Abstract
Controlling the doping profile in solar cells emitter and front/back surface field is mandatory to reach high efficiencies. In the current state of the art, these doped layers are made by dopant diffusion at around 900°C, which implies potential temperature induced damages in the c-Si absorber and for which a precise control of doping is difficult. An alternative solution based on boron-doped epitaxial silicon layers grown by plasma-enhanced chemical vapor deposition (PECVD) from 200°C using SiF4/H2/Ar/B2H6 chemistry is reported. The structural properties of the doped and undoped epitaxial layers were assessed by spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). The incorporation of boron has been studied via plasma profiling time of flight mass spectrometry (PP-TOFMS) and secondary ion mass spectrometry (SIMS) measurements. The boron-doped epitaxial layers revealed excellent structural and electrical properties even for high carrier concentrations (>1019cm-3). Sheet resistances between 100 and 130 Ω/sq can been obtained depending on the thickness and the doping concentration, which is within the range of targeted values for emitters in c-Si solar cells. Electrochemical capacitance voltage (ECV) revealed a uniform doping profile around 3.1019 cm-3 and by comparing with SIMS measurement a doping efficiency around 50% has been found.
Highlights
To produce high efficiency crystalline silicon solar cells it is necessary to limit recombination losses at the front and rear sides of the crystalline silicon wafer
In the common process flow used for nPERT solar cells, p-type emitter is made by BBr3 diffusion but requires numerous processing steps[1] such as boron gettering and post-oxidation to limit formation of boron rich layers at the surface which reduce minority carriers lifetime.[2]
We have demonstrated that several microns can be obtained by RF-plasma-enhanced chemical vapor deposition (PECVD) at 200◦C.12
Summary
Because of the low temperature growth (about 200◦C), no thermal stress is induced in the c-Si bulk and the diffusion of impurities is reduced during the junction formation It allows the formation of shallow junctions along with the possibility of introducing a gradient of doping in the epi-layer for an optimization of the doping profile. In particular Cariou et al.[13] widely studied and optimized silicon epitaxy by PECVD using SiH4/H2 chemistry They achieved a 8.5% efficient solar cell using ∼5 μm thin epi-layers as absorber, showing the high quality of the material.[12] Labrune et al reported on a 14.2% efficient solar cell with a p-type low temperature emitter.[14] no device exceeding an efficiency of 15% has been reported in literature so far, mostly due to the presence of defects in the epi-layers. We show the first and successful results of boron-doped LTE films with SiF4/H2/Ar chemistry for the formation of p-type layers in crystalline silicon solar cells
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