Page 39 - Solar Power in Building Design The Engineer's Complete Design Resource

TYPES OF SOLAR CELLS TECHNOLOGIES 9

Solar cell manufacturing and packaging process Currently, solar cells
essentially are manufactured from mono crystalline, polycrystalline amorphous and
thin-film-based materials. A more recent undisclosed solar technology, known as
organic photovoltaics, is also currently under commercial development. Each of the
technologies have unique physical, chemical, manufacturing, and performance char-
acteristics and are best suited for specialized applications.
In this section we will discuss the basic manufacturing principles, and in subsequent
chapters we will review the production and manufacturing process of several solar
power cell technologies.

Introduction to monocrystalline and polycrystalline silicon cells The
heart of the most monocrystalline and polycrystalline photovoltaic solar cells is a crys-
talline silicon semiconductor. This semiconductor is manufactured by a silicon purifi-
cation process, ingot fabrication, wafer slicing, etching, and doping which finally
forms a PNP junction that traps photons, resulting in the release of electrons within
the junction barrier, thereby creating a current flow.
The manufacturing of a solar photovoltaic cell in itself is only a part of the process
of manufacturing a 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 prop-
er conductivity, electrical insulation, and mechanical strength.
One of the most important materials used in sealing solar cells is the fluoropolymer
manufactured by DuPont called Elvax. This chemical compound is manufactured
from ethylene vinyl acetate resin. It is then extruded into a film and used to encapsu-
late 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 1.5 depicts various stages of monocrys-
talline solar power manufacturing process.
Another chemical material manufactured by DuPont, called Tedlar, is a polyvinyl
fluoride film that is coextruded with polyester film and applied to the bottom of
silicon-based photovoltaic cells as a backplane that provides electrical insulation and
protection against climatic and weathering conditions. Other manufacturing compa-
nies such as Mitsui Chemical and Bridgestone also manufacture comparable products
to Tedlar which are widely used in the manufacture and assembly of photovoltaic
panels.
Another important product manufactured by DuPont chemical is Solamet, which is
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 the solar rays.
A dielectric silicon-nitride product used in photovoltaic manufacturing creates
a sputtering effect that enhances silicon to trap 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.