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

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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


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12



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E. Sachs, J. Cryst. Growth 82 (1987) 117

Slide adapted from A.A. Istratov, Siltronic

Similar technique:


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Source: www.siliconsultant.com . Disclaimer: Certain numbers above do not reflect actual commercial production values, nor the opinion of the presenter.


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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

http

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/psd

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ages

/zin

cbg.

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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


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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


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


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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Cadmium Telluride (CdTe)

Light

Cadmium Telluride (CdTe)

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CdTe

CdTe Characteristics

Advantages:- Technology developed for application on glass BIPV.- Radiation hardness.

Challenges:- Cadmium- Marketability (Greenpeace opposed)

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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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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


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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.


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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/

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