Solar Energy 101

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Solar Energy 101

• Peter Bermel
• Advisor John D. Joannopoulos
• Tim Heidel
• Advisor Marc Baldo
• March 4, 2008

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Outline

• Introduction to solar energy
• Solar thermal
• Solar photovoltaics
• Organic (soft) materials
• Inorganic (hard) materials
• Solar market
• Conclusion

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Inorganic Photovoltaics

• Device design
• Major categories
• Silicon
• Single crystal
• Polycrystalline
• Amorphous silicon
• Microcrystalline
• Micromorph
• CIGS
• Cadmium telluride

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Inorganic PV Device Design

• Solid-state device, based on diode
• Optical design maximizes absorption
• Absorption generates photocarriers
• Electric field brings photocarriers to terminals

electric field
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Monocrystalline Silicon PV

• One of the first, and still dominant, cell
  technologies
• Advantages
• Process is mature
• Relatively high efficiencies
• Disadvantages
• High materials usage
• High costs
• Batch processing

Czochralski process for creating monocrystalline
silicon ingots
Ingots are then sawed into individual wafers
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Polycrystalline Silicon PV

• Manufacturing improvement decreases
• Costs
• Kerf loss
• Disadvantages
• Lower electronic quality
• Increased fragility

Evergreens string ribbon process
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Amorphous Silicon PV

• First commercial thin-film technology
• Advantages
• Monolithic deposition
• Low materials usage
• Disadvantages
• Large bandgap
• Low material quality

Sanyos Amorton a-Si PV cell
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Microcrystalline Silicon PV

• Emerging commercial thin-film technology
• Advantages
• Broader absorption spectrum (vs. a-Si)
• Disadvantages
• Light trapping problematic in near-IR

CSG Solars nc-Si PV cell
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Micromorph PV

• Combines microcrystalline and amorphous silicon
• Can be double or triple junction
• Covers whole solar spectrum more efficiently ?
  20 efficiency improvement

Schematic of triple-junction cell design
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CIGS (Copper Indium Gallium Diselenide)

• Engineered for direct bandgap at target
  wavelength
• Promising efficiencies up to 20
• Sticking point manufacturing processes
• Vacuum deposition
• Inkjet-style printing

CIGS cell diagram (from AIST, Japan)
CIGS manufacturing (from AIST, Japan)
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CdTe (Cadmium Telluride)

• Advantages
• Direct bandgap at target wavelength
• Inexpensive fabrication process
• Disadvantages
• Susceptibility to degradation
• Cd toxic

CdTe cell diagram (from Awakura lab, Kyoto
University)
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Best Research-Cell Efficiencies (NREL)
Spectrolab
Multijunction ConcentratorsThree-junction
(2-terminal, monolithic)Two-junction
(2-terminal, monolithic) Crystalline Si
CellsSingle crystalMulticrystallineThin
Si Thin Film TechnologiesCu(In,Ga)Se2CdTe
Amorphous SiH (stabilized) Emerging PVOrganic
cells
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Spectrolab
Japan Energy
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NREL/ Spectrolab
NREL
28
NREL
UNSW
UNSW
24
UNSW
Spire
UNSW
NREL Cu(In,Ga)Se2 14x concentration
UNSW
Stanford
Spire
UNSW
ARCO
Georgia Tech
20
NREL
Efficiency ()
Sharp
Georgia Tech
Westing- house
Varian
NREL
NREL
NREL
16
UniversitySo. Florida
NREL
No. Carolina State University
NREL
AstroPower
Euro-CIS
Boeing
Solarex
ARCO
Boeing
Kodak
12
Boeing
UnitedSolar
AMETEK
Masushita
United Solar
AstroPower
Kodak
Boeing
8
Monosolar
Photon Energy
RCA
Solarex
University California Berkeley
Boeing
Princeton
University of Maine
4
RCA
RCA
UniversityKonstanz
RCA
RCA
RCA
NREL
RCA
0
2000
1995
1990
1985
1980
1975
026587136
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Solar Market
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Solars Growing Rapidly

• Solars transitioning from niche to mainstream at
  CAGR 40
• 10 billion in sales in 06
• Drivers
• Rising energy costs
• Increased subsidies
• Environmental concerns
• Energy security
• Improved manufacturing

Solar markets growth (from Travis Bradford)
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Solar Market Share
Fraction of modules manufactured in 2006
(2.5 GW total)
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Photovoltaic Learning Curve
Courtesy Dan Arvizu (Director, NREL)
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Silicon Supply Issues

• Silicon supply is constrained reasons
• Explosive demand growth
• Capital and time-intensive process of building
  new plants
• Few companies have necessary expertise
• Hoarding?
• New supplies coming online later this year

Polysilicon price inflation (from Travis Bradford)
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Solar Investments

• 1 billion of VC investments in 07
• Most deals in thin-film space
• CIGS Nanosolar, Miasole, Heliovolt, Solibro,
  Solopower, Solyndra
• Thin-film silicon CSG Solar, Flexcell,
  Optisolar, Signet
• II-VI materials Sunovia, EPIR
• Other deals Stion, Infinia

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Market Growth Projections

• Overall market projected to grow at 30 tripling
  to 30 billion by 13
• Thin films growing from niche to a quarter of
  overall supply

3 scenarios for PV sales growth (from Travis
Bradford)
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Decreasing Costs of Solar
Courtesy Dick Swanson (CTO, Sunpower)

• Potential for huge drop in installed costs of
  solar system through variety of sources
• Solar power on track to match grid prices in a
  variety of locations grid parity

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Conclusion

• Two mechanisms to convert photons into power
• Technological landscape is diverse in terms of
  applications, maturity, and costs
• Solar market should continue growing rapidly
• Thin-films expected to be major players in the
  near-term
• Grid parity possible within a decade

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Thank You!

• Any questions for Tim or Peter?

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(No Transcript)
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Introduction to Solar Energy

• Objective convert photons into electricity as
  efficiently as possible
• Approaches
• Concentration of light ? steam ? turbine
• Absorbed photons ? free electrons ? electricity

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Solar Energy

• Clean, abundant energy source
• Solar spectrum close to black-body spectrum for
  T5800K
• Crystalline silicon has near-optimal band gap
• Absorption of silicon limited in near IR

Solar irradiance spectrum (on earth)
c-Si absorption spectrum
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Photonic Crystals (PhCs)
1D
2D
3D

• Periodic dielectric media reflect certain
  wavelengths and transmit others
• Certain structures display a full 3D photonic
  band gap (PBG)

periodic dielectric structures
PBG for diamond structure
Joannopoulos et al., Photonic Crystals (2007)
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Omnidirectional Reflectors

• 1D PhC with special index values
• Brewster angle outside light line of air leads to
  reflection at
• All angles, and
• All polarizations, for
• Frequencies in omnidirectional band gap

Projected bandstructure for omnidirectional
reflector
Joannopoulos et al., Photonic Crystals (2007)
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Current Light Trapping Schemes

• Traditional light trapping based on geometric
  optics
• Aluminum back reflector absorptive
• Effective path length enhancement limited to 2n2
• Perfect scattering not achieved in experiments
• Texturing can introduce electrical losses

Front surface texturing
c-Si, index n
Aluminum back reflector
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Two New Solar Cell Designs

• Wave optics targets key wavelengths in c-Si
• Two designs
• DBR reflector grating
• Photonic crystal

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Mechanisms of Light Trapping

• Reflection introduces stronger reflection than
  before
• Diffraction introduces coupling to guided modes
  in silicon
• Mode characteristics modeled with simple model

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Absorption Spectrum (2 ?m-thick c-Si)

• Slight edge for PhC Less scattering loss
  extra absorbing region

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Efficiency Enhancement for Gratings
For optimized parameters, 2D grating efficiency
enhancement ranges from 7 at 128 mm up to 35 at
2 mm