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

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

ASU-UA-NAU Student Solar Conference 04/01/2014C. Honsberg 2

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)


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NREL,2012 Renewable Energy Data Book

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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Learning Curves for Photovoltaics


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• 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

Potential for PV in the US

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


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• PV technology must be high efficiency, efficient use of materials, scalable, reliable, and enable path for future improvements

• High efficiency; overcome limits; thin

Present State of PV: efficiencies

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Fraction of Efficiency Achieved

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 >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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What do efficiency calculations tell us?

Approaches to high efficiency:1. Concentrate sunlight. “One sun” = 1kW/m2, max

concentration ~46,000.• No entropy penalty for concentrating sunlight, but

etendue limits to acceptance angle and concentration.

2. Optically split solarspectrum (i.e. tandem)– No entropy penalty– Efficiency controlled by

existence of materials

3. Beneficially circumventone of the assumptionsin thermodynamics

# junctions in solar cell

1 junction

2 junction

3 junction

1 sun

30.8%

42.9%

49.3%

junction 68.2%

Max con.

40.8%

55.7%

63.8%

86.8%

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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– Metamorphic;Ge-GaInAs-GaInP– Metamorphic; GaInAs-GaAs-GaInP

• Band gaps for 4-tandem arepoorly lattice matched;5 band gapsand six band-gaps are better matched


Ge-based tandem solar cells

• Metamorphic solar cell reached 40.7% at ~200X.

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

Experimental GaAsSb/InAs QD material

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

GaAsSb (20nm)

GaAsSb (20nm)

S.I. GaAs substrate

InAs QDsδ-doping

GaAs (50nm)

(b)

(c) 8nm

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.

Experimental GaAsSb/InAs QD material

0 50 100 150 20080

90

100

110

120

130

140 Sample 1 (undoped) Sample 2 (2 electrons per dot) Sample 3 (4 electrons per dot) Sample 4 (6 electrons per dot)

F

WH

M (

meV

)

Temperature (K)

(d)

0 50 100 150 200 250

1.06

1.08

1.10

1.12

1.14

1.16

Sample 1 (undoped) Sample 2 (2 electrons per dot) Sample 3 (4 electrons per dot) Sample 4 (6 electrons per dot)

En

erg

y (e

V)

Temperature (K)

(c)


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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)

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

ASU-UA-NAU Student Solar Conference 04/01/2014 C. Honsberg 30

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Questions?

Photovoltaics for the Terawatt Challenge

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