Transcript
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”
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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
• • • • • • • •
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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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