Study on the properties and optical emission spectroscopy of the intrinsic silicon thin film in silicon heterojunction solar cells

Abstract

The intrinsic silicon thin film for passivation of the crystalline silicon wafer surfaces in silicon heterojunction cells was prepared by very high fregucency plasma enhanced CVD (VHF-PECVD). Plasma emission versus time was recorded by optical emission spectroscopy (OES) during the silicon thin film deposition. Results show that the Hα* and SiH* signals stabilize soon (about 25 s after deposition) under the optimized deposition conditions, and the variation of SiH*/Hα* ratio is little, thus avoiding the structure non-uniformity of silicon film during the growth. The reason is that the SiH4 back diffusion is avoided owing to SiH4 being not fully depleted. The study of the influence of the deposition parameters on steady-state plasma emission spectra and properties of silicon films shows that as the SiH4 concentration increases, the Hα* decreases and the SiH* increases, the silicon film will transit from microcrystalline to amorphous, and the good passivation effect can be achieved in the amorphous silicon film. Hα* and SiH* increase firstly and then decrease with the deposition pressure, the decrease of Hα* and SiH* under high pressure can be attributed to a high polymer formation which is not beneficial to the formation of high quality silicon film, and therefore the passivation effect of silicon films decreases under high pressures. Hα* and SiH* increase with power density, and are saturated when the power density is 150 mW/cm2; for this the quality and passivation effect of the silicon film begin to decrease, the passivation effect of the silicon film at a power density of 50 mW/cm2 is poor, which may be due to the low concentration of atomic H being unable to fully passivate the dangling bonds at the silicon surface.