Page 54 - Alternative Energy Systems in Building Design

SOLAR CELL MANUFACTURING AND PACKAGING TECHNOLOGIES 31

The manufacture of a solar PV cell in itself is only one part of the process of manu-
facturing a whole solar panel product. To manufacture a functionally viable product
that will last over 25 years requires that the materials be specially assembled, sealed,
and packaged to protect the cells from natural climatic conditions and to provide proper
conductivity, electrical insolation, and mechanical strength.
One of the most important materials used in sealing solar cells is the fluoropolymer
Elvax manufactured by DuPont. This chemical compound is manufactured from ethyl-
ene vinyl acetate resin. It is then extruded into a film and used to encapsulate the silicon
wafers that are sandwiched between tempered sheets of glass to form the solar panel.
One special physical characteristic of the Elvax sealant is that it provides optical clarity
while matching the refractive index of the glass and silicon, thereby reducing photon
reflections. Figure 3.5 depicts various stages of the monocrystalline solar power manu-
facturing process.
Another chemical material manufactured by DuPont, Tedlar, is a polyvinyl fluoride
film. It is extruded with polyester film and applied to the bottoms of silicon-based pho-
tovoltaic cells as a backplane that provides electrical insolation and protection against
climatic and weathering conditions. Other manufacturing companies, such as Mitsui
Chemical and Bridgestone, also manufacture comparable products to Tedlar that are
used widely in the manufacture and assembly of photovoltaic panels.
Another important product manufactured by DuPont chemical is Solamet, a silver
metallization paste used to conduct electric currents generated by individual solar
silicon cells within each module. Solamet appears as micronwide conductors that are
so thin that they do not block solar rays. A dielectric silicon-nitride product used in
photovoltaic manufacturing creates a sputtering effect that enhances silicon, trapping
sunlight more efficiently. Major fabricators of polycrystalline silicon are Dow Corning
and General Electric in the United States and Shin-Etsu Handotai and Mitsubishi Material
in Japan.
Because of the worldwide silicon shortage, the driving cost of solar cells has become
a limiting factor for lowering the manufacturing costs. At present, silicon represents
more than 50 percent of manufactured solar panels. To reduce silicon costs, at present,
the industrial trend is to minimize the wafer thickness from 300 to 180 μm.
It should be noted that the process of ingot slicing results in 30 percent wasted mate-
rial. To minimize this material waste, General Electric is currently developing a tech-
nology to cast wafers from silicon powder. Cast wafers thus far have proven to be
somewhat thicker and less efficient than the conventional sliced silicon wafers.
However, they can be manufactured faster and avoid the 30 percent waste produced by
wafer sawing.

Thin-film solar cell technology The core material of thin-film solar cell tech-
nology is amorphous silicon. This technology, instead of using solid polycrystalline
silicon wafers, uses silane gas, which is a chemical compound that costs much less
than crystalline silicon. Solar cell manufacturing involves a lithographic-like process
in which the silane film is printed on flexible substrates such as stainless steel or Plexiglas
material in a roll-to-roll process.