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.

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.

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.

Broadband transparent conductive oxide layers with high electron mobility (μ_e) are essential to further enhance crystalline silicon (c-Si) solar cell performances. Although metallic cation-doped In₂O₃ thin films with high μ_e (>60 cm² V^-1 s^-1) have been extensively investigated, the research regarding anion doping is still under development. In particular, fluorine-doped indium oxide (IFO) shows promising optoelectrical properties; however, they have not been tested on c-Si solar cells with passivating contacts. Here, we investigate the properties of hydrogenated IFO (IFO:H) films processed at low substrate temperature and power density by varying the water vapor pressure during deposition. The optimized IFO:H shows a remarkably high μ_e of 87 cm² V^-1 s^-1, a carrier density of 1.2 × 10²⁰ cm^-3, and resistivity of 6.2 × 10^-4 ω cm. Then, we analyzed the compositional, structural, and optoelectrical properties of the optimal IFO:H film. The high quality of the layer was confirmed by the low Urbach energy of 197 meV, compared to 444 meV obtained on the reference indium tin oxide. We implemented IFO:H into different front/back-contacted solar cells with passivating contacts processed at high and low temperatures, obtaining a significant short-circuit current gain of 1.53 mA cm^-2. The best solar cell shows a conversion efficiency of 21.1%.

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.