The Basic Solar Cell
How can you tell a good solar cell from a bad one? If you’re not an electrical engineer, this is probably a difficult question to answer.
When most people think of solar cells, they picture a large panel in the desert, turning sunlight into electricity. While there are ways to store and save electricity from solar power, the basic function is to use the sun’s rays to power your appliances and electronics.
The basic structure of a solar cell is a semiconductor, similar to copper, that is neither an insulator nor a conductor. When light hits the solar cell, the electrons in the semiconductor are excited, allowing them to flow freely and create electricity.
As it turns out, however, only about thirty percent of the light is converted to electricity. The rest is reflected, and the reflected light becomes heat. This is why the actual cells are not exposed to the air, but they are covered with glass, plastic, or a mesh to keep the insulating materials out of the way.
This process works best when the sun is hitting the solar cell directly. However, the cell’s semiconductor can absorb light from a wide variety of different angles. This means that a single solar cell can still produce electricity from a cloudy day or from an area that is not directly in the sun.
The Evolution of Solar Cell Materials
Solar cells have come a long way from being made of heavy and slightly expensive glass. Nowadays, solar cells are made up of lightweight, extremely thin sheets of silicon that can be used in a vast variety of applications. Silicon is also the most popular element used in solar cells and solar panels because of its availability – silicon is the second most abundant element found in the earth's crust.
Some of the problems that come with using silicon as a material for solar cells are:
- Silicon-crystalline wafers alone are harder to produce than other materials
- Replacing high-cost silicon materials involves a significant amount of expensive electricity
- The ability of silicon to absorb light is limited by Shockley-Queisser limit
A couple of decades ago, photovoltaic scientists and technology started working on replacing silicon for other materials that are lighter, more suitable or less costly. Their efforts have produced a number of materials that are used in the production of solar cells in place of silicon, including cadmium telluride, gallium arsenide, and copper-indium-diselenide.
Researchers are also working on an organic material that can be thin film-printed into a substance that can absorb the sun's energy and create electricity. Scientists are hoping to extend the invention held in this substance to allow the production of affordable solar cells that are also stable, lightweight and flexible.
Different Solar Cell Materials
Silicon is the most common material used for solar cell production. On a complete solar cell, silicon is used for the cell’s base and the cell’s top. These two layers make up the outside of the solar cell that you see.
Surrounding the silicon is a layer of glass and then a layer of plastic. The plastic acts as a covering to protect the silicon from the environment. The silicon can be hazardous if it comes into contact with things in the air, like moisture or oxygen. The plastic covers are used to protect the silicon, and the cells, from these dangers.
The glass covers the solar cell to increase the amount of power that is produced. Since glass is clear, the sun is able to pass through it to power the solar cell.
Silicon cells are not the only type of cells being made for harnessing the sun’s power. There are a few other materials that also work well for producing clean solar power. A few of the materials being used for solar cells include:
Crystalline solar cells are panels that have a specially treated crystalline silicon that mimics the way natural crystals are formed.
This allows the solar cells to capture more sunlight which in turn, increases the overall power output.
Polycrystalline Solar Panels
The majority of solar panels installed in the US is made of polycrystalline silicon. The sandwiching of thin round slices of silicon between glass sheets makes up the panel. If you put your hand on a solar panel, you can feel the glass panels. The glass sheets are another component of the panel that that help it to keep its shape but also act as a surface for the sun’s rays to be absorbed into the silicon. In the manufacturing process, many silicon slices are used to make a panel, so it may have the word “modules” attached to the end of the name. One solar panel = one module. In a few words, a module = 3-5 solar cells. Your final count will be based on the wattage of the panel.
Amorphous Silicon Solar Panels
The most common material used for solar panels is called amorphous silicon. This type of silicon is very cheap and can be used to make very low quality solar panels. The silicon used for solar panels is very thin and used to make solar cells. The energy produced by solar panels is directly proportional to the number of solar cells used within the system.
However, the most common material used for commercial production of solar panels and solar cell production is crystalline silicon. Solar panels made of crystalline silicon are more expensive, but they are much more efficient in converting energy and giving out more power per square foot.
The production of solar panels using crystalline silicon requires a highly controlled environment and high temperatures, which makes them very expensive and less efficient. There are also many other types of silicon, some of which are used for constructing solar panels.
Other types of silicon are called that, because the crystalline structure is unorganized and elongated. The unorganized crystalline structure gives a lower output power and higher material shrinkage.
Cadmium Telluride (CdTe)
Thin films of cadmium telluride are some of the most commonly used materials in sun cells. Alongside copper indium gallium selenide (CIGS), cadmium telluride is easy and cheap to produce, has decent light absorption and conversion efficiency, and has the added benefits of being more durable and flexible than many other solar cells.
Cadmium telluride is a mixture of two compounds, cadmium telluride itself, and another compound called cadmium selenide. These two compounds both form crystals that are very similar and have a 100% incorporation within each other.
Copper Indium Gallium Selenide – CIGS
Since scientists began developing solar cells, the material used to make them has been anything but consistent. Early cells were made of crystal or various metal or germanium pieces, while cell types developed over the last 75 years were varied.
The constantly changing materials faced a problem, however. Efficiency. These cells could only reach only a maximum efficiency rate of about 11%.
Then, in the 1980s, researchers at the University of California, (Berkeley), discovered the perfect material for the construction of solar cells: copper indium gallium selenide. The more commonly known abbreviation for this material is "CIGS".
Copper Indium Gallium Selenide, is a chemical compound arranged in a semiconductor crystal lattice. It is a common material used in modern solar cells.
With the introduction of CIGS, scientists reached a significantly higher level of efficiency. The goal, to reach 20%, seemed to be in reach at long last.
Solar panel efficiency was so high, in fact, that the US and other foreign governments have been subsidizing and even giving away panels to the public to help reduce their massive reliance on non-renewable resources.
Organic Photovoltaic – OPVs
Organic photovoltaic or OPV are a relatively new addition to photovoltaics. The way it works is very similar to the organic process which produces energy for photosynthesis.
In photosynthesis, energy is obtained from the sun's light and stored as chemical energy. In organic photovoltaics, light is converted directly into electrical energy.
The most common use for organic photovoltaic cells may be in the development of solar paper or solar cloth. Solar clothes, also known as solar textiles use the technology to develop clothing or sheets that are thin, lightweight, flexible and waterproof, making them ideal to endure multiple uses or washings. Solar clothing has already made manufacturing breakthroughs, and it’s just one area of many that are discovering the many benefits this technology has to offer.
Perovskite Solar Cell
So what is a perovskite solar cell? The perovskite is a naturally occurring mineral, usually found in the form of a grey, light-weight rock. Because it is made of three-dimensional crystals, this mineral was first used as a ceramic coating on hand-made art pieces.
The perovskite mineral, named after Lev V. Perovski, who originally discovered the mineral in the early 1840s, has a unique property. It has the amazing ability to multiply sunlight. It can absorb light and create electricity.
The uniqueness of the mineral attracted scientists, who wanted to work with perovskite to make a solar cell so we can harness its power. Scientists also thought it might be possible to make perovskite out of a material that is less expensive than silicon.
Scientists spent more than 50 years researching their possibilities. Eventually, they came up with a number of techniques to make perovskite solar cells. The main difference between silicon and perovskite is that perovskite is less efficient at creating electricity. Some of the other important differences are that perovskite is easier to manufacture and it is more flexible.
Other Solar Cell Technologies
Solar cells can be made of a variety of materials and can be refined in different ways to give various efficiencies. The main types of solar cells that are currently in production and in use are monocrystalline silicone, Ag-CdTe (silver and cadmium telluride), crystalline silicon with a thin layer of metal deposited on top, and amorphous silicon.
But so far, the preferred solar cell material is silicon, a material with a high surface to volume ratio. Silicon can be bought from aluminosilicate (found in common glass) or as quartz sand. This is why the most common material for making solar cells is the same material which may have been used to build or create other items.
Other cell types include organic cells, dye-sensitized cells, organic thin-film cells, and hybrid polymer cells. These can be used for specific applications like space equipment, certain military equipment, or anything that is not exposed to much variation in temperature.
In this chapter, we talked about various methods of creating solar cells. If you had to pick one method to use to make your cells, then the most economical would be to use the castable process.
Other methods of making the cells, like making the various types of wafers, are more costly. Remember, cost is one of the main concerns when making solar cells. Making the cells using the casting method means you are able to get the “recipe” down resulting in many more consistent and functional cells.
The wetting and drying method, while consistent, is also more time-consuming—which means a higher cost. If you want to go the more involved, wetting and drying method, then you would probably need to have a machine ready to go. This way you can quickly manufacture different types of bifacial and multijunction cells to see what works best.
The two other methods are single-crystal silicon and silicon-tin. These both produce high-quality cells, but it really depends on your budget and the intended purpose of your cells. Both are rather involved methods, and the costs go up from there. But if you have the funding and would like to pursue a long-lasting solar cell, then they are definitely strong options.