High-efficiency solar cells for large-scale electricity generation

Sarah Kurtz, Jerry Olson, John Geisz, Daniel Friedman, Willliam McMahon, Aaron Ptak, Mark Wanlass, Alan Kibbler, Charlene

Kramer, Kris Bertness, Scott Ward, Anna Duda, Michelle Young, Jeff Carapella, Myles Steiner

Dozens of others at Spectrolab, Emcore, and elsewhereSeptember 26, 2008

UC Merced Solar SymposiumNREL/PR-520-44201

Outline• Solar is growing very fast • Optical concentration

– Reduces semiconductor material– Allows use of more efficient (expensive) cells

• The physics high-efficiency solar cells – Why using multiple junctions increases efficiency– Success of GaInP/Ga(In)As/Ge cell – New designs (GaInP/GaInAs/GaInAs) reach 40.8%

• Potential for large-scale solar electricity generation

Growth of photovoltaic industry

~0.1% of electricity now comes from solar - extrapolates to > 5% in 2020

competitive with conventional electricity for 0.1% - 1% of market; more in futureRogol, PHOTON International August 2007, p 112.

Industry growth is currently constrained by Si availability, motivating us toreduce semiconductor material

Front Solar cell

Back

Concentration:1. Reduces semiconductor use2. Allows use of higher efficiency cell (higher system efficiency)

Thin film

Concentrator

Why multijunction? Power = Current X Voltage

5x1017

4

3

2

1

0

Sol

ar s

pect

rum

43210

Photon energy (eV)

Band gapof 0.75 eV

5x1017

4

3

2

1

0

Sol

ar s

pect

rum

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 a material that has a band gap equal to the photon energy

in

4 5 6 7 8 91

2 3 4

Energy (eV)

Multijunction cells use multiple materials to match the solar spectrum

Multijunction cells use multiple materials - many possible combinations

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Ban

dgap

(eV)

6.16.05.95.85.75.65.55.4

AlP

AlAsGaP

GaAs

GaSb

InP

InAs

Ge

Si

Lattice Constant (Å)

Champion solar-cell efficiencies -Multijunction achieves highest

40.8% Geisz

(NREL)

Historical perspective

• In 1984:– Multijunction approach is predicted to

achieve higher efficiency– AlGaAs/GaAs is considered lowest-risk

approach to implementing multijunction– Lattice mismatched combinations

considered higher risk approach– GaInP alloys were “known” to be unstable

• Jerry Olson proposed GaInP/GaAs

Success of multijunction cells using GaInP

Mars Rover powered by multijunction cells

This very successful space cell is currently being engineered into systems

for terrestrial use

40

30

20

10

0

Effi

cien

cy (%

)

20052000199519901985Year

NREL invention of GaInP/GaAs solar cell tandem-

poweredsatelliteflown

commercialproduction oftandem

productionlevels reach300 kW/yrfor space

3-junction concentrator 40.8% Geisz2008

Dan David Prize 2007

Olson, Kurtz

High-efficiency GaInP/Ga(In)As/Ge cells

Ge bottom cell and substrate

GradeGaInAs middle cell

GaInP top cell

Metal40.7% King, 2006

1.0 eV

1.8 eV

1.3 eV

0.7 eV

New research: from 40% to 50%

Ge bottom cell and substrate

GaInAs middle cellGaInP top cell

Metal

40.4% King, 2006

1.9 eV

1.4 eV

0.7 eV

Lattice matched Lattice mismatched

Inverted GaInP/Ga(In)As/GaInAs solar cell champion: 40.8%

1.8 eV GaInP

1.3 eV GaAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

• MOVPE growth on GaAs substrate.• Inverted structure.• Mounted on Si or glass handle.• GaAs substrate selectively removed.• Front-side processing completes cell.

Advantages:• Higher efficiency• Reuse of substrate or use of impure

substrate can reduce cost

1.8 eV GaInP

1.3 eV GaAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

Inverted GaInP/Ga(In)As/GaInAs solar cell champion: 40.8%

• MOVPE growth on GaAs substrate.• Inverted structure.• Mounted on Si or glass handle.• GaAs substrate selectively removed.• Front-side processing completes cell.

Advantages:• Higher efficiency• Reuse of substrate or use of impure

substrate can reduce cost

1.8 eV GaInP

1.3 eV GaInAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

Inverted GaInP/Ga(In)As/GaInAs solar cell champion: 40.8%

Handle

• MOVPE growth on GaAs substrate.• Inverted structure.• Mounted on Si or glass handle.• GaAs substrate selectively removed.• Front-side processing completes cell.

Advantages:• Higher efficiency• Reuse of substrate or use of impure

substrate can reduce cost

Solar for large-scale electricity generation

Convergence of utilities and photovoltaic companies?

• Recent trends– Multi megawatt systems becoming

common– Dominance of building integration is no

longer assumed– More systems are ground mounted– More systems are tracked

• Interest from utilities is growing

How can multijunction cells make a difference?

• Use less semiconductor material– Capital investment can be less– If other costs can be reduced, then may be able to

achieve very low cost• High efficiency has potential to generate the most

electricity per area• Commercial investment involves more than 50

companies with total investment on order of $1B

• Deployment in 2008 expected to be ~ 1 MW

Electricity costs are going up, but PV is still expensive

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5

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Ove

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ost (

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Wind PV

source: http://www.eia.doe.gov/oiaf/aeo/assumption/pdf/electricity.pdf#page=3

Scrubbedcoal

IGCC ConvCombust

AdvNuclear

Windoffshore

Solarthermal

Operating costs depend on fuel cost

Summary• Photovoltaic industry is doubling every two

years• Using concentration may help the solar

industry grow even faster • Multijunction cells provide the path to high

efficiency; > 40% and are still increasing• Solar industry is moving into large-scale,

utility applications; multijunction cells may hasten this trend

Flying high with high efficiencyCells from Mars rover

may soon provide electricity on earth

High efficiency, low cost, ideal for large systems