Solar Cell Materials





Solar cell efficiency. Image by NREL.

Solar cell efficiency. Image by NREL.

There are many different materials used to produce solar cells. These range from the common crystalline silicon to the exotic and experimental organic polymer cells. Each of these materials is suited to specific roles as they all have unique advantages and disadvantages. As solar technology continues to advance the face of solar power will be continually changing but you can get a glimpse of the future here. The graph to the right shows how solar cell efficiency and technology has been progressing over the last decades.

Monocrystalline Silicone Solar Cells (c-Si)

Monocrystalline solar cell. Image by Ersol.

Monocrystalline solar cell. Image by Ersol. License: GNU FDL

Monocrystalline solar cells use silicone with a single homogeneous crystalline lattice. This results in higher performance when compared to other solar cell materials but that comes at significantly higher prices. Large silicon crystals are rarely found naturally and expensive to produce artificially. In spite of this higher cost monocrystalline solar cells still get a lot of use in military and scientific endeavors where the increased performance is considered to be worth the cost. The homogeneous crystalline lattice results in an even external coloring to the cell making them easy to spot as you can see in the image to the left.

Polycrystalline Silicon Solar Cells (c-Si)

Polycrystalline solar wafer. Image by Georg Slickers.

Polycrystalline solar wafer. Image by Georg Slickers. License: CC BY-SA 2.5

Polycrystalline solar cells use silicone wafers that are actually formed from multiple smaller silicon crystals. This uneven crystal lattice caused by the grain boundaries between silicon crystals results in a reduced performance when compared with monocrystalline cells. As you might expect though it is also much cheaper. Currently around half of all polycrystalline silicon is used in the production of solar cells making it the workhorse of consumer solar power all around the world. These type of solar cells are easily noticed by the multicolored silicon crystal flakes that make up the silicone wafer in a given cell as shown in the image to the left.

Amorphous Silicon Solar Cells (a-Si)

Thin film flexible solar photovolatics. Image by Fieldsken Ken Fields.

Thin film flexible solar photovolatics. Image by Fieldsken Ken Fields. License: CC BY-SA 3.0

Amorphous silicone is a non-crystalline form of silicon that enables thin films to be deposited onto other surfaces at low (for silicon) temperatures. Although amorphous silicone solar cells are much less powerful than conventional silicon cells this easy depositing enables simpler and less expensive construction. Another major advantage is that amorphous photovoltaic cells use around 1% of the silicon that traditional mono/poly crystalline cells use. The layer of silicon deposited ranges from only a couple nanometers to dozens of micrometers. This extremely thin layering allows for multiple layers to be stacked with each tuned to absorb different frequencies of light. Amorphous silicon solar cells are also capable of flexing when deposited on flexible surfaces.

Cadmium Telluride Photovoltaics (CdTe)

Cadmium telluride photovoltaic array. Image by NREL.

Cadmium telluride photovoltaic array. Image by NREL.

Similar to amorphous silicon are cadmium telluride (CdTe) photovoltaics in that they are both thin film solar cells. CdTe solar cells are appealing because they have a higher efficiency rate than amorphous silicon, the band gap (essentially what portion of sunlight a material absorbs) is a near perfect match to the solar spectrum for generating electricity, and the manufacturing process is significantly cheaper than amorphous silicon cells. This increased efficiency and cheaper manufacturing process combined with the low material consumption of thin film solar cells has made CdTe photovoltaics the only thin film photovoltaic that is cheaper than traditional crystalline silicon solar cells in most applications.

Copper Indium Gallium Selenide Solar Cells (CIGS)

Copper-Indium selenide solar cell. Image by NREL.

Copper-Indium selenide solar cell. Image by NREL.

Copper-Indium-Gellium-Selenide (CIGS) solar cells currently hold the highest efficiency record among thin film solar cells at 20.3% compared to 16.7% for CdTe solar cells and 12.5% for amorphous silicon solar cells. Although CIGS photovoltaics have obtained the highest efficiency among thin film solar cells the production cost associated with the vacuum processes required keeps them prohibitively expensive for consumer/industrial use. Recently other methods involving electroplating, sintering, and other lower cost depositing methods have reduced cost significantly but efficiency is reduced to 10%-15% at which point they have to compete with CdTe which is very low cost.

Gallium Arsenide Multijunction Solar Cells (GaAs)

Gallium arsenide multijunction solar cells on Mars rover. Image by Rtphokie.

Gallium arsenide multijunction solar cells on Mars rover. Image by Rtphokie. License: CC BY-SA 3.0

Gallium-Arsenide multijunction solar cells currently hold the record for the highest efficiency obtained by any solar cell at 42.4%. A multijunction solar cell is simply one that uses multiple P-N junctions to absorb more of the light spectrum by having multiple layers that are designed to absorb different frequencies as photons pass through. This high efficiency and performance afforded by multijunction photovoltaics does not come cheap though. Even as the demand for GaAs multijunction solar cells has been increasing at a fast pace the technology is still only used for things like solar powered vehicles such as the Mars Rovers or in solar power competitions.

Dye Sensitized Solar Cells (DSSC)

Multiple dye sensitized solar cells. Image by Sastra.

Multiple dye sensitized solar cells. Image by Sastra. License: US PD

DSSC are made using inexpensive equipment and materials making them among the cheapest solar cells you can get. DSSC also tolerate mechanical stress, such as hail, better than conventional cells and can be made flexible. Different than P-N junction cells, DSSC instead using a titanium dioxide layer coated with a photosensitive dye, typically ruthenium-polypyridine, and a platinum layer. In between these two layers is an electrolyte solution. The dye absorbs a photon and an electron enters the TiO2. The electron from the dye is replaced from the electrolyte which takes from the platinum. An external pathway generates power by allowing the TiO2 electron to rejoin the platinum.

Organic Photovoltaics

Organic photovoltaic cell. Image by US DOE.

Organic photovoltaic cell. Image by US DOE.

Organic photovoltaics or polymer solar cells are a newcomer to the solar landscape. They are simply solar cells made from organic semiconductors which, as you might guess, get their name from the organic elements used to construct them. Currently efficiency is low, around 8%, but these cells offer extreme flexibility and alleviate a lot of the concerns about disposing of a solar cell at the end of it’s life. There is hope that these cells will eventually be produced using a roll to roll printing method that will greatly reduce price as well. Polymer photovoltaics rely on an effective field created with a heterojunction between dissimilar materials.

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