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Jordan University of Science and Technology Department of applied Physics. Advisor: Dr. Adnan Shariah. Solar cells [ Operation principles and testing ]. Ghassan Mohammad Masadeh. Table of content. Introduction.
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Jordan University of Science and TechnologyDepartment of applied Physics Advisor: Dr.Adnan Shariah Solar cells [Operation principles and testing] Ghassan Mohammad Masadeh
Introduction Solar cells are devices in Which sun light releases electric charges so they can move freely in a semiconductor and ultimately flow through an electric load, such as a light bulb or a motor . The phenomenon of producing voltages and currents in this way is known as the photovoltaic effect (PV e ffect).
The PV effect was discovered in 1839 by French physicist Becquerel. It remained in the laboratory until 1954. When Bell laboratories produced the first silicon solar cell.
Solar cells are already being used in terrestrial applications where they are economically competitive with alternative sources. Examples are powering communications equipment ,pumps, and refrigerators located far from existing power lines . The first of the economic forces the rising price of conventional sources particularly those employing fossil fuels. continues automatically, in part because the resource is limited.
The second reducing the cost of electricity from solar cell system is the subject of world wide research and development efforts today. To increase the economic attractiveness of the solar cell option: • - increase cell efficiencies • - reduce cost of producing cells modules. • - devise new cell or system designs for lower total cost per unit power out put.
Semiconductors: Semiconductors are crystals that in their pure state are resistive, but when the proper impurities are added this process is called doping in trace amounts often measured in parts per billion, display much lower resistance along with other interesting and useful properties. Depending on the selection of impurities added, semiconductor materials of two electrically different types: n-type and p-type.
p-n junction: The basic structure formed by the intimate contact of p-type and n-type semiconductors n-type semiconductor: A semiconductor type in which the density of holes in the valence band is exceeded by the density of electrons in the conduction band. P-type semiconductor : A semiconductor type in which the density of electrons in the conduction band is exceeded by the density of holes in the valence band.
The most important physical phenomena employed in all solar cells are very similar to the classical p-n junction. When light is by the junction the energy of the absorbed photons is transferred to the electron and hole both free to move. These particles diffuse through the semiconductor and ultimately encounter an energy barrier that permits charged particles of one sign to pass but reflects those of the other sign. The charge carriers in the junction region create a potential gradient, get accelerated under the electric field and circulate as the current through an external circuit.
The current from the cell may pass directly through the load or it may be changed first by the power, conditioning equipment from those provided by the cell, other subsystems that may also be used include energy storage devices such as batteries and concentrating lenses or mirror that focus the sunlight onto a smaller to and hence less costly semiconductor cell.
Performance of solar cells An important feature of solar cells is that the voltage of the cell does not depend on its size, and remains fairly constant with changing light intensity. However, the current in a device is almost directly proportional to light intensity and size.
Figure below shows example I / V curves for a single cell as a function of light input
A solar cell's power output can be characterized by two numbers a maximum Open Circuit Voltage Voc measured at zero output current and a short circuit current Isc Where: Voc = k T/ q ln [(IL /I o)+1] And I = I o [ exp.(qv/kT)-1] - IL I L = q AG (L e+ W + L h ) And the power can be computed via this equation: P = I * V
As you might then expect, a combination of less than maximum current and voltage can be found that maximize the power produced. This condition is called "maximum power point”
the single crystal silicon cell Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell. To do this, it will share electrons with four of its neighbor silicon atoms. except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.
A solar cell has silicon with impurities other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell would not work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.
In solar cells applications this characteristic is usually drawn inverted about the voltage axis. The cell generates no power in short-circuit or open-circuit. The cell delivers maximum power P max when operating at a point on the characteristic where the product IV is maximum. This is shown graphically below where the position of the maximum power point represents the largest area of the rectangle shown.
