1. Photovoltaics for the Terawatt Challenge
Christiana Honsberg Department of Electrical Computer and Energy Engineering
Director, QESST ERC Arizona State University

2. Outline Terawatt Challenge Future prospects Education What is it?
Photovoltaics for the TW challenges
Importance of rapid growth
Recent milestones in PV
But what about …..
Myths of photovoltaics: land area; efficiency; energy payback time; materials availability; time to impact; duck curves, etc
Future prospects
Education
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3. Terawatt Challenge
Terawatt Challenge: Encapsulates the dichotomy surrounding energy– essential for improved quality of life, but also tied among the most serious global challenges.
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4. Terawatt Challenge Why is compound annual growth rate important?
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5. Terawatt Challenge
In the nearly two decades since the TW challenge paper, renewables have reached multiple milestones
In US, renewable compound annual growth rate 4.8% from (NREL data)
NREL,2012 Renewable Energy Data Book
ASU-UA-NAU Student Solar Conference /01/2014 C. Honsberg

6. Photovoltaic Milestones
Germany, Spain, Italy have yearly installed PV capacity > yearly increase in electricity demand.
In Germany, PV is 50% of summer peak electricity demand
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7. Learning Curves for Photovoltaics
PV learning curves show compound annual growth rate (CAGR) of ~30% over the last several decades
Extending the growth rates shows ability of PV (renewables more generally if these are included) to make a substantial impact on electricity generations
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8. Potential for PV in the US

9. Photovoltaic Milestones
ASU – reached 50% of total electricity supplied by PV
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10. Arizona Context
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11. Photovoltaics “FAQ” Energy payback time Land use Cost
What do you do at night for power?
Materials availability
For silicon, limitation is silver in grids, which cause a limitation at 2 TW
Availability subject to efficiency, thickness
APS Tutorial Nanostructured Photovoltaics C. Honsberg

12. Duck Curves Power after sun goes down a concern for utilities.
Can mitigate by load management.
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13. PV for the Terawatt Challenge
PV technology must be high efficiency, efficient use of materials, scalable, reliable, and enable path for future improvements
High efficiency; overcome limits; thin
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14. Present State of PV: efficiencies

15. Fraction of Efficiency Achieved
APS Tutorial Nanostructured Photovoltaics C. Honsberg

16. Types of PV Systems
Optical configuration of photovoltaic systems: One-sun or flat plate; concentrating systems; tracking
APS Tutorial Nanostructured Photovoltaics C. Honsberg

17. Scope of QESST ERC
4/13/2018
QESST Second Site Visit - Overview

18. Multiple Junction (Tandem) Solar Cells
Concentration or stacking multiple solar cells increases efficiency
To reach >50% efficiency, need ideal bandgap 6-stack tandem, (assuming ~75% of detailed balance limit).
Hard to get compatible materials with the right bandgaps.
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19. What do efficiency calculations tell us?
Approaches to high efficiency:
Concentrate sunlight. “One sun” = 1kW/m2, max concentration ~46,000.
No entropy penalty for concentrating sunlight, but etendue limits to acceptance angle and concentration.
Optically split solar spectrum (i.e. tandem)
No entropy penalty
Efficiency controlled by existence of materials
Beneficially circumvent one of the assumptions in thermodynamics
# junctions in solar cell
1 junction
2 junction
3 junction
1 sun h
30.8%
42.9%
49.3%
 junction
68.2%
Max con. h
40.8%
55.7%
63.8%
86.8%
APS Tutorial Nanostructured Photovoltaics C.Honsberg

20. Tandem Solar Cells
Key issue for III-Vs: need precisely controlled band gaps which are lattice matched
“Missing” low band gap material
Approaches:
Lattice matched; Ge-GaAs-GaInP
Metamorphic;Ge-GaInAs-GaInP
Metamorphic; GaInAs-GaAs-GaInP
Band gaps for 4-tandem are poorly lattice matched;5 band gaps and six band-gaps are better matched
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21. Ge-based tandem solar cells
Metamorphic solar cell reached 40.7% at ~200X.
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22. Carrier-Selective Contacts
Carrier-selective contacts enable ideal VOC
Shouldn’t the external contacts contact the fermi levels.
In the diagram, the inversion is too strong

23. CSC Implementation: a-Si/c-Si solar cell
Demonstrated 746 mV on 50 µm wafers

24. InAs QDs on GaAsSb barriers
InAs QDs achieved on GaAsSb material
Increasing Sb composition decreases QD size and increases QD density
InAs QDs on GaAs (5 ML) / GaAs1-xSbx (5nm) buffer layers with x = 23%, with density 2.6 x 106 cm-2
InAs QDs on GaAs
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25. Experimental GaAsSb/InAs QD material
Doping of QD layers to control occupancy of the QD.
GaAsSb (20nm)
S.I. GaAs substrate
InAs QDs
δ-doping
GaAs (50nm)
(b)
(c)
8nm
GaAsSb/GaAs interface

26. Tandem Solar Cells
Monolithic III-V tandem solar cells; Series connected; three junctions
High efficiency used in high concentration, two-axis tracking systems
High concentration means small area (and lower cost) needed for solar cells
Trade balance of systems and solar cell cost.
APS Tutorial Nanostructured Photovoltaics C.Honsberg

27. Experimental GaAsSb/InAs QD material

28. Path for Continual Improvement
Ideal solar cell consists of a light-trapped, thin solar cell
Nanostructured surfaces allow light trapping and advanced concepts (e.g., multiple exciton devices)
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29. Student Led Pilot Line
Silicon pilot line capabilities for interaction among students, industry and researchers
10 Fulton Undergraduate Research Initiative Projects
2 honors thesis
4 capstone projects

30. Questions?
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