high efficiency multijunction solar cells for large scale solar electricity generation kurtz

7/31/2019 High Efficiency Multijunction Solar Cells for Large Scale Solar Electricity Generation Kurtz

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High-efficiency, multijunction solarcells for large-scale solar

electricity generationSarah Kurtz

APS March Meeting, 2006

Acknowledge: Jerry Olson, John Geisz, Mark Wanlass, Bill McMahon, Dan Friedman,Scott Ward, Anna Duda, Charlene Kramer, Michelle Young, Alan Kibbler, Aaron Ptak,Jeff Carapella, Scott Feldman, Chris Honsberg (Univ. of Delaware), Allen Barnett (Univ.of Delaware), Richard King (Spectrolab), Paul Sharps (EMCORE)


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Outline Motivation - High efficiency adds value

The essence of high efficiency

Choice of materials & quality of materials Success so far - 39%

Material quality

Avoid defects causing non-radiative recombination High-efficiency cells for the future

Limited only by our creativity to combine high-qualitymaterials

The promise of concentrator systems


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Photovoltaic industry is growing

Growth would be even faster if cost is reduced and availability increased

1500

1000

500

0W

orldwidePVsh

ipments(MW)

1999 2000 2001 2002 2003 2004 2005

Year

~$10 Billion/yrData from thePrometheus Institute


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To reduce cost and increase availability:reduce semiconductor material

Front Solar cell

Back

Thin film

Concentrator

A higher efficiencycellincreases the value of

the rest of the system

Goal: Cost dominated by

Balance of system


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Detailed balance:Elegant approach

for estimating efficiency limit

Balances the radiative transfer between the sun (black body)and a solar cell (black body that absorbs Ephoton > Egap), thenuses a diode equation to create the current-voltage curve.

Shockley-Queisser limit: 31% (one sun); 41% (~46,200 suns)


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Why multijunction?Power = Current X Voltage

5x1017

4

3

2

1

0

Solarspectrum

43210

Photon energy (eV)

Band gapof 0.75 eV

5x1017

4

3

2

1

0

Solarspectrum

43210

Photon energy (eV)

Band gapof 2.5 eV

High current,but low voltage

Excess energy lost to heat

High voltage,but low current

Subbandgap light is lost

Highest efficiency: Absorb each color of light with amaterial that has a band gap equal to the photon energy


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Detailed balance for multiple junctions

Depends on Egap, solar concentration, & spectrum.Assumes ideal materials

100

80

60

40

20

0Detailed

BalanceEfficien

cy(%)

108642

Number of junctions or absorption processes

One sun limit

One sun

Concentration limit

Maximum ConcentrationMarti & Araujo

1996 Solar EnergyMaterials and SolarCells 43 p. 203


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Achieved efficiencies - depend

more on material quality80

60

40

20

0

Efficie

ncy(%)

54321

Number of junctions

Detailed balance

One sun

Detailed balanceMaximum concentration

Amorphous

Polycrystalline

Single-crystal

Single-crystal(concentration)

Polycrystalline


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Efficiency(%)

200019951990198519801975

Multijunction ConcentratorsThree-junction (2-terminal,

monolithic)

Two-junction (2-terminal,

monolithic)

Crystalline Si CellsSingle crystal

MulticrystallineThick Si Film

Thin Film TechnologiesCu(In,Ga)Se2CdTe

Amorphous Si:H (stabilized)

Nano-, micro-, poly- Si

Multijunction polycrystallineEmerging PV

Dye cells

Organic cells(various technologies)12

8

4

0

16

20

24

28

32

36

2005

40

Best Research-Cell Efficiencies

Year


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p4 5 6 7 8 9 1 2 3 4

Energy (eV)

Ge 0.7 eV

GaInAs 1.4 eV

GaInP 1.9 eV

Efficiency record = 39%2005 R. King, et al, 20th European

PVSEC

Multijunction cells use multiplematerials to match the solar spectrum


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40

30

20

10

0

Efficienc

y(%)

20052000199519901985Year

NREL inventionof GaInP/GaAs

solar celltandem-poweredsatelliteflown

commercialproduction oftandem

productionlevels reach

300 kW/yr

3-junction concentrator

Success of GaInP/GaAs/Ge cell

QuickTime and aTIFF (LZW) decompressor

are needed to see this picture.

Mars Rover powered bymultijunction cells

This very successful space cell iscurrently being engineered into systems

for terrestrial use

39%!


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Solar cell - diode model

Collect photocarriers at built-in field before they recombine.

Fermilevel

Deplet ionwidth

p-typematerial

n-type material

light


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Types of recombination

Auger

Radiative Non-radiative - tied to material quality


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Non-radiative recombinationgenerates heat instead of electricity

Heat

Shockley - Read - Hall recombination

Large numbers of phonons are required

when E is large -- probability of transitiondecreases exponentially with E Trap fills and empties; Fermi level is critical

Ec

Ev

Trap

Heat


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Defects - problems and solutions Defects that cause states near the middle of

the gap are the biggest problem These tend to be crystallographic defects

(dislocations, surfaces, grain boundaries)

use single crystal Perfect single-crystal material has defects

only at edgesTerminate crystal with a material that forms bonds

to avoid unpaired electrons

Build in a field to repel minority carriers


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Solar cell schematic to showsurface passivation

Fermi

level

Passivat ing

windowDeplet ion

width

Passivat ing

back-surface field

light

n-type p-type


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Summary about high efficiency

High efficiency cell makes rest of systemmore valuable

Minimize non-radiative recombination

Use single crystal Get rid of surfaces with passivating layers

With these ground rules, how do we combinematerials? - Lots of research opportunities


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Many available materials

By making alloys, all band gaps can be achieved

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Bandg

ap

(eV)

6.46.26.05.85.65.4Lattice Constant ()

AlP

AlAs

AlSb

GaP

GaAs

GaSb

InP

InAs

InSb

Ge

Si


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Ways to make a single-crystal alloy

Ordered Random Quantum

wells

Quantum

dots

Challenges: avoid forming defects while controlling structure collect photocarriers


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Strain is distributed uniformlyMobility is determined by band structureNeed driving force for ordering, or growthis impossible

Relatively easy to growAlloy scattering is usually small; mobilityis decreased slightly

Collection of photocarriers usually

requires a built-in electric fieldGrowth is typically more complex,especially to avoid defects and tocontrol sizes of quantum structures


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Strain is distributed uniformlyMobility is determined by band structureNeed driving force for ordering, or growthis impossible

Relatively easy to growAlloy scattering is usually small; mobilityis decreased slightly

Collection of photocarriers usually

requires a built-in electric fieldGrowth is typically more complex,especially to avoid defects and tocontrol sizes of quantum structures


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GaInP/GaAs/Ge cell is latticematched

2.8

2.4

2.0

1.61.2

0.8

0.4

0.0

Bandgap

(eV)

6.46.26.05.85.65.4

Lattice Constant ()

AlP

AlAs

AlSb

GaP

GaAs

GaSb

InP

InAsInSb

Ge

Si

Ga0.5In0.5P

GaAs

Ge


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1.0

0.9

0.8

0.7

0.6

0.5

Open-circuitvoltage(V)

1.401.351.301.251.201.15

Bandgap (eV)

GaAs

Ga(In)NAs

n-on-p p-on-n

GaAsGaNAs

GaInNAs

New lattice matched alloys

GaInNAs is candidatefor 1-eV material, but

does not give idealperformance

Lattice matched approach iseasiest to implement, but is

limited in material combinations

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Bandgap(e

V)

6.05.85.65.4

Lattice Constant ()

AlP

AlAsGaP

GaAs

GaSb

InP

InAs

Ge

Si

Ga0.5In0.5P

GaAs

Ge

GaInAsN


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Method for growingmismatched alloys

11 mm

GaAsPGaAsPstep gradestep grade

220DF

GaAsGaAs0.70.7PP0.30.3

GaPGaP

XTEMXTEM

Larger

lattice constant

Smaller

lattice constant

Step

gradeconfinesdefects

SiGe (majority-carrier)devices are now common,but mismatched epitaxial

solar cells are in R&D stage


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High-efficiency mismatched cell

Ge bottom celland substrate

Grade

GaInAs middle cell

GaInP top cell

Metal38.8% @ 240 suns

R. King, et al 2005, 20th European PVSEC

1.8 eV

1.3 eV

0.7 eV

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Bandgap

(eV)

6.05.85.65.4

Lattice Constant ()

AlP

AlAsGaP

GaAs

GaSb

InP

InAs

Ge

Si

Ga0.4In0.6P

Ga0.9

In0.1

As

Ge


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Inverted mismatched cell

GaInAs bottom cell

Grade

GaAs middle cellGaInP top cell

Metal

GaAssub

strate

Substrate

removed

aftergrow

th

37.9% @ 10 sunsMark Wanlass, et al 2005

1.9 eV1.4 eV

1.0 eV

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Bandg

ap

(eV)

6.05.85.65.4

Lattice Constant ()

AlP

AlAsGaP

GaAs

GaSb

InP

InAs

Ge

Si

Ga0.5In0.5P

GaAs

Ga0.3In0.7As


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Mechanical stacks

32.6% efficiency @ 100 suns

1990 L. Fraas, et al 21st PVSC, p. 190

GaAs cell

GaSb cell4-terminals

Easier to achieve high efficiency, but more difficult ina system because of heat sinking and 4-terminals

Wafer bonding provides pathway to monolithic structureA. Fontcuberta I Morral, et al, Appl. Phys. Lett. 83, p. 5413 (2003)


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Summary Photovoltaic industry is growing > 40%/year

High efficiency cells may help the solar industrygrow even faster

Detailed balance provides upper bound (>60%)

for efficiencies, assuming ideal materials Single-crystal solar cells have achieved the

highest efficiencies: 39%

Higher efficiencies will be achieved when waysare found to integrate materials while retaininghigh crystal quality


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Flying high with high efficiencyCells from Mars rovermay soon provideelectricity on earthQuickTime and a

TIFF (LZW) decompressorare needed to see this picture.

QuickTime and aTIFF (LZW) decompressor

are needed to see this picture.

High efficiency, low cost,

ideal for large systems

QuickTime and aIFF (LZW) decompressoreeded to see this picture.

high efficiency multijunction solar cells for large scale solar electricity generation kurtz

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