Lecture 2 Solar Cell theory: pn junctions under Illumination Homojunctions Open-circuit voltage, short-circuit current, fill factor, IV curve, Solar-toelectric Conversion Efficiency Carrier Generation and recombination Defects and Minority carrier diffusion Solutions to the diffusion equation under spatially-homgeneous and Inhomogeneous generation 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
Example: PV-Roof and Front, 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 14
Alwitra Solar-foil 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 15
Example: Sports-Center Tübingen 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 16
Example: Fire-brigade 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 17
Example: BP Showcase 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 18
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
Solar-roof shingle 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 22
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 )
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 %) 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 30
Energía Fotovoltaica Efecto Fotovoltaico LUZ 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. CELDA SOLAR Voltaje fotogenerado Corriente eléctrica fotogenerada
pn junction physics
electron-energy energy-states in solids: energy absorption and emission conduction-band - x - E F h + x + h Generation Recombination valence-band 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 34
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: 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: 5 IIIB IVB VB B 14 15 Si P 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 35
N - Doping crystal view energy-band view conduction-band Si Si - Si Si P + - Si Si E F - - - - - P + P + P + P + P + majority carriers Donator level Si Si Si n-conducting Silicon valence-band 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 36
P - Doping crystal energy-band view conduction band Si Si + Si Si Si B - + Si Si Si Si E F Acceptor level B - B - B - B - B - + + + + + majority carriers p-conducting Silicon valence-band 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 37
p/n-junction without light Band pattern view depletion-zone U d - - - - P + P + P + P + P + - Diffusion - B - B - B - B - B - E F + + + + + + Diffusion n type region E d + - internal electrical field p type region 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 38
irradiated p/n-junction band pattern view (absorption p-zone) E = h depletion-zone U d - - - - P + P + P + P + P + - photocurrent - B - B - B - B - B - E F + + + + + + n type region E d + - Internal electrical field p type region 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 39
p/n junction with irradiation crystal view + + + + + + + + + + + + + + h - + + + + + + + + + + + + p-silizium - + + + + + + + + + + + + - diffusion - - + -+ -+ + - + - + - + - + - + - + - + - + - + - -+ + - + - + - + - + - + - + - + - + - + - + - - - - - - - - - - - - - n-silizium - - - - - - - - - - - - - - - - - - - - - - - - - + - E electrical field depletion zone 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 40
The real Silicon Solar-cell Front-contact Antireflectioncoating h - n-region p-region ~0,2µm ~300µm + + + + + + + + + + - - - - - - - - - - + depletion zone Backside contact 6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 41
6.6.06-8.6.06 Clemson Summer School Dr. Karl Molter / FH Trier / molter@fhtrier.de 45
We now calculate the lightgenerated minority (diffusion) current
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