2012 tus lecture 4

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2012 tus lecture 4

  1. 1. Lecture 4Solar Cells: Theory I
  2. 2. 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
  3. 3. Celdas Solares
  4. 4. LA TECNOLOGIA FOTOVOLTAICA ESTACONTEMPLADA PARA APLICACIÓNAUTONAMA. ELECTRIFICACION RURAL,BOMBEO DE AGUA, ILUMINACION DECARRATERAS, MONITOREO DE NIVELS DEAGUA EN RIOS ETC. SON ALGUNOSEJEMPLOSESTA TECNOLOGIA CONVIERTE LAENERGIA SOLAR DIRECTO A ENERGIAELECTRICA DC UTILIZABDO MODULOSFOTOVOLTAICOS (CELDAS SOLARES)EL MATERIAL DE CONSTRUCCION DECELDA SOLAR SE LLAMA“SEMICONDUCTOR”
  5. 5. Example: PV-Roof and Front, Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 16 trier.de
  6. 6. Alwitra Solar-foil Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 17 trier.de
  7. 7. Example: Sports-Center Tübingen Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 18 trier.de
  8. 8. Example: Fire-brigade Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 19 trier.de
  9. 9. Example: BP Showcase Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 20 trier.de
  10. 10. 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
  11. 11. 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
  12. 12. Building Integrated PV• Roof tile replacements• Solar glass• Thin film bonded to single-ply membrane roofing material
  13. 13. Solar-roof shingle Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 24 trier.de
  14. 14. 25 m2
  15. 15. 300 m2
  16. 16. 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”)
  17. 17. Energía Fotovoltaica Efecto Fotovoltaico LUZ SOLAREl Efecto Fotovoltaico (FV): es la generación de un voltaje enlas terminales de un captadorsolar cuando éste es iluminado. Si CELDAa las terminales del captador se le SOLARconecta un aparato eléctrico, porejemplo, un foco, entonces el focose enciende debido a la corriente Voltaje fotogeneradoeléctrica que circula por él. Esta esla evidencia física del fenómenofotovoltaico. Corriente eléctrica fotogenerada
  18. 18. 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 %) Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 33 trier.de
  19. 19. energy-states in solids: Band-Pattern Atom Molecule/Solid • • • • • • • • energy-states Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 35 trier.de
  20. 20. energy-states in solids: Insulatorelectron-energy conduction-band Fermi- bandgap EG level EF (> 5 eV) valence-band Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 36 trier.de
  21. 21. energy-states in solids : metal / conductorelectron-energy Fermi- level EF conduction-band Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 37 trier.de
  22. 22. energy-states in solids: semiconductorelectron-energy conduction-band Fermi- bandgap EG level EF ( 0,5 – 2 eV) valence-band Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 38 trier.de
  23. 23. energy-states in solids: energy absorption and emissionelectron-energy conduction-band - x - EF h h + x + Generation Recombination valence-band Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 39 trier.de
  24. 24. doping of semiconductorsIn 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 canbe achieved by doping of semiconductors: IIIB IVB VB„Doping“ means in this case the replacement oforiginal atoms of the semiconductor by different ones 5(with slightly different electron configuration). BSemiconductors like Silicon have four covalent 14 15electrons, doping is done e.g. with Boron or Si PPhosphorus: Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 40 trier.de
  25. 25. N - Doping crystal view energy-band view conduction-band Si Si Si - - - - - - - majority carriers P+ P+ P+ P+ P+ Si Si+ P Si EF Donator level Si Si Si n-conducting Silicon valence-band Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 41 trier.de
  26. 26. P - Doping crystal energy-band view conduction band Si Si + Si Si B Si- + Si EF Acceptor level B- B- B- B- B- + + + + + majority carriers Si Si Si p-conducting Silicon valence-band Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 42 trier.de
  27. 27. p/n-junction without light Band pattern view depletion-zone Diffusion - Ud - - - - - P+ P+ P+ P+ P+ EF B- B- B- B- B- + + + + + + Diffusion Ed + - p – type region n – type region internal electrical field Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 43 trier.de
  28. 28. irradiated p/n-junction band pattern view (absorption p-zone) E = h depletion-zone photocurrent - Ud - - - - - P+ P+ P+ P+ P+ EF B- B- B- B- B- + + + + + + Ed + - p–type region n–type region Internal electrical field Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 44 trier.de
  29. 29. p/n–junction with irradiation crystal view h + + + + + + + + + + + + + + - + + + + + + + + + + + + + p-Silizium - + + + + + + + + + + + + + diffusion + - - + + + + + + + + +- +- + + - - - - - - - - + - - - - - - - - - + + + + + + + + +- +- +- E electrical field - - - - - - - - - - - - - - - n-Silizium - - -- - - - - - - - - - - - - - - - - - - - - - depletion zone - Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 45 trier.de
  30. 30. The real Silicon Solar-cell Front-contact -Antireflection- h n-regioncoating p-region ~0,2µm + + + + + + + + + + + - - - - - - - - - - ~300µm depletion zone Backside contact Clemson Summer School 6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 46 trier.de
  31. 31. Practical Considerations
  32. 32. This is for an “ideal cell”. In reality, there are other effects which can often be accounted for by introduction of amultiplier “A” (larger than 1) in front of the kT/q term on the right.
  33. 33. 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.
  34. 34. We now solve this differential equation under various boundary conditions: 1)uniform generation, semi-infinite geometry2) generation decaying exponentiallywith position, semi-infinite geometry3)uniform generation, finite thickness4)generation decaying exponentially with position, finite thickness
  35. 35. n=0 x=0 xN P
  36. 36. Heterojunctions
  37. 37. Efficiency Losses in Solar Cell1 = Thermalization loss2 and 3 = Junction and contact voltage loss4 = Recombination loss
  38. 38. Clemson Summer School6.6.06 - 8.6.06 Dr. Karl Molter / FH Trier / molter@fh- 80 trier.de

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