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Solar Cell(Photovoltaics)

: 20075418 Ju Dae-Hyun

Solar Cell (PV)

Light Electricity

http://www.ci.tacoma.wa.us/power/greenpower/green-images/cf-light.jpghttp://www.ci.tacoma.wa.us/power/greenpower/green-images/cf-light.jpg

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Nonrenewable Enargy

(Renewable Energy)

(Nonrenewable Energy)

Renewable

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What is a Solar Cell?

A structure that converts solar energy directly to DC electricenergy.

It supplies a voltage and a current to a resistive load (light,battery, motor).

Power = Current x Voltage=Current2 x R= Voltage2/R

It is like a battery because it supplies DC power.

It is not like a battery because the voltage supplied by the cellchanges with changes in the resistance of the load.

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Basic Physics of Solar Cells

Silicon (Si) is from group 4 of the periodtable. When many Si atoms are in closeproximity, the energy states form bands offorbidden energy states.

One of these bands is called the bandgap(Eg) and the absorption of light in Si isa strong function of Eg.

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The Sun daily provides about 10 000 times more energy tothe Earth than we consume

Photovoltaic technology directly converts solar energy intoelectricity

No moving parts

no noise

no emissions

long lifetime Large industrial potential - cost reductions needed

Feedstock for PV industry is silicon - the second most abundant element in the crust of the Earth

The Sun as Energy Source

http://www.irish-energy.ie/images/lighting.jpghttp://sprg.ssl.berkeley.edu/jgr/sun.gif

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Solar Energy status

Market is exploding

The solar industry is very profitable

Lack of highly purified silicon (polysilicon)

Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

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Actual Growth vs. Historic Forecasts

Actualmarket

development

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Solar Energy status

Market is exploding

The solar industry is very profitable

Lack of highly purified silicon (polysilicon)

Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

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159

435

857

1705

2454

0

200

400

600

800

1000

1200

1400

1600

1800

2001 2002 2003 2004 2005

(MNOK)

- Gross revenue development

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Solar Energy status

Market is exploding

The solar industry is very profitable

Lack of highly purified silicon (polysilicon)

Cost of solar electricity is too high, R&D focus

on reducing cost and increasing efficiency

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Solar Grade Silicon Supply-Demand

(MT/year)

0

5 000

10 000

15 000

20 000

25 000

2002 2003 2004 2005 2006 2007 2008 2009 2010

SOG Polysilicon supply SOG Polysilicon demand

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Solar Energy status

Market is exploding

The solar industry is very profitable

Lack of highly purified silicon (polysilicon)

Cost of solar electricity is too high, R&D fo

cus on reducing cost and increasing effici

ency

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Cost reductions existing technologies

Thinner wafers

- Wire sawing

- Laser cutting and etching Higher efficiencies

- Semiconductor technologies on single

crystal wafers (examples Sanyo / SunPower) Thin film technologies (flat panel display)

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Public incentives are important

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Cost goals for third generation solar cells

Efficiency and cost projections for first-, second- and third generation photovoltaic

technology (wafers, thin-films, and advanced thin-films, respectively)Source: University of New South Wales

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Next generation technology

Silicon nanostructures

Bandgap engineering of silicon.

Applications could be tandem solar cells and energy selective contacts for hot carrier solar cells.

Fabrication of silicon nanostructures consisting o

f quantum well and quantum dot super lattices toachieve band gap control

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Understanding cell efficiency

SOLAR SPECTRUM AM 1,5

(1000 watt/m2)

0

2

4

6

8

10

12

14

16

18

200 300 400 500 600 700 800 900 1000 1100 1200 1300

wavelength, nm

Irradiance,

watt/m2

Irradiation AM 1,5

Useful irradiation (c-Si)

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Hot carrier Cells

This concept tackles the major PV loss mechanism of thermalisationof carriers.

The purpose is to slow down the rate of photoexcited carrier coolingcaused by phonon interaction in the lattice to allow time for the carriers to be collected whilst they are still hot, and hence increasing the voltage of a cell.

Next generation technology (cont.)

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Thermoelectric solar cells

Application of the concept of energyselective elec

tron transport used in hot carrier solar cells, to develop thermo electrics and thermo-ionics devices.

Next generation technology (cont.)

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Polysilicon Wafer Solar Cell Solar Module

Chemical Process

(purification)

Casting

Cutting

Surface Treatment Assembly

Systems

Installation

Operation

The PV Value Chain (multi-crystalline)

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Prices are actually increasing

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How does solar energy work?

Solar Electric or Photovoltaic Systems convert some of the energy in

sunlight directly into electricity. Photovoltaic (PV) cells are made

primarily of silicon, the second most abundant element in the earth'scrust, and the same semiconductor material used for computers. When

the silicon is combined with one or more other materials, it exhibits

unique electrical properties in the presence of sunlight. Electrons are

excited by the light and move through the silicon. This is known as the

photovoltaic effect and results in direct current (DC) electricity. PV

modules have no moving parts, are virtually maintenance-free, and

have a working life of 20 - 30 years.

Silicon Solar cell

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PhotovoltaicsMost current solar cells are photovoltaicTypically made from silicon or amorphous silicon.

Typical efficiency ~ 12%.Best efficiency ever in laboratory: ~30%.Theoretical maximum, including concentrating light: 43%

Generic design: doped pnjunction.Photons come in and photoionize donors.Built-in electric field at junction causes carriers to flow,building up a potential (voltage) btw the pand nsides.Clearly one can play with different band gap systems toarrive at materials with different absorption spectra.

Also, good mobility of charge essential for this to work well -trapping of charge or poor mobility will kill efficiency.

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Principle p-n Junction Diode.

Silicon Solar cell

Ref. Soft Condensed Matter physics group in

univ. of Queenland

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http://localhost/var/www/apps/Program%20Files/QuickTime/QuickTimePlayer.exe

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p n

p

n

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:Si ingot 330m 2cm x 2cmSurface cleaning

Texturing :chemical v-groove

p-n junction :

POCl3(900C)

ITOincreasing minorty carrier correction, ARC

Poly-Si Solar cell Making process

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Back Surface FieldDeposition Al and Ag ohmic-contact

Forward surface Electrode

Anti-reflection coating(ARC) TiO2

deposition

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H2 diffusiondangling bond H2 bonding Decreasing recombination

Measure

H2 H2 H2 H2

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- An individual PV cell typically produces between 1 and 2 watts

Solar Cell, Module, Array

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decrease the area of solar cell material being used in a system

Concentrator collectors

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Uses for Solar Energy

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Solar Home Systems

Space

Water

PumpingTelecom

Main Application Areas Off-grid

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Residential Home

Systems (2-8 kW)PV Power Plants

( > 100 kW)

Commercial Building

Systems (50 kW)

Main Application Areas

Grid Connected

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Solar energy will become the most important and cost-efficient energy source in the future.

The present lack of silicon feedstock is promoting a rapid development of next generation technology.

Immediate actions are taken to cut thinner wafers and increase cell efficiencies for crystalline silicon.

New thin film technologies are being developed

Stronger influence from semiconductor industry will accelerate thedevelopment of better technologies

Nanosilicon and other third generation technologies may offer a long-term solution for the future solar energy technology.

Conclusions