Physics of instability of thin film Si and (Si,Ge) alloy solar cells

The physics of instability of thin film Si solar cells is strongly dependent upon the nature of the Si, whether it is amorphous or nano(also called micro)crystalline. The amorphous phase is much more unstable than the nanocrystalline phase. The instability of the amorphous Si , which is really an alloy of Si and H, and H, is primarily due to the poor microstructure of the material. The amorphous material is not homogeneous, but is rather composed of an inhomogeneous mixture of randomly distributed Si-H bonds, and localized voids which are full of Si-H double bonds, or H atoms in close proximity to each other. The instability depends critically upon the presence of these voids and clustered H. In contrast, nanocrystalline Si, which also has H at the grain boundaries, is much more stable. However, some structural forms of nanocrystalline Si are unstable because of the penetration of impurities such as oxygen into the structure, or poorly bonded Si-H bonds at large-grain grain boundaries. The techniques for making both amorphous and nanocrystalline Si based solar cell materials more stable are discussed in this paper. In general, the alloys such as amorphous (Si,Ge) are more unstable because their structure is more complicated and more inhomogeneous than that of Si.