Ab Initio Study of Interfaces in Halide Perovskites and Prediction of Two-Dimensional Ferroelectric Materials

Ab initio method is widely used as an approach for atomic-level understanding of mechanisms and a cost-effective tool for exploring target materials with advanced properties. This dissertation harnesses the strengths of ab initio method to investigate Metal Halide Perovskites (MHPs) and predict new two-dimensional (2D) ferroelectric (FE) materials. MHP-based solar cells (PSCs) have witnessed a remarkable increase in power conversion efficiencies (PCEs) reaching over 25% for laboratory-scale devices. However, there are still challenges in application because of its lack of long-term stability and reliability, which requires further understanding of microstructure for refining manufacturing techniques. Here ab initio calculation was performed to study intra-grain interfaces in MHPs based on the experimental evidence. The calculation proved the general existence of facile moving ferroelastic domain boundaries in MHPs arising from the soft-bonding characters. These domain boundaries are lack of polarization. Other intra-grain interfaces such as compositional interfaces, stacking-fault, and twin grain boundaries are mostly benign to the PSC performance without inducing deep gap states. 2D FE and anti-ferroelectric (AFE) materials hold significant potential for advancing computing paradigms like neuromorphic computing, in-memory computing, and neuromorphic visual sensing. FE and AFE materials are suitable for developing nonvolatile and volatile devices separately. However, the currently identified choices of 2D FE and AFE materials with stable polar ordering at room temperature are very limited. More efforts in exploring potential 2D FE/AFE materials are required for further application in real devices. a new family of 2D ferroelectric materials NbOX2 (X = Cl, Br, I) was identified through a combination of data-mining techniques and DFT calculation. The 2D AFE phase is then stabilized through strain engineering. This 2D FE and AFE material show stable in-plan polar configurations under room temperature and the polar ordering can exist in the monolayer, making its application without the limitation of thickness. Therefore, this family of 2D materials is a promising candidate in the applications of real electronic devices