Toward BaSi₂/Si Heterojunction Thin‐Film Solar Cells: Insights into Heterointerface Investigation, Barium Depletion, and Silicide‐Mediated Silicon Crystallization

Abstract

for a conveniently synthesized material with minimal toxicity and adequate availability, has triggered a research focus toward barium silicide (BaSi 2). This is regarded as a low-cost alternative to conventional absorber materials. [1,2] The semiconducting BaSi 2 is stable in the ambient condition, [3] and exhibits an orthorhombic crystal structure. The Si atom is covalently bound with three neighboring Si atoms forming the characteristic tetrahedron [Si 4 ] 4− , then with Ba 2+. [4,5] Besides the essentially elemental abundance, BaSi 2 possesses a bandgap E g ≈ 1.3 eV with a light absorption coefficient (α) over 10 5 cm −1 at the visible light region. [6,7] Its potential also stems from excellent charge transport properties, i.e., a long minority carrier lifetime τ (≈10-27 µs), and the corresponding long diffusion length L (≈10 µm). [8-10] Despite the great potential of BaSi 2 , there is still a huge gap between the fabrication of materials and the realization of efficient solar cells. Preliminarily computational researches have established various BaSi 2 homojunction and heterojunction solar cell architectures. Despite its intrinsically moderate n-type nature (electron concentration n = 10 15-10 17 cm −3), the conductivity of BaSi 2 can be modified by external doping. Dopants, such as P, Sb, Ga, Cu, and As, can enhance the electron concentration to the range of 10 19-10 20 cm −3 , while B, Al, Ag, In, etc., would alter it to a p-type conductivity. [11-18] Such bipolar conductivity of BaSi 2 facilitates homojunction architectures. Theoretically, the conversion efficiency (η) of an n-p BaSi 2 homojunction solar cell can reach 22.5-25%. [1,19] However, controllable BaSi 2 doping processes were currently carried out only by molecular beam epitaxy (MBE) with in situ coevaporation or ex situ ion implantation of dopants. [14,16] Regardless of expensive and complex equipment involved in processes as well as a restriction of c-Si substrate for depositions, additional high-temperature annealing was needed after the doping process, which caused issues such as the segregation of dopants. [14] In the attempt to obtain BaSi 2 homojunction solar cells, n +-BaSi 2 (20 nm)/p-BaSi 2 (500 nm)/p +-BaSi 2 (50 nm) diodes experimentally exhibited an extremely low η of ≈0.1% that could be caused by high volume of defects. [20] To this end, heterojunction architectures, which have an inherent advantage of being free from doping, exhibit a great promise for BaSi 2 solar cell development. The low lattice

Determining the Annealing Temperature Dependency of Wetting and Mechanical Features on Fe3Si Films

“…Subsequently, the crystalline orientation shifted further with a higher activation energy from annealing at 900 • C. The β (202)/(220) evolved dramatically, causing a superposition between itself and Si(111). At the same time, several peaks of β (400), β (331), β (422), and β (333) appeared at 34.72, 45.20, 49.80, and 58.90 • , respectively [22,29]. When annealing further, the sample at the phase above the ideal Si concentration of the D03 structure can retain the phase from the boundary of 26% at RT to 31% at 1200 • C, even when an absurd phase transition occurs [5].…”

Recent Progress Toward Realization of High‐Efficiency BaSi2 Solar Cells: Thin‐Film Deposition Techniques and Passivation of Defects

“…[15,21] In this review, we focus on the recent progress of BaSi 2 to bridge the gap between the present status and that described in previous review articles. [14,15] Great progress has especially been made in thinfilm deposition techniques [22][23][24][25][26][27][28][29][30][31] and understanding the surface structure of BaSi 2 epitaxial films, [32] the reactions at the air/BaSi 2 and BaSi 2 /Si interfaces during post annealing, [33][34][35][36][37] and understanding the importance of satisfying the stoichiometry. [38,39] By positron annihilation spectroscopy, [39] it has been revealed that the electron concentration increases to %10 18 cm À3 with increasing vacancy-type defects [38] when the Ba/Si atomic ratio is far from the stoichiometric ratio.…”