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
Improvement of crystalline silicon surface passivation by hydrogen plasma treatment
A completely dry low-temperature process has been developed to passivate 3.3 Ω cm p-type crystalline silicon surface with excellent results. Particularly, we have investigated the use of a hydrogen plasma treatment, just before hydrogenated amorphous silicon carbide (a-SiCx:H) deposition, without breaking the vacuum. We measured effective lifetime, τeff, through a quasi-steady-state photoconductance technique. Experimental results show that hydrogen plasma treatment improves surface passivation compared to classical HF dip. Seff values lower than 19 cm s−1 were achieved using a hydrogen plasma treatment and an a-SiCx:H film deposited at 300 °C.
Reaction mechanism in chemisorption kinetics: nitrogen on the {100} plane of tungsten
Kinetic models are developed for the formation of a chemisorbed overlayer on a homotattic surface with fourfold symmetry, taking into account the existence of short range order in the overlayer. It is shown that where the pairwise lateral interaction energy between nearest neighbour adatoms is large (repulsive or attractive), and the overlayer is consequently highly ordered, dissociative adsorption follows a pseudo first-order rate mechanism. Accurate sticking probabilities as a function of surface stoichiometry are reported for the interaction of nitrogen with the {100} plane of tungsten, obtained by a molecular beam technique, over a wide range of both surface and gas temperatures. The results are shown to be in quantitative agreement with the kinetic model, allowing for initial trapping into a mobile physisorbed state, and consistency is established with structural (low-energy electron diffraction) studies.