Solar Cells: Energy for the Future

1. Solar Cells:Energy for the Future

2. Basic Solar Cell Design DOE - Solar Energy Technologies Program National Renewable Energy Laboratory Page2

3. Measures of Efficiency • Short Circuit Current • 40~50mA/cm2 • “Illumination” current • Open Circuit Voltage • 500~700mV • Fill Factor • “Square area” • 0.7-0.85 • Efficiency • Production: 10-15% • Laboratory: 20-25% Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications” Page3

4. Efficiency Losses • Light reflection • Silicon • Electrical contact coverage • Cell thickness • Lower collection probability away from depletion region • Recombination • Defect states • Wavelength of Light • Material dependent • Material resistances • Both bulk and contact • Temperature • Metal and semiconductor dependence Page4

5. Silicon – Various Types DOE - Solar Energy Technologies Program • Single-crystal silicon • Czochralski • Float-zone • Polycrystalline silicon • Ribbon • Amorphous silicon Evergreen Solar Technology Page5

6. Materials -Silcon • Silicon • Indirect bandgap Eg = 1.142eV • Low absorptivity • Photon travels farther before absorbed • >100µm thick • Photon + Phonon absorption processes (indirect) • Recombination • Dominated by defects • Impurities and surface states Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications” Page6

7. Materials-Silicon • Silicon (continued) • Doping (~1016 cm-3) • P-type: Boron • Trace amounts in Cz growth process • N-type: Phosphorus • POCl3 + oxygen gas stream in heated furnace to oxidize Si • Diffusion of P from oxide into Si • Contacts • Vacuum evaporation • Three layers • Ti for good Si adherence • Ag for high conductivity • Pd barrier layer inbetween • Sintering at high T (500-600°C) for low resistance and high adherence Page7

8. Materials- Silicon • Contacts (continued) • Back is completely covered • Metal grid on front • Antireflective Coating • Vacuum evaporation • Various oxides of Si, Al, Ti, Ta… • Encapsulation • Structural back for support and moisture resistance • Al, Steel, Glass • Transparent front for light transmission • Glass Page8

9. Typical Silicon Cell Design Single and Polycrystalline Silicon The Solarserver Forum Amorphous Silicon DOE - Solar Energy Technologies Program Page9

10. Improving Silicon Cell Design (I) • Textured top surface • Selective etching to couple light into cell • Surface passivation • SiOx or SiNX • Restores bonding state of dangling surface Si bonds • Back Surface Field • Low recombination velocity interface • Screen print Al and fire to alloy Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications” Page10

11. Improving Silicon Cell Design (II) • Layer thickness • Thinner = lower light absorption • Carrier diffusion length and surface passivation important • If high recombination, then want thinner • Contact placement • Both on back: ~25% efficiency Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications” Handbook of Photovoltaic Science and Engineering Page11

12. Silicon Cell Efficiency Wikipedia.org Page12

13. Costs Handbook of Photovoltaic Science and Engineering Page13

14. Highest efficiency Many processing techniques Purity = Process dependent Expensive Circular cells Huge market High waste (ingot) Excellent electrical properties Structure Comparison Single Crystalline Polycrystalline • Cheaper than single crystalline • Less efficient • More easily formed into squares • High waste Page14

15. Advantages/Disadvantages of Silicon • Second most abundant element in the crust • Well-developed processing techniques • Huge market for crystalline Si • Highest efficiency ADVANTAGES DISADVANTAGES • Need thick layer (crystalline) • Brittle • Limited substrates • Expensive single crystals • Some processing wasteful Page15

16. Other Inorganic Solar Cells • Amorphous Si-based Solar Cells • Cu(InGa)Se2 Solar Cells • Cadmium Telluride Solar Cells • GaAs • InN Solar Cells Page16

17. Motivation for Other Materials • Graph of Semi-conductor band gap vs. Efficiency • A band gap of ~1.4eV matches the photon energies where the sun’s spectral intensity is strongest • GaAs is an example of a material with an optimal band gap • Silicon Band Gap is 1.1 eV, not optimal • This explains why there is a maximum in efficiency for single layer devices Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications” Page17

18. Amorphous Si Solar Cells Amorphous Silicon Semiconductor • First made 1974 • Plasma deposited • Doping • p-type: B2H6 • n-type: PH3 • Hydrogen helps properties • hydrogenated amorphous silicon (a-Si:H) • Alloying changes the band gap • Ge, C, O, or N • Ge used for bilayer devices Page18

19. a-Si:H: Photodiode Design • Photodiode: three layers • (typical example) • 20 nm p-type layer • Few hundred nm intrinsic layer • 20 nm n-type layer • Built-in E-Field • ~ 104 V/cm • Voc • Varies with band gap • Band gap varies with alloying Handbook of Photovoltaic Science and Engineering: Depiction of an a-Si:H photodiode Page19

20. a-Si:H: Photodiode Design • Direction of incoming light • Photons reach p-type first • Asymmetry in the drift of holes and electrons • Power drop if lighted from the n-type side • Width of Intrinsic layer • Thicker cells do not absorb much more light • Best thickness around 300nm (power saturates) Handbook of Photovoltaic Science and Engineering: Computer calculation of Power vs. Intrinsic Layer Thickness for different absorption coefficients. Solid symbols indicate illumination through the p-layer. Open Symbols indicate illumination through the n-layer Page20

21. a-Si:H: Cell Design Handbook of Photovoltaic Science and Engineering: Design of the cell • Two types of cell design • Superstrate (left): better for applications in which the glass substrate can be an architectural element • Substrate (right): Substrate can be flexible Stainless Steel • Substrate affects the properties of the first photodiode layer deposited Page21

22. Advantages of a-Si:H • Technology simple and inexpensive compared to crystalline technology • Still need to lower costs • Absorbs more light: need less material than c-Si • Better high temperature stability than c-Si • Band gap: • variable, 1.4-1.8 eV • Efficiency ~15% Handbook of Photovoltaic Science and Engineering: IV curves for amorphous silicon solar cells at two different times Page22

23. Further Advantages • Must be hydrogenated • Low efficiency • Poor electrical properties • High light absorption • Very little needed (~1/100th) • Produced at lower T • Many substrates • Low cost Disadvantages Page23

24. Advantages of Other Materials • Cu(InGe)Se2 (CIGS) • Thin film: easy fabrication, low cost • Band gap: variable, 1.0-1.2 eV • High efficiency – up to 18.8% • High radiation resistance • Can take large variations in composition without appreciably affecting the optical properties • Cadmium Telluride (CdTe) • Also Thin Film • Band gap in optimal range: 1.5eV • Efficiencies of about 7% Page24

25. Advantages of Other Materials • GaAs • Band gap in the optimal range: 1.4 eV • Efficiencies of >20% shown (1982) • InN • Optical band gap is also a good match to the sun’s spectrum: can tune the band gap • This means that multiple layers can be used to absorb different wavelengths and the crystal structures won’t mismatch • Band gap: 0.7 eV • Large heat capacity, resistant to radiation • many defects but this does not affect light emitting diodes of the same material Page25

26. Dye Sensitized Solar Cell (Grätzel Cell) • Overall power conversion efficiency of 10.4% has been attained (US National Renewable Energy Laboratory) • General Structure: • Glass • Transparent Conductor (ITO) • Semiconducting Oxide (TiO2) • Dye • Electrolyte • Cathode (Pt) • Glass M. Grätzel, “Dye Sensitized Solar Cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145–153 (2003) Page26

27. Components (I) • Mesoporous oxide films: • Network of tiny crystals measuring a few nanometers across. • Can be TiO2, ZnO, SnO2, Nb2O5, CdSe • Exceptional stability against photo-corrosion • Large band gap (>3eV) = transparency for large part of spectrum SEM of the surface of a mesoporous anatase film prepared from a hydrothermally processed TiO2 colloid. M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001). Page27

28. Components (II): The dye Dye absorbs light and generates current in the entire visible spectrum M. Grätzel, “Dye Sensitized Solar Cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145–153 (2003) Page28

29. Components (III) • Mesoscopic pores • filled with a semiconducting or a conducting medium (such as a p-type semiconductor, a polymer, a hole transmitter or an electrolyte) • Traditional electrolyte material consists of iodide (I-) and triiodide (I3-) as a redox couple. M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001). Page29

30. DSSC: Operation • Mesoporous dye-sensitized TiO2, receives electrons from the photo-excited dye • Oxidized dye in turn oxidizes the mediator in electrolyte • Mediator is regenerated by reduction at the cathode. M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001). Page30

31. DSSC: Degradation • Photo-chemical or chemical degradation of the dye (e.g. desorption of the dye, or replacement of ligands by electrolyte species or residual water molecules) • Direct band-gap excitation of TiO2 (holes in the TiO2 valence band act as strong oxidants) • Photo-oxidation of the electrolyte solvent, release of protons from the solvent (change in pH) • Dissolution of Pt from the counter-electrode in contact with electrolyte • Adsorption of decomposition products onto the TiO2 surface. J. Halme, “Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests,” Helsinki University of Technology, Masters Thesis (2002). Page31

32. DSSC: Benefits • Relatively cheap to fabricate • the expensive and energy-intensive high-temperature and high-vacuum processes needed for the traditional devices can be avoided • Can be used on flexible substrates • Can be shaped or tinted to suit domestic devices or architectural or decorative applications. • Stable even under light soaking for more than 10,000 h (with certain conditions/materials that are less efficient). M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001). Page32

33. DSSC: Drawbacks • Efficiencies not yet commercially competitive with Si-based alternatives. • Degradation still an issue • EC Cell cycles important to operation • Encapsulation necessary • High temperature stability a problem • Production only at small scale Page33

34. DSSC: Costs $0.40/Wp at 5% module efficiency (Zweibel 1999) J. Halme, “Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests,” Helsinki University of Technology, Masters Thesis (2002). Page34

35. Organic Heterojunction Solar Cells Efficiency of 3.5% has been achieved Bilayer P.Peumans, S.Uchida, S.R.Forrest. Nature, 425, 158 (2003). Bulk Heterojunction Page35

36. Summary of PV & PEC cells M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001). Page36

37. Photovoltaic Efficiency Comparison SPIE Magazine of Photonics Applications and Technologies Page37

38. Environmental Impact – CO2 Emissions • PV will be responsible for the displacement of millions of metric tons of CO2 per year, even under the most modest estimates V Fthenakis, S Morris PREDICTIONS OF FUTURE PV CAPACITY AND CO2 EMISSIONS' REDUCTION IN THE US. 2003 Page38

39. According to economic models, PV will result in the reduction of NOx, soot, and SO2 Environmental Impact – Other Pollutants V Fthenakis, S Morris Page39