Low Temperature Back-Surface-Field (BSF) Technology for Crystalline Silicon (c-Si) Thin Film Solar Cells Based on Heterojunctions between Boron-Doped P-Type Hydrogenated Amorphous Silicon and c-Si
We propose a low temperature back-surface field (BSF) technology for crystalline silicon (c-Si) thin film solar cells. The BSF structure has a heterojunction between a p-type c-Si substrate and a boron (B)-doped p-type hydrogenated amorphous silicon (a-Si:H) layer deposited on the back surface of the c-Si substrate at 200degC. The back-surface recombination velocity, Sb, of minority carriers can be reduced to less than 1000 cm/s in this structure, while the value of S b is 106 cm/s when a B-doped p-type layer is epitaxially grown on the c-Si substrate. We have clarified, from internal photoemission (IPE) and attenuated-total-reflection Fourier transform infrared (ATR-FTIR) spectroscopy measurements, that possible reasons for the observed low value of Sb are (1) the band lineup of the heterojunction (HJ) between B-doped p-type a-Si:H and c-Si and (2) the hydrogen passivation of defects in B-doped a-Si:H
Sticking and recombination of the SiH3 radical on hydrogenated amorphous silicon: The catalytic effect of diborane
The deposition rate of hydrogenated amorphous silicon films in SiH4 glow-discharge is drastically enhanced upon addition of B2H6 when the gas-phase concentration exceeds 10−4. This cannot be attributed to gas-phase reactions and must be interpreted as an increase of the sticking probability of the dominant SiH3 radical. However, the total surface loss probability (β) of SiH3 which includes both sticking (s) and recombination (γ) increases only above 10−2 B2H6 concentration, which reveals that between 10−4 and 10−2 the ratio sβ increases. A precursor-state model is proposed in which SiH3 first physisorbs on the H-covered surface and migrates until it recombines, or chemisorbs on a free dangling bond site. At a typical deposition temperature of 200° C, the only mechanism of creation of dangling bonds in the absence of B2H6 is precisely the recombination of SiH3 as SiH4 by H abstraction, which limits the sticking probability to a fraction of β. This restriction is overcome with the help of hydroboron radicals, presumably BH3, which catalyze H2 desorption.
Characterized of glow-discharge deposited a-Si:H
Abstract The study of hydrogenated amorphous silicon, a-Si:H has become an active and large subfield of growing interest in noncrystalline semiconductors. This paper reviews recent successes as well as difficulties encountered in interpreting some fundamental physical properties of a-Si:H prepared by glow-discharge deposition. This material appears to contain structural and compositional heterogeneities which depend on the preparation conditions and affect its mechanical and electronic properties. These are related to the concentration and bonding configurations of hydrogen. Determinations of the density and distribution of localized gap states by means of field effect and capacitance measurements are discussed as well as evidence for and against the presence of surface states. Optical absorption near the absorption edge is described as are conductivity measurements. The latter reveal many interesting puzzles. Staebler and Wronski discovered that the dark conductivity and the photoconductivity can be changed significantly by strong exposure to light. Moreover, space charge layers adjacent to the substrate interface and near the free surface can strongly influence the electronic properties of a-Si:H films, especially those having low densities of gap states. The pre-exponential factor of the conductivity increases with its activation energy in accordance with the Meyer-Neldel rule regardless of whether the changes in dark conductivity are produced by doping, light exposure or space charge layers. Attempts are made to identify causes for the scatter of data on some film properties reported by various laboratories. Our present understanding of the recombination processes involved in photoluminescence and photoconductivity is presented. We conclude that a-Si:H films of superior quality can be expected when the preparation conditions become more fully understood.