From Innovation to Commercialization – the Story of

Solar Cells

Subhendu GuhaUnited Solar Ovonic

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Pearson, Chapin and Fuller, 1954Inventor of Si solar cell Bell lab document

1839 : Becquerel observed photovoltaic action in an electrolytic cell1876: Adams and Day discovered PV effect in solid Selenium1925: Czochralski grew single crystal silicon1940-1950: Golden era of semiconductor research including invention of pnjunction and transistor1954: First silicon solar cell demonstrated with 4.5% efficiency

New York Times - 1954

“…the beginning of a new era, leading eventually to the realization of harnessing the almost limitless energy of the sun for the uses of civilization.”

Evolution of Invention of Solar Cell

Phases of Commercialization

1956 Searching for ApplicationsDuring the first years after the discovery of the silicon solar cell, its prohibitive cost kept it out of the electrical power market. Desperate to find commercial outlets for solar cells, novelty items such as toys and radios run by solar cells were manufactured and sold as this advertisement illustrates.

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Late 1950s - Saved by the Space Race

Dr. Hans Ziegler advocated for powering satellites with silicon solar cells. Solar cells used in Vanguard satellite

Early 1970s - The First Mass Earth Market

Solar cells power navigation warning lights and horns on most

off-shore gas and oil rigs throughout the world

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1980s - Electrifying the UnelectrifiedA common sight in French Polynesia:

solar modules on thatched roofs

1980s - Solarizing the ElectrifiedSolar electric modules cover the rooftops of

this apartment complex in Bremen, Germany

Phases of Commercialization

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Shipment Growth and Price Reduction

PV is a $50 billion business today; the shipment has gone up 3000 times and price has come down by a factor of 20 in the last three decades

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1970 1975 1980 1985 1990 1995 2000 2005 2010

PV m

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Topics to Discuss

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•Semiconductor physics

•Solar cells

•Different materials for solar cells

•Thin film silicon solar cell

•Building-integrated photovoltaic

•Future direction

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Physics of Semiconductor

Intrinsic semiconductor n-type semiconductor p-type semiconductor

PN junction

Physics of Solar Cell

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Photons are absorbed to create free carriers; these are transported to the contacts

Light createselectron-hole pair

You can connect several solar cells in series and encapsulate to complete the module

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Requirement for high efficiency solar cell

•Optimum bandgap to match the solar spectrum•High quality material so that the electron-hole pairs can be transported to the contacts without recombination

Si GaAs CdTe

Materials for High Efficiency Cells

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Global Shipment by Technology

Source: PV News, May 2011

Silicon technology still dominates the market United Solar is the third largest thinfilm silicon solar cell manufacturer

Other Technologies are Gaining Traction

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Announced production Capacities - 2010

Major Players

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Sharp SolarPowerKyoceraBP SolarQ-CellsMitsubishiSolarWorldPanasonic (Sanyo)Schott SolarIsofotonMotechSuntechEvergreen SolarJA Solar

United SolarKanekaFuji ElectricSharpMitsubisihiSchott SolarTronyEPVPowerFilmAMAT licenseesOrelikonlicensees

NanosolarAvancisSolar FrontierWurthSolarGlobal SolarHonda Soltec

First SolarAntec SolarAbound SolarPrimeStar SolarCalyxo

There are currently more than 300 companies developing or producing solar cells.With prices continuing to decrease, and more companies entering themarket, many small companies and start-ups are likely to fail

C-Si or pc-Si Thin Film Si CIGS CdTe

Ref: Carlson, APS Meeting, 2010

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Global Cell Production

U.S. lags behind in both production and deployment

Manufacturing of Silicon Solar Cell

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Growth of polysilicon

chunks/grains

Deposition of anti-reflection coating and

sintering

Interconnect and

encapsulateApply junction boxes and test

Screen-printing/evapo

ration of contacts

Growth of silicon ingots

Slicing into wafers and

etching

Diffusion of impurities

Ship

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Cell process steps and structure

Silicon Solar Cell

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High Efficiency Devices

BURIED CONTACT BACK CONTACT

PERL (PASSIVATED EMITTER)

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CdTe Solar Cell

Recognized as a semiconductor with near-ideal bandgap match to solar spectrum

• 1960’s : Solar cells made by GE, Matsushita, Monosolar

• 1981 : Kodak enters the field with 10% efficiency

• 1992 : University of South Florida demonstrates 15% cell

• 2002 : 7% products available from First Solar

• 2009 : First Solar emerges as the world’s largest PV manufacturer

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Glass

Tin Oxide

CdS

CdTe

Interface layer

Metal

Wet chemical process*

Closed space sublimation, vapor transport*

Sputtering*

* Other processes are also used

CdTe Cell Structure

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Monolithic Module

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CIGS Solar Cell

Of all the thin film technologies, CIGS has received a great deal of efficiency because of high efficiency obtained in the laboratory. Manufacturing has been a challenge. Degradation due to moisture is another issue

• 1973 : First thin film CIS solar cell demonstrated•1980’s: Boeing leads efforts in CIS cells; ARCO Solar joins the race•1990’s: NREL demonstrates high efficiency solar cells•2000 – 2010: Many companies enter the field

Manufacturing process•Co-evaporation•Sputtering•Sputtering followed by selenization•Electroplating•Ink-growth

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Zinc Oxide

CdS

CIGS

Mo

Metal/glass

Wet chemical process*

Co-evaporation, sputtering, plating*

Sputtering*

* Other processes are also used

Cell Structure and Manufacturing

Manufacturing: Laser-integrated or cell interconnected

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1969: First report of amorphous silicon (a-Si) thin film deposited by glow-discharge decomposition of silane: Chittick, STL, U.K.

1974: Report by Walter Spear of University of Dundee that a-Si has low defect states in the band gap

1975: Report by Walter Spear that a-Si can be doped n-type or p-type

1976: First solar cell made at RCA laboratory by David Carlson (2% efficiency)

1977: Report of light-induced degradation of a-Si by Dave Staebler and Chris Wronski of RCA

1979: First a-Si alloy solar cell for calculators introduced in the market

1981: ECD/Uni-Solar enters the field

2010 : 1300 MW global manufacturing

Amorphous Silicon

From Innovation to Commercialization

NREL validation

2 MW Machine

Prototype Machine

0.5 MW Machine

5 MW Machine Auburn Hillsfacility (1&2) 60MW

Greenville 120 MW

Building-integrated(BIPV) product

Acquisition ofSolar Integrated

Technologies

1981 1986 1991 1994 1996 1997 2003 2007 2009

More than 65 issued U.S. Patents

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Advantages• Low material cost• Short energy pay back

time• Superior high

temperature performance

• Environmentally safe• Rugged and flexible

products

Challenges• Light-to-electricity

conversion efficiency• Manufacturability

Amorphous Silicon

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GROWTH OF AMORPHOUS SILICON USING HYDROGEN DILUTION

The best material is grown with hydrogen dilution of the active gas. As the hydrogen dilution increases, there is a transition from amorphous to nanocrystalline structure. The highest quality materials for both the nanocrystalline and amorphous phases are obtained near the edge of this transition. Materials grown on both sides of the edge are receiving a great deal of attention for solar cell applications. 2

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IRERDA

Hyd

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Silane Concentration [%]

a-Si:H regimeµc-Si:H regime

SiH4 --- Si + 2H2Deposition of amorphous SiH alloy

HEATER

GAS (SiH4)

SUBSTRATE

TO VACUUM

RF POWER

Amorphous Silicon

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Amorphous Materials

• Unlike crystals, amorphous or disordered materials do not have any long-range order. There is no periodicity in the arrangement of the atoms.

° °• ° • °•

° •° °• °•

° °• ° • °•

° °• °• •°Crystals °Amorphous •

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What Does Disorder Cause?

• Weak bonds, dangling bonds, band tails

- these defects impede carrier transport

• Facilitates efficient light absorption

- allows use of thin film

How to Improve Efficiency?

• Have better order with more stable structure - Role of hydrogen dilution• Use multijunction cells to facilitate better absorption

Blue

Green

Red

Reflector

Nano -crystalline

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Amorphous Silicon Alloy Triple-Junction Cell Processor

Six rolls of stainless steel, each 2.5 km long, processed in a single run in 65 hours.

Manufacturing

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Small area machine 2” by 2” substrate Large area machine 15” by 14” substrate

Large-area machine (3 14” webs) Roll-to-roll production machine

From Lab to Production

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United Solar- A Differentiated Product

Conventional Solar Cells UNI-SOLAR® Laminates

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Competitive Advantages

Photo courtesy Solar Integrated

Low-impact solar roof solutionLightweight, durable, flexible Ideal for Building Integrated (BIPV)Easy to installRemovableNew lightweight BAPV application

GM Facility / Zaragoza, Spain / 11.8 MW Enel Green Power / Nola, Italy / 25 MW

Tesco | Fresh & Easy / Riverside, CA / 2 MW Posco Warehouse / Pohang, South Korea / 1 MW

UNI-SOLAR Largest Rooftop Solar Installations

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Improved Light Trapping: Back Reflector

Improved Light Trapping

Anti-reflective coating

Blue light-absorbing cell

Green light-absorbing cell

Red light-absorbing cell

Back reflector

Stainless steel substrate

Cross-section of a solar cell

Back reflector

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Nano Technology

Nano Technology replaces green and red light-absorbing layers

• Compatible with a-Si alloy deposition

• Ideal for middle and bottom cells of multi-junction structure

• Improved light absorption and no light-induced degradationof nano layers has resulted in conversion efficiency of 12% in in the lab

Anti-reflective coating

Blue light-absorbing cell

Green light-absorbing cell

Red light-absorbing cell

Back reflector

Stainless steel substrate

Results in greater stability and higher conversion efficiency

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INCENTIVE DRIVEN GROWTH

Global Shipment of PV

Grid parity

Cost per kW hour (in constant 2005 US dollars)

Source: Solar America Initiative

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Challenge for PV--How to Reach Grid Parity

Cost of solar electricity is decreasing every year. We are on our path to grid parity.

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Problems with Conventional Fuel

• PollutionThe power plants emit mercury and sulphur dioxide resulting in acid rain. There is particulate (soot) emission, too. The pollution causes diseases having a severe impact on the economy.

• Global WarmingThe emission of greenhouse gases like CO2 and NOx lead to global warming; research studies attribute many of the recent severe weather calamities to global warming.

• Energy PovertyThere are 2 billion people in the world without access to electricity. Distributed power in the form of renewables like PV is the only option for them.

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“In the end, more than they wanted freedom, they wanted a comfortablelife-and they lost both comfort and freedom. When the Athenians wantednot to give to society but for society to give to them, when the freedomthey wished for most was freedom from responsibility, then Athensceased to be free” – Edith Hamilton