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The majority of PV solar cells are made from crystalline silicon,either single crystal or multi-crystalline silicon.This account for around 90% of the current production.The remainder comprises various forms of thin film solar cells where the absorber material is deposited onto a suitable substrate such as glass.The absorber materials for thin film cells are either amorphous/polycrystalline silicon or compound semiconductors such as cadmium telluride,gallium arsenide,copper indium diselenide etc.Organic solar cells in the form of either dye sensitized solar cells or polymer absorbers are a newer low cost PV option that until recently has been confined to the laboratory but a recent investment by G24i in Wales is set to produce large volumes of flexible,dye sensitized,solar cells for niche applications.
The common characteristic to all the inorganic absorber materials is that they are semiconductors where absorption of light results in excitation band as illustrated in Fig.1.
Unfortunately,not all the light can be absorbed in this way as the low energy photons,corresponding to the longer wavelengths of solar radiation will be insufficient to excite an electron hole pair.Selecting a very narrow band gap semiconductor absorber will result in more photons being absorbed but less voltage being produced.To achieve maximum power there is a compromise between the current (photons absorbed) and the voltage of the cell.This will result in a predicted optimum band gap for the absorber of around 1.3 eV and a maximum theoretical efficiency under normal terrestrial solar light of around 30%[1-3].Unfortunately this is only part of the process as the electrons excited into the conduction band in p-type semiconductors are unstable and will tend to recombine,releasing heat.A junction is needed to draw the photo-generated electrons into n-type material where they are stable and extract an electric current via an external circuit.The n-type side of the junction can be made of the same material or a different semiconductor material to the absorber.
The dominance of the PV solar market by silicon is partly historical but is also a result of the development of silicon for the electronics industry.PV solar energy has been able to ride on the back of the developments in electronics until recently when more high purity silicon was being produced for PV solar than for electronics.However,the absorption coefficient of silicon is poor ,because it is not a direct band gap semiconductor,which is why direct band gap compound semiconductors have been developed gap compound semiconductors have been developed for the thin film applications.In reality this means that crystalline silicon cells require approximately 200 m,whereas direct..Band gap semiconductors require only 1-2 m thickness of absorber.Thia factor will be discussed later in the context of cost of PV solar modules.
The majority of PV solar cells are made from crystalline silicon,either single crystal or multi-crystalline silicon.This account for around 90% of the current production.The remainder comprises various forms of thin film solar cells where the absorber material is deposited onto a suitable substrate such as glass.The absorber materials for thin film cells are either amorphous/polycrystalline silicon or compound semiconductors such as cadmium telluride,gallium arsenide,copper indium diselenide etc.Organic solar cells in the form of either dye sensitized solar cells or polymer absorbers are a newer low cost PV option that until recently has been confined to the laboratory but a recent investment by G24i in Wales is set to produce large volumes of flexible,dye sensitized,solar cells for niche applications.
The common characteristic to all the inorganic absorber materials is that they are semiconductors where absorption of light results in excitation band as illustrated in Fig.1.
Unfortunately,not all the light can be absorbed in this way as the low energy photons,corresponding to the longer wavelengths of solar radiation will be insufficient to excite an electron hole pair.Selecting a very narrow band gap semiconductor absorber will result in more photons being absorbed but less voltage being produced.To achieve maximum power there is a compromise between the current (photons absorbed) and the voltage of the cell.This will result in a predicted optimum band gap for the absorber of around 1.3 eV and a maximum theoretical efficiency under normal terrestrial solar light of around 30%[1-3].Unfortunately this is only part of the process as the electrons excited into the conduction band in p-type semiconductors are unstable and will tend to recombine,releasing heat.A junction is needed to draw the photo-generated electrons into n-type material where they are stable and extract an electric current via an external circuit.The n-type side of the junction can be made of the same material or a different semiconductor material to the absorber.
The dominance of the PV solar market by silicon is partly historical but is also a result of the development of silicon for the electronics industry.PV solar energy has been able to ride on the back of the developments in electronics until recently when more high purity silicon was being produced for PV solar than for electronics.However,the absorption coefficient of silicon is poor ,because it is not a direct band gap semiconductor,which is why direct band gap compound semiconductors have been developed gap compound semiconductors have been developed for the thin film applications.In reality this means that crystalline silicon cells require approximately 200 m,whereas direct..Band gap semiconductors require only 1-2 m thickness of absorber.Thia factor will be discussed later in the context of cost of PV solar modules.
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