Microstructure and open-circuit voltage of n−i−p microcrystalline silicon solar cells

Abstract

A series of microcrystalline silicon n−i−p solar cells has been deposited by very high frequency plasma enhanced chemical vapor deposition at various values of silane to hydrogen source gas ratio and on two different substrate types. Relationships between microstructure and electrical characteristics of these solar cells are investigated by transmission electron microscopy, atomic force microscopy, and I(V) measurements. A mixed phase (so-called heterophase) layer consisting of amorphous plus microcrystalline material is observed at the bottom of the solar cell and identified here as one of the key microstructural features of the device: the relationship between the crystalline nuclei density and the heterophase layer thickness is presented as well as its relationship with the open-circuit voltage (Voc). The effects of substrate roughness and of silane to hydrogen gas ratio used for the fabrication of the device on the heterophase layer are evidenced. These observations underline the importance of the first stages of growth of the intrinsic (i) layer for the fabrication of high- Voc n−i−p microcrystalline solar cells.