Extended Time-Resolved Terahertz Spectroscopy for Photovoltaic Material Analysis

Time-resolved terahertz spectroscopy (TRTS) is a crucial technique to study the lifetime and charge carrier dynamics in materials suitable for photovoltaic applications. This technique, utilizing THz probes generated by fs lasers, provides insights into charge carrier dynamics, including lifetimes and recombination processes, with picosecond resolution. In TRTS, an optical laser excites the sample while THz waves probe its transient conductivity. Although perovskites and organic photovoltaics are known for their long carrier lifetimes, as reported in published literature, the TRTS's capability to study these materials is traditionally constrained to a few nanoseconds due to physical stage limitations. Our current setup includes a pump source with interchangeable laser diodes and electronic synchronization between the pump and probe, extending the observable timeframe from 2 nanoseconds to hundreds of microseconds, with a probe resolution of 30 microseconds. A wider observation window helps analyze the dynamics of these materials, allowing us to probe various depths with the interchangeable diode laser. This enables a detailed investigation of decay dynamics and differentiation between surface and bulk recombination processes through transient pump-probe measurements. By extending the temporal window, we can capture longer-lived processes and obtain a more comprehensive understanding of the material's behavior over time, including how charge carriers move and recombine in different regions of the sample. We utilize transient pump-probe measurements and frequency-dependent analysis to monitor THz signal changes over time and frequency, applying the Drude model to derive parameters like scattering time. By adjusting the laser spot size and the duty cycle of the pump diode laser, we enhance detection signals. Our steady-state measurements also address materials with shorter lifetimes by illuminating the sample. This balances carrier generation and recombination rates, allowing us to study carrier dynamics even below the minimum resolution limit of transient techniques. We are currently focused on conducting further measurements on a silicon wafer, for which we have already generated reference data for parameters such as lifetime and mobility to match with published information. Our next step is to extend these studies to perovskite and organic photovoltaic materials. However, we recognize that there are resolution limitations in our current setup, and we are considering substituting the diode laser with an Nd: YAG nanosecond laser. This modification would enable us to study charge carrier dynamics and overall recombination behavior in photovoltaic materials with longer lifetimes, achieving nanosecond resolution within the hundreds of microseconds observation window.