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.@en

As a potential absorber candidate for high-efficient solar cell applications, BaSi₂ films are confronted with issues of surface oxidation associated with the higherature annealing. Herein, BaSi₂ films are deposited by the sputtering technique. A vacuum annealing process is subsequently carried out to crystallize sputtered BaSi₂ films. Raman spectroscopy is used to study surface structures and crystalline quality. Elemental depth profile is measured by Auger Electron spectroscopy to understand the compositions of films. Optical and electrical properties are further investigated to reveal the effects of annealing condition. Applying vacuum annealing condition can effectively suppress diffusions of Ba and ensures a stochiometric BaSi₂ layer. However, surface oxidation still occurs even in the vacuum environment owing to the high reactivity of Ba. Further attempts to prevent BaSi₂ surface oxidation may focus on the combination of other methods, such as capping layer and reducing atmosphere, with vacuum (or low-pressure) annealing condition.

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Regarded as a promising candidate for absorber material in photovoltaic applications, BaSi2 confronts the challenge of high-quality material synthesis via low-cost processes. Here, we fabricated BaSi2 thin films through the industrially applicable sputtering technique with the face-to-face annealing (FTFA) approach. The employment of the FTFA approach leads to an improvement of the sputtered BaSi2 from perspectives of surface homogeneity and crystal quality. Various covers are applied in the FTFA, including BaSi2, glass, and Si, which causes alterations in the film's electrical and optical properties. These impacts of the FTFA method on sputtered BaSi2 films stem from two aspects, i.e., heat redistributions caused by the variation of thermal networks, and interfacial interactions within the confined space between the cover and the film. The FTFA approach provides a facile strategy for minimizing the impacts of BaSi2 surface oxidation during high-temperature processes. These results and findings can push forward the material development of BaSi2 and its photovoltaic applications.

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The knowledge of the structural and compositional details of Si/BaSi₂/Si heterostructure annealed at high temperature is a prerequisite for BaSi₂ application in heterojunction thin-film solar cells. For this purpose, Si/BaSi₂/Si heterostructures deposited by magnetron sputtering with different Si layer thickness are submitted to systematic structural and compositional characterizations. Results reveal a BaSi₂/Si heterointerfacial variation caused by surface oxidation and Ba diffusion at the high temperature. Its effects on the optical and electrical properties of Si/BaSi₂/Si heterostructure are presented. The outcomes of this work can be extended to BaSi₂ deposited by other techniques, and generate substantial advantages in BaSi₂ development ranging from improvement on material qualities and eventual deployment in thin-film solar cells.

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Barium di-silicide (BaSi₂) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi₂ thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi₂ layers, in a comparison with high quality BaSi₂ films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi₂ films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi₂ films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi₂ films, as judged from DB-PAS.

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Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.@en

Regarded as a promising absorber material for solar cell applications, Barium disilicide (BaSi2) is still confronted with issues related to surface oxidation. Here, we use a-Si.H deposited by plasma-enhanced chemical vapor deposition as capping layer to prevent surface oxidation of sputtered BaSi2 films. Based on crystalline quality and optical properties characterizations, thin a-Si.H capping cannot sufficiently prevent surface oxidation. Conversely, oxidation of a-Si.H layer in turn promotes Ba diffusion and Si isolation. Applying a thicker a-Si.H capping layer (more than 20 nm) can suppress such effect. The multi-materials capping layer can also be regarded as potential strategy to prevent surface oxidation of BaSi2.

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