Ultrafast spectroscopy is enabling the study of carrier dynamics at femtosecond timescales, providing insights into energy transfer mechanisms in novel semiconductors like perovskites and transition metal dichalcogenides (TMDs). Recent experiments using pump-probe spectroscopy have revealed carrier relaxation times as short as 100 fs in methylammonium lead iodide (MAPbI3), with recombination rates varying between 10^9 and 10^12 s^-1 depending on material quality. These findings are critical for optimizing photovoltaic efficiency, which has already surpassed 25% in perovskite-based solar cells.
Time-resolved terahertz spectroscopy (TRTS) has emerged as a powerful tool for probing carrier mobility in TMDs such as MoS2 and WS2. Studies have shown mobilities exceeding 200 cm^2/Vs at room temperature, with scattering times on the order of picoseconds. TRTS has also revealed anisotropic carrier transport properties in these materials, which are essential for designing high-performance field-effect transistors (FETs). For example, MoS2 FETs fabricated using TRTS-optimized layers exhibit on/off ratios greater than 10^8 and subthreshold swings below 70 mV/decade.
Ultrafast spectroscopy is also being used to investigate exciton dynamics in two-dimensional (2D) semiconductors. In monolayer WSe2, exciton lifetimes have been measured at approximately 1 ps, with binding energies exceeding 500 meV due to strong Coulomb interactions. These properties make WSe2 a promising candidate for optoelectronic applications such as light-emitting diodes (LEDs) and photodetectors. Recent work has demonstrated external quantum efficiencies above 80% in WSe2-based LEDs using ultrafast spectroscopy-guided design principles.
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