EML4450L12

EML4450L12 - S ustainable E nergy S cience and E ngineering...

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Unformatted text preview: S ustainable E nergy S cience and E ngineering C enter Photovoltaic Effect: An Introduction to Solar Cells Text Book: Sections 4.1.5 & 4.2.3 References: The physics of Solar Cells by Jenny Nelson, Imperial College Press, 2003. Solar Cells by Martin A. Green, The University of New South Wales, 1998. Silicon Solar Cells by Martin A. Green, The University of New South Wales, 1995. Direct Energy Conversion by Stanley W. Angrist, Allyn and Beacon, 1982. S ustainable E nergy S cience and E ngineering C enter Photovoltaic Effect Solar photovoltaic energy conversion: Converting sunlight directly into electricity. When light is absorbed by matter, photons are given up to excite electrons to higher energy states within the material (the energy difference between the initial and final states is given by h ). Particularly, this occurs when the energy of the photons making up the light is larger than the forbidden band gap of the semiconductor. But the excited electrons relax back quickly to their original or ground state. In a photovoltaic device, there is a built-in asymmetry (due to doping) which pulls the excited electrons away before they can relax, and feeds them to an external circuit. The extra energy of the excited electrons generates a potential difference or electron motive force (e.m.f.). This force drives the electrons through a load in the external circuit to do electrical work. S ustainable E nergy S cience and E ngineering C enter The solar cell is the basic building block of solar photovoltaics. The cell can be considered as a two terminal device which conducts like a diode in the dark and generates a photovoltage when charged by the sun. Pn-Junction Diode When the junction is illuminated, a net current flow takes place in an external lead connecting the p-type and n-type regions. The light generated current is superimposed upon the normal rectifying current-voltage characteristics of the diode. The power can be extracted from the device in a region shown in the fourth quadrant. S ustainable E nergy S cience and E ngineering C enter The solar cell is the basic building block of solar photovoltaics. When charged by the sun, this basic unit generates a dc photovoltage of 0.5 to 1.0 V and, in short circuit, a photocurrent of some tens of mA/cm 2 . Since the voltage is too small for most applications, to produce a useful voltage, the cells are connected in series into modules , typically containing about 28 to 36 cells in series to generate a dc output of 12 V . To avoid the complete loss of power when one of the cells in the series fails, a blocking diode is integrated into the module. Modules within arrays are similarly protected to form a photovoltaic generator that is designed to generate power at a certain current and a voltage which is a multiple of 12 V ....
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This note was uploaded on 10/22/2011 for the course EML 4450 taught by Professor Greska during the Fall '06 term at FSU.

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EML4450L12 - S ustainable E nergy S cience and E ngineering...

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