When individual solar cells are combined to form a module and wired in a series, which is an array, they have the ability to capture energy from sunlight and convert it to electricity.
Electricity is created because of the semi-conducting silicon within the individual solar cells. The semi-conducting properties of silicon contains both a metal and an insulator.
Negative N-type silicon contains phosphorus and produces free electrons. Positive P-type silicon contains boron which produces the positive charge in the cells needed to produce electricity.
The sunlight absorbed by the cells excites the N-type and P-type electrons which travel to electrodes in the solar cell creating an electrical current.
Useable electricity isn’t produced unless a demand or “load” and other components are added to the system.
Each individual solar cell is constructed by layering different component materials together. Individually, these components cannot generate electricity. However, when combined in a specific configuration, they generate and conduct electricity from sunlight.
Imagine a solar cell as a framed picture. Enclosed in a metal frame and sandwiched in between the clear, protective cover glass and a sturdy backing material would be the picture. Solar cell construction is similar. The difference would be to substitute our picture with the electrical current generating properties of the solar cell core.
It isn’t economical to use this individually designed cell configuration in an active solar power application. In reality, a number of these individual solar cell cores are connected in series to form a solar panel. The panel, containing multiple solar cell cores would be framed in metal and covered with clear glass and a backing support material.
Let’s dissect the actual components used in a photovoltaic solar cell from top to bottom.
Individual Solar Cell Cross Section Diagram
The Top Cover Layer of the solar cell uses clear glass or plastic and is part of the enclosure. The clear cover lets sunlight through to the cells, protects the cells, adds rigidity and is encased by a sturdy metal frame.
The Contact Grid is made of conductive metals. The contact grid collects the electrons generated in the semi-conductive cell core and transmits them out through the contact grid to an electronic device or a “load” as electrical voltage. The surface area of the contact grid is designed to be large enough to collect as many electrons as possible without sacrificing or blocking the sunlight absorbing surface area of the solar cell’s core.
Antireflective Coatings are used to help avoid the incoming sunlight from being reflected back off the solar cell. Only by capturing and absorbing as much of the incoming sunlight as possible can a solar cell maximize its electricity generating capacity. Since only spectral solar light is processed during the photovoltaic effect, the antireflective coating helps increase absorption over the entire solar spectrum and aides in the absorption of sunlight when the cells aren’t oriented to optimum sun angles.
There are two common techniques for applying antireflective coatings to solar cells. One is to cover them with a thin film of silicon monoxide. Another process is to texture or “rough up” the surface of the cell by chemically etching it and forming tiny scratches that resemble cones and pyramids. The cones and pyramids redirect the sunlight down into the cell core instead of allowing it to reflect back off the panel.
In the future, both these techniques may be replaced by a new, highly efficient antireflective coating process called nanotechnology. Nanotechnology uses seven thin layers of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle and stacked one on top of the other. According to tests by the FCC have shown this process to absorb over 96% of the sunlight it receives.
The N-Type Semiconductor Silicon layer produces the negatively charged electrons needed to conduct an electrical current. The conduction is made possible through a process called doping. An N-type silicon semiconductor is doped when the impurities of phosphorus atoms are added. This creates extra negatively charged electrons. These extra negative electrons combine with holes created in the doped P-type semiconductor’s positive electrons creating an electrical current which flows through to the contact grid.
The N-P Junction lies in between the top N-type semiconductor and the bottom P-type semiconductor. It is the absorber layer or the core of the semiconductor layers. The junction is created at the point where the doped N-type semiconductor negative electrons and the doped P-type semiconductor positive electrons meet and the transfer of electricity takes place through the photovoltaic effect.
The P-Type Semiconductor Silicon layer produces the positively charged electrons needed to conduct an electrical current. Like the N-type semiconductor, the P-type semiconductor is also doped. The P-type semiconductor is doped with boron. This creates holes in the positive field. The holes created are filled by the excess negatively charged electrons of the N-type semiconductor layer. The electric current created by this transfer of atoms flows through the N-type semiconductor to the contact grid and eventually to an electrical device or “load.”
The Back Contact layer is made of metal, seals off the bottom of the cell, adds rigidity and acts as a conductor. When multiple cells are connected in series, all the combined components are encased in an aluminum metal frame. This is known as a panel or a module.
By: Rick Contrata
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