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Solar%20Cells:%20Energy%20for%20the%20Future

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Solar Cells: Energy for the Future Basic Solar Cell Design Measures of Efficiency Short Circuit Current 40~50mA/cm2 Illumination current Open Circuit Voltage ... – PowerPoint PPT presentation

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Title: Solar%20Cells:%20Energy%20for%20the%20Future


1
Solar CellsEnergy for the Future
2
Basic Solar Cell Design
DOE - Solar Energy Technologies Program
National Renewable Energy Laboratory
3
Measures of Efficiency
  • Short Circuit Current
  • 4050mA/cm2
  • Illumination current
  • Open Circuit Voltage
  • 500700mV
  • 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
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

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

Evergreen Solar Technology
6
Materials -Silcon
  • Silicon
  • Indirect bandgap Eg 1.142eV
  • Low absorptivity
  • Photon travels farther before absorbed
  • gt100µm 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
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 (500-600C) for low
    resistance and high adherence

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

9
Typical Silicon Cell Design
Single and Polycrystalline Silicon
The Solarserver Forum
Amorphous Silicon
DOE - Solar Energy Technologies Program
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
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
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
13
Costs
Handbook of Photovoltaic Science and Engineering
14
Structure Comparison
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

15
Advantages/Disadvantages of Silicon
ADVANTAGES
DISADVANTAGES
  • 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

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

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 suns 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
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-SiH)
  • Alloying changes the band gap
  • Ge, C, O, or N
  • Ge used for bilayer devices

19
a-SiH 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-SiH photodiode
20
a-SiH 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
21
a-SiH 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

22
Advantages of a-SiH
  • 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
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

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

25
Advantages of Other Materials
  • GaAs
  • Band gap in the optimal range 1.4 eV
  • Efficiencies of gt20 shown (1982)
  • InN
  • Optical band gap is also a good match to the
    suns spectrum can tune the band gap
  • This means that multiple layers can be used to
    absorb different wavelengths and the crystal
    structures wont 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

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, 145153 (2003)
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 (gt3eV)
  • 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).
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, 145153 (2003)
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).
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).
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).
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).
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

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

36
Summary of PV PEC cells
M. Grätzel, Photoelectrochemical cells, Nature,
414, 338 (2001).
37
Photovoltaic Efficiency Comparison
SPIE Magazine of Photonics Applications and
Technologies
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
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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