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SOLAR CELLS

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Introduction:

Photovoltaic cells or solar cells are the devices

used in photovoltaic conversion i.e. when solar

radiation falls on these devices, it is converteddirectly into dc electricity. The first solar cell

was built by CharlesFritts, who coated the

semiconductor selenium with an extremelythin layer ofgold to form the junctions.

http://en.wikipedia.org/wiki/Goldhttp://en.wikipedia.org/wiki/Gold

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Advantages:

No moving parts, require littlemaintenance, and work quite

satisfactorily with beam or diffuseradiation. Readily adapted for varying power

requirements. Environmentally friendly source of

electricity.

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Description: Single crystal silicon cell.

p-type, n-type silicon and a

junction.

Metal electrodes (Ti-Ag

solder).

Front metal electrode.

Back metal electrode.

Anti-reflection coating and a

thin transparent

encapsulating sheet on the

top surface.

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Solar Module:

In order to obtain highervoltages and currents, individual

cells are fixed side by side on a

suitable back-up board andconnected in series and parallel

to form a module or solar panel

In turn a number of PV modules

are interconnected to form an

array.

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Principle of working of a solar cell:(1) Creation of pairs of positive

and negative charges(electrons-hole pairs) in the

solar cell by absorbed solar

radiation.

Photons of sunlight. Semiconductor materials.

Energy bands-valence and

conduction bands.

Band gap energy.

Electron-hole pairs.

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(2) Separation of the positive and negative charges by a

potential gradient within the cell. Electron-hole pair. Separated if potential

gradient exists.

Obtained by sandwiching of

p-type & n-type silicon. p-type_silicon doped with

boron. n-type_silicon dopedwith phosphorous.

Energy levels, jump inenergy levels.

Existence of potentialgradient, flow of directelectric current.

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Efficiency factors:

(1) Maximum power point: A solar cell may operate over a

wide range ofvoltages (V) and currents (I). By increasing theresistive load on an irradiated cell continuously from zero (ashort circuit) to a very high value (an open circuit) one candetermine the maximum-powerpoint, that is, the load forwhich the cell can deliver maximum electrical power at that

level of irradiation. Vm x Im = Pm in watts.(2) Energy conversion efficiency: The maximum conversion

efficiency of a solar cell is given by the ratio of the maximumuseful power to the incident solar radiation.

(3) Fill factor:Another defining term in the overall behavior of asolar cell is the fill factor(FF). This is the ratio of themaximum power point divided by the open circuit voltage(Voc) and the short circuit current (Isc).

http://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Current_%28electricity%29http://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Open_circuithttp://en.wikipedia.org/wiki/Maximum_power_theoremhttp://en.wikipedia.org/wiki/Maximum_power_theoremhttp://en.wikipedia.org/wiki/Wattshttp://en.wikipedia.org/wiki/Fill_factorhttp://en.wikipedia.org/wiki/Fill_factorhttp://en.wikipedia.org/wiki/Wattshttp://en.wikipedia.org/wiki/Maximum_power_theoremhttp://en.wikipedia.org/wiki/Open_circuithttp://en.wikipedia.org/wiki/Short_circuithttp://en.wikipedia.org/wiki/Current_%28electricity%29http://en.wikipedia.org/wiki/Voltage

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Materials and efficiency:

Various materials have been investigated for solar cells. There are two

main criteria - efficiency and cost. Efficiency is a ratio of the electricpower output to the light power input. By far the most common

material for solar cells is crystalline silicon. Crystalline silicon solar

cells come in three primary categories:

Single crystal or monocrystalline wafers: Most commercialmonocrystalline cells have efficiencies on the order of 14%; the

SunPower cells have high efficiencies around 20%. Single crystal cells

tend to be expensive, and because they are cut from cylindrical ingots,

they cannot completely cover a module without a substantial waste of

refined silicon. Most monocrystalline panels have uncovered gaps at

the corners of four cells. Sunpowerand Shell Solarare among the main

manufacturers of this type of cells.

http://en.wikipedia.org/wiki/Siliconhttp://www.sunpowercorp.com/html/http://www.sunpowercorp.com/html/http://en.wikipedia.org/wiki/Royal_Dutch_Shellhttp://en.wikipedia.org/wiki/Royal_Dutch_Shellhttp://en.wikipedia.org/wiki/Royal_Dutch_Shellhttp://www.sunpowercorp.com/html/http://en.wikipedia.org/wiki/Silicon

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Poly or multi crystalline made from cast ingots - large crucibles of

molten silicon carefully cooled and solidified. These cells are cheaper

than single crystal cells, but also somewhat less efficient. However,

they can easily be formed into square shapes that cover a greater

fraction of a panel than monocrystalline cells, and this compensates for

their lower efficiencies.

Ribbon silicon formed by drawing flat thin films from molten silicon

and has a multicrystalline structure. These cells are typically the leastefficient, but there is a cost savings since there is very little silicon

waste since this approach does not require sawing from ingots.

These technologies are wafer based manufacturing. In other words,

in each of the above approaches, self supporting wafers of ~300micrometres thick are fabricated and then soldered together to form a

module.

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Thin film approaches are module based. The entire module substrate

is coated with the desired layers and a laser scribe is then used to

delineate individual cells. Two main thin film approaches are

amorphous silicon and CIS:

Amorphous silicon films are fabricated using chemical vapor

deposition techniques, typically plasma enhanced (PE-CVD). These

cells have low efficiencies around 8%.

CIS stands for general chalcopyrite films of copper indium selenide

(CuInSe2)While these films can achieve 11% efficiency, their costs

are still too high.

There are additional materials and approaches. For example,

Sanyo has pioneered the HIT cell. In this technology, amorphous

silicon films are deposited onto crystalline silicon wafers.

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Cost reduction: Developing innovative manufacturing techniques (like the EFG

process), which speed up the production process, reducematerial wastage & yield large size cells.

Development of thin film solar devices which require much less

material and, if possible, use material which is inherently

inexpensive. For example Cadmium sulphide-cadmium tellurideand Copper Indium Diselenide solar cells.

Significant cost reduction is achieved by the use of

concentrators to focus the sunlight on high efficiency solar

cells. The concentration is achieved by using either linear or

circular Fresnel lenses or parabolic or paraboloid concentrators

which focus along a line or at a point, concentration ratios

ranging from 10 to 1000 being used.

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Applications: In spite of the high initial cost, photovoltaic systems are being used

increasingly to supply electricity for many applications requiringsmall amounts of power. Their cost-effectiveness increases with the

distance of the location (where they are to be installed) from the main

power grid lines. Some applications for which PV systems have been

developed are,

(1) Pumping water for irrigation and drinking and electrification for

remote villages for providing street lighting and other community

services.

(2) Telecommunication for the post and telegraph and railway

communication network.

In addition, in developed countries solar cells are being used

extensively in consumer products and applications.

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Conclusion:

There are currently many research groups active in this field

around the world.Much of the research is focussed on making solar cells cheaper

and/or more efficient, so that they can more effectively compete withother energy sources, including fossil energy. One way of doing this isto develop cheaper methods of obtaining silicon that is sufficiently

pure. Silicon is a very common element, but is normally bound insilica sand. Another approach is to significantly reduce the amount ofraw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approachto reducing the cost of electricity from solar cells.

The invention ofconductive polymersmay lead to thedevelopment of much cheaper cells that are based on inexpensiveplastics, rather than semiconductor grade silicon. However, allorganic solar cellsmade to date suffer from degradation upon exposureto UV light, and hence have lifetimes which are far too short to beviable.

http://en.wikipedia.org/wiki/Researchhttp://en.wikipedia.org/wiki/Fossil_energyhttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Conductive_polymershttp://en.wikipedia.org/wiki/Conductive_polymershttp://en.wikipedia.org/w/index.php?title=Organic_solar_cells&action=edithttp://en.wikipedia.org/w/index.php?title=Organic_solar_cells&action=edithttp://en.wikipedia.org/wiki/UVhttp://en.wikipedia.org/wiki/UVhttp://en.wikipedia.org/w/index.php?title=Organic_solar_cells&action=edithttp://en.wikipedia.org/wiki/Conductive_polymershttp://en.wikipedia.org/wiki/Sandhttp://en.wikipedia.org/wiki/Fossil_energyhttp://en.wikipedia.org/wiki/Research

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Reference: 1. B. O'Regan, M. Gratzel,Nature, Vol.353, 1991

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