Alta and Spectrum-splitting III-V Multijunction Solar Cells

Harry A. Atwater California Institute of Technology and Alta Devices

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Ultrahigh Efficiency – Radiative Emission and Photon Recycling Full Spectrum Approach – Carrier Thermalization Holographic Spectrum Splitting Polyhedral Specular Reflector Light Trapping Filtered Concentrator

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Inefficient use of spectrum is largest loss mechanism

Polman  and  Atwater,  Nature  Materials  (2012).   Harry Atwater [email protected]

InterSolar July 9th, 2013

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Multijunction Architectures Spectrum Splitting Independent Connect

Monolithic Series Connect

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Lattice match or slight lattice mismatch -> constrains choice of subcell bandgaps Current matching reduces energy yield Fewer subcells (2-4)

Harry Atwater [email protected]

InterSolar July 9th, 2013

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More subcells (6-15) Almost optimal bandgaps Independent electrical connections More complex assembly Spectral splitting optics required

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Full Spectrum Research History

Record cell design VHESC design

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DARPA initiative for Very High Efficiency Solar Cells (VHESC) Dichroic filters DuPont, η=38.5% spectrum-splitting record (2010)

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A. Barnett PVSC (2006); A. Barnett et al. Prog. in PV. (2009) Harry Atwater [email protected]

InterSolar July 9th, 2013

Patented in 1987 Fraunhofer prototype showed 34.3% efficiency outdoor unconcentrated 2 single junction and 1 dual junction cell connected in series Patent WO 87/01512 B. Mitchell et al. Prog. Photovolt.: Res. Appl. (2010)

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New Developments Commoditization of complexity ~ 1 m2 integrated optoelectronics: Visio 50” 1080p Flat Panel LCD à >106 pixels @ $500/m2 All perfect

Harry Atwater [email protected]

InterSolar July 9th, 2013

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New Developments Lower cost solar cells and cell assembly: •  Epitaxial liftoff •  Transfer printing •  Pick-and-place assembly

Yoon et al, Rodgers Group, UIUC Nature (2010) Harry Atwater [email protected]

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New Developments •  Single junction cells driven to high ERE •  Cost of III-V cells driven toward flat plate $/m2 GaAs Epitaxial Liftoff

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InterSolar July 9th, 2013

High Throughput GaAs Growth

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Balance between Solar Absorption and Light Extraction high internal quantum efficiency (internal fluorescence yield)  photons internally absorbed and re-radiated many times

IQE ~ 1 R~1 Conditions for high ERE: IQE~1 and R~1

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InterSolar July 9th, 2013

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Single crystal thin film GaAs solar cells and modules iPhone cover 1.1 W

Man portable charger

• Cell efficiency η = 28.8% @ 1 Sun AM 1.5G

ERE = 48%

• Module efficiency of 23.2% • IQE > 0.96 • Economical III-V flat plate ($1/Wp) PV System) Kayes et al, Alta Devices PVSC (2011, 2012) Harry Atwater [email protected]

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Premises •  No conceptual or theoretical obstacle to >50% module efficiency •  Multijunction solar cells most viable ultrahigh efficiency option to mitigate carrier thermalization (compared to MEG or hot carrier collection). •  Lifted off III-V cells will be available from 0.74 – 2.0 eV with ERE 0.1-5% and costs of 1-5x Si cell $/m2. •  Complex, large-area optoelectronics can be commoditized •  Concentrating solar 2-axis tracking commercially available

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Efficiency with Many Subcells

7/12/13 Harry Atwater

[email protected]

InterSolar July 9th, 2013

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7 Junction Subcell Choices

In0.53Ga0.47As-

ERE# (Simulated)# 2.56%-

ERE# (Record)# 2%-

0.94#

In0.71Ga0.29As0.62P0.38-

0.33%-

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1.15# 1.42# 1.58# 1.84# 2.13#

In0.87Ga0.13As0.28P0.72GaAsAl0.1Ga0.9AsGa0.52In0.48PAl0.20Ga0.32In0.48P-

0.40%8.51%-5.8%0.19%0.08%-

548.5% 22.5%58%5-

Eg#(eV)#

III)V#Alloy#

0.74#

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Single junction ELO cells lattice-matched to GaAs or InP Device modeling: average ERE = 0.3% and 90% of ideal Jsc

Warmann, et. al,, PVSC (2013)

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Full Spectrum Optical Designs

Atwater, et. al,, PVSC (2013)

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Four-way Holographic Spectrum Splitting

•  8 lattice-matched subcells •  Materials grown on GaAs and InP

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Commercially available gratings 3 x 4 gratings: tractable design problem

Escarra, et. al,, PVSC (2013)

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InterSolar July 9th, 2013

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Holographic Splitting Design Cycle

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InterSolar July 9th, 2013

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Power Efficiency Diffraction Efficiency Using simple gratings: •  Not ideal “top-hat” spectral slices •  2nd order diffraction issues, especially for blue light •  Optical efficiency: 77%

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InterSolar July 9th, 2013

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Two-axis Concentration @ 90% abs & 1% ERE 42.3% Stacked holographic optical elements Hollow trough compound parabolic concentrator (CPC) Height: 172 mm

Total concentration: 672X Solid Polymer CPCs Height: 72 mm relative losses from the CPCs (4%), reflections at interfaces (5%), electrical series resistance (2%), and DC-to-DC conversion (2%)

Four dual-junction cells: 1mm x 7.7mm Two-terminal module efficiency37.1%

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Full Spectrum Optical Designs

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Polyhedral Specular Reflector Eg  (eV)  

III-­‐V  Alloy  

Substrate  

2.15  

Al0.20Ga0.32In0.48P  

GaAs  

1.84  

Ga0.51In0.49P  

GaAs  

1.58  

Al0.1Ga0.9As  

GaAs  

1.42  

GaAs  

GaAs  

1.15  

In0.87Ga0.13As0.28P0.72  

InP  

0.94  

In0.71Ga0.29As0.62P0.38  

InP  

0.74  

In0.53Ga0.47As  

InP  

Cell modeling: average ERE of 0.3% and 90% of ideal Jsc Eisler, et. al,, PVSC (2013) Harry Atwater [email protected]

InterSolar July 9th, 2013

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Subcell 7

Subcell 6

Subcell 5

Subcell 3 Subcell 4

Subcell 2

Subcell 1

Shortpass Filters Prevent Parasitic Loss

shortpass filters available near the applicable wavelengths; aperiodic alternating SiO2 and Nb2O3 multillayers. Harry Atwater [email protected]

InterSolar July 9th, 2013

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Polyhedral Specular Reflector Prototype Spectra

Optical Efficiency = 80.4%

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Calculated Efficiencies with Concentrator

Imperfect filters & electrical losses.

•  Tradeoff between concentration and optical efficiency •  Higher index parallelepiped gives higher optical efficiency

ERE of 0.3% and Jsc 90% of ideal Harry Atwater [email protected]

InterSolar July 9th, 2013

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Polyhedral Specular Reflector Prototype

Proof of concept design incorporating 6 subcells

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Full Spectrum Optical Designs

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Light Trapped in a Textured Slab

High Concentration Optic

Angle Restrictor Omnidirectional Filter Multijuction Solar Cell

Reflector

Multijunction Solar Cells

Light trapped by total internal reflection and angle restrictor Light scatters in the slab until it enters solar cell or escapes. Kosten, et. al,, PVSC (2013) Harry Atwater [email protected]

InterSolar July 9th, 2013

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Light Trapping Filtered Concentrator •  Low index slab allows for nearly omnidirectional filter performance •  Initial designs give 45% receiver efficiency at ~150 suns •  Estimated two-terminal module efficiency of 38% •  Improved tracking accuracy enables much higher concentration and small efficiency boost

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Full Spectrum Team

Cristofer Flowers

Dr. Matt Escarra

Emily Kosten

Harry Atwater [email protected]

Emily C. Warmann Carissa Eisler

John Lloyd InterSolar July 9th, 2013

Sunita Darbe

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Web Resources on Photonic Design in PV http://www.lmi.caltech.edu/

The  "Light-­‐Material  InteracAons  in  Energy  Conversion"   Energy  FronAer  Research  Center  (LMI-­‐EFRC)  is  a  naAonal   resource  for  fundamental  opAcal  principles  and   phenomena  relevant  to  solar  energy  conversion,  and  for   design  of  the  opAcal  properAes  of  materials  and  devices   used  for  energy  conversion.    

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InterSolar July 9th, 2013

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Plenty of Room at the Top •  PV designs with 6-15 high radiative efficiency lifted off subcells to mitigate carrier thermalization

•  Three designs with 37-40% realistic two-terminal module efficiency potential with available components Support gratefully acknowledged

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InterSolar July 9th, 2013

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Supporting Slides

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InterSolar July 9th, 2013

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Complexity? Death by a thousand cuts

Tied down by many small problems

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InterSolar July 9th, 2013

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Two-way Holographic Splitting U. Minnesota

U. Arizona

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Two multiplexed volume phase gratings 100 suns concentration, 2 cells, η≈45% from detailed balance

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Holograms to split concentrated light 28% efficiency expected GaAs and Si subcells

D. Zhang, et al., Proc. SPIE (2012).

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InterSolar July 9th, 2013

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Photovoltaic Cavity Concentrator

•  Hollow Cavity •  Array of Single Junction Cells •  Filters (Rugate or Multilayer

U. Ortabasi

Harry Atwater [email protected]

US Patent Application US20030213514 A1, May 17th 2002

InterSolar July 9th, 2013

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Efficiency with Many Subcells

Contours: 90% optical efficiency and 95% electrical system efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Efficiency with Many Subcells

Contours: 90% optical efficiency and 95% electrical system efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Efficiency with Many Subcells

Contours: 90% optical efficiency and 95% electrical system efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Efficiency with Many Subcells

Contours: 90% optical efficiency and 95% electrical system efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Efficiency with Many Subcells

Contours: 90% optical efficiency and 95% electrical system efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Tracking Accuracy

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Performance of holograms sensitive to incident angle Concentrator has acceptance angle of +/- 2° Tracking accuracy of +/- 1.5° (1500x) necessary

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Calculated Efficiencies with Concentrator

•  Tradeoff between concentration and optical efficiency •  Higher index parallelepiped gives higher optical efficiency

Harry Atwater [email protected]

InterSolar July 9th, 2013

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Free carrier absorption Imperfect mirror

Optical Efficiency

Parasitic Absorption Losses

Parallelepiped Refractive Index

Parasitic Losses (%)

•  Free carrier absorption in GaAs cell ~8-10% •  >70% efficiency filters to obtain greater than 90% optical efficiency

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InterSolar July 9th, 2013

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Textured Slab Multipass Optical Model sin2θ/n2=probability of escape per pass

θ= restrictor acceptance angle n=slab refractive index

h≈w

f = fraction of light on correct cell (1/# of cells) 0.7/0.93 eV

1.23/1.06 eV

1.6/1.42 eV

2.25/1.84 eV

w

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Assume slab as thick as cells are wide, ideal filters Absorption on the first pass = f On the second pass = (1- f)(1- sin2θ/n2)f, etc. Overall absorption

Harry Atwater [email protected]

f = 1− (1− f )(1− sin 2 θ / n 2 )

InterSolar July 9th, 2013

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Achieving Efficient Light Absorption Acceptance Angle for Glass Slab!

>90% light directed to correct cells

27.3˚!

40.4˚!

52.6˚!

66.5˚!

2-4 multijunction subcells give high optical efficiency with a glass slab and reasonable acceptance angle Harry Atwater [email protected]

InterSolar July 9th, 2013

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Reducing Slab Escape 2 cells 2 cells 90% light abs. 3 cells 3 cells 4 cells 4 cells

Acceptance angle