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From Innovation toCommercialization the Story of
Solar Cells
Subhendu Guha
United 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 newera, leading eventually tothe realization of
harnessing the almostlimitless energy of the sunfor the uses of civilization.
Evolution of Invention of Solar Cell
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Phases of Commercialization
1956 Searching for Applications
During the first years after the
discovery of the silicon solar cell,its prohibitive cost kept it out of theelectrical power market. Desperateto find commercial outlets for solarcells, novelty items such as toysand radios run by solar cells were
manufactured and sold as thisadvertisement illustrates.
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Late 1950s - Saved by theSpace Race
Dr. Hans Ziegler advocated for
powering satellites with siliconsolar cells. Solar cells used inVanguard satellite
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Early 1970s - The First Mass
Earth MarketSolar cells power navigation
warning lights and horns on mostoff-shore gas and oil rigs
throughout the world
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1980s - Electrifying the Unelectrified
A common sight in French Polynesia:solar modules on thatched roofs
1980s - Solarizing the Electrified
Solar electric modules cover the rooftops ofthis apartment complex in Bremen,
Germany
Phases of Commercialization
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Shipment Growth and Price Reduction
PV is a $50 billion business today; the shipment has gone up3000 times and price has come down by a factor of 20 in the last
three decades
1
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100
1000
10000
100000
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100
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1970 1975 1980 1985 1990 1995 2000 2005 2010
PVm
oduleprice($/W)
Years
MW
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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
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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 inseries and encapsulate to completethe module
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Requirement for high efficiency solar cell
Optimum bandgap to match the solar spectrumHigh quality material so that the electron-hole pairs can be transported tothe contacts without recombination
Si GaAs CdTe
Materials for High Efficiency Cells
http://upload.wikimedia.org/wikipedia/commons/4/4c/ShockleyQueisserFullCurve.svghttp://upload.wikimedia.org/wikipedia/commons/4/4c/ShockleyQueisserFullCurve.svg -
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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
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Other Technologies are Gaining Traction
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Tota
l
Total
Total
Total
Glass
Glass
Glass
Glass
Flexible
Flexible F
lexible
Flexible
0
1000
2000
3000
4000
5000
6000
a-Si CdTe CIGS Other
Announced2012
Capacity(MW)
Announced production Capacities - 2010
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Major Players
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Sharp
SolarPowerKyoceraBP SolarQ-CellsMitsubishiSolarWorldPanasonic
(Sanyo)Schott SolarIsofotonMotechSuntechEvergreen Solar
JA Solar
United Solar
KanekaFuji ElectricSharpMitsubisihiSchott SolarTronyEPV
PowerFilmAMAT licenseesOrelikon
licensees
Nanosolar
AvancisSolar
FrontierWurth
SolarGlobal
Solar
HondaSoltec
First Solar
Antec SolarAbound SolarPrimeStar SolarCalyxo
There are currently more than 300 companies developing or producing solarcells.
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
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Manufacturing of Silicon Solar Cell
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Growth of
polysiliconchunks/grains
Deposition ofanti-reflectioncoating and
sintering
Interconnectand
encapsulate
Apply junctionboxes and test
Screen-printing/evapo
ration ofcontacts
Growth ofsilicon ingots
Slicing into
wafers andetching
Diffusion ofimpurities
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
1960s : 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 worlds 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 efficiencybecause of high efficiency obtained in the laboratory. Manufacturing hasbeen a challenge. Degradation due to moisture is another issue
1973 : First thin film CIS solar cell demonstrated1980s: Boeing leads efforts in CIS cells; ARCO Solar joins the race
1990s: NREL demonstrates high efficiency solar cells2000 2010: Many companies enter the field
Manufacturing processCo-evaporationSputteringSputtering followed by selenizationElectroplatingInk-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-dischargedecomposition of silane: Chittick, STL, U.K.
1974: Report by Walter Spear of University of Dundee that a-Si has low defectstates 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 ChrisWronski 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
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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 backtime
Superior hightemperatureperformance
Environmentally safe
Rugged and flexibleproducts
Challenges
Light-to-electricityconversion efficiency
Manufacturability
Amorphous Silicon
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GROWTH OF AMORPHOUS SILICONUSING HYDROGEN DILUTION
The best material is grown withhydrogen dilution of the activegas. As the hydrogen dilutionincreases, there is a transitionfrom amorphous tonanocrystalline structure. Thehighest quality materials forboth the nanocrystalline andamorphous phases areobtained near the edge of this
transition. Materials grown onboth sides of the edge arereceiving a great deal ofattention for solar cellapplications. 2
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1 10 100
IR
ERDA
HydrogenContent[at.-%]
Silane Concentration [% ]
a-Si:H regimec-Si:H regime
SiH4 --- Si + 2H2Deposition of amorphous SiH alloy
HEATER
GAS(SiH4)
SUBSTRATE
TOVACUUM
RFPOWER
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 theatoms.
Crystals Amorphous
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What Does Disorder Cause?
Weak bonds, danglingbonds, band tails
- these defects impede
carrier transport Facilitates efficient
light absorption
- allows use of thin film
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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 ofstainless steel,each 2.5 km
long, processedin a single run in65 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
U it d S l A Diff ti t d P d t
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United Solar- A Differentiated Product
Conventional Solar Cells UNI-SOLAR
Laminates
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Competitive Advantages
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Competitive Advantages
Photo courtesy Solar Integrated
Low-impact solar roof solution
Lightweight, durable, flexible
Ideal for Building Integrated (BIPV)
Easy to install
Removable
New lightweight BAPV application
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UNI-SOLAR Largest Rooftop Solar Installations
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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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I d Li ht T i B k R fl t
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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
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Nano Technology
Nano Technology replacesgreen 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% inin 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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2000
3000
4000
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7000
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2004 2005 2006 2007 2008 2009
JAP
ITA
ROE
USA
ROW
Germa
nySpain
MW
INCENTIVE DRIVEN GROWTH
Global Shipment of PV
Challenge for PV Ho to Reach Grid Parit
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Grid parity
Cost per kW hour
(in constant2005 US dollars)
Source: Solar America Initiative
$0.00
$0.20
$0.40
$0.60
$0.80
$1.00
1990 2000 2010 2020
Year
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 inacid rain. There is particulate (soot) emission, too. Thepollution causes diseases having a severe impact on theeconomy.
Global WarmingThe emission of greenhouse gases like CO2 and NOx lead toglobal warming; research studies attribute many of the recentsevere weather calamities to global warming.
Energy Poverty
There are 2 billion people in the world without access toelectricity. Distributed power in the form of renewables like PVis 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 Athens