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Solar Cells. The electrical conductivity of semiconductors. Conductivity increases as T decreases. Low T sensors!. Extrinsic conduction. The density of free electrons for n doping. ionization energy necessary to release electrons from the donor atoms. Effective state density.
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The electrical conductivity of semiconductors Conductivity increases as T decreases. Low T sensors!
The density of free electrons for n doping ionization energy necessary to release electrons from the donor atoms. Effective state density
For silicon crystals with a temperature of T = 300 K, NL = 3.22•1019 cm–3 ED = 0.044 eV for phosphorus atoms as donors.
The density of free electrons for ptype semiconductors A small amount of energy can release a freely moving hole. the effective state density in the valence band density of the acceptors
(NV = 1.83•1019 cm–3 for silicon at T = 300 K, and EA = 0.045 eV for boron)
The charge neutrality within the boundaries dn and dp of the space charge region in the n-type and p-type semiconductor region leads to:
When electrons are lifted from the valence band into the conduction bandand thus released from the atom in the space charge region, the electric fieldwill pull them into the n-region.
Similarly, generated holes will move into thep-region. This can be explained in the energy band model by band bending inthe space charge region.
the solar cell can only convert a part of the photonenergy into electrical current. • For photon energies smaller than the band gap,the energy is not sufficient to promote an electron from the valence band tothe conduction band. This is the case for wavelengths above:
Not all the energy of photons with wavelengths near the band gap isconverted to electricity. • The solar cell surface reflects a part of the incominglight, and some is transmitted through the solar cell. • Furthermore, electronscan recombine with holes. In other words, they can fall back to the valenceband before they are converted to electricity.
The solar cell only uses an amount of energy equal to the band gap of thehigher energy of photons with lower wavelengths. Excess energy, i.e. energyabove the band gap equivalent, is passed on to the crystal in the form of heat.
Hence, the share of the usable energy mainly depends on the wavelength andthe band gap. • The external quantum collecting efficiency ηext(λ) is thelikelihood that an incident photon generates an electron–hole pair. It is closely related to the spectral response, which is a measure of the part of the energyconverted into charge carriers.
Spectral response external quantum collecting efficiency
In the absence of an external field, i.e. if a solar cell is short-circuited, thephotocurrent IPh is generated. This current can be calculated using the solarcell area A, the spectral sensitivity S and the spectrum of sunlight E(λ).
The irradiance E absorbed by the semiconductor is a share of the incoming irradiance E0. It depends on the thickness d of the semiconductor and thematerial-dependent absorption coefficient α:
Comparison of semiconductors • GaAs has anabsorption coefficient for light with a wavelength of about 1 μm of α(GaAs) app. 630 mm–1, whereas this value decreases to α(Si) app. 7.2 mm–1 for silicon.
For both semiconductors to absorb the same amount of light, the silicon will have to be 87.5 times thicker than a GaAs semiconductor. The wavelengthdependence of the absorption coefficients must be considered for an exactcalculation.
Crystalline silicon solar cells should have a thickness of at leastabout 200 μm for high absorptions.
PRODUCTION OF SOLAR CELLS AND SOLAR MODULES • Crystalline silicon solar cells Various semiconductor materials are suited to solar cell production; however, silicon is the most commonly used material today.
Silicon can mainly be found in quartz sand (SiO2). The following reduction process extracts silicon from the quartz sand at high temperatures of about1800°C (3272°F): Metallurgical grade silicon MG-Si, 98%Si
However, silicon gained by this process also has significant impurities. Silicon used by the computer industry is so-called electronic-grade silicon (EG-Si) for the production of semiconductor devices. Its impurity level is below 10–10 per cent.
SOG-Si • This high purity is not necessary for solar cell production, in which solargrade silicon (SOG-Si) is commonly used. Nevertheless, purification processes are needed for the production of SOG-Si.
Silicon is mixed with hydrogen chloride or chloric acid (HCl) in the silaneprocess. An exothermic reaction produces trichlorosilane (SiHCl3) andhydrogen (H2):
Trichlorosilane is liquid at temperatures of 30°C. Multiple fraction distillationsare used to remove the impurities. The chemical vapour deposition (CVD)process is used for silicon recovery.
Silicon is deposited as a thin silicon rod at temperatures of 1350°C (2462°F), when the trichlorosilane is brought intocontact with high-purity hydrogen:
The end product is a high-purity silicon rod with diameters of up to 30 cm(about 12 inches) and lengths up to 2 m (about 80 inches). These rods can beused for the production of polycrystalline solar cells, which consist of anumber of crystals, rather than a single crystal. The crystals of polycrystallinesilicon are differently oriented and separated by grain boundaries. Theyintroduce some efficiency losses.
To increase solar cell efficiency, monocrystalline material can be producedfrom polycrystalline material applying the Czochralski or float zone process. • Seeding a single crystal at high temperatures transforms the polycrystallinesilicon to the desired monocrystalline silicon. No grain boundaries are presentin the resulting material and thus losses within a solar cell are reduced.
The silicon slices, or so-called wafers, are cleaned and doped in thefollowing steps. • Hydrofluoric acid removes any saw damage. Phosphorus andboron are used for doping silicon to create the p-n junction. • Gaseous dopantsare mixed with a carrier gas such as nitrogen (N2) or oxygen (O2) forgasdiffusion, and this gas mixture flows over the silicon wafers. The impurityatoms diffuse into the silicon wafer dependingon the gas mixture, temperatureand flow velocity. Etching cleans the surface of the doped semiconductor.
Finally, cell contacts are added. A screen printing process adds the frontand rear contacts. Materials for the contacts are metals or alloys of aluminiumor silver. The rear contact usually covers the whole cell area. Thin contactfingers are used for the front contacts, because they obstruct and reflectsunlight. Only a minimum of the cell’s surface should be covered by contactsin order to optimize light capture.
Finally, an antireflective coating is added to the solar cell. This coatingreduces reflection at the metallic silicon surface. Titanium dioxide (TiO2) ismostly used for the coating and gives the solar cell its typical blue colour.
Solar Cell Structure and Front View of a CrystallineSilicon Solar Cell
Module sizes • To avoid climatic damage several solar cells withan edge length between 10 and 21 cm are combined in a solarmodule for cell protection. Many modules are made up of 32–40 cells; however,other module sizes with significantly more or fewer cells exist.
Thin film modules • Besides crystalline silicon, thin film modules hold promise for the cells of the future. They can be made of amorphous silicon and other materials such ascadmium telluride (CdTe) or copper indium diselenide (CuInSe2 or CIS).
Thinfilm modules can be produced usinga fraction of the semiconductor material necessary for crystalline modules andthis promises lower production costs in the medium term.
ELECTRICAL DESCRIPTION OF SOLAR CELLS • Simple equivalent circuit • A photovoltaic solar cell is a large area diode. It consists of an n-type and ptype doped semiconductor with a resulting space charge layer. Typically, anon-irradiated solar cell has nearly the same behaviour as a diode. Therefore,a simple diode can describe the equivalent circuit.
