Civil Engineering Reference
In-Depth Information
direct electrical charge (DC). This direct charge then, through utilization of an
inverter can be converted to an alternative charge (AC) which would be suitable for
building electricity needs.
Photovoltaic cells main design structures are homo-junction devices, hetero-
junction devices, p-i-n and n-i-p devices and multi-junction devices. Photovoltaic
cells have also been categorized under two main types of Crystalline Silicone (wafer
based) and Thin Film. Subcategories of Crystalline Silicone type are Poly-
Crystalline and Mono-Crystalline, while subcategories of the Thin Film type are
Amorphous-Si (a-Si), Tandem (a-Si/microcrystalline), CIGS (Copper Indium
Gallium Selenite), and CdTe (Cadmium Telluride).
Crystalline silicon is a homo-junction device that is made of one single material
of crystalline silicon in which one side has positive and the other side has negative
charges. When light absorbs in this material create free electrons. The freed electrons
move from one side to other and therefore generate an electrical current. In crystal-
line silicone technology single slices of silicone undergoes numerous manufacturing
processes and then array of these single slices are put together in order to create a
crystalline silicone panel. These panels then will be located on the roof or other
building surfaces facing the sunlight to use its energy and generate electricity.
Generally crystalline cells are made of pure silicon of one hundred and fi fty to
two hundred (150-200) microns thick, while the thin fi lms are made by depositing
semiconductor layers onto glass or other specifi c material (mainly stainless steel).
Thin fi lm layers are approximately three tenth to two (0.3-2) micrometer thick.
In thin-fi lm technology which uses a hetero-junction device the junction is
formed from different semiconductors, such as copper indium gallium selenite
(CIGS). Copper Indium Gallium Selenite is a direct bandgap semiconductor mate-
rial that can absorb the sun energy with very high effi ciency level. Due to possibility
of using different semiconductor as the positive and negative ends, devices that use
this technology provide higher effi ciency than other devices using homo-junction
technology.
As it was said earlier other thin-fi lm technologies exist as well. An amorphous
silicon (a-Si) thin-fi lm technology uses a p-i-n (positive a-Si, intrinsic silicon, and
negative a-Si) structure, while a cadmium telluride (CdTe) uses a n-i-p (negative
zinc telluride (ZnTe), intrinsic CdTe, and positive cadmium sulfi de (CdS)) structure.
Each of these devices are made of three layers of semiconductors that each have one
of the positive, negative and neutral charges, with the neural layer located in the
middle of the other two layers.
Another type of thin-fi lm photovoltaic cells is Multi-junction device. These
devices also called cascade or tandem and are made of multiple layers of
semiconductor junctions, which by capturing different levels of energy of light in
multiple layers provide higher effi ciency devices. In this structure multiple stacks of
single junction cells are installed on top of each other. The order of installation from
top to bottom is from higher to lower band-gap, in which the upper level captures
the highest energy light photons and the bottom level captures the lowest energy
light photons.
