Barium Disilicide for Photovoltaic Applications: Thin-Film Synthesis and Characterizations

Energy and materials have been assigned with great significance for the development of society over the past centuries. For the sake of environmental sustainability, earth-abundant and eco-friendly materials for energy utilization have been gaining increasing attention. Among them, barium disilicide (BaSi2) possesses attractive optical and electrical properties, enabling its potential for achieving low-cost and high-efficiency thin-film solar cells. This research provides a systematical investigation on sputtered BaSi2 ranging from thin-film fabrication to properties characterizations. Chapter 1 gives a general introduction about solar energy and photovoltaics. The prospects and challenges of thin-film solar cell technology are discussed. Chapter 2 is a literature review of BaSi2, including material structure, optical and electrical properties, thin-film fabrications, and recent advancements in BaSi2-based solar cell development. Chapter 3 lists experimental methods used in this research including deposition techniques and material characterization methods. Chapter 4 exhibits the fabrication of poly-crystalline BaSi2 films via sputtering with subsequent high-temperature annealing in N2 atmosphere. The film thickness uniformity is determined by the target-to-substrate distance. The surface oxidation during high-temperature annealing results in the inhomogeneous structure of sputtered BaSi2 films. An oxidation-induced structural transformation mechanism of BaSi2 is proposed, which describes the complex reactions and elemental diffusion within the BaSi2 film at high-temperature conditions. Chapter 5 explores the effects of vacuum annealing condition on sputtered BaSi2 film properties. The vacuum annealing method enables the BaSi2 crystallization at 600 °C, and decreases the thickness of the surface oxide layer from ∼200 nm (in N2 atmosphere) to ∼100 nm. In Chapter 6, a face-to-face annealing (FTFA) approach is applied for the post-growth treatment of sputtered BaSi2 films, which improves surface composition homogeneity and crystal quality of sputtered BaSi2. By employing various covers for FTFA including BaSi2, silicon, and glass, a transition of conductivity type from n- to p-type is observed. Thermal resistance analysis is carried out to understand the mechanism of the FTFA method and its impacts on the film crystallization process and properties. Chapter 7 investigates the interface properties of Si/BaSi2/Si hetero-structures serving as the fundamental for the development of BaSi2/Si heterojunction solar cells. The effects of Si layer thickness on the composition and structure of Si/BaSi2/Si under high temperature conditions are analyzed. A thick Si layer (dSi › 20 nm) can effectively suppress the surface oxidation and elemental diffusion during the high-temperature annealing. The process of structure and composition variations of Si/BaSi2/Si samples consist of the oxidation of deposited Si layer, growth of the oxide layer, Ba diffusion and depletion, as well as Si isolation and crystallization. These interfacial phenomena lead to the complex structure and composition of Si/BaSi2/Si heterostructures. Conclusions of this thesis and outlook for the future development of the material and devices are listed in Chapter 8. Recommendations are given for high-quality BaSi2 film fabrications and solar cell development. This thesis provides insights into BaSi2 films from perspectives of thin-film depositions via sputtering and property characterizations. These results and knowledge shed light on fabrications of BaSi2 films for the goal of efficient BaSi2-based solar cells.