Photovoltaics direct conversion of sunlight to electricity

1. Photovoltaicsdirect conversion of sunlight to electricity _____________________________________קובץ זה נועד אך ורק לשימושם האישי של מורים למתמטיקה, פיזיקה, כימיה וביולוגיה ולהוראה בכיתותיהם. אין לעשות שימוש כלשהו בקובץ זה לכל מטרה אחרת, ובכלל זה: שימוש מסחרי, פרסום באתר אחר (למעט אתר בית הספר בו מלמד המורה), העמדה לרשות הציבור או הפצה בדרך אחרת כלשהי של קובץ זה או חלק ממנו.

2. Why Photovoltaics?

3. Noise and pollution free

4. Abundant, indigenous resource.

5. Distributed and remote applications

6. Reliable. Low operating costs

7. History of solar cells • 1839: Edmund Becquerel: photovoltaic effect • 1883: Charles Fritts: first solar cells made from selenium wafers • 1921: Albert Einstein: Nobel Prize for theories about photoelectric effect. • 1954: Bell Labs announces the invention of the first modern silicon solar cell. • 1957: Hoffman Electronics achieved 14% efficient cell. • 1964: The Nimbus spacecraft was launched with a 470-W PV array. • 1985: Solar cell more than 20% efficient.

8. Solar Energy Conversion Toward 1 Terawatt, 2008 , D. Ginley et al.

9. Semiconductors Metals Insulators Titanium Oxide Silicon Magnesium Semiconductors/ insulators Metals

10. Fermi Energy • The chemical potential of an electron (at absolute zero) • Only electron with energies greater than the Fermi energy may be acted on and accelerated in the present of an electric field= free electrons.

11. Silicon- a semiconductor In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current. This makes a silicon crystal an insulator rather than a conductor.

12. Current- made by moving holes and electrons

13. p- doping n- doping You can change the behavior of silicon and turn it into a conductor by doping it. In doping, you mix a small amount of an impurity into the silicon crystal.

14. What happens when you connect the two parts?

15. Built in potential P/N junc. N P

16. e - h+ e - h+ load The Photovoltaic (PV) effect: Light Absorption + Carrier Generation + Carrier Collection charge separation

17. Light Photoelectric effect P/N junc. N P

18. PV device:

19. Current – Voltage diagram Power = Current Voltage V = IR C- OPV- open circuit R= ∞ A- Short circuit- R=0

20. Efficiency of a solar cell depends on: Light absorption↑ (band gap) Conduction↑ Recombination↓ Current Voltage

21. Improve efficiency

22. How do Solar Cells work?

23. n h e e - - high energy photon - partial loss e e - - Energy h n p p - - type type useable photo- n n - - type type voltage h h + + low energy photon-total loss Energy Losses in single p-n junction solar cell

24. Quantum conversiona Threshold process Solar Energy Spectrum Infra- visible ultra- -Red -violet (IR) (UV)

25. Most solar energy is “wasted” as heat

26. Absorption Spectra of Some Promising PV Materials 106 105 a-SiGe:H a-Si:H CuInSe2 104 Absorption coefficient  (cm-1) CdTe 103 1.82 eV Crystalline Si 102 101 1.0 1.5 2.0 2.5 Photon energy hw (eV) 026587272 Thin-Film Solar Cells, Y. Hamakawa, Springer

27. The ideal cell 1. No reflection anti-reflective coating 2. No recombination high quality semiconductor, thin film 3. Maximum absorbance of light band gap of 1.1-1.4eV, thick film? 4. Maximum conduction/ minimum resistance high quality of semiconductor

28. Quantum dot Cells (optics!) 3d generation 2nd generation 1st generation, Si next generations • 4.5 6 15 • nm Nanocrystalline TiO2 Cheaper, simpler 15-20 nm m cm Development of Solar Cells

29. Si,Ge GaAs InP CdTe CuInSe2 Phenylene- Vinylidene ++ Ru dye-TiO2 Current types of PV devices Primarily based on solid-state electronic material systems • First genetarion: • Single or multi-crystal • Polycrystalline • Amorphous thin film • Second generation: • CdTe • CIGS • GaAs • Third generation: • Quantum dots technology • Multi-junction cell • Non-semiconductor technology- polymer cells • Thermophotonics • Nanosolar

30. 41.1% Multi-junction 25% Crystalline Silicon Thin films- 20% CIGS 16.7% CdTe 6.4% organic SC

31. Crystalline Silicon 25% efficiency Wafers cost accounts for over 50% of the total module cost

32. TFSC- Thin Film Solar Cell • Deposition of one or more thin layers of PV material on a substrate (usually glass). • Mass production- CdTe- cadmium telluride. CIGS- copper indium gallium selenide. TFSi- thin film silicon. (Dye sensitized)

33. Why TFSC? • A big part of the expence is the price of the substrate - as in silicon wafer. • Thin film- reducing the width of the substrate from micron to nanometers- minor diffusion length.

34. Chopra et al. 2004

35. TFSC- Thin Film Solar Cell- CIGS Highest efficiency tricky to deposit Cd toxic In scarce n-type p-type TCO= transparent conducting oxide 20% efficiency

36. TFSC- Thin Film Solar Cell- CdTe Easily deposited Cd toxic Te scarce 16.7% efficiency

37. What can we do? Better utilization of sunlight: Photon management

38. Photon Management multi-junction device structures

39. Third generation • Efficient- Infrared, UV spectrum • Light • Flexible • Compact

40. Multi-junction PV Thermodynamic Efficiency Limits non-concentrated Sunlight (AM 1.5)

41. Organic solar cells • Absorption of photons leads to the creation of bound electron-hole (excitation) pairs with a binding energy of 0.5 eV. • The excitation pairs have to diffuse to dislocation sites where their charge can be seperated and transported to the contacts. • Only about 30% of incident light is absorbed - polymers have bandgaps of above 2 eV.

42. Organic solar cells • Cheap!! • Less than 100nm thick • Processing performed at room temperature - can be preformed on flexible substrates, cheaper methods.

43. DISC- dye sensitized solar cell

44. Definition of efficiency: • Highest measured cell efficiencies: • ~ 35% III-V triple junction cell • (41% IV + III-V cells, with concentration) • ~ 25% single crystalline Si • ~ 20% single junction PX thin films (CIGS, Si) • ~ 11% dye sensitized solar cells (DSSC) • ~ 6.5% organic PV

45. From solar cell to cell array

48. How it’s made- solar panels