2012 tus lecture 4

Lecture 4

Solar Cells: Theory I

Lecture 4. Solar cells: Motivation (examples) and Theory pn junctions under illumination

Homojunctions

Open-circuit voltage, short-

circuit current

IV curve, fill factor, solar-to-

electric conversion efficiency

Carrier generation and

recombination

Defects and minority carrier

diffusion

Current due to minority carrier

diffusion:

Solution to the diffusion

differential equation under

Spatially-homogeneous

generation, and

under Inhomogeneous

generation

Effect of an electric field

Heterojunctions

Celdas Solares

LA TECNOLOGIA FOTOVOLTAICA ESTA CONTEMPLADA PARA APLICACIÓN AUTONAMA. ELECTRIFICACION RURAL, BOMBEO DE AGUA, ILUMINACION DE CARRATERAS, MONITOREO DE NIVELS DE AGUA EN RIOS ETC. SON ALGUNOS EJEMPLOS

ESTA TECNOLOGIA CONVIERTE LA ENERGIA SOLAR DIRECTO A ENERGIA ELECTRICA DC UTILIZABDO MODULOS FOTOVOLTAICOS (CELDAS SOLARES)

EL MATERIAL DE CONSTRUCCION DE CELDA SOLAR SE LLAMA

“SEMICONDUCTOR”

6.6.06 - 8.6.06 Clemson Summer School

Dr. Karl Molter / FH Trier / [email protected]

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Example: PV-Roof and Front,



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Alwitra Solar-foil



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Example: Sports-Center Tübingen



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Example:

Fire-brigade



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Example: BP Showcase

Crystalline Silicon

• Polycrystalline Si – Made from melting Si into ingots, slicing off

wafers

– Cell efficiencies of 14% - 15%

– Widest use

• Monocrystalline Si – “Grown” crystals, more uniform structure

– Higher cell efficiencies (17% - 22%)

– Higher cost and better space utilization

• Most often manufactured in framed modules

Amorphous Thin-Film Si

• Si solution layered onto various substrates

• Conversion efficiencies of 9% to 12%

• Some framed module products, others bonded to flexible roofing materials

• Very uniform appearance, but less effective space utilization

• Less costly to produce than crystalline modules

Building Integrated PV

• Roof tile replacements • Solar glass • Thin film bonded to single-ply membrane

roofing material



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Solar-roof shingle

25 m2

300 m2

Concentrating Solar Panels

• Fresnel lenses in tracking panels concentrate light 500:1 on smaller amount of Si (Xerox PARC Research)

• Tracking mirrors focus sunlight on stationary Si (Energy Innovations “Sunflower”)

Energía Fotovoltaica

Efecto Fotovoltaico

LUZ SOLAR

Voltaje fotogenerado

Corriente eléctrica fotogenerada

CELDA

SOLAR

El Efecto Fotovoltaico (FV): es la generación de un voltaje en las terminales de un captador solar cuando éste es iluminado. Si a las terminales del captador se le conecta un aparato eléctrico, por ejemplo, un foco, entonces el foco se enciende debido a la corriente eléctrica que circula por él. Esta es la evidencia física del fenómeno fotovoltaico.



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History

• 1839: Discovery of the photoelectric effect by Bequerel

• 1873: Discovery of the photoelectric effect of Selen (change of electrical resistance)

• 1954: First Silicon Solar Cell as a result of the upcoming semiconductor technology ( = 5 %)



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energy-states in solids: Band-Pattern

Atom Molecule/Solid

ene

rgy-

stat

es

• • • • • • • •



36

energy-states in solids: Insulator

electron-energy

conduction-band

valence-band

Fermi- level EF

bandgap EG

(> 5 eV)



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energy-states in solids : metal / conductor

electron-energy

conduction-band

Fermi- level EF



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energy-states in solids: semiconductor

electron-energy

conduction-band

valence-band

Fermi- level EF

bandgap EG

( 0,5 – 2 eV)



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energy-states in solids: energy absorption and emission

electron-energy

conduction-band

valence-band

EF

+

-

h

Generation

+

-

h

Recombination

x

x



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doping of semiconductors In order to avoid recombination of photo-induced charges and to „extract“ their energy to an electric-device we need a kind of internal barrier. This can be achieved by doping of semiconductors:

IIIB IVB VB

Si

14

B

5

P

15

„Doping“ means in this case the replacement of original atoms of the semiconductor by different ones (with slightly different electron configuration). Semiconductors like Silicon have four covalent electrons, doping is done e.g. with Boron or Phosphorus:



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N - Doping

Si Si

Si

Si

Si

Si

Si

Si

Si

P+

-

n-conducting Silicon

-

crystal view

conduction-band

valence-band

EF

- - - - -

P+ P+ P+ P+ P+

majority carriers

Donator level

energy-band view



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P - Doping

Si Si

Si

Si

Si

Si

Si

Si

Si

p-conducting Silicon

B- +

+

crystal

conduction band

valence-band

EF B- B- B- B- B-

majority carriers

Acceptor level

+ + + + +

energy-band view



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p – type region

EF B- B- B- B- B-

+ + + +

n – type region

- - - -

P+ P+ P+ P+ P+

p/n-junction without light Band pattern view

+

-

- Diffusion

+

Diffusion

internal electrical field

+ - Ed

Ud

depletion-zone



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p–type region

EF B- B- B- B- B-

+ + + +

n–type region

- - - -

P+ P+ P+ P+ P+

irradiated p/n-junction band pattern view (absorption p-zone)

+

-

+

photocurrent

Internal electrical field

+ - Ed

Ud

depletion-zone E = h

-



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p/n–junction with irradiation crystal view

n-Silizium

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

- - - - - - - - - - - -

p-Silizium

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

+

- diffusion

-

+

electrical field E - - - - - - - - - - - - + + + + + + + + + + + +

+

-

h

-

+

-

-

-

+

depletion zone



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Antireflection- coating

The real Silicon Solar-cell

~0,2µm

~300µm

Front-contact

Backside contact

n-region

p-region

-

+

h

depletion zone

- - - - - - - - - - + + + + + + + + + +

Practical Considerations

This is for an “ideal cell”. In reality, there are other effects which can often be accounted for by introduction of a

multiplier “A” (larger than 1) in front of the kT/q term on the right.

Next we calculate the light-generated short circuit current for using the relevant differential equations.

Consider the p-region where the minority carriers are electrons.

Also assume that the minority current is diffusion-dominated.

We now solve this differential equation under various boundary

conditions: 1)uniform generation, semi-infinite

geometry 2) generation decaying exponentially with position, semi-infinite geometry 3)uniform generation, finite thickness 4)generation decaying exponentially

with position, finite thickness

n=0 x=0 x

N P

Heterojunctions

Efficiency Losses in Solar Cell

1 = Thermalization loss

2 and 3 = Junction and contact voltage loss

4 = Recombination loss



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2012 tus lecture 4

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