1. Solar Cells: Energy for the Future

2. Basic Solar Cell Design
DOE - Solar Energy Technologies Program
National Renewable Energy Laboratory
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3. Measures of Efficiency
Short Circuit Current
40~50mA/cm2
“Illumination” current
Open Circuit Voltage
500~700mV
Fill Factor
“Square area”
Efficiency
Production: 10-15%
Laboratory: 20-25%
Green, Martin A. “Solar Cells – Operation Principles, Technology and System Applications”
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4. Efficiency Losses Light reflection Electrical contact coverage
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
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5. Silicon – Various Types
DOE - Solar Energy Technologies Program
Single-crystal silicon
Czochralski
Float-zone
Polycrystalline silicon
Ribbon
Amorphous silicon
Evergreen Solar Technology
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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”
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7. Materials-Silicon Silicon (continued) Contacts 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 ( °C) for low resistance and high adherence
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8. Materials- Silicon Contacts (continued) Antireflective Coating
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
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9. Typical Silicon Cell Design
Single and Polycrystalline Silicon
The Solarserver Forum
Amorphous Silicon
DOE - Solar Energy Technologies Program
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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”
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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
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12. Silicon Cell Efficiency
Material
Laboratory Efficiency [%]
Production Efficiency [%]
Single crystal silicon
~ 24
14-17
Polycrystalline silicon
~ 18
13-15
Amorphous silicon
~ 13
5-7
Wikipedia.org
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13. Handbook of Photovoltaic Science and Engineering
Costs
Handbook of Photovoltaic Science and Engineering
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14. Structure Comparison Highest efficiency Many processing techniques
Single Crystalline
Polycrystalline
Highest efficiency
Many processing techniques
Purity = Process dependent
Expensive
Circular cells
Huge market
High waste (ingot)
Excellent electrical properties
Cheaper than single crystalline
Less efficient
More easily formed into squares
High waste
High light absorption
Very little needed
Produced at lower T
Many substrates
Must be hydrogenated
Low efficiency
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15. Advantages/Disadvantages of Silicon
Second most abundant element in the crust
Well-developed processing techniques
Huge market for crystalline Si
Highest efficiency
Need thick layer (crystalline)
Brittle
Limited substrates
Expensive single crystals
Some processing wasteful
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16. Other Inorganic Solar Cells
Amorphous Si-based Solar Cells
Cu(InGa)Se2 Solar Cells
Cadmium Telluride Solar Cells
GaAs
InN Solar Cells
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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 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”
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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
Amorphous Silicon as a semiconductor discovered in 1973 in Dundee Scottland by Walter Spear and Peter LeComber
Made by Carlson at RCA in Princeton. Findings reported in 1976 by Carlson and Wronski. (5,6,7)
Figure 12.1 Current density versus voltage under solar illumination for a very early single-junction amorphous silicon solar cell (Carlson and Wronski [5]) and from a recent “triple-junction” cell (Yang, Banerjee, and Guha [8]). The stabilized efficiency of the triple-junction cell is 13.0%; the active area is 0.25 cm2
Alloying: Ge used in bilayer cells
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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
Figure 12.3 In a pin photodiode, excess electrons are donated from the n-type to the p-type layers, leaving the charges and electric fields illustrated. Each photon absorbed in the undoped, intrinsic layer generates an electron and a hole photocarrier. The electric field causes these carriers to drift in the directions shown. pin diodes are incorporated into solar cells in either the superstrate or substrate designs. For amorphous silicon–based cells, photons invariably enter through the p-type window layer as shown here
Handbook of Photovoltaic Science and Engineering:
Depiction of an a-Si:H photodiode
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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)
Figure Computer calculation of the power output from a pin solar cell as a function of intrinsic layer thickness. The differing curves indicate results for monochromatic illumination with absorption coefficients from 5000/cm to /cm; for typical a-Si:H, this range corresponds to a photon energy range from 1.8 to 2.5 eV (cf. Figure 12.2). Solid symbols indicate illumination through the p-layer and open symbols indicate illumination through the n-layer. Incident photon flux 2 × 1017/cm2s; no back reflector
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
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21. Handbook of Photovoltaic Science and Engineering:
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
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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, eV
Efficiency ~15%
Handbook of Photovoltaic Science and Engineering:
IV curves for amorphous silicon solar cells at two different times
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23. Further Advantages High light absorption Very little needed (~1/100th)
Produced at lower T
Many substrates
Low cost
Disadvantages
Must be hydrogenated
Low efficiency
Poor electrical properties
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24. Advantages of Other Materials
Cu(InGe)Se2 (CIGS)
Thin film: easy fabrication, low cost
Band gap: variable, 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%
18.8% for .5 cm2, 13.4% for 3459 cm2
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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
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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)
M. Grätzel, “Dye Sensitized Solar Cells,” Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 4, 145–153 (2003)
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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).
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28. Components (II): The dye
A single layer of dye molecules absorb less than one percent of the incoming light.
Only dye in contact with semiconductor can inject electrons.
Mesoporous Semiconductor has a surface area available for dye chemisorption over a thousand times that of a flat, unstructured electrode of the same size.
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)
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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).
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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.
mesoporous dye-sensitized TiO2, receives electrons from the photo-excited dye (which is thereby oxidized).
Oxidized dye in turn oxidizes the mediator (redox species dissolved in electrolyte).
Mediator is regenerated by reduction at the cathode (by electrons circulated through the external circuit).
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
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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.
Most have been linked to degradation of electrolyte. – some can be alleviated by use of different solvents, or adding other things.
Causes are temp, light, and UV based
dye molecule must sustain at least 108 redox cycles of photo-excitation, electron injection and regeneration, to give a device service life of 20 years.
Can be achieved with certain solvents for electrolyte formulation (valeronitrile, or γ-butyrolactone)
J. Halme, “Dye-sensitized nanostructured and organic photovoltaic cells: technical review and preliminary tests,” Helsinki University of Technology, Masters Thesis (2002).
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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).
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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
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34. DSSC: Costs $0.40/Wp at 5% module efficiency (Zweibel 1999)
tin oxide coated glass is about $10/m2 (Zweibel 1999),
suggesting $20/m2, or $0.40/Wp at 5% module efficiency,
$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).
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35. Organic Heterojunction Solar Cells
Bilayer
P.Peumans, S.Uchida, S.R.Forrest. Nature, 425, 158 (2003).
Bulk Heterojunction
Light Absorption (creation of exciton (electron hole pair)
Exciton diffusion (diff length very short, therefore use bulk heterojunction)
Dissociation at juntion
Made by spin coating or evaporating solution of ETL and HTL – evaporating out the solvent, and annealing it to phase separate…
performance of the cell declined rapidly within hours of exposure to sunlight
Efficiency of 3.5% has been achieved
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36. Summary of PV & PEC cells
M. Grätzel, “Photoelectrochemical cells,” Nature, 414, 338 (2001).
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37. Photovoltaic Efficiency Comparison
SPIE Magazine of Photonics Applications and Technologies
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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
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39. Environmental Impact – Other Pollutants
According to economic models, PV will result in the reduction of NOx, soot, and SO2
V Fthenakis, S Morris
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