1. Introduction
The photovoltaic (PV) solar energy industry has been growing
very fast in the last a few years. Crystal silicon (c-Si) PV panel
production is the dominant technology because of the high
efficiency and very well developed manufacturing technology.
However, with the high demands for solar energy of terawatt
scales, new technologies capable of providing low production
costs which are environment friendly and have no-limitation in
raw materials are needed. Thin film silicon PV is one of the
potential technologies to meet the requirements. Hydrogenated
amorphous silicon (a-Si:H) and silicon germanium alloy
(a-SiGe:H) based thin film silicon have been used as the absorber
layers in multi-junction solar cells. The high defect density and low
carrier mobility in thin film amorphous silicon materials, especially
a-SiGe:H, cause a-Si:H and a-SiGe:H based solar cells having
lower efficiency than c-Si solar cells. Although 14.6% initial and
13.0% stable active area efficiencies were attained in 1997 by
United Solar,1) the a-Si:H/a-SiGe:H/a-SiGe:H solar laminate
product still shows a rated stable aperture-area efficiency around
8.2%. The lower efficiency in the product than the lab devices is
because of various limitations such as high deposition rate, less
reflective Al/ZnO back reflector (BR), and non-uniformity over a
large area deposition.