Solar Cells
In 1886, the American solar cell pioneer Charles Fritts concluded that,
“…the supply of solar energy is both without limit and without cost and will continue to stream down on earth after we exhaust our supplies of fossil fuels.”
This statement seems of more importance nowadays than ever before even though we know that the technology for harvesting solar energy is neither without limits nor without costs.
In 1886, the American solar cell pioneer Charles Fritts concluded that,
“…the supply of solar energy is both without limit and without cost and will continue to stream down on earth after we exhaust our supplies of fossil fuels.”
This statement seems of more importance nowadays than ever before even though we know that the technology for harvesting solar energy is neither without limits nor without costs.
Solar Cell History
Alexandre-Edmond Becquerel is usually credited as the first to demonstrate the photovoltaic effect. Becquerel was interested in photographic images and his pioneering photoelectric experiments in 1839 were done with liquid and not solid state devices. Platinum electrodes, coated with
AgCl or AgBr, were illuminated in acidic solution and a small photocreated voltage was registered. It would take almost 50 years from Bequerel’s first photovoltaic observation until the first solar cell was designed. It was the American inventor Charles Fritts who fabricated a solar cell in 1883 based on the photosensitive material selenium.
The efficiency was low, less than 1% but there was a market for the device in light sensors for cameras. The real breakthrough for solar cells came from researchers at the Bell Laboratories in 1954.
Based on their results they concluded that, “The direct conversion of solar radiation into electrical power by means of a photocell appears more promising as a result of recent work on silicon p-n junctions”. An overall solar energy efficiency of 6% was achieved, which was the real start for photovoltaics as a power generator. Since then considerable effort has been made to boost the efficiency and lower the fabrication cost of solar cells. In 1961 Shockley and Queisser published an analysis that has been described as the most elegant theoretical work in photovoltaics to date that puts an upper limit on the performance of solar cells with one extraction mechanism. The limit is fundamental and originates from the fact that photons with energy less than the bandgap are not absorbed and the excess energy in photons with energy larger than the band gap is lost to phonon vibrations (heat). This upper solar-to-electrical conversion efficiency is 32% at room temperature, regardless of likely future improvements in material quality or device design of photovoltaics with one extraction mechanism.
AgCl or AgBr, were illuminated in acidic solution and a small photocreated voltage was registered. It would take almost 50 years from Bequerel’s first photovoltaic observation until the first solar cell was designed. It was the American inventor Charles Fritts who fabricated a solar cell in 1883 based on the photosensitive material selenium.The efficiency was low, less than 1% but there was a market for the device in light sensors for cameras. The real breakthrough for solar cells came from researchers at the Bell Laboratories in 1954.
Based on their results they concluded that, “The direct conversion of solar radiation into electrical power by means of a photocell appears more promising as a result of recent work on silicon p-n junctions”. An overall solar energy efficiency of 6% was achieved, which was the real start for photovoltaics as a power generator. Since then considerable effort has been made to boost the efficiency and lower the fabrication cost of solar cells. In 1961 Shockley and Queisser published an analysis that has been described as the most elegant theoretical work in photovoltaics to date that puts an upper limit on the performance of solar cells with one extraction mechanism. The limit is fundamental and originates from the fact that photons with energy less than the bandgap are not absorbed and the excess energy in photons with energy larger than the band gap is lost to phonon vibrations (heat). This upper solar-to-electrical conversion efficiency is 32% at room temperature, regardless of likely future improvements in material quality or device design of photovoltaics with one extraction mechanism.
Today we are indeed aware that fossil fuels such as coal, oil and natural gas are finite resources. Excessive worldwide usage has also introduced conflicts and modern concepts such as global warming and smog alerts. Currently, as much as 80% of all energy consumed comes from chemical energy stored in fossil reserves.
Any major changes in energy consumption are not foreseen in the near future. Instead, the global energy demand is expected to increase by up to 60% and CO2 emissions by 70% by 2020.
This is going to put further pressure on an already stressed energy system. Therefore, it is of great importance to develop new alternative energy sources not based on fossil fuels. Solar energy has many advantages; it is an abundant resource and it is renewable. The most sophisticated and favorable form of solar energy is solar photovoltaic. Solar photovoltaic, which means “light-electricity”, is the direct conversion of sunlight into electricity without moving parts, pollution or noise.
A solar cell can be described as a device that under illumination is charged and works as a “battery” as long as the solar cell is kept under illumination.
No matter what materials are used in the solar cell, the origin of this effect is the same. The incoming light is absorbed by the semiconductor and electrons are excited from a low-energy state to a high-energy state. To prevent the excited electron to recombine back to its initial energy state it is crucial that the excitation is followed by a lateral selective extraction. The excited electrons are transported from the high-energy state to the high electron energy contact (negative potential) simultaneously as the vacated low-energy states are replenished from the low energy (positive) contact. These two basic mechanisms, absorption and extraction, create the necessary potential difference over the device and under load are the driving force for the photocurrent.
Any major changes in energy consumption are not foreseen in the near future. Instead, the global energy demand is expected to increase by up to 60% and CO2 emissions by 70% by 2020.
This is going to put further pressure on an already stressed energy system. Therefore, it is of great importance to develop new alternative energy sources not based on fossil fuels. Solar energy has many advantages; it is an abundant resource and it is renewable. The most sophisticated and favorable form of solar energy is solar photovoltaic. Solar photovoltaic, which means “light-electricity”, is the direct conversion of sunlight into electricity without moving parts, pollution or noise.
A solar cell can be described as a device that under illumination is charged and works as a “battery” as long as the solar cell is kept under illumination.
No matter what materials are used in the solar cell, the origin of this effect is the same. The incoming light is absorbed by the semiconductor and electrons are excited from a low-energy state to a high-energy state. To prevent the excited electron to recombine back to its initial energy state it is crucial that the excitation is followed by a lateral selective extraction. The excited electrons are transported from the high-energy state to the high electron energy contact (negative potential) simultaneously as the vacated low-energy states are replenished from the low energy (positive) contact. These two basic mechanisms, absorption and extraction, create the necessary potential difference over the device and under load are the driving force for the photocurrent.
Solar Cells Today
Today, after 50 years of research, the solar cell market is totally dominated by silicon cells due to their good efficiency and stability. However, they are still too expensive to be a good competitor to conventional electricity generation. The cost for a complete grid connected photovoltaic system of 12.5% is about 5.5 USD per peak Watt, a price that must be reduced by at least one order of magnitude to be competitive. The high cost is mainly due to high-energy demand for purifying SiO2 o high quality Si, which in combination with low material yield during fabrication leads to a high fabrication cost. There are three types of silicon solar cells, single crystal.
No comments:
Post a Comment