The Influence of High Doping on Electronic and Optical Properties in Tungsten Oxide
- Author(s): Wang, Wennie
- Advisor(s): Van de Walle, Chris G.
- et al.
Perovskites are a well-known class of materials with rich physics and a wide variety of applications. One such perovskite is tungsten oxide, which is a well-known chromogenic material used in smart windows and other display technologies. Due to its open crystal structure, it is possible to incorporate high concentrations of dopants. In this dissertation, we seek to understand the influence of dopants on atomic and electronic structure, as well as transport and optical properties using density functional theory.
First, we examine alkali doping and incorporating the oxygen vacancy in cubic and monoclinic tungsten oxide. We investigate the relative stabilities of different charge states and its implications on electrical properties, such as conductivity and electrochromism. Our results suggest that both alkali dopants and oxygen vacancies are shallow donors, and we discuss its implications for device development.
Next, we study the effect of charge doping. Tungsten trioxide has been experimentally shown to transform from the monoclinic symmetry to cubic symmetry with increasing monovalent doping. Our calculations show that electron doping primarily drives the phase transformation. We characterize the phase transformation from low to high symmetry, quantify the energetics of this transformation, and elucidate on the mechanism of these structural changes.
Building on our insights on the electronic behavior of dopants and defects, we study the influence of doping concentration on transport. Understanding the transport properties of these carriers is critical in many of the device applications for which tungsten oxide is used. We investigate the role of electron-phonon scattering in electron transport, and discuss the effects of spin-orbit coupling.
Finally, we examine the influence of doping concentration and structural distortions on optical absorption. We explore crystalline and disordered structures to demonstrate why these structural changes can enhance absorption at a microscopic level. Carrier-induced direct absorption is shown to wholly explain the drop in transmittance and coloration in electrochromism. Our findings shed light on how electronic features can be optimized for improved display and energy technologies.