First-principles Electronic Structure and Defect Physics in Photovoltaic and Neuromorphic Materials

Abstract

The last decade has witnessed the increasing demand for renewable energy and new information technology, largely due to the reduction in the conventional nonrenewable energy resources and the von Neumann bottleneck in the classical computing architecture for data storage, transfer, and processing. While a wide range of materials have been explored for thin-film photovoltaics, each of these solar absorbers suffers from different kinds of issues. In contrast, initial neural circuits based on conventional complementary metal-oxide-semiconductor processors are energy inefficient for neuromorphic computing. Materials with dynamic behavior such as nonlinear conductance switch could directly emulate neural elements.

In this dissertation, the fundamental physical and chemical properties of several emerging functional materials were investigated, including quasi-one-dimensional antimony chalcogenides as solar absorbers for photovoltaic applications, and strongly correlated transition metal oxides and two-dimensional transition metal dichalcogenides as phase-change materials for neuromorphic computing. The competing superior electronic structure and complex defect chemistry in antimony chalcogenides were systematically studied. Furthermore, antimony chalcogenide thin-film solar cell via close space sublimation is compatible with the current thin-film manufacturing process. The power conversion efficiency of Sb₂Se₃ with graphite as electrodes has been improved from 4% to 7% via interfacial engineering.

The second part of the dissertation is focused on the fundamental understanding and rational design of neuromorphic materials using first-principles approaches. Comprehensive study has demonstrated VO₂ devices closely emulated neural circuits due to the metal-insulator transition (MIT). However, the mechanism of its MIT is still under debate and the phase transition temperature is too low for practical applications. To understand the MIT in VO₂, we studied the structural, electronic and magnetic properties of both the semiconducting (M1) and metallic (R) phases based on different DFT functionals. We then investigated the intrinsic and extrinsic defects in M1 VO₂. Based on theoretical findings, we rationalized materials design rule for further modulating its transition temperature in VO₂. The last part of the dissertation is focused on the theoretical study on the few-layer transition metal dichalcogenides, where we revealed the role of interlayer sliding in the piezoelectric and ferroelectric effect in 2D semimetals with potential application in low-power neuromorphic computing.