Title: Time-Resolved Spectroscopy Study on Carrier and Exciton Dynamics in Organo-Lead Iodide Perovskites
Description: Recent discoveries of highly efficient solar cells based on methylammonium lead iodide (MAPbI3) perovskites (three dimensional, 3D, structure) attract a surge in research activity on the photo-generated carriers and how carrier and/or exciton interact in these materials. Understanding the photo-carrier dynamics and interactions as well as the nature of the trap states are crucial for elucidating the working mechanisms of perovskite solar cells. Lead iodide perovskites can also be prepared in two-dimensional (2D) structures, which are essentially self-assembled quantum wells. Questions remain on whether the photo-excited species are free carriers or excitons and how they interact and recombine. The nature of trap states and how to minimized them in these materials are also unclear. In this thesis, the carrier/ exciton interactions and the trap states in 3D and 2D lead iodide perovskites and the Auger recombination in 3D perovskites are studied with ultrafast Time-Resolved Transient Absorption (TA) Spectroscopy.

The first part of this thesis is the carrier generation and carrier/carrier interaction study in 3D MAPbI3 along with a comparative study on the exciton/carrier interaction in 2-dimentional (2D) lead iodide perovskites (Chapter 4 and 5). The major photo-generated species are charge carriers in 3D perovskites and excitons in 2D perovskites. Upon high photon energy excitation, the hot electrons and holes are created instantaneously which induce a red-shift on the band-edge optical transition in 3D perovskites while a broadening effect on the 1S exciton in 2D perovskites. The red-shift is the result of the Stark effect from the hot carriers and the broadening comes from the scattering by the carriers. The band-edge carriers in 3D perovskite recombine following two-molecular recombination at low density and Auger recombination at higher density. In 2D perovskite, we observed a blue-shift in 1S exciton transition due to the localized exciton-exciton interaction. The 6th chapter is the discussion on the below-gap trap states, depending on the dimensionality and the organic/inorganic interfaces.

We observed trap states in both 3D and 2D perovskites below the optical band-gap, and in 2D perovskites the trap states increase with the decrease of the quantum well thickness. With the help of surface sensitive UPS and temperature dependent PL measurements, we concluded the trap states localize at the “soft” organic/inorganic interfaces, which in 3D are the grain boundaries and surfaces and in 2D are the barrier/well interfaces. Aside from the TA studies on perovskites, Time-Resolved Second Harmonic Generation (TR-SHG) study on the transient electric field in neat C70 film and CuPc/C70 bilayer film are reported at the end of the thesis. TR-SHG has been applied to study the interfacial electric field generation at donor/acceptor interface but the total SHG signal may have contributions from the donor, the acceptor and the interface. All of these contributions need to be considered in order to fully understand the TR-SHG signal. With ultrafast laser excitation with ~100 fs time scale, we observed an internal E-field generated in C70 film due to charge drift and diffusion, with ~ 10 ps rise time.

For CuPc/C70 bilayer film, an additional interfacial E-field appears with a time constant of ~0.1 ps due to charge separation at the donor/acceptor interface. The E-Field induced SHG signal from these two E-fields interfere with each other giving rise to the overall SHG, which is dependent on both the probe polarization and the film thickness.