The development of clean energy resources as alternatives to oiI has
become one of the most important challenges for modern science and
technology. The obvious motivation for these efforts is to reduce the
air polIution resulting from the mass consumption of fossil fuels and to
protect the ecological cycles of the biosystems on Earth. Analyses of
future energy usage envision that the energy structure in the 21st
century will be characterized as a "Best Mix Age" involving different
renewable energy forms. Among the wide variety of renewable energy
projects in progress, photo- voltaics is the most promising as a future
energy technology. It is pollution- free and abundantly available
everywhere in the world, even in space, and can also operate with
diffuse light. However, a major barrier impeding the devel- opment of
large-scale bulk power applications of photovoltaic systems is the high
price of solar cell modules. Therefore, reduction of the costs of solar
celIs is of prime importance. To achieve this objective, tremendous R&D
efforts have been made over the past two decades in a wide variety of
technical fields ranging from solar-cell materials, cell structure, and
mass production pro- cesses to the photovoltaic systems themselves. As
the result, about an order of magnitude cost reduction has been achieved
in the past 10 years.