Tuning the Spin Texture in Organic-Inorganic Halide Perovskites

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Non-Technical Description: Organic-inorganic perovskite materials are attracting a lot of attention as novel optoelectronic materials, in particular for photovoltaic applications. They are becoming a viable source for clean power generation, as the efficiency of perovskite solar cells gets close to that of crystalline silicon. As with any new technology, there are fundamental challenges that need to be addressed; there is a need to find alternate ways of improving the efficiency and stability of perovskite materials. As sunlight is absorbed in a photovoltaic material, charges are produced. Some factors that improve the efficiency of solar cell materials involve suppression of charge recombination and increasing the distance that charges can travel in the material (diffusion length) before being collected. Perovskite materials allow the manipulation of recombination pathways by tuning interatomic distances, which in turn impacts the magnetic behavior of the charge carriers, known as spin. This research interfaces the areas of perovskite materials and high pressure techniques, and provides a novel way of tuning charge carrier lifetimes and diffusion lengths by influencing the spin-dependent property of the charges. The project has a direct impact on improving the efficiency and stability of perovskite materials for photovoltaic applications. The connection between advanced concepts in science and education is reinforced by involving undergraduate students in advanced magneto-optical experiments to probe spin-dependent phenomena in perovskite films. The project plays a vital role in training future scientists; undergraduate, graduate, and postdoctoral researchers gain expertise in a multidisciplinary range of technical skills. The international scope provides US students an exciting opportunity to work with researchers from Europe and South Africa. A diversity intensive seminar course on "Science under Pressure" is designed, which includes discussions on the physics of materials under high pressure. Technical Description: The project focuses on high pressure techniques for tuning the spin texture effect in alkali halide organic-inorganic perovskite materials. The spin texture of the conduction and valence bands may be controlled by changing the interatomic distances, resulting in different spin helicities. The main objective of the research is a fundamental understanding of the ramifications of spin-orbit coupling in perovskite materials. The research validates the prediction of the Rashba spin-splitting, which result in both spin-allowed and spin-forbidden recombination channels. The project employs: a) chemical vapor deposition of methylammonium Pb-halogen perovskite films including replacement of Pb and organic cations with Cs and other molecular cations in collaboration with the University of Western Cape, South Africa; (b) optical, structural, and magneto-transport studies under ambient conditions; (c) Raman scattering under high pressure to monitor the phonons associated with the octahedral cage of the perovskite structure; (d) X-ray diffraction under pressure for correlating structural and optical properties; (e) photoreflectance spectroscopy under pressure; and (f) magnetoresistance measurements under high pressure. The research team aims to develop protocols for the analysis of a non-trivial Berry's phase from magnetoresistance measurements. An additional facet of the project addresses the simultaneous presence of ferroelectric and topological order in a class of inorganic halide perovskites under a strain field. The results of this project are relevant for various practical applications. The presence of topological states and the prospect of tuning the spin texture effect in hybrid alkali halide perovskites open up schemes for designing materials with improved performance in photovoltaic applications and high speed electronics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.