ABSTRACT
The utilization offemtosecond laser pulses in optical spectroscopy in the 1990s provided new tools for research addressing the fastest photochemical and photo-physical reactions in photoactive materials. The high-performance of perovskite polycrystalline absorbers is the result of their greater light absorption, leading to ultra-fast and efficient formation of mobile charges, slowly recombining to a ground state. This chapter provides basic description of time-resolved pump-probe transient absorption and terahertz spectroscopies. It presents experimental results, which have helped to decipher the origin of the optical properties responsible for the high overall efficiency of perovskite-based solar cells. In femtosecond transient absorption spectroscopy, the central wavelength of excitation is adjusted to the difference between the ground and the excited electronic states in the studied system. Spectroscopic experiments have shed light on the nature and role of the trap states in perovskite thin polycrystalline films. There are a wide variety of traps sources for electrons and holes in perovskites, including elemental vacancies, substitutions, and interstitials.
