Probing carrier and phonon dynamics in halide perovskites using ultrafast terahertz spectroscopy
Date of Issue2018-08-02
School of Physical and Mathematical Sciences
The development of photovoltaic devices based on semiconductors requires a clear understanding of charge-carrier and phonon dynamics on an ultrashort time scale after a photoexcitation. Transport processes are related to lattice vibration modes via carrier–phonon coupling, which determines the unique properties of the material. As a promising next-generation solar cell material, lead halide perovskites show excellent photovoltaic performances, which triggered a flurry of intense research. Terahertz (THz) radiation can be absorbed by free charge carriers, therefore is suitable to probe the transport properties with a contact-free manner. Moreover, the low-lying optical phonon modes lie in THz range as well. Therefore, terahertz time-domain spectroscopy (THz-TDS), especially the time-resolved terahertz spectroscopy (TRTS), is an excellent tool for probing carrier and phonon dynamics in halide perovskites. Three work will be presented. (1) Temperature evolution of low-frequency phonon modes in MAPbBr3 is performed over a range of 20–300 K. We observed phonon mode splitting at the orthorhombic–tetragonal phase transition temperature. Furthermore, both transverse-optical and longitudinal-optical (LO) phonon modes at 300 K are identified from the THz-TDS spectra. We further use these LO phonon mode frequencies to calculate carrier mobility using Feynman’s polaron theory. (2) TRTS study on FA0.85Cs0.15Pb(I0.97Br0.03)3 thin films shows that upon the photoexcitation, both photo-generated charge carriers and photo-induced change in optical phonon modes contribute to the photoconductivity. We observe a delicate interplay between carriers, phonons and excitons that govern the photo-physical phenomena of the perovskites. Our data show that the combination of phonon emission during carrier relaxation, phonon-assisted exciton dissociation and photo-enhanced carrier screening, ensure a large and long-lived free-carrier population in the system, especially at room temperature. (3) Organic-ligand treated and pristine MAPbI3-xClx thin films are comparatively studied by TRTS. The dynamics of photo-generated carriers in both thin films are investigated, and modelled by the Drude model. Both samples show high carrier mobility (100s cm2V−1s−1) and long diffusion length (several microns).