mit iap pv 2007-lecture-2signallake.com/signallake.com/innovation/buonassisijan07.pdfi. advanced...
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Instructors / Facilitators:David Danielson, MIT Andrew Gabor, Evergreen SolarJoel Conkling, MIT Adam Lorenz, Evergreen SolarMichael Rogol, MIT Edward Kern, IrradianceProf. Rebecca Henderson, MIT Tonio Buonassisi, Evergreen Solar
Schedule (7-8:30P, 66-144):Jan 9 (T): Intro to PV: Basics of Energy & PVJan 11 (H): PV Technologies: Latest Trends, Feedstock to DevicesJan 16 (T): PV Manufacturing on the Multi-GW ScaleJan 18 (H): PV Modules: Importance of PackagingJan 23 (T): Downstream PV: BOS and InstallationJan 25 (H): PV Industry: Supply, Demand, Price and Profits
Special Events:Jan 11 (H), 1-4P, E51-145: Technology Strategy: SunPower Case StudyJan 17 (W), 7-9P, Muddy Charles: MIT PV Community Social
Course Website: http://web.mit.edu/mit_energy/resources/classes.html
Email List: Contact David Danielson, [email protected], to sign-up for this course and be added to the course email list.
Course Details
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Course Mission Statement:To provide a forward-looking statement of emerging trends
in PV technology, policy, and economics, in context of a growing (renewable) energy market.
Lecture 2
PV TechnologiesLatest Trends, Materials to Devices
Tonio Buonassisihttp://homepage.mac.com/buonassisi
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1. A few notes about efficiency2. Materials
a. C-Sib. Ingot mc-Sic. Ribbon mc-Sid. Sheet mc-Sie. Thin Films
i. Material abundancesii. Cdiii. Amorphousiv. HITv. CIGSvi. CdTevii. Heterostructures
f. Die-sensitized Cellsg. Nanoh. Organici. Advanced Concepts
Outline
Efficiency of a solar cell is an important parameter
100% efficiency(impossible to achieve)
33% efficiency(space-grade solar cells)
20% efficiency(monocrystallinesilicon solar cells) 10% efficiency
(amorphoussilicon solar cells)
powerlight incident power electrical generated =η
VeryExpensivematerial
Expensivematerial
Relatively Inexpensive material
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A Note about “Efficiency”
Technical Terms:- Solar Conversion Efficiency- External Quantum Efficiency- Internal Quantum Efficiency
Solar Conversion Efficiency
η = Power OutPower In
= Jmp ⋅ Vmp
ΦF
= FF ⋅ Jsc ⋅ Voc
ΦF
Typical values are 12–20% for established technologies, <10% for most emerging technologies.
η and ΦF: Vary with illumination intensity (e.g., 1 Sun)
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External Quantum Efficiency
EQE = Electrons OutPhotons In
Typical peak values are 60–90%, depending on reflectivity.
EQE highly wavelength- and illumination-dependent!
Internal Quantum Efficiency
IQE = Electrons OutPhotons In( )⋅ 1− Reflectivity( )
Typical peak values are 80–98%, depending on reflectivity.
EQE highly wavelength- and illumination-dependent!
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When an efficiency quoted, think about:
- What “efficiency” is being measured?- What is the nature of the light being used?
- What spectrometer to simulate solar spectrum?- If monochromatic, what wavelength?- What intensity (photon flux)?
- What used car are they trying to sell me?
W.U. Huynh, Science 295 (2002) 2425
An example of honest efficiency reporting
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Solar CellEfficiencyTables
M.A
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ModuleEfficiencyTables
M.A. Green, Prog. Photovolt: Res. Appl. 14 (2006) 45
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Silicon Photovoltaics
Worldwide PV Production
IEA-PVPS, Report IEA-PVPS T1-15:2006
PV News, PV Insider’s Report, Feb. 2001IEA-PVPS: Report IEA-PVPS T1-15:2006
US Market Share:1980: 75%1990: 33%2000: 26%2005: 10%
• Sustained 25-40% industry growth rates.• PV now a $10+ bi industry.
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1980
1990
2000Mono
-SiMulti-Si
300 MWp / year
Thin film (a-Si)Ribbon-Si
1000 Roofs Program, D
Residential Roof Program, JPN
100 000 Roofs Program, DRenewable Energy Law, D
Development of the Global PV Market
courtesy of Gerhard Willeke, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
Noteworthy: 2006 1,000,000 roof program, CA PUC
Silicon is the second most abundant element on Earth after oxygen (28% of the Earth’s crust). Its most familiar forms are sand and quartzite (the latter one is more pure).
MulticrystallineSilicon
MonocrystallineSilicon
Human-made Monocrystalline Silicon Human-made Multicrystalline Silicon
Silicon in Nature: It’s everywhere!
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- Sharp, Kyocera, BP Solar, Shell Solar
Slide from A.A. Istratov, Siltronic
Slide from A.A. Istratov, Siltronic
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Slide from A.A. Istratov, Siltronic
Slide from A.A. Istratov, Siltronic
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Slide from A.A. Istratov, Siltronic
Slide from A.A. Istratov, Siltronic
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Slide from A.A. Istratov, Siltronic
Slide from A.A. Istratov, Siltronic
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Slide from A.A. Istratov, Siltronic
Slide from A.A. Istratov, Siltronic
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E. Sachs, J. Cryst. Growth 82 (1987) 117
Slide adapted from A.A. Istratov, Siltronic
Similar technique:
Slide from A.A. Istratov, Siltronic
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Slide from A.A. Istratov, Siltronic
Source: www.siliconsultant.com . Disclaimer: Certain numbers above do not reflect actual commercial production values, nor the opinion of the presenter.
Slide from A.A. Istratov, Siltronic
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Materials Availability
Alex Freundlich: http://www.rio6.com/proceedings/RIO6_181106_MA_1730_Freundlich.pdf
Plenty of (oxidized) silicon in the Earth’s crust, but…
Not enough silver! New solar cell contact materials needed.
Other
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Diversity in the PV Market
Hybrid (nano)
SpheralSolar
Future technologies must consider:• Cost ($/kWh)• Resource availability• Environmental impact
Amorphous Silicon
Organics
Copper IndiumDiselenide (CIS)
CadmiumTelluride
SunPowerBack-contacted
Dye-sensitizedCells
HeterojunctionCellsSilicon Ribbon
Silicon Sheet
Record laboratory efficiencies of various materials
NOTE: These are record cell efficiencies under ideal conditions (25°C,~1000 W/m2)! Actual commercially-available silicon solar cells are typically 14-17%
efficient. Modules are typically around 11-13%.
L.L. Kazmerski, Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135
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Thin Films
Advantages- 1 µm layers less material used potential cost decrease.- Potential for lower thermal budget potential cost decrease.- Potential for roll-to-roll deposition on flexible substrate.
- Technology transfer with TFT, flat panel display industry.- Good for BIPV applications.- Radiation hardness.
Disadvantages- Lower efficiencies potentially larger module costs.- Potential for capital-intensive production equipment.- Potentially scarce elements sometimes used.- Spatial uniformity a challenge during deposition.
Thin Films
Building-integrated solutions
Roll-to-roll deposition of µm-sized layers potentially high throughput, large-area deposition, and cheap.
Advantages:
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Radiation hardness of different compounds
Space payloads cost ~$1400–$6000/pound (~$2866–$13228/kg) Key parameter not $/W. Instead, it’s W/kg and reliability!
Grain Size and Efficiency
R.B. Bergmann, Appl. Phys. A69 (1999) 187
See also:T.F. Ciszek, J. Cryst Growth237-239 (2002) 1685
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Heterostructuresand Lattice Matching
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To prevent interface recombination and achieve high carrier mobilities, atoms in the different layers must line up (adjacent hetero-epitaxial layers must be lattice matched). Otherwise, defects form at these interfaces.
An good example of a heteroepitaxialsystem is Ge / GaAs / InGaP / AlAs, in order of increasing bandgap.
B.A
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MaterialAbundances
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Environmental Concerns: Cadmium
Arguments Against:- Suspected carcinogen.- Industrial emissions tightly regulated, esp. in E.U.
- Cradle-to-grave requirement.
Arguments in Favor:- By-product of Zn, Cu mining [1].
- “Better to tie it up in CdTe than dump it in the ground.”- “Negligible” Cd released during fires [2].- “Public fear a perception issue” [3].- CdTe is a stable compound.
- Much less Cd released per kWh than a battery [4].- Safe production.- Full recycling guaranteed (by law in Europe).
[1] http://www.firstsolar.com[2] V.M. Fthenakis et al., Proc. 19th EU-PVSEC (Paris, France, 2004); Paper 5BV.1.32[3] http://www.nrel.gov/cdte[4] V.M. Fthenakis, Renewable and Sustainable Energy Reviews 8 (2004) 303.
Record laboratory efficiencies of various materials
NOTE: These are record cell efficiencies under ideal conditions (25°C,~1000 W/m2)! Actual commercially-available silicon solar cells are typically 14-17%
efficient. Modules are typically around 11-13%.
L.L. Kazmerski, Journal of Electron Spectroscopy and Related Phenomena 150 (2006) 105–135
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Advantages:- Potentially very cheap, low-temperature.
Challenges:- Overcoming the Staebler–Wronski effect (SWE)- Uniform (thickness, quality, grain size) film deposition.- TCO expensive.- Challenges to scaling
B. Rech and H. Wagner, Appl. Phys. A 69 (1999) 155
Amorphous Silicon (a-Si)
Energy Band Diagram of a-Si
B. Rech and H. Wagner, Appl. Phys. A 69 (1999) 155
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a-Si heterostructures
B. Rech and H. Wagner, Appl. Phys. A 69 (1999) 155http://www.sandia.gov/pv/images/PVFSC36.jpg
B. Rech and H. Wagner, Appl. Phys. A 69 (1999) 155
Staebler–Wronski effect (SWE)
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http
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The a-Si (µ-Si) nc-Si transition……is determined by deposition temperature…
150°C
133°C
…ambient gas content, and other factors.
E. Srinivasan and G.N. Parsons, J. Appl. Phys. 81 (1997) 2847See also:
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Heterojunction with Thin Intrinsic layer (HIT) Cells
Advantages:- Less surface recombination.- Higher maximum voltages (Voc > 710 mV).- Efficiency less temperature sensitive.- High efficiencies (21.5% on 100 cm2 cell)
Challenges:- Deposition: doping, nano-to-micro-crystalline phase transition- Optimizing the c-Si and a-Si interface, low-damage plasma.
http://www.sanyo.co.jp/clean/solar/hit_e/hit.html
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Energy Band Diagram of HIT Cell
M. Taguchi et al., Prog. Photovolt: Res. Appl. 13 (2005) 481.
Temperature Dependence of HIT Cells
M. Taguchi et al., Prog. Photovolt: Res. Appl. 13 (2005) 481.
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CIS and its variants
Se,S
Basic Facts:- CIS = Copper Indium Diselenide = CuInSe2 = Chalcopyrite- Zincblende-like structure- Record efficiencies: 19.2% lab; 13.4% large area
Thin-film polycrystalline CIS
http://level2.phys.strath.ac.uk
A. Klein, Proc. 31st IEEE PVSC(Lake Buena Vista, FL, 2005) p.205
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L. Weinhardt, C. Heske et al.,Appl. Phys. Lett. 84 (2004) 3175
CIS Band Structure Debated
A. Klein, Proc. 31st IEEE PVSC(Lake Buena Vista, FL, 2005) p.205
CIS Characteristics
Advantages:- High efficiencies (18+%)- “Most proven technology” among thin films
Challenges:- Defects, Interface States are complex, poorly understood.- Replace n-type emitter with Cd-free material.
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A. Klein, Proc. 31st IEEE PVSC(Lake Buena Vista, FL, 2005) p.205
Cadmium Telluride (CdTe)
Light
Cadmium Telluride (CdTe)
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A. Klein, Proc. 31st IEEE PVSC(Lake Buena Vista, FL, 2005) p.205
CdTe
CdTe Characteristics
Advantages:- Technology developed for application on glass BIPV.- Radiation hardness.
Challenges:- Cadmium- Marketability (Greenpeace opposed)
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Alex Freundlich: http://www.rio6.com/proceedings/RIO6_181106_MA_1730_Freundlich.pdf
Materials Availability
Most experts agree: not enough In, Te to produce TW of PV.
Development of new TCO materials may reduce costs.
Tandem (Heterostructure) Cells
- Stack of lattice-matched materials with decreasing bandgaps.- Spectrolab Cells: GaInP2/GaAs/Ge. Effmax=32%, Effave=28%. 375 kW in orbit!- Theoretical efficiency limit for infinite tandem cell: 86.8%- Heteroepitaxial growth slow and expensive!
http://www.spectrolab.com/DataSheets/TNJCell/utj3.pdf
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Alex Freundlich: http://www.rio6.com/proceedings/RIO6_181106_MA_1730_Freundlich.pdf
Materials Availability
Most experts agree: not enough Ge to produce TW of PV.
Development of new low-bandgap materials.
Organic PV
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Slide from Ilan Gur, UC Berkeley
Slide from Ilan Gur, UC Berkeley
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Slide from Ilan Gur, UC Berkeley
Slide from Ilan Gur, UC Berkeley
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Slide from Ilan Gur, UC Berkeley
Slide from Ilan Gur, UC Berkeley
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Slide from Ilan Gur, UC Berkeley
Electrical properties of polymers:
- Electrical conductivity in the range of 10-12-104 Ω-1cm-1
- Oxidizing agents (e.g., I2, Br2, AsF5) act as p-type dopants (add holes to conduction band of conjugated polymer).
- Reducing agents (e.g., Li, Ca) act as n-type dopants (add electrons to conduction band of conjugated polymer).
- Bound excitons are created by light; these diffuse to an interface, where they dissociate.
Heeger et al., Phys. Rev. Lett. 39 (1977) 1098See also:
K. Horie et al., Molecular Photonics (2000) 116
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Pros and Cons of Organic PV
PROS:- Cheap- Low materials consumption- Synthesis in solution, inkjet, screen printing, spin-on processes- Bandgap tunable- Low T annealing
CONS:- Low efficiency (low hole mobility: carrier hopping).- Most organic PV degrades when exposed to UV.
New field: Defect-tolerant and self-repairing materials
SS
SS
Regioregular P3HT
n/4
Glass
Al
ITO
CdSe/P3HT Blend
100nm
Nanocrystal Polymer Solar Cells vac
Polymer
3.0
5.35
4.46.2
Nanocrystalh+
e-
I.P.E.A.
Slide from Ilan Gur, UC Berkeley
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Hybrid Organic-Semiconductor
CdSe nanocrystals: W.U. Huynh, Science 295 (2002) 2425Advantages:- Synthesis in solution, inkjet, screen printing- Bandgap tunable- Spin-on process- Low T annealing
Challenges:- Low carrier mobility in organic material- Organic degrades over time
New concept: All-semiconductor “hybrid” cellGur et al., Science 310 (2005) 462.
- Low efficiencies under 1 Sun illumination.
Die-sensitized cells
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Charge transfer events in die-sensitized solar cells
J.-E. Moser, Nat. Mater. 4 (2005) 723
1. Absorption (Good)2. Charge transfer (Good)
3, 5. Recombination (Bad)4, 6. Redox (Good)
Die-sensitized cells
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http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf
Organics and Chemical Energy
Goals:
Mimic natural processes with high efficiencies to produce compounds beneficial to humankind.
Most research at a very fundamental level.
future designs [3rd generation pv].
Advantages:- Theoretical η > 85%
Challenges:- Practical implementation difficult.
- Appropriate materials and technologies not currently developed.
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Future Designs: Multiband materials
- Electrons can be excited either in one step by a single high-energy photon (1), or by a combination of steps using two lower-energy photons (3+2).
- Theoretical efficiency limit for n-band multiband cell: 86.8%
Challenges:- Practical implementation difficult.- Low carrier mobilities in highly defective materials.- High recombination rates (step down).- N- and P-type doping.
M.A. Green, Physica E 14 (2002) 65
Future Designs: Hot carrier cells
- Thermalization (pathway 1, left) accounts for a large efficiency loss, especially in small-bandgap materials.
- Hot carrier cells aim to collect carriers before they decay from an excited state. Carriers either move very quickly, and/or are inhibited from decaying. Band structure and contacts must also be properly designed.
- Theoretical efficiency limit for hot carrier cell: 86.8%.
Challenges:- Practical implementation difficult.- Must compete with highly-efficient processes (e.g., thermalization).
M.A. Green, Physica E 14 (2002) 65
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Future Designs: Multiple Exciton Generation (MEG)
- One photon creates multiple electron-hole pairs, each with the energy of the bandgap. Thermalization losses are avoided.
- Physical mechanisms: Raman luminescence, impact ionization.- Theoretical η limit for these devices, with bandgap 0 eV: 85.9%
Challenges:- Practical implementation difficult.- Must compete with highly-efficient processes (e.g., thermalization).
R.D. Schaller, V.I. Klimov, Phys. Rev. Lett. 92, 186601 (2004)Recent pubs by R.J. Ellingson
Future Designs: Photon Cascade
- High energy photons are converted into multiple lower-energy, bandgap-matched photons. Thermalization losses reduced.
Challenges:- Practical implementation, conversion layer material choice.- Conversion layer must directionally emit lower-energy photons into, not
away from, the absorber material. Self-assembled nanorod arrays?
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• A few notes about efficiency• Materials
– C-Si– Ingot mc-Si– Ribbon mc-Si– Sheet mc-Si– Thin Films
• Material abundances• Cd• Amorphous• HIT• CIGS• CdTe• Heterostructures
– Die-sensitized Cells– Nano– Organic– Advanced Concepts
Outline
thanks.
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Addendum:
• In response to audience questions, here are links to sources concerning health, safety, and environmental impacts of PV:– http://www.pv.bnl.gov/– http://externe.jrc.es/– http://lea.web.psi.ch/– http://www.ecn.nl/