The Anatomy and Function of the Modern Photovoltaic (PV) Cell

Solar cells have been around for a long time now, and for the most part have always operated the same basic way. In short, a photovoltaic cell is simply a device that converts sunlight into usable electric current through a process known as the photovoltaic effect. The first solar pv cell was built over 120 years ago, made of selenium and gold, and was only 1% efficent at the time.

Cross-section of a photovoltaic PV cell
 
 

The theory behind photovoltaics is relatively simple. Cells are made of semiconductors such as silicon, covered in glass, and placed into direct sunlight. As the sun's rays strike the cell, photons in the sunlight are absorbed by the semiconductor material (usually two types of material are used). In reaction, electrons are dislodged from the atoms making up the material. PV cells are constructed to channel and direct the flow of these electrons, and as they move they create an electric field due to the imbalance of charge on either side of the semiconductor materials. A diode is established, and electric current flows across the gap between the two layers of silicon. As positive and negative sides of the solar cell are created, it acts as a type of battery. Electrodes are connected to each side of the cell to harness this newly created energy.

While the overall process of the pv cell remains unchanged, advances in solar technology have made tremendous gains in both materials and design. Front contacts of solar cells are serrated or corrugated to allow for maximum exposure to the sun. Anti-reflective coatings applied to pv cell surfaces allow for more photons to be absorbed than before. Glass encasements have been made stronger and longer lasting, while remaining clear enough to transmit the most sunlight possible. Bigger still, breakthroughs in semiconductor technology have made the electrical output of the modern photovoltaic cell higher and more energy efficient than it's ever been.

Types of Photovoltaic Cells and New Materials for Modern Solar Energy Technology

Single Junction PV Cells - This first generation of photovoltaic cell methodology requires larger surface areas for each cell and is therefore more labor-intensive to produce. Silicone cells using single junction technology have been theorized to max out at 33% of limiting efficiency, with little room for future improvement in design. These cells are generally made by vacuum deposition - a process used to layer the crystalline silicon atoms as closely together as possible over the surface of the cell.

Thin-Film Cells - The use of new materials has improved the cost effectiveness of producing a much improved second generation solar pv cell. By applying these materials in a thinner film, the overall thickness of each cell is substantially smaller. Cadmium telluride and amphorous or micromorphous silicon substitutes are responsible for lessening the production costs and making this type of technology more viable for home solar use. PV cell efficiency remains the same however - only the cost is changed.

Multijunction PV Cells - This newer type of photovoltaic cell is designed to maximize the efficiency of all types of solar photons bombarding the structure. Made up of layered thin-film materials, various semi-conductor materials are used for each layer. Each of these layers is then optimized to absorb different colors in the light spectrum, thereby making the most of photon exposure.

Silicon Nanotechnology - Improvements in the construction of the semiconductor layers have lead to the silicon atoms being layered in the nanotextured range, meaning that more atoms can be placed in the same or smaller surface area. This process can greatly improve the performance of PV cells manufactured this way, but is a lot a more expensive.

Dye-Sensitized Solar Cells - In this exciting new solar technology, pv cells are manufactured using microscopic molecules of photosensitive dye. Rather than the semiconductive material supplying the electrons, the dye itself provides them and sends the flow of the electricity through the semiconductor. Because these dye molocules are arranged in a 3-Dimensional matrix, more of their surface area gets exposed to the sun's photon emissions.

3-D Solar Photovoltaic Cells - Making the most of sunlight, these emerging types of PV cells are made up of structures physically designed to capture as much sunlight as possible. Looking like tiny stacked Lego-brick cities, these cells boost efficiency through harnessing more photons than flat PV surfaces.

Costs of Solar Cells and Panels - Dollars per Watt & Efficiency Percentages

Alternative Home Energy

Individual PV cells have been traditionally weighed at a cost of US dollars per watt. When choosing a solar panel system it's important to take this cost into consideration, and it's the only way to calculate the payback period of solar equipment. Sunlight energy conversion efficiency is another important measurement for PV cells, determining just how much of the sun's photon energy can be converted to electricity.

First-generation solar cells have been priced as low as $1.00 per watt. Second-generation cell technology has cut that number in half. The lowest-cost multi-crystalline solar modules have been priced at just over $4.00 a watt, with thin-film modules going for a dollar less. These prices have dropped drastically in recent years, and will continue to fall as technological advances are made. PV cell efficiency has gone from an average 17% in 1988 to 24% in 2000. Triple-junction solar technology has produced efficiency ratings near 30%, with a record near 43% efficiency set in the summer of 2007. As these cells continue to become more efficient, solar energy and pv solar arrays emerge as a viable alternative to many current fuel sources.


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