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
6
5
4
3
2
1
0
Ove
rnig
ht c
ost (
$/W
)
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