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Characteristics of a Photovoltaic Cell.

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Introduction

Characteristics of a Photovoltaic Cell

        Henri Becquerel was the first man to associate the production of electricity with

light. Seeing that many materials in a circuit reacted to light with varying amounts of

electricity, in 1839 he coined the expression ‘photovoltaic’: ‘photo’ for light and voltaic for

‘voltage’.                                                                                 

          Photovoltaic cells work by converting light energy into ‘Electromotive force’

(EMF) or Voltage in a circuit. The solar cell is the most important component in a

photovoltaic device and is made of silicon. The semiconductor in the solar cell can be used

for generating electricity. By placing such cells in panels, enough energy can be created for

practical use. First used practically used in the development of spacecraft, it was the energy

crisis of the 1970s that extended its use across the market.

How Do They Generate Electricity?

       Using the principal of ‘Band Gap theory’ scientists realised that by introducing

controlled amounts of impurities called ‘dopants’ into the matrix of the semiconductor, the

density of free electrons (the electrons able to ‘jump’) could be manipulated. This was

possible because the dopants are similar in structure and valence (the shell structure of the

atom) to fit into the matrix and have one electron more or less than the semiconductor

(silicon). If the dopant is phopherus for example, there will be 5 valence electrons, and a

negative (n-type)

...read more.

Middle

load. They protect the battery from overcharge or excessive discharge.

*        Battery - Batteries store the energy generated by the solar modules.

*        Inverter - Inverters convert DC (direct current) electricity into AC (alternating

current) to run many common appliances and equipment.

Band Gap Theory

        ‘Band Gap Theory’ is the basis for the functioning of the Photovoltaic cell. Using

layers of oppositely charged semiconductors, the principal of creating an electrical field or

‘gradient’ between the valence band and conduction band allows electrons to ‘jump’ across

the junction and transfer a current through the cell. The energy needed from the photons

striking the semiconductor layers is called ‘photon energy’ and must be at least equal to the

‘band gap energy’ in the covalent bond holding the electrons in place in the valance band.

This diagram illustrates the functioning of a semiconductor in comparison to an insulator or a

metal. In a giant metallic structure (most metals) the ‘gap’ is non-existent; the electrons are

free to travel in a ‘cloud’ or ‘sea’ between the bonded atoms. The opposite is apparent in

an insulator. Here the gap between the valence and conduction bands is too large for

electrons to freely ‘jump’ between them, and so electrical current cannot flow. A

semiconductor however draws a balance between the two extremes. Here, the two bands

...read more.

Conclusion

to an alternating voltage source ES with series impedance ZS (comprising resistance RS and reactance

XS), maximum power transfer to the load occurs when ZT is equal to ZS* (the complex conjugate of ZS)

such that RT and RS are equal and XT and XS are equal in magnitude but of opposite sign (one inductive

and the other capacitive).

When a load impedance ZT (comprising variable resistance RT and constant reactance XT) is connected

to an alternating voltage source ES with series impedance ZS (comprising resistance RS and reactance

XS), maximum power transfer to the load occurs when RT is equal to the magnitude of the impedance

comprising ZS in series with XT:

RT = |ZS + XT| = (RS2 + (XS + XT)2)½

Note that if XT is zero, maximum power transfer occurs when RT is equal to the magnitude of ZS:

RT = |ZS| = (RS2 + XS2)½

When a load impedance ZT with variable magnitude and constant phase angle (constant power factor) is

connected to an alternating voltage source ES with series impedance ZS, maximum power transfer to the

load occurs when the magnitude of ZT is equal to the magnitude of ZS:

(RT2 + XT2)½ = |ZT| = |ZS| = (RS2 + XS2)½

       The place of maximum power on these I-V graphs

       http://www.scolar.org.uk/html/school-info/ks4/ks4-2c.html#

Efficiency of cell = 12-15% efficient

       For the short circuit when you use EMF for the voltage, and take current you can

find the internal resistance.

...read more.

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