Application of low hydrogen-diluted and low gaseous-pressure monosilane plasma to fast deposition of solar-cell-grade microcrystalline silicon

Abstract

A low hydrogen-diluted and low gaseous-pressure monosilane plasma (hydrogen to monosilane flow rate ratio of 3.3 ≤ [H 2 ]/[SiH 4 ] ≤ 10, 80 Pa) was applied to the fast chemical-vapor deposition of solar-cell-grade hydrogenated microcrystalline silicon (c-Si:H) by using an ultrahigh-vacuum very-high-frequency (105 MHz) hollow-electrode-enhanced glow-plasma transfer technique (VHF-HEEPT). The deposition of a well-crystallized and photosensitive c-Si:H thin film with a preferential <110> crystal orientation was achieved at a growth rate of 5.0 nm/s and a [H 2 ]/[SiH 4 ] ratio of 3.3 by applying a VHF power of 100 W (the highest available power in this work). The same deposition conditions were applied to grow the i (intrinsic)-layer of an n−i−p-type single-junction solar cell. The photo-conversion efficiency of the solar cell was 3.4% with no degradation of the open-circuit voltage (0.44 V) and its fill factor was 54.7%. The crystallographic structure of the solar cell revealed that the i-layer (c-Si:H) consisted of almost the same <110>-preferential crystallite and columnar structure as that of the solar cell, in which the i-layer was deposited under high hydrogen-diluted conditions ([H 2 ]/[SiH 4 ] = 26.7). These results indicate that a low hydrogen-diluted and low gaseous-pressure monosilane plasma is applicable to the fast thin-film deposition of solar-cell-grade c-Si:H by using the VHF-HEEPT system. The depth profiles of phosphorus and oxygen concentrations demonstrated that the i-layer of the solar cells contained larger amounts of these elements than the single i-layer deposited on a glass substrate, especially in the boundary region with the substrate. Further development of both the HEEPT system and film deposition conditions would be effective for improving the performance of solar cells.