Page 53 - Alternative Energy Systems in Building Design

30 SOLAR POWER SYSTEM PHYSICS AND TECHNOLOGIES

Solar Cell Electronics

An electrostatic field is produced at a PN junction of a solar cell by impinging photons
that create 0.5 V of potential energy, which is characteristic of most PN junctions and
all solar cells. This miniscule potential resembles in function a small battery with
positive and negative leads. These are then connected front to back in series to achieve
higher voltages.
For example, 48 solar cell modules connected in series will result in 24 V of output.
An increase in the number of solar cells within the solar cell bank will result in a high-
er voltage. This voltage is employed to operate inverters that convert the dc power into
a more suitable ac form of electricity. In addition to the previously discussed PN-
junction device, solar cells contain construction components, for mechanical assem-
bly purposes, that are laid over a rigid or flexible holding platform or a substrate, such
as a glass or a flexible film, and are interconnected by microthin, highly conductive
metals. A typical solar panel used in photovoltaic power generation is constructed
from a glass supportive plate that houses solar PV modules, each formed from sever-
al hundred interconnected PN devices. Depending on the requirements of a specific
application, most solar panels manufactured today produce an output of 6, 12, 24, or
48 V dc. The amount of power produced by a solar panel, expressed in watts, repre-
sents an aggregate power output of all solar PN devices. For example, a manufacturer
will express various panel characteristics by voltage, wattage, and surface area.

Solar Cell Manufacturing and

Packaging Technologies

Solar cell technologies at present fall into three main categories: monocrystalline
(single-crystal construction), polycrystalline (semicrystalline), and amorphous silicon
thin film. A more recent undisclosed solar technology, known as organic photovoltaics,
is also currently under commercial development. Each of these technologies has unique
physical, chemical, manufacturing, and performance characteristics and is best suited
for specialized applications. This section will discuss the basic manufacturing prin-
ciples, and subsequent chapters will review the production and manufacturing process
of several solar power cell technologies.

MONOCRYSTALLINE AND POLYCRYSTALLINE SILICON CELLS
The heart of most monocrystalline and polycrystalline PV solar cells is a crystalline sili-
con semiconductor. This semiconductor is manufactured by a silicon purification process,
ingot fabrication, wafer slicing, etching, and doping, which finally forms a PNP junction
that traps photons. This results in the release of electrons within the junction barrier, there-
by creating a current flow.