Optical-spin dynamics in organic-inorganic hybrid lead halide perovskites

• Main Author: Giovanni, David
• Other Authors: Nripan Mathews
• Format: Theses and Dissertations
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Materials; DRNTU::Science::Physics::Optics and light
• Online Access: http://hdl.handle.net/10356/72769
• Institution: Nanyang Technological University

Low temperature (< 150°C) solution-processed organic-inorganic hybrid lead halide perovskites are an emergent class of material which has attracted the full attention of the academic community. Apart from its striking optoelectronic properties and economical production cost, the chemical versatility for facile properties-tuning is its primary appeal. Recent works have demonstrated the potential of this material system for light-harvesting (i.e. solar cell) and light-emitting (e.g. LEDs, lasers, etc.) devices with ground-breaking efficiencies. Moreover, this perovskite system also exists in with various dimension of connectivity from 0D to 3D, which provides an additional platform for tunability. Researchers in the field, especially optical spectroscopists, have focused their study on understanding why lead halide perovskites work so well. Despite from the worldwide efforts to elucidate the fundamental physics behind lead halide perovskite system, it still has untapped potential. Owing to its heavy-element components, the spin-orbit coupling (SOC) in this material system is known to be huge. Since in normal condition light does not interact with electron’s spin, SOC becomes a prerequisite to optically access the spin degree of freedom in a material. Ability to optically manipulate the spin degree of freedom via SOC will enable us to realize novel opto-spintronics technology using lead halide perovskites. Herein, the spin photophysics in lead halide perovskites has not been fully explored and understood, which therefore forms the focus of the thesis. Here, time-resolved transient absorption spectroscopy was used as the main technique to explicate the transient dynamics of photoexcited carriers and spin polarization in lead halide perovskite system. Our study spans across the multi-dimensional structural phase of lead halide perovskites from 2D, Ruddlesden-Popper (RP) phase, and 3D systems. To understand the optical-spin dynamics, the first half of the thesis focused on elucidating the dynamics of the photoexcited species in the conventional lead halide perovskites. In the 3D system (i.e. CH3NH3PbI3) where the main photoexcited species are free carriers, we determined the branching ratio of multiple relaxation pathways of these photoexcited free carriers. On the other hand, in the 2D and RP phase (up to n = 4) system, we discovered that the photoexcited species is dominated by excitons. We determined the origin of their transient spectral features and investigated the relaxation mechanism of these photoexcited excitons. The second half of the study then proceeded to elucidate the optical-spin dynamics in these multi-dimensional lead halide perovskites. Our results show that the J- (or spin-) polarized carriers/excitons can be photoexcited in this multi-dimensional perovskites by using circularly polarized light. This photoexcited spin-polarization decays within the first 10 ps of the dynamics, which to some extent can be tuned by the film morphology or its dimensional phase. In the 3D system, we observed that the spin-polarized carriers relax via Elliot-Yafet mechanism, but with a weak spin-phonon coupling; and ultra-large Faraday rotation up to 10°/μm. On the other hand, in the 2D system, we observed a strong coupling between the photons and exciton spin states, which manifested in spin-selective optical Stark effect. This coupling strength can be tuned by simply varying the dielectric contrast between the organic and inorganic layers in the 2D perovskites. We also noted the peculiar behavior of the spin relaxation in the RP phase (up to n = 4), which is in contrast to the current understanding of exciton spin relaxation mechanism in conventional III-V or II-VI semiconductor system; implying that another mechanism or higher order process might be dominant in lead halide RP perovskites. Importantly, our study does not only lay down the fundamentals of spin photophysics in lead halide perovskites but also demonstrates its potential for opto-spintronic applications.