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©Denis Lenardic
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07:03
This page was last time updated on
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Solar cells
"The Sun, with all the planets revolving around it, and depending on
it, can still ripen a bunch of grapes as though it had nothing
else in the Universe to do."
(Galileo Galilei)
Solar cells
(source/copyright: Hahn-Meitner-Institut
Berlin)
Solar cells are in fact large area semiconductor diodes. Due to photovoltaic effect energy of light (energy of photons) converts into electrical current. At p-n junction, an electric field is built up which leads to the separation of the charge carriers (electrons and holes). At incidence of photon stream onto semiconductor material the electrons are released, if the energy of photons is sufficient. Contact to a solar cell is realised due to metal contacts. If the circuit is closed, meaning an electrical load is connected, then direct current flows. The energy of photons comes in "packages" which are called quants. The energy of each quantum depends on the wavelength of the visible light or electromagnetic waves. The electrons are released, however, the electric current flows only if the energy of each quantum is greater than WL - WV (boundaries of valence and conductive bands). The relation between frequency and incident photon energy is as follows:
Where there is: h - Planck constant (6.626·10-34 Ws2), v - frequency (Hz)
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Solar cell features |
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Crystalline Silicon Solar Cells
Amorphous Silicon Solar Cells |
Solar cell testing |
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Solar cell models |
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The simplest solar cell model consists of diode and current source connected parallelly. Current source current is directly proportional to the solar radiation. Diode represents PN junction of a solar cell. Equation of ideal solar cell, which represents the ideal solar cell model, is:
Where is: IPh - photocurrent (A), IS - reverse
saturation current (A) (aproximately range 10-8/m2),
V - diode voltage (V), VT - thermal voltage (see equation
below), VT = 25.7 mV at 25°C, m - diode ideality factor = 1...5 x VT (-) (m = 1 for ideal diode)
Thermal voltage / VT / ( V ) can be calculated with the following
equation:
Where is: k - Boltzmann constant = 1.38 x 10-23 J/K, T -
temperature ( K ), q - charge of electron = 1.6 x 10-19 As
The working point of the solar cell depends on load and solar insolation. In the picture, I-U characteristics at short circuit and open circuit conditions can be seen. Very important point in I-U characteristics is Maximal Power Point - MPP. In practice we can seldom reach this point, because at higher solar insolation even the cell temperature increases, and consequently decreasing the output power. As a measure for solar cell quality fill-factor - FF is used. It can be calculated with the following equation:
Where is: Impp - MPP current ( A ), Vmpp - MPP voltage ( V ), Isc - short cirquit current ( A ), Voc - open cirquit voltage ( V ) In the case of ideal solar cell fill-factor is a function of open cirquit parameters and can be calculated as follows (Stone, see literature below):
Where is: voc - voltage calculated with equation below ( V )
Where is: k - Boltzmann constant = 1.38 x 10-23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10-19 As, m - diode ideality factor ( - ), Voc - open cirquit voltage ( V )
For additional explanations and further solar cell models description
please see literature below (Quaschning, Stone, Wagner for example).
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Solar cells related web sites |
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Q Cells - Production of solar cells explained in a short movie "How is a solar cell produced?"
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ProjectSol - On these pages you can take a interesting journey into a photovoltaic cell. You will learn how the silicon atoms convert sunlight into electricity. Languages: |
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New technologies - spherical solar cells |
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Spheral Solar Power - Spheral Solar Power cells produce electricity at considerably lower cost than conventional solar technology, and on a cost-par with fossil-fuel based electricity in many regions of the world. Languages:
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Kyosemi Corporation - provides high reliability products of opto-electronics to be used for optical communications, ATM, auto-vending machines, OA/FA equipments, precision optical instruments and hazard prevention equipments.
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New technologies - organic photovoltaic cells and photoelectrolysis |
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Global Photonic Energy Corporation - Incorporated in 1994 Global Photonic Energy Corporation, Inc. is a renewable energy technology development company. GPEC is harnessing photonic energy (Sunlight) using small-molecule organic materials to produce electricity and hydrogen - or - "Photo Fuel™".
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Literature and more information |
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