Part I
The term "Photovoltaic"
The term “Photovoltaic” or abbreviated “PV” is a combination of two words - “photo”, meaning light, and “voltaic”, meaning electricity. Thus, photovoltaic technology, the scientific term used to describe solar energy, involves the generation of electricity from light. More precisely, solar cells or solar photovoltaic arrays convert light from the sun directly into electricity.
Photovoltaic cells and modules
A photovoltaic cell consists of at least two layers of semiconductor material (prevalently silicon). One layer is positively charged, the other negatively (see Figure 2). The sun's photons strike the semiconductor material, freeing electrons from the negative layer through an external circuit and back into the positive layer (flow of electrons).
Figure 1: Basic Solar cell construction
This flow produces an electrical current. PV technology is modular in nature fostering great flexibility and utilization. Figure 2 shows that a PV module, the building block of a PV system, is comprised of individual PV cells. These PV cells are sealed and interconnected in a weatherproof frame making up the PV module. They are robust, reliable and usually guaranteed by module producers to perform 20-25 years.
Figure 2: PV cells - the building block for PV modules
Again PV modules can be wired again in series or parallel (a string of modules) to generate more electricity. Importantly to know is that every single PV module can work independently, whether it measures 2 square centimetres or 2 square metres emphasizing on the flexibility of the PV technology to tailor energy demand.
Types of PV cells
Crystalline silicon is still the mainstay of most PV cells. The two standard types are monocrystalline and polycrystalline (see Figure 3 below). Another emerging cell technology is called Thin film, where the silicon is deposited in thin layers on a low cost backing such as glass, stainless steel or plastic. Three types of thin film modules are currently commercially available. These are manufactured from amorphous silicon (a-Si), copper indium diselenide (CIS, CIGS) and cadmium telluride (CdTe).
Thin film technology is regarded to be “the” cell technology in the future, as it holds several advantages over standard crystalline silicon cells such as:
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lower material consumption of the world’s scarce silicon resources
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better performance = higher maximum yield = at higher cell temperatures!
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lower weight of cells = lower weight of PV modules = ease of installation
...and a smoother visual appearance...see Figure 3 below
   
Figure 3: Types of PV cells
(From the left to the right: amorphus, thin-film, polycrystalline and monocrystalline)
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