under electrochemical control (EC-TAS) enables direct tracking of ultrafast excited-state dynamics while external bias is applied to the system. By combining femtosecond pump–probe spectroscopy with potentiostatic control, we can follow how the changes in the electric field influence trap filling and recombination pathways on the sub-nanosecond timescales.

Ultrafast spectroelectrochemical measurements are carried out at the ELI ALPS Research Institute using the HR1 (high repetition rate) laser-driven TAS setup. The system provides 10 fs pump pulses at 515 nm central wavelength (SHG of 1030 nm fundamental) at 100 kHz with adjustable excitation power between 5 and 50 mW. Under applied electrochemical bias in the range of −0.4 V to +1.2 V vs Ag/AgCl, we monitor TAS signals and directly observe how exciton decay pathways evolve with potential.

We use 2D transition-metal dichalcogenides (e.g., MoS₂ and WSe₂) as model systems that exhibit strong excitonic effects and layer-dependent optoelectronic properties. These materials are promising candidates as active materials in photodetectors, photovoltaics, and photoelectrodes. In these devices, interfacial processes and carrier lifetime critically determine performance. The carrier dynamics of the thin film electrodes were evaluated in different electrolytes (e.g., Bu₄NPF₆ and LiPF₆ acetonitrile) to examine how the ionic environment, in particular the presence of Li⁺ ions, modifies interfacial electric fields, trap occupation, and bias-dependent carrier dynamics under working conditions.

We systematically investigate the transient response and observe clear bias-driven modifications in exciton decay dynamics. Potential-dependent measurements reveal systematic, reversible changes in the extracted lifetime and the relative contributions of decay components. Charge–discharge (CDC) experiments confirm the stability and reproducibility of these electrochemically induced variations in both lifetime and component amplitudes, demonstrating that the excited-state dynamics can be permanently and reversibly altered in these systems. Excitation-fluence-dependent measurements further show that the decay lifetimes and relative pathway contributions depend on the excitation density. We further examine how these trends evolve, demonstrating that both the ionic environment and material-dependent electronic structure significantly influence interfacial trap states and ultrafast carrier dynamics. Overall, this work establishes EC-TAS as a powerful platform for probing and actively tuning ultrafast carrier dynamics under realistic operating conditions, providing direct insight into bias-controlled interfacial processes relevant to optoelectronic device performance.