Ultrafast carrier dynamics in metal-halide perovskites by terahertz spectroscopy

Solution-processed metal-halide perovskites (MHPs) have gained tremendous research attention in recent years owing to their superior photovoltaic performance with a certified solar-to-electricity power conversion efficiency over 25%. Employing ultrafast terahertz time-domain spectroscopy, this thesis investigates the photoexcited charge carrier dynamics and charge transport in newly developed MHPs, focusing on all-inorganic black γ-phase CsPbI₃ and lead-free double perovskite Cs₂AgBiBr₆. We first unveil that large polarons form in black γ-phase CsPbI₃ polycrystalline thin films with a high local mobility up to 270 ± 44 cm²V^-1s^-1, and polaron-longitudinal optical phonon interaction governs the charge transport. With further increasing the photon injection, we conduct the first experimental quantification of the Mott polaron density (~10¹⁸ cm^-3) in lead halide perovskites which is in line with theoretical estimations based on the Feynman polaron model. This effect is found to be universal and independent of the constituted cations and anions, representing an intrinsic property of lead halide perovskites. Above the Mott density, excess photoinjected carriers annihilate quickly within tens to hundreds of ps to form a stable, long-lived Mott polaron state. In the lead-free double perovskite Cs₂AgBiBr₆, we reveal excess energy-dependent highly conductive hot carriers, originating primarily from the quasi-ballistic transport of hot holes. By constructing Cs₂AgBiBr₆ double perovskite/graphene heterostructure, we achieve an optical control over hole transfer in both direction and efficiency. These fundamental insights play a vital role in determining and optimizing the efficiency of optoelectronics of the new generation perovskites.