Photovoltaics for the Terawatt Challenge

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Photovoltaics for the Terawatt Challenge

• Christiana HonsbergDepartment of Electrical
  Computer and Energy Engineering
• Director, QESST ERCArizona State University

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Outline

• Terawatt Challenge
• 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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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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Terawatt Challenge

• Why is compound annual growth rate important?

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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 2000-2012 (NREL data)

NREL,2012 Renewable Energy Data Book
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Photovoltaic Milestones

• Germany, Spain, Italy have yearly installed PV
  capacity gt yearly increase in electricity demand.
• In Germany, PV is 50 of summer peak electricity
  demand

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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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Potential for PV in the US
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Photovoltaic Milestones

• ASU reached 50 of total electricity supplied
  by PV

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Arizona Context
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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

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Duck Curves

• Power after sun goes down a concern for
  utilities.
• Can mitigate by load management.

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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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Present State of PV efficiencies
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Fraction of Efficiency Achieved
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Types of PV Systems

• Optical configuration of photovoltaic systems
  One-sun or flat plate concentrating systems
  tracking

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Scope of QESST ERC
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Multiple Junction (Tandem) Solar Cells

• Concentration or stacking multiple solar cells
  increases efficiency
• To reach gt50 efficiency, need ideal bandgap
  6-stack tandem, (assuming 75 of detailed
  balance limit).
• Hard to get compatible materials with the right
  bandgaps.

APS Tutorial Nanostructured Photovoltaics
C.Honsberg 18
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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 solarspectrum (i.e. tandem)
• No entropy penalty
• Efficiency controlled by existence of materials
• Beneficially circumventone of the assumptionsin
  thermodynamics

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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
• MetamorphicGe-GaInAs-GaInP
• Metamorphic GaInAs-GaAs-GaInP
• Band gaps for 4-tandem arepoorly lattice
  matched5 band gapsand six band-gaps are better
  matched

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Ge-based tandem solar cells

• Metamorphic solar cell reached 40.7 at 200X.

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Carrier-Selective Contacts

• Carrier-selective contacts enable ideal VOC

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CSC Implementation a-Si/c-Si solar cell

• Demonstrated 746 mV on 50 µm wafers

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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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Experimental GaAsSb/InAs QD material

• Doping of QD layers to control occupancy of the
  QD.

GaAsSb/GaAs interface
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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 meanssmall area (and lower
  cost) needed for solarcells
• Trade balance of systemsand solar cell cost.

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Experimental GaAsSb/InAs QD material
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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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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

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• Questions?