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Solar power, along with wind, wave, and geothermal energy sources, is seeing accelerating interest in renewable energy research. Once installed, solar panels produce zero emissions as they convert sunlight directly into energy via the photovoltaic effect, bypassing the turbine system found in every other commercial energy source.
The Photovoltaic Effect
Light is converted into electricity by the photovoltaic effect. When light is incident to the cell, the absorbed energy excites bound electrons. This allows them to jump their atomic bonds and become free. The free electrons travel through the material, and the resulting current is harnessed when conductors are attached to either side of the cell. Because there are no moving parts, including turbines, maintenance fees are lower and there is zero fuel use.
The photovoltaic effect requires a material that is light sensitive. Over the last 175 years, researchers have noted the photovoltaic properties of several different materials. The first solar cells of 1880 were just one percent efficient - revolutionary for the time. These first efforts were constructed with gold-coated selenium.
Progress stalled at that mark for decades. Albert Einstein's accurate description of the photovoltaic effect in 1905 was a major benchmark in solar power development, but it took until the 1950s for an actual improvement to be seen in solar panels. In 1956, Gordon Pearson, Darryl Chapin, and Cal Fuller first used silicon to produce a solar cell, achieving a much-improved efficiency of 4% and introducing silicon as a key material in solar energy production.
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Silicon in Solar Cells
The silicon that you'll now find in a solar cell is highly processed. The material is sourced in silica mines, which are often found in regions with heavy quartz concentrations. The silica is refined to reach metallurgical grade. This process takes place in an electric arc furnace, where carbon is used to release the oxygen in the silica quartzite, resulting in a more consistent silica makeup. However, metallurgical purity doesn't cut it for a photovoltaic cell.
High efficiency rates will boost the amount of energy released by the cell, so the purity of the photovoltaic-capable material is of utmost importance. The metallurgical grade silicon is exposed to hydrochloric acid and copper, which produce trichlorosilane gas. Hydrogen is then used to reduce this gas to silane gas, which is in turn heated to make molten silicon.
Pure silicon is crystalline - a structure necessary for photovoltaic cells. The purity level of silicon at this state is anywhere from 99.99999% to 99.9999999% pure. Silicon can be arranged into either a monocrystalline structure, which boasts the highest efficiency rates as well as the highest cost, or a polycrystalline shell.
Processing Crystalline Silicon for Solar Cells
Polycrystalline shells are made by melting various silicon crystals together, making them cheaper than monocrystalline setups. Polycrystalline cells are also far less energy intensive to produce — a positive for those who decide to go solar for environmental reasons.
Once the silicon (both monocrystalline and polycrystalline) has been properly prepared, it is treated, or 'doped', with phosphorous and boron to form a semiconductor. Semiconductors are materials that conduct electricity in radios, computers, televisions, and other everyday devices. Their conductivity lies between that of a conductor and an insulator.
After this stage, the basic properties of the solar panel are present. However, before the panel can be installed, several further procedures are completed. First, the semiconducting silicon disks are coated with titanium dioxide. This makes them less reflective, limiting risks for aircraft.
The silicon discs are then arranged in a frame, which is often made of aluminum. Aluminum’s low weight allows for easy installation, and increases the number of structures that can easily support solar panels. Each cell is protected by either silicon rubber or butyryl plastic. Glass then covers the cell, unless it will be used in space on a satellite, in which case, plastic is chosen instead.
Processing Solar Cells - Environmental and Health Concerns
When assesing solar panels as a key energy resource, it is important to weigh up any concerns. One of the issues confronting the solar industry is that many of the materials used to produce solar panels can be hazardous. Some potential issues include:
- Sawing silicon into discs for use creates silicon dust called kerf, with up to 50% waste. Kerf can be inhaled by workers, causing severe respiratory problems.
- Silica gas is highly explosive, and has been known to spontaneously combust.
- Silicon production reactors are cleaned with sulfur hexafluoride, which is the most potent greenhouse gas per molecule according to the Intergovernmental Panel on Climate Change. It also can react with other chemicals to produce sulfur dioxide, which is responsible for acid rain.
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The Future of Solar Cells
Recent research has been aimed at increasing the efficiency rate of photovoltaic cells and solar arrays. While many strides have been made since the first solar cell was built back in 1880, average efficiencies still lie well below 30 percent, with many cells barely topping 10 percent efficiency. If solar power is to take off in the coming century, serious improvements in their efficiency need to be realized.
Perovskites in Solar Cells
Recent research from has led to a cheap way to augment the photovoltaic properties of silicon. This method calls on the use of perovskites. Perovskites can be made of several different materials, but lead is a popular choice. They have a particular crystalline structure similar to that of calcium titanate.
In one study by researchers at Stanford, solar cell efficiency was boosted from 11.4 percent to 17 percent by adding a perovskite cell. That's an increase of over fifty percent. However, when they added the same structure to a silicon cell with an efficiency of 17 percent, results were far less drastic: an increase of 0.9 percent efficiency. Perovskites will continue to be the focus of much future research into solar cells.