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 elsewhere

September 26, 2008

UC Merced Solar Symposium

NREL/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 future

Rogol, PHOTON International August 2007, p 112.
Industry growth is currently constrained by 

Si availability, motivating us to

reduce semiconductor material

Front Solar cell

Back

Concentration:

1. Reduces semiconductor use

2. Allows use of higher efficiency cell (higher system efficiency)

Thin film

Concentrator
Why multijunction? 

Power = Current X Voltage

5x1017

4

3

2

1

0

Solar spectrum

0 1 2 3 4

Photon energy (eV)

Band gap

of 0.75 eV

5x1017

4

3

2

1

0

Solar spectrum

0 1 2 3 4

Photon energy (eV)

Band gap

of 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 9

1

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

Bandgap (eV)

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

AlP

AlAs

GaP

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

Efficiency (%)

1985 1990 1995 2000 2005

Year

NREL invention 

of GaInP/GaAs

 solar cell tandempowered

satellite

flown

commercial

production of

tandem

production

levels reach

300 kW/yr

for space

3-junction concentrator 40.8% 

Geisz

2008

Dan David Prize 

2007 

Olson, Kurtz

High-efficiency 

GaInP/Ga(In)As/Ge cells

Ge bottom cell 

and substrate

Grade

GaInAs middle cell

GaInP top cell

Metal

40.7% 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 cell

GaInP 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

6

5

4

3

2

1

0

Overnight cost ($/W)

Wind PV

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

Scrubbed

coal

IGCC Conv

Combust

Adv

Nuclear

Wind

offshore

Solar

thermal

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 efficiency

Cells from Mars rover 

may soon provide 

electricity on earth

High efficiency, low cost, 

ideal for large systems