Time resolved terahertz spectroscopy
Terahertz (THz) waves are long wavelength infrared light that are sensitive to electron or charge carrier motions. We use pump-probe techniques to probe carrier dynamics. The pump pulses excite charge carriers in a material and the charge carriers interact with the THz probe pulses. The transmission of the THz pulse is sensitive to material’s conductivity, which is determined by the density of charge carriers and their mobility. By monitoring the transmission of THz pulses as a function of time and frequency, we gain information on how long carriers stays in the excited state and how mobile these carriers are. Combining THz spectroscopy with other optical pump-probe techniques, we learn about the charge carrier generation and transport dynamics in materials. Examples of materials we have studied include: hyperdoped silicon for advanced photovoltaic applications, hybrid perovskites as efficient broadband light emitters, and tin sulfide earth abundant solar cell materials.
We use ultrafast lasers to generate short THz pulses. By propagating light pulses through different paths, the speed of light and the path length differences determine the difference of arrival times between the pump and the probe pulses. The THz radiation we generate is short in time and broad in frequency spectrum. This allows us to achieve sub-picosecond time resolution in studying carrier dynamics and also to probe a broad frequency response in charge carrier motions.
Carrier lifetime in solar cells
We use time resolved THz spectroscopy to study carrier lifetime. It’s a non-contact conductivity probe with sub-picosecond time resolution. For a solar cell to be efficient, excited carrier lifetime needs to be sufficient long so charge can be extracted from the material. The figure below shows carrier lifetime in thin film solar cells and their cell efficiencies. Bulk silicon is included for comparison. For emerging solar cell materials in early stage development, information on how different processes improve carrier lifetime or passivate defected surfaces can guide further material development.
For example, we studied sulfur hyperdoped silicon. Lower left figure shows density of carriers as a function of time after absorbing a pump light pulse. The series of curves represent materials processed under different conditions. With information on carrier lifetimes, we calculated the figure of merit and identified optimized material parameter.
In addition to carrier lifetime, charge carrier mobility determines the dynamics of carrier motions. How do defects, disorders, and grain boundaries affect charge carrier motions? For example, we used THz spectroscopy to probe local carrier mobility in polymer organic solar cells and found that local carrier mobility is orders of magnitude higher than long-range carrier mobility. In addition, material properties revealed by THz spectroscopy and optical pump-probe techniques are important in many different material systems, such as phase change, ferromagnetic, ferroelectric, and two-dimensional materials.