Chap 4 : Photovoltaic (Solar Cells) - pin mat troi

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Ánh sáng, với năng lượng photon của nó, có thể cung cấp năng lượng để đưa một electron lên mức năng lượng cao hơn. Năng lượng photon được cho theo: Đủ năng lượng để đưa electron quay quanh E∞ cũng được gọi là năng lượng ion hóa (hiệu ứng quang điện bên ngoài). Tế bào quang điện chủ yếu là chuyển đổi cho các photon điện của ánh sáng nhìn thấy được, tia cực tím và hồng ngoại, do đó, nội bộ hiệu ứng hình ảnh xác định ảnh hưởng của ánh sáng trong một tế bào năng lượng mặt trời....

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Nội dung Text: Chap 4 : Photovoltaic (Solar Cells) - pin mat troi

再再再再 Chap 4 : Photovoltaic (Solar Cells) 劉再再再再再再再再再再再
Introduction The history of photovoltaics goes back to the year 1839, when Becquere discovered the photovoltaic effect, but no technology was available in the 19th century to exploit this discovery. The semiconductor age only began about 100 years later. After Shockley had developed a model for the pn junction, Bell Laboratories produced the first solar cell in 1954; the efficiency of this, in converting light into electricity, was about 5%. Photovoltaics offers the highest versatility among renewable energy technologies. Theoretically, PV systems could cover the whole electricity demand of most countries in the world. 再再再再再再再再再 2/70
Introduction Worldwide, the installed photovoltaics capacity and the share of electricity generated by PV are still low, despite impressive market growth. The political environment and magnitude of market introduction programmes will determine the future of this technology. 再再再再再再再再再 3/70
The photo effect Light, with its photon energy, can provide the energy to lift an electron to a higher orbit. The photon energy is given by : h⋅ c E=  The energy sufficient to lift the electron to orbit E∞ is also called the ionization energy (external photoelectric effect). photovoltaic cells mainly convert to electricity photons of visible, ultraviolet and infrared light, therefore, the internal photo effect determines the effect of light in a solar cell. 再再再再再再再再再 4/70
The photo effect The highest fully occupied band is called the valence band The next highest band, which can be partially occupied or totally empty, is called the conduction band The space between VB and CB is called the forbidden band The energy gap between the band is called the band gap 再再再再再再再再再 5/70
The photo effect Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. 再再再再再再再再再 6/70
The photo effect Every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the N-type and P-type silicon are in contact. Right at the junction, electrons and holes mix and form a barrier, equilibrium is reached, and an electric field separating the two sides is formed. This electric field acts as a diode. 再再再再再再再再再 7/70
The photo effect When light, in the form of photons, hits solar cell, its energy frees electron-hole pairs. Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. 再再再再再再再再再 8/70
Principle of solar cells Not all the energy of photons with wavelengths near the band gap is converted to electricity. The solar cell surface reflects a part of the incoming light, and some is transmitted through the solar cell. Further more, electrons can recombine with holes. The spectral response is given by : e⋅  S ( ) = ext ( ) h⋅ c 再再再再再再再再再 9/70
Principle of solar cells 再再再再再再再再再 10/70
Solar Cell Materials Solar cells can be made from a wide range of semiconductor materials. They are: Silicon (Si)—including single-crystalline Si, multicrystalline Si, and amorphous Si Polycrystalline thin films—including copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon Single-crystalline thin films—including high-efficiency material such as gallium arsenide (GaAs) 再再再再再再再再再 11/70
Solar Cell Materials The crystallinity of a material indicates how perfectly ordered the atoms are in the crystal structure. Silicon, as well as other solar cell semiconductor materials, can come in various forms: single-crystalline, multicrystalline, polycrystalline, or amorphous. In a single-crystal material, the atoms making up the framework of the crystal are repeated in a very regular, orderly manner from layer to layer. In contrast, in a material composed of numerous smaller crystals, the orderly arrangement is disrupted moving from one crystal to another. One classification scheme for silicon uses approximate crystal size and also includes the methods typically used to grow or deposit such material. 再再再再再再再再再 12/70
Solar Cell Materials Type of Silicon Abbreviation Size Range Deposition Method Single-crystal silicon sc-Si >10cm Czochralski, float zone Multicrystalline silicon mc-Si 1mm-10cm Cast, sheet, ribbon Polycrystalline silicon pc-Si 1mm-10mm Chemical-vapor deposition Microcrystalline silicon mc-Si
Solar Cell Materials Absorption The absorption coefficient of a material indicates how far light having a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed. Solar cell material has an abrupt edge in its absorption coefficient. The reason is that light whose energy is below the material's bandgap cannot free an electron. And so, it isn't absorbed. 再再再再再再再再再 14/70
Solar Cell Materials Bandgap The bandgap of a semiconductor material is an amount of energy. Specifically, it's the minimum energy needed to move an electron from its bound state within an atom to a free state. This free state is where the electron can be involved in conduction. The lower energy level of a semiconductor is called the "valence band.“ And the higher energy level where an electron is free to roam is called the "conduction band." The bandgap (often symbolized by Eg) is the energy difference between the conduction band and valence band. 再再再再再再再再再 15/70
Solar Cell Materials 再再再再再再再再再 16/70
Solar Cell Types (Silicon) Single-Crystal Silicon To create silicon in a single-crystal state, we must first melt high- purity silicon. We then cause it to reform or solidify very slowly in contact with a single crystal "seed.“ The silicon adapts to the pattern of the single-crystal seed as it cools and gradually solidifies. Not surprisingly, because we start from a seed, we say that this process is "growing" a new rod (often called a "boule") of single crystal silicon out of molten silicon. Several different processes can be used to grow a boule of single- crystal silicon. The most established and dependable processes are the Czochralski (Cz)method and the float-zone (FZ) technique. We also discuss "ribbon-growth" techniques. 再再再再再再再再再 17/70
Solar Cell Types (Silicon) Single-Crystal Silicon The most widely used technique for making single-crystal silicon is the Czochralski process, in which a seed of single crystal silicon contacts the top of molten silicon. As the seed is slowly raised, atoms of the molten silicon solidify in the pattern of the seed and extend the single- crystal structure. 再再再再再再再再再 18/70
Solar Cell Types (Silicon) Multicrystalline Silicon Multicrystalline silicon devices are generally less efficient than those of single-crystal silicon, but they can be less expensive to produce. The multicrystalline silicon can be produced in a variety of ways. The most popular commercial methods involve a casting process in which molten silicon is directly cast into a mold and allowed to solidify into an ingot. The starting material can be a refined lower-grade silicon, rather that the higher-grade semiconductor grade required for single-crystal material. The cooling rate is one factor that determines the final size of crystals in the ingot and the distribution of impurities. The mold is usually square, producing an ingot that can be cut and sliced into square cells that fit more compactly into a PV module. 再再再再再再再再再 19/70
Solar Cell Types (Silicon) Amorphous Silicon Amorphous solids, like common glass, are materials whose atoms are not arranged in any particular order. They don't form crystalline structures at all, and they contain large numbers of structural and bonding defects. But they have some economic advantages over other materials that make them appealing for use in solar electric, or photovoltaic (PV), systems. Today, amorphous silicon is common in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators. 再再再再再再再再再 20/70