Crystalline Silicon Solar Cells - International Centre for ...indico.ictp.it/event/a04253/session/7/contribution/4/material/0/0.pdf · • 1954 First silicon solar cell at Bell Laboratories

Workshop on "Physics for Renewable Energy"

October 17 - 29, 2005

301/1679-2

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Crystalline Silicon Solar Cells

A. Goetzberger Fraunhofer Institut for Solar Energy Systems

Germany

ICTP Workshop 2005

Crystalline Silicon Solar Cells

Adolf Goetzberger

Fraunhofer-Institut fürSolare Energiesysteme ISE

Workshop on Physics for'RENEWABLE ENERGY'

October 17 - 29, 2005

Miramare-Trieste, Italy

ICTP Workshop 2005

Contents

1. History2. Physics of silicon solar cell3. Characteristics and equivalent circuit4. Materials5. Solar cell structures, loss mechanisms6. High efficiency cells7. Importance of efficiency8. Future developments

ICTP Workshop 2005

History of silicon solar cells

• 1839 first photovoltaic effect discovered by Edmond Becquerel• 1904 physical explanation by Albert Einstein• 1954 First silicon solar cell at Bell Laboratories by Chapin, Fuller and

Pearson. 6% efficiency which was soon increased to 10%• 1961 first fundamental theory by Shockley and Queisser based on

detailed balance.• 1991 first high efficiency silicon cell (?<20%) by M. Green.

• ……..

• ……..

ICTP Workshop 2005

Properties of silicon as a solar cell material

• Advantages– Unlimited supply of raw material– Well developed materials and device technology– Well developed understanding of physics– High solar cell efficiency– Well established long term solar cell stability

• Disadvantages– Low light absorption coefficient because of indirect band structure– Large thickness of material required– High cost of silicon wafers– At present shortage of solar grade silicon

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Carrier distribution at pn junction

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Diffusion gradients of minority carriers

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Front Contact

Emitter

Back Contact

Anti Reflection Layer

Back Surface Field

Base

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Solar cell characteristics

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Fundamental relations

I = I0(exp(VA/VT) -1)

I = I0(exp(VA/VT) -1) - IL

Voc = VT ln (IL / I0 + 1)

Efficiencylight

ocsc

light

m

P

VFFI

P

V==

mIη

I0=Diode saturation current

VA = Applied voltage

VT = Therm. Voltage (const)

IL = Light induced current

ICTP Workshop 2005

Equivalent circuit of solar cell

ICTP Workshop 2005

Crystal pulling apparatus Si casting apparatus

seed holder

seed

crystal neck

shoulder (cone)

thermalshield

heater

cruciblesusceptor

crucible

silicon meltAr + SiO + CO Ar + SiO + CO

(SiO)

Ar Ar

(SiO)

<Si>

crucible shaft

(Si)

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Multiple wire saw

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ICTP Workshop 2005

High efficiency solar cell

Metal grid

p-base

Aluminium

SiO2

Local back surface field

Emitter

Inverted pyramids

SiO2

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The buried contact solar cell

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The point contact solar cell

n+-diffusion

n-base

passivation layer

n-contact

n+-diffusion

p+-diffusion

p-contact

contact holesin passivation layer

passivation layer

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The hetero junction solar cell (HIT) by Sanyo

TCO

a – Si : H (n+, p+)

c – Si (p, n)

metal

a – Si : H (p+, n+)

grid

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The emitter wrap through solar cell

p-contact

n-contact

n+-diffusion

p-base

via hole (open)

passivation layer

n+-diffusion

ICTP Workshop 2005

The metal wrap through cell

p-contact

n-grid

n+-diffusion

p-base

via hole

(filled with metal)

passivation layer

n+-diffusion

n-bus

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Crys. Si + Modules

0

5

10

15

20

25

30

1940 1960 1980 2000 2020 2040 2060

Year

Eff

icie

nc

y Si cell eff.

Prod. modules

Lim. Eff.= 21%

η(t) = ηL(1 – exp((a0-a)/c))

ICTP Workshop 2005

Long term efficiency development

Efficiency for all technologies

0

5

10

15

20

25

30

35

40

1940 1960 1980 2000 2020 2040 2060

year

effic

iency

Cryst. Si

CIGS

a-Si

Org. Cell

Lim. Eff. = 42%

Thin Si

3rd gen. PV

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Market growth of solar cell technologies

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Learning curve for module prices

ICTP Workshop 2005

Future developments

• Short term: Lower cost– Thinner wafers– Further enhancement of efficiency by adapting high efficiency

techniques to production– New solar cell structures

• Long term– Crystalline thin film cells– Spectrum conversion to utilize solar spectrum more completely

• Crystalline silicon cells will dominate the market for a very long time