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Master in Materials Science:Master in Materials Science:

Materials for Micro and Nano TechnologiesMaterials for Micro and Nano Technologies

Class Presentation onClass Presentation on

Photovoltaics (Solar Electricity)Photovoltaics (Solar Electricity)ByBy

Sana UllahSana UllahStudent Master in Materials ScienceStudent Master in Materials Science

IUSSIUSS

PaviaPavia -- ItalyItaly

June 12, 2008June 12, 2008



SummarySummary

1.1. What is Photovoltaics (PV)What is Photovoltaics (PV)2.2. Basis of PV OperationBasis of PV Operation3.3. Solar CellsSolar Cells

3.1 Solar Cell Structure3.1 Solar Cell Structure3.2 Absorption of Light 3.2 Absorption of Light 3.3 Generation of Electricity3.3 Generation of Electricity

4.4. Different Cell TypesDifferent Cell Types5.5. Efficiency LimitationsEfficiency Limitations6.6. Goals of onGoals of on--going researchgoing research7.7. Potential of PVPotential of PV



What is Photovoltaics ?What is Photovoltaics ?

Technology of generating direct current electrical powerTechnology of generating direct current electrical power

from semiconductors when they are illuminated byfrom semiconductors when they are illuminated by

photonsphotons

Solar Cell (an Individual photovoltaics element) is usedSolar Cell (an Individual photovoltaics element) is usedto generate this powerto generate this power

Sources: Handbook of Photovoltaic Science and Engineering. John Wiley & Sons, Ltd. ISBN: 0Sources: Handbook of Photovoltaic Science and Engineering. John Wiley & Sons, Ltd. ISBN: 0--471471--4919649196--99



Basis of PV OperationBasis of PV Operation

Solar Cell is an indiviudual PV element.Solar Cell is an indiviudual PV element.

Electrons from valence band are moved to conductionElectrons from valence band are moved to conductionband by application of energy exceeding band gapband by application of energy exceeding band gap

This energy needed to free the electrons could beThis energy needed to free the electrons could be

supplied by photonssupplied by photons




Solar CellSolar Cell

model of a crystalline solar cellmodel of a crystalline solar cell

Sources: The Solarserver Forum for Solar Energy, www.solarserver.deSources: The Solarserver Forum for Solar Energy, www.solarserver.de



Solar Cell StructureSolar Cell Structure

Consists of two layers of semiconductor material, one pConsists of two layers of semiconductor material, one p--type and one ntype and one n--type sandwiched to form a pntype sandwiched to form a pn--junction junction

Metal grid allows light fall on semiconductorMetal grid allows light fall on semiconductor

Antireflection layer increases amont of light transmitted Antireflection layer increases amont of light transmitted




Absorption of Light Absorption of Light

Creation of electronCreation of electron--hole pair via absorption of sunlight hole pair via absorption of sunlight

Both the total energy and momentum of particlesBoth the total energy and momentum of particlesinvolved must be conservedinvolved must be conserved

Direct excitation from valence to conduction band isDirect excitation from valence to conduction band iscalled fundamental absorptioncalled fundamental absorption




Absorption of Light Absorption of Light Direct bandDirect band--gap semiconductorsgap semiconductors

In direct band gap semiconductors, like GaAs, GaInP, CdTe, etc.In direct band gap semiconductors, like GaAs, GaInP, CdTe, etc.both energy and momentum are conserved in the transitionboth energy and momentum are conserved in the transition

Photon absorption in a direct band gap semiconductor for an incident photonPhoton absorption in a direct band gap semiconductor for an incident photon

withwith energyenergy hh == EE2 2 EE11 > E> EGG




Absorption of Light Absorption of Light Indirect bandIndirect band--gap semiconductorsgap semiconductors

Light absorption is facilitated by either absorption or emission of aLight absorption is facilitated by either absorption or emission of aPhononPhonon

Photon absorption in an indirect band gap semiconductor for a photon with energyPhoton absorption in an indirect band gap semiconductor for a photon with energy hh < E< E2 2 EE1 and1 anda photon with energya photon with energy hh > E> E2 2 EE1. Energy and momentum in each case are conserved by the1. Energy and momentum in each case are conserved by theabsorption and emission of a phonon, respectivelyabsorption and emission of a phonon, respectively




Generation of ElectricityGeneration of Electricity

Electrons are pumped by photons from valence band to the conductionElectrons are pumped by photons from valence band to the conduction

band. There they are extracted by a contact selective to the conductionband. There they are extracted by a contact selective to the conductionband (an nband (an n--doped semiconductor) at a higher (free) energy and delivereddoped semiconductor) at a higher (free) energy and deliveredto the outside world via wires, where they do some useful work, then areto the outside world via wires, where they do some useful work, then arereturned to the valence band at a lower (free) energy by a contact selectivereturned to the valence band at a lower (free) energy by a contact selectiveto the valence band ( a pto the valence band ( a p--type semiconductor)type semiconductor)




Different Cell TypesDifferent Cell Types

About 90% of the PV production consists of Silicon Semiconductor because of About 90% of the PV production consists of Silicon Semiconductor because of

tremendous scientific and technical infrastructure availabletremendous scientific and technical infrastructure availableSilicon bandSilicon band--gap of 1.1 eV is almost optimum to make a goodgap of 1.1 eV is almost optimum to make a goodsolar convertersolar converterSi is one of the most abundant minerals in the earths crust Si is one of the most abundant minerals in the earths crust

Three types of cells according to the technologiesThree types of cells according to the technologies

1.1. Mono Crystalline SiliconMono Crystalline Silicon

2.2. Multi Crystalline SiliconMulti Crystalline Silicon3.3. Amorphous Silicon Amorphous Silicon





Distribution of the PV market by technologies








Efficiency LimitationsEfficiency Limitations

Different semiconductors or combination are suited only for specificDifferent semiconductors or combination are suited only for specificspectral rangesspectral ranges

Certain amount of energy is transformed into heat Certain amount of energy is transformed into heat

There are optical losses, such as shadowing or reflectionThere are optical losses, such as shadowing or reflection

Electrical resistance losses in semiconductors and connecting cablesElectrical resistance losses in semiconductors and connecting cables

Disrupting influence of contamination, surface effects, crystal defects,Disrupting influence of contamination, surface effects, crystal defects,

etc.etc.




Goals of current researchGoals of current research

1.1. Use less expensive semiconductor materials. These tend to be lessUse less expensive semiconductor materials. These tend to be lesspure and less perfect with improved performancepure and less perfect with improved performance

2.2. Even with this poorer material keep a high production yield, that is,Even with this poorer material keep a high production yield, that is,reduce the number of cells or modules rejected by the qualityreduce the number of cells or modules rejected by the qualitycontrol.control.

3.3. Increase material utilization by reducing waste in semiconductor andIncrease material utilization by reducing waste in semiconductor andcell fabricationcell fabrication

4.4. Increase solar cell flux on the solar cells by using concentratorsIncrease solar cell flux on the solar cells by using concentratorswithout increasing cost or optical losses too much. In this way, lesswithout increasing cost or optical losses too much. In this way, lesssemiconductor material is used.semiconductor material is used.

5.5. Increase solar radiation utilization by absorbing more of theIncrease solar radiation utilization by absorbing more of thespectrum efficientlyspectrum efficiently

6.6. Increase speed and throughput of manufacturing processesIncrease speed and throughput of manufacturing processes

7.7. Simplify processing steps (this reduces fabrication costs andSimplify processing steps (this reduces fabrication costs andincreases the yield) and reduceincreases the yield) and reduce equipment costsequipment costs

8.8. Reduce costs and improve reliability of auxiliary elements.Reduce costs and improve reliability of auxiliary elements.




Current Use and Potential of PV : a comparisonCurrent Use and Potential of PV : a comparison

ResourceResource Current useCurrent use TechnicalTechnicalpotentialpotential

TheoreticalTheoreticalpotentialpotential

HydroPowerHydroPower 99 5050 147147

BiomassBiomassenergyenergy

5050 >276>276 29002900

SolarSolar

energyenergy

0.10.1 >1575>1575 3900 0003900 000

WindWindenergyenergy

0.120.12 640640 60006000

Units: exajoule per yearFor comparison: global primary energy demand 402 exajoule / annum




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