Hundreds of solar cells (also called photovoltaic cells) make up a solar photovoltaic (PV) array. Solar cells are the components of solar arrays that convert radiant light from the sun into electricity that is then used to power electrical devices and heat and cool homes and businesses. Solar cells contain materials with semiconducting properties in which their electrons become excited and turned into an electrical current when struck by sunlight. While there are dozens of variations of solar cells, the two most common types are those made of crystalline silicon (both monocrystalline and polycrystalline) and those made with what is called thin film technology.
Silicon Solar Cells
The majority of the solar cells on the market today are made of some type of silicon – by some estimates, 90% of all solar cells are made of silicon. However, silicon can take many different forms. Variations are most distinguished by the purity of the silicon; purity in this sense is the way in which the silicon modules are aligned. The greater the purity of the silicon molecules, the more efficient the solar cell is at converting sunlight into electricity. The majority of silicon based solar cells on the market – about 95% – are comprised of crystalline silicon, making this the most common type of solar cell. But there are two types of crystalline – monocrystalline and polycrystalline.
Monocrystalline Silicon Solar Cells
Monocrystalline solar cells, also called “single crystalline” cells are easily recognizable by their coloring. But what makes them most unique is that they are considered to be made from a very pure type of silicon. In the silicon world, the more pure the alignment of the molecules, the more efficient the material is at converting sunlight into electricity. In fact, monocrystalline solar cells are the most efficient of all; efficiencies have been documented at upwards of 20%.
Monocrystalline solar cells are made out of what are called “silicon ingots,” a cylindrically shaped design that helps optimize performance. Essentially, designers cut four sides out of cylindrical ingots to make the silicon wafers that make up the monocrystalline panels. In this way, panels comprised of monocrystalline cells have rounded edges rather than being square, like other types of solar cells.
Beyond being most efficient in their output of electrical power, monocrystalline solar cells are also the most space-efficient. This is logical since you would need fewer cells per unit of electrical output. In this way, solar arrays made up of monocrystalline take up the least amount of space relative to their generation intensity.
Another advantage of monocrystalline cells is that they also last the longest of all types. Many manufacturers offer warranties of up to 25 years on these types of PV systems.
The superiority of the monocrystalline cells comes with a price tag – in fact, solar panels made of monocrystalline cells are the most expensive of all solar cells, so from an investment standpoint, polycrystalline and thin film cells are often the preferred choice for consumers. One of the reasons monocrystalline cells are so expensive is that the four sided cutting process ends up wasting a lot of silicon, sometimes more than half.
Polycrystalline Solar Cells
Polycrystalline solar cells, also known as polysilicon and multisilicon cells, were the first solar cells ever introduced to the industry, in 1981. Polycrystalline cells do not go through the cutting process used for monocrystalline cells. Instead, the silicon is melted and poured into a square mold, hence the square shape of polycrystalline. In this way, they’re much more affordable since hardly any silicon is wasted during the manufacturing process.
However, polycrystalline is less efficient than its monocrystalline cousin. Typically, polycrystalline solar PV system operated at a 13-16% efficiency – again, this is due to the fact that the material has a lower purity. Due to this reality, polycrystalline is less space-efficient, as well. One other drawback of polycrystalline is that has a lower heat tolerance than monocrystalline, which means they don’t perform as efficiently in high temperatures.
Thin Film Solar Cells
Another up and coming type of solar cell is the thin film solar cell with growth rates of around 60% between 2002 to 2007. By 2011, the thin film solar cell industry represented approximately 5% of all cells on the market.
While many variations of thin film products exist, they typically achieve efficiencies of 7-13%. However, a lot of research and development is being put into thin film technologies and many scientists suspect efficiencies to climb as high as 16% in coming models.
Thin film solar cells are characterized by the manner in which various type of semi-conducting materials (including silicon in some cases) are layered on top of one another to create a series of thin films.
The major draw of thin film technologies is their cost. Mass production is much easier than crystalline-based modules, so the cost of mass producing thin film solar cells is relatively cheap. The product itself is also flexible in nature, which is leading to many new applications of solar technologies in scenarios where having some type of flexible material is advantageous. Another perk is that high heat and shading have less of a negative impact on thin film technologies. For these reasons, the thin film market continues to grow.
One major drawback is that thin film technologies require a lot of space. This makes them less of an ideal candidate for residential applications where space become an issue; as a result, thin film is taking off more in the commercial space. And thin film solar cells have a shorter shelf life than their crystalline counterparts, which is evidence by the shorter warranties offered by manufacturers.
Thin film technology using various photovoltaic substances, including amorphous silicon, cadmium telluride, copper indium and gallium selenide. Each type of material is suitable for different types of solar applications.
Amorphous Silicon Solar Cells
Thin film solar cells made out of amorphous silicon are traditionally used for smaller-scale applications, including things like pocket calculators, travel lights, and camping gear used in remote locations. A new process called “stacking” that involves creating multiple layers of amorphous silicon cells have resulted in higher rates of efficiency (up to 8%) for these technologies; however, it’s still fairly expensive.
Cadmium Telluride Solar Cells
Cadmium Telluride is the only of the thin-film materials that have been cost-competitive with crystalline silicon models. In fact, in recent years, some cadmium models have surpassed them in terms of their cost-effectiveness. Efficiency levels result in a range of 9-11%.
Copper Indium Gallium Selenide Solar Cells
Copper Indium Gallium Selenide cells have demonstrated the most promise with respect to their efficiency levels that range from 10-12%, somewhat comparable to crystalline technologies. However, these cells are still in the nascent stages of research and have been commercial deployed on any wide scale. That said, the technology is most used in larger or commercial applications.