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High Efficiency Photovoltaics: g yMeeting the Terawatt Challenge

Harry AtwaterThomas J. Watson Laboratories of Applied Physicspp yCalifornia Institute of Technology

Photovoltaics for Energy SupplyLimits to Photovoltaic EfficiencyLimits to Photovoltaic EfficiencyPV Technology Comparison: Si, Thin Films, Concentrators, NanostructuresMultijunction PV: Path to Ultrahigh EfficiencyNanostructures in PhotovoltaicsNanostructures in Photovoltaics

3/15/09H. Atwater Caltech

PV Land Area RequirementsAll US Primary Energy

4 1 Terawatts w/ 10% modules

All US Electricity800 Gigawatts w/ 10% modules

4.1 Terawatts w/ 10% modules

3/15/09H. Atwater Caltech

Crystalline Silicon vs. other Solar TechnologyN th t Si t ll h DOE t l Now appears that c-Si can eventually reach DOE cost goals

Thin film modules can get there first, but efficiency limits, materials issues Innovative and disruptive technologies must have film-like cost/area and

>20% efficiencyy

c-Si Cant Scale Module and BOS?!

~2020$1/W

Module and BOS?!

Thin Films,Others

(1 GW) (10 GW)100000

(100 GW)

3/15/09H. Atwater Caltech

Thin Film Solar CellsMaterials: CdTe CuIn Ga Se poly-Si amorphous SiMaterials: CdTe, CuInxGa1-xSe2, poly-Si, amorphous Si

Low Density of UnpassivatedGrain boundaries

High Density of passivatedGrain boundaries

p+

E

p+

E

Inactive

areasGl S b t t

nn+

Gl S b t t

nn+

Glass Substrate Glass Substrate

CdTe Thin Film Q4 2008 $1.08/Watt56% gross margin

3/15/09H. Atwater Caltech

56% gross margin750 MW production capacity

PV Resources: MaterialsRelative abundance of elements vs. atomic number*

8 1000x108x

*from P.H. Stauffer et al, Rare Earth Elements - Critical Resources for High Technology, USGS (2002)

PV Materials by Production and Reserve

0.3 1.5 50(2000) (2005) (2020)

PV Feedstock Production and Reserve Base Annual PV Production (GigaWatts/year)

PV Feedstock Consumption World Production

(1000s of ton/year)

Reserve Base*

(1000s of ton/year)

FeedstockMaterial (1000s of ton/year)

(2000) (2005) (2020)

Si (c-Si) 1 000 abundant 4 (1) 15 (2) 150 (3)

Te (4) (CdTe) 0.3 (Cu) 47 0.030 0.15 5

In (4) (CIGS) 0.5 (Zn) 6 0.030 0.15 5

Ga(5) (GaAs) 184 (Al) >1100 0.008 0.041 1.4

As(5) (GaAs) 59 1100 0.008 0.041 1.4

*: Resources that are currently economic, marginally economic and some of those that are currently subeconomic

Material use in module production (grams / Watt): (1) 13, (2) 10, (3) 3, (4) 0.1, (5)0.025

Sources: US Geological Survey 2004 (http://minerals.usgs.gov/minerals/pubs/mcs/),M.A. Green, Prog. Phot. 14 (2006) 743-751; G. Willeke, Fraunhofer Institut

Si and Te Data From G. Willeke, Fraunhofer ISE

Cost/Efficiency of PV Technology Argues for High Efficiency

BOSLimit

~18% efficiency

3/15/09H. Atwater Caltech

Single Junction Solar CellsSingle Junction Solar Cells

Depletedn-type p-type

Metal grid Antireflective layer

Sunlightjunctionregion region

n-type layerIncreasing

Energy

EC

e-h+p-type layer

EF

Energy

EV

Back Metal Contact

Vbi

05/08/2008

h > EG Electron Hole

PV Figures of MeritPV Figures of Merit

Current Maximum Power Output:

FFVJVJP OCSCMMM ==Density

Dark

Light

Maximum Power Output:

MM

VJVJFF = VOC

Light

VM

Fill Factor:

OCSCVJVOC

Voltage

JL

VM

Efficiency:

JSC

JLJM

incident

M

PP

=

05/08/2008

Maximum Solar Cell EfficienciesTheoretical95% C t ff 1 T/T T 300 K T

MeasuredReferences 95% Carnot eff. = 1 T/Tsun T = 300 K, Tsun

5800 K93% Max. eff. of solar energy conversion

= 1 TS/E = 1 (4/3)T/Tsun (Henry)

ReferencesC. H. Henry, Limiting efficiencies of ideal single and multiple energy gap terrestrial

solar cells, J. Appl. Phys., 51, 4494 (1980). W. Shockley and H. J. Queisser, Detailed Balance Limit of Efficiency of p-n Junction

Solar Cells, J. Appl. Phys., 32, 510 (1961). J. H. Werner, S. Kolodinski, and H. J. Queisser, Novel Optimization Principles and

Efficiency Limits for Semiconductor Solar Cells, Phys. Rev. Lett., 72, 3851 (1994). M. Green, K. Emery, D. L. King, Y. Hisikawa, W. Warta, "Solar Cell Efficiency Tables

(Version 27)", Progress in Photovoltaics, 14, 45 (2006)

72% Ideal 36-gap solar cell at 1000 suns (Henry)(Version 27) , Progress in Photovoltaics, 14, 45 (2006)

R. R. King et al., "Pathways to 40%-Efficient Concentrator Photovoltaics," Proc. 20th European Photovoltaic Solar Energy Conf., Barcelona, Spain, 6-10 June 2005.

R. R. King et al., "Lattice-Matched and Metamorphic GaInP/ GaInAs/ Ge Concentrator Solar Cells," Proc. 3rd World Conf. on Photovoltaic Energy Conversion, May 11-18, 2003, Osaka, Japan, p 622.

A. Slade, V. Garboushian, "27.6%-Efficient Silicon Concentrator Cell for Mass Production," Proc. 15th Int'l. Photovoltaic Science and Engineering Conf., Beijing, China, Oct. 2005.

56% Ideal 3-gap solar cell at 1000 suns (Henry)50% Ideal 2-gap solar cell at 1000 suns (Henry)

3-gap GaInP/GaAs/Ge cell @240suns (Fraunhofer)

,R. P. Gale et al., "High-Efficiency GaAs/CuInSe2 and AlGaAs/CuInSe2 Thin-Film

Tandem Solar Cells," Proc. 21st IEEE Photovoltaic Specialists Conf., Kissimmee, Florida, May 1990.

J. Zhao, A. Wang, M. A. Green, F. Ferrazza, "Novel 19.8%-efficient 'honeycomb' textured multicrystalline and 24.4% monocrystalline silicon solar cells," Appl. Phys. Lett., 73, 1991 (1998).

44% Ultimate eff. of device with cutoff Eg: (Shockley, Queisser)43% 1-gap cell at 1 sun with carrier multiplication

(>1 e-h pair per photon) (Werner,

3-gap GaInP/GaAs/Ge cell @240suns (Fraunhofer)

41.1 %

3-gap GaInP/GaAs/Ge cell @ 1 sun (Spectrolab)32 0% ( p p p ) ( ,Kolodinski, Queisser)

37% Ideal 1-gap solar cell at 1000 suns (Henry)

31% Ideal 1-gap solar cell at 1 sun (Henry)

32.0%

1-gap solar cell (Si, 1.12 eV) @92 suns (Amonix) 27.6%

1 gap solar cell (GaAs 1 424 eV) @1 sun (Kopin) g p ( y)30% Detailed balance limit of 1 gap solar cell at 1

sun (Shockley, Queisser)

1-gap solar cell (GaAs, 1.424 eV) @1 sun (Kopin) 25.1%

1-gap solar cell (silicon, 1.12 eV) @1 sun (UNSW)25.0% Richard King3/15/09H. Atwater Caltech

Detailed Balance Limit for Solar Cell Efficiencyy

absorptionof sunlight radiative

emission

Cell

emission

1 5AM dJ 1.511

AM radJ N Nq

= Sh kl d Q i (1961)

generation rateby sunlight

radiativerecombination

charge carrierflux

Shockley and Queisser (1961) by sunlightabsorptionrecombinationrate

3/15/09H. Atwater Caltech

Open Circuit Voltage Offset from Bandgap: Photon EntropyF = H TS

2.0 800VocEg from EQE

Voc of solar cells with wide range of bandgaps and

qVoc = Eg T kln = Eg - kT ln 46,200 = Eg 10.7 kT

1.5

oc (

V)

600

700

W/(m

2 . e

V))

g(Eg/q) - Vocradiative limitAM1.5D, low-AOD

wide range of bandgaps and comparison to radiative limit

1.0(Eg/

q) -

Vo

400

500

oton

Ene

rgy

(W

aInA

s

p)

0 5q, V

oc, a

nd

200

300

y pe

r Uni

t Pho

d-A

lGaI

nP

GaA

s4

- eV

GaI

nAs

o-G

aInP

AlG

aInA

s

d-A

lGaI

nPd-

GaI

nP

d-A

lGaI

nP

0.97

-eV

GaI

nAs

GaI

nNA

s1.

10-e

V G

a

1.24

-eV

GaI

nAs

1.30

-eV

GaI

nAs

Ge

(ind

irect

gap

AlG

aInA

s

0 0

0.5

Eg/q

0

100

200

Inte

nsityG1.4

0.00.6 1 1.4 1.8 2.2 2.6 3

Bandgap Eg (eV)

0

Richard King3/15/09H. Atwater Caltech

Single-junction Cells vs. Multijunctions

GaAsEG = 1.42 eV

1.2

1.4

1.6

m2

nm)

h < EG 0.40.6

0.8

1.0

ower

Den

sity

(W/m

h EG h > EG 500 1000 1500 2000 25000.00.2

Po

Wavelength (nm)

1 0

1.2

1.4

1.6

m2

nm)GaInP (1.90 eV)

0.4

0.6

0.8

1.0

Pow

er D

ensi

ty (W

/mGaAs (1.42 eV)

GaInAsP (1.05 eV)

3/15/09H. Atwater Caltech

500 1000 1500 2000 25000.0

0.2

P

Wavelength (nm)

GaInAs (0.60 eV)

Concentrator Photovoltaics: An Approach to ReapConcentrator Photovoltaics: An Approach to Reap Benefit from Expensive Ultrahigh Efficiency Cells

UMUWA SOLAR POWER STATIONUMUWA SOLAR POWER STATION

3/15/09H. Atwater Caltech

Cost per Watt and Levelized Cost of Electricity in Flat Plate vs. Concentrator Systemy

Detailed Balance ModelsAssumptions:

Perfect absorption of incident photons Photo-current loss through radiative reemission:

P. Wurfel, Journal of Physics C: Solid State Physics 15, 3967-3985 (1982)

Detailed Balance of First Subcell

JC ll 1 1.511

AM radJ N Nq

= Cell 1

Cell 2

Detailed Balance of nth Subcell

1.51

AM rad radnn n

J N N N= + Cell n

Cell n-1

qCell n+1

Detailed Balance: Ge-based Triple-junction

E1 (GaInP)

E2 (GaAs)

Isoefficiency Plot

2 50

Independently Connected Series Connected

Ge(0.67 eV)

0.250.30

0.35

0.25

0.40

0 450.50

0.520.51

0.532.25

2.50

0.400.45

0.45

0.510.54 0.50

1.75

2.00

(eV)

0.200 53

1.50

E 1

100x AM1.5300 K

1.00 1.25 1.50 1.75 2.00 2.25 2.50

0.53

1.00 1.25 1.50 1.75 2.00 2.251.00

1.25 300 K

E2 (eV)

Detailed Balance: GaInP/GaAs-Based Four-Junction Isoefficiency Plot

AlInGaP(1 90 V)1 06

1.08

1.10100x AM1.5

300 K

GaAs(1.42 eV)

E (InGaAsP)

(1.90 eV)0.52

0.541.02

1.04

1.06

eV)

E3 (InGaAsP)

E4 (InGaAs)

0.50

0.45 0.40

0.55

0.96

0.98

1.00 E 3 (e

0.35

0.30

0.25

0.20

0 90

0.92

0.94 Narrow-gap bottom subcell eases current-matching requirements for E3I ffi i t0.50 0.55 0.60 0.65 0.70 0.75 0.80

0.90

E4 (eV)

Iso-efficiency contours atmospheric absorption

Detailed Balance: Summary

O ti l S b llConfiguration

Optimal Subcell Bandgaps (eV)

Maximum Efficiency

Ge-Based Triple-Junctionp(Series connected)

1.90, 1.42, 0.67 46.3%

Si-Based Triple-Junction(Series connected) 2.00, 1.49, 1.12 51.2%(Series connected) , ,

Optimal Four-Junction (Series connected) 2.00, 1.49, 1.12, 0.72 57.9%

GaInP/GaAs-Based Four-Junction (Series connected) 1.90, 1.42, 1.02, 0.60 54.9%

Efficiency vs. Bandgap Variation in 4 Junction Cell0 7

0.6

0.7

= 0.579 (2.00 eV, 1.49 eV, 1.12 eV, 0.72 eV)n AlGaInP

n+GaAs

Contact

Cap

Windown AlGaInP

n+GaAs

Contact

Cap

Window

0.4

0.5

ency

n InGaAsP

p AlGaInP

n AlGaInP Emitter

Base

Tunnel Junction

Emittern InGaAsP

p AlGaInP

n AlGaInP Emitter

Base

Tunnel Junction

Emitter

0 2

0.3

Effic

ie E1E2

n Sip GaAs

p InGaAsP Base

Transferred Layer

Bonded Interface

Emitter

Base

n Sip GaAs

p InGaAsP Base

Transferred Layer

Bonded Interface

Emitter

Base

0.1

0.2 2 E3 E4

p Si

p+ Sin+ Ge

p Ge

Emitter

Backside Field

Bonded Interface

Base

Base

p Si

p+ Sin+ Ge

p Ge

Emitter

Backside Field

Bonded Interface

Base

Base

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.30.0

E (eV)

p

Contact

p

Contact

Variation of efficiency of optimal 100 sun AM1.5 series-connected four-junction solar cell with changes of each subcell bandgap. Each subcell is varied independently, maintaining the other subcells at their optimum bandgap of 2.00, 1.49, 1.12, and 0.72 eV respectively.

Improvements in Solar Cell EfficienciesFraunhofer ISE

US DOE3/15/09H. Atwater Caltech

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1 91.9

1.4

0.670.67

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1 91.9

1.4

0.670.67

05/08/2008

Lattice Matched and Metamorphic 3-Junction Cell Cross-Sections

n+-Ga(In)As

contact

ARn+-GaInAs

contact

AR

p-GaInP base

p-AlGaInP BSF

n-GaInP emittern-AlInP windown Ga(In)As

GaInP top cell p-GaInP base

p-AlGaInP BSF

n-GaInP emittern-AlInP windown GaInAs

n-Ga(In)As emitter

p++-TJn++-TJ

p-Ga(In)As base

Wide-bandgap tunnel junctionn-GaInP window

Ga(In)As middle cell

n-GaInAs emitter

p++-TJn++-TJ

p-GaInAs base

n-GaInP window

p-GaInP BSF

n-Ga(In)As buffer

p++-TJn++-TJ

Tunnel junction

Buffer region

p-GaInP BSF

p-GaInAsstep-graded buffer

p++-TJ

n+-Ge emitter

p-Ge baseand substratecontact

nucleation

Ge bottom celln+-Ge emitter

p-Ge baseand substratecontact

nucleationn++-TJ

3/15/09 H. Atwater Caltech

Lattice-Matched (LM) Lattice-Mismatchedor Metamorphic (MM)

Richard King

EQE and PL of Subcells Matched to 1%-In and 8%-In GaInAs

90

100 7500

70

80

cien

cy (

%)

6000

nten

sity

40

50

60

uant

um E

ffic

3000

4500

min

esce

nce

I(a

rb. u

nits

)EQE, lattice-matchedEQE, metamorphic

PL, lattice-matched

PL t hi

20

30

40

Exte

rnal

Qu

1500

3000

Phot

olum

PL, metamorphic

0

10

350 450 550 650 750 850 9500

3/15/09 H. Atwater Caltech

Wavelength (nm)

1.890 1.813 eV 1.414 1.305 eV

Richard King

High Efficiency GaInP/GaInAs/Ge Triple Junction

AM1 5 Direct AM1.5 Direct, Low-AOD standard spectrum

SpectrolabGaInP/ GaInAs/ Ge Cell

SpectrolabGaInP/ GaInAs/ Ge Cell

0.269 cm2aperture area

Voc = 3.089 V Jsc = 3.377 A/cm2FF = 88.24%Vmp = 2.749 V

Voc = 3.089 V Jsc = 3.377 A/cm2FF = 88.24%Vmp = 2.749 V

39.0% record efficiency,236 suns 25C

Efficiency = 39.0% 2.3%

236 suns (23.6 W/cm2) intensity0.2691 cm2 aperture area25 1C, AM1.5D, low-AOD spectrum

Efficiency = 39.0% 2.3%

236 suns (23.6 W/cm2) intensity0.2691 cm2 aperture area25 1C, AM1.5D, low-AOD spectrum236 suns, 25 C , , p, , p

3/15/09 H. Atwater Caltech Richard King

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1 91.9

1.4

0.670.67

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1.8

1 3

0.67

1.3

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1 91.9

1.4

1.0

05/08/2008

Metamorphic InGaAs Buffer Layersp y

Step Graded Buffer Linearly Graded Bufferp

P. Kidd et al. Journal of Crystal Growth (1996)

Metamorphic 3J Solar Cells

p InGaAs n+ GaAs

GaInPmetamorphic buffer

n/pGaInP/AlInP

DH

n/pGaInAs/GaInP

DH

GaInP

metamorphic buffer n/pGaAs/GaInP

DH

n/pGaAs/GaInP

DH GaInPmetamorphic buffer

n/pGaInAs/GaInP

DH

n/pGaInP/AlInP

DH

GaAs

GaInPGaAs p InGaAs

Handle Substrate

Effi i 37 9%M.W. Wanlass et al, Proceedings of the 4th WCPEC (2006)

Efficiency = 37.9%

Record Efficiency 3J

n+ GaAsn/p

GaInP

n+ GaAsn/p

GaInPEfficiency 3J Solar Cells

GaAs Buffer

n/pGaAs

n/pInGaAs Buffer

n/pGaAs

n/p

Lattice Matched Lattice Mismatched

Ge

Ge n/pInGaAs

Lattice Matched Lattice Mismatched

VOC 3.054 V 2.911 V

JSC 0.1492 A/W 0.1596 A/W

FF 0 881 0 875FF 0.881 0.875

Concentration 454 suns 326 suns

Efficiency 41.1% 40.8%

40.1% 40.7%

Fraunhofer NREL

R.R. King et al. APL 90 183516 (2006) Spectrolab Spectrolab

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

2.0

1.49

0.72

1.12

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

2.0

1.49

0.72

1.12

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

2.0

1.49

0.72

1.12

05/08/2008

Band Gap versus Lattice Constant

02 24 4 6 8 10 12 14Percent Lattice Mismatch from GaAs [%]

1 901.90

1.42

0 60

1.02bondedinterconnection 0.60interconnection

05/08/2008

Subcell Integration by Bondingg y g

G A

GaInPh = 1.9eVGaAsh = 1.42eVInGaAsPh = 1.05eV

InGaAsh = 0.72eV

InP

Wafer bonding:High-performance solar cells:

Si Substrate

Non-lattice-matched materials integration

Misfit defect isolation at only

Multi-junction, current-matched tandem monolithic solar cell

Optimal bandgap sequence bonded interfaceachievable

4 Junction Solar Cell Device via Bonding/Layer Transfer

n GaInPn AlInGaP

n+ GaAsContactCap

Window

Emitter

n GaAs

p GaInP

n GaInP

a = 5.66

Emitter

Base

Tunnel Junction

EmitterSeries-connected

GaInP 1.90

GaAs 1 42

n InGaAsPn+ InPp+ Ge

p GaAs

Tunnel JunctionTransferred Layer

Emitter

Base

Series connected GaInP/GaAs/

InGaAsP/InGaAs

Band Gap (eV)

GaAs 1.42

InGaAsP 1.02

InGaAs 0.60

n InGaAs

p InGaAsP

n InGaAsP

a = 5.8 Tunnel Junction

Emitter

Emitter

Base

n+ Sip+ InP

p InGaAs

Transferred Layer

Base

hl h h i C lif i i f h l (200 )

Detailed Balance Efficiency = 54.9%

p Sia = 5.43

Contact

Support SubstrateJ.M. Zahler Ph.D. Thesis, California Institute of Technology (2005)

Wafer Bonding/Layer Transfer Process

1. Ion implantation: Defects / Internal

Surfaces

H+80 keV H+ 1x1017 cm-2250oC, 10 minSurfaces

Pressure2. Bond formation:

Smooth particle-

,

1.

Smooth, particle-free surfaces

Surface activation3. Thermal processing:

25 nm

device sub

3. Thermal processing: Bond strengthening Exfoliation

4 Result:

device sub.2.

4. Result: Thin, uniform

transferred filmhandle sub. 4.

3.4/25/08H. Atwater Caltech

InGaP/GaAs/Ge/SiO2/Si Two Junction Cells Fabricated by Wafer Bonding/Layer Transfer

50

y g y

As-Transferred Ge/Si After Ge wet etch & CMP After InGaP/GaAs Growth

50mm

and Cell Processing

-0.020

100

AM 1.5D Light I-V AM 1.5D Spectral ResponseBulk Ge Ge/Si

Cell Data

-0.010

-0.012

-0.014

-0.016

-0.018

nsity

(A/c

m2 )

50

60

70

80

90 Epi-ready Ge Ge/Si Template Ge/Si Template

(%)

Bulk Ge Control

Ge/Si Template

Jsc 0.016 0.016

0 000

-0.002

-0.004

-0.006

-0.008

Cur

rent

Den

Ge/Si Template Ge/Si Template Epi-ready Bulk Ge Wafer

10

20

30

40

EQ

E

Voc 2.22 1.97

FF 0.77 0.79

ff 28% 2 %0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.000

Bias (V)400 500 600 700 800 900

0

Wavelength (nm)

Eff ~28% ~25%

3/15/09H. Atwater Caltech

InGaAs/InP/Si Low Bandgap Cell with Performance Equivalent to State-of-the-Art InGaAs/InP

-0 2

-0.1

0.0

Bulk InP Reference Bulk InP + RIE + Wet Etch InPOI + RIE + Wet Etch InPOI + RIE + CMP + Wet Etch

I G A B (3 1 1017 3)

n+ InGaAs Emitter (0.3m; 5x1018cm-3)

n+ InGaAsTi/Pt/Au

n InP Window (0.05m; 1-2x1018cm-3)(1x1019cm-3; 0.05m)

I G A B (3 1 1017 3)

n+ InGaAs Emitter (0.3m; 5x1018cm-3)

n+ InGaAsTi/Pt/Au

n InP Window (0.05m; 1-2x1018cm-3)(1x1019cm-3; 0.05m)

I G A B (3 1 1017 3)

n+ InGaAs Emitter (0.3m; 5x1018cm-3)

n+ InGaAsTi/Pt/Au

n InP Window (0.05m; 1-2x1018cm-3)(1x1019cm-3; 0.05m)

Performance Equivalent to State of the Art InGaAs/InP

-0.4

-0.3

0.2

Cur

rent

(mA

)

n+ InP Buffer / Spreading Layer (1m; 1x1019cm-3)

Ti/Pt/AuInGaAs Contact Layer

InGaAs Tunnel Junction

p InGaAs Base (3m; 1x1017cm-3)

p+ InP BSF (0.05m; 1-2x1018cm-3)

InP Etch Stop Layer (0.1m; 1x1018cm-3)

n+ InP Buffer / Spreading Layer (1m; 1x1019cm-3)

Ti/Pt/AuInGaAs Contact Layer

n+ InP Buffer / Spreading Layer (1m; 1x1019cm-3)

Ti/Pt/AuInGaAs Contact Layer

InGaAs Tunnel Junction

p InGaAs Base (3m; 1x1017cm-3)

p+ InP BSF (0.05m; 1-2x1018cm-3)

InP Etch Stop Layer (0.1m; 1x1018cm-3)InGaAs Tunnel Junction

p InGaAs Base (3m; 1x1017cm-3)

p+ InP BSF (0.05m; 1-2x1018cm-3)

InP Etch Stop Layer (0.1m; 1x1018cm-3)

0.0 0.1 0.2 0.3 0.4-0.7

-0.6

-0.5InP Template

SiO2

Si

InP Template

SiO2

Si

InP Template

SiO2

Si

Voltage (V)Cell Description Jsc(mA/cm2) Voc(mV) Fill Factor

Bulk InP Reference Cell -59.7 329 0.686Bulk InP + RIE + wet etch 61 7 342 0 685Bulk InP + RIE + wet etch -61.7 342 0.685InPOI #1(InPOI + RIE + wet etch) -62.7 338 0.675

Process Eliminates InP Substrate CostTemplate Fabrication Cost ~ Epi Cost

3/15/09H. Atwater Caltech

Zn3P2 - An Earth Abundant Semiconductor

Zn + (P4)n

500C

650C

200C

850C

Zn + P4

650 C 850 C

Zn3P2

Energy Gaps Direct

Zn3P2 (tetragonal)

cmEnergy Gaps Direct

and Indirect nearly aligned @ ~1.3 eV

1 cm diameterZinc Phosphide not commercially available have to grow crystals

Zn3P2 Energy Bands andScientific/Technical Approach

Zn3P2:ZnS:ZnO Band Alignment

Heterojunction Cell

Lifetime at direct gap:

ZnSZn3P2 ZnO:Al

AM 1.5 G IlluminationLifetime at direct gap: diffusion length ~10 um.

3 2

Ec = 0.2 eV

VOC ~ 0.8 V

Ev = 2.3 eV

Si Wire Array Solar Cells1/ 1/

t

n-type

~

1/ 1/

p-type

Traditional planar

~L

solar cell based on arrays of Si wires features:

Traditional planar, single junction solar cell

Idealized radial junctionwire solar cell

~L

Orthogonalize light absorption and photocarrier collection Retain efficiencies competitive with planar, crystalline Si solar cells Compatible with low minority carrier diffusion length Compatible with low minority carrier diffusion length Si wire arrays formed by SiCl4 chemical vapor deposition Can be formed into flexible that are peeled off template Si

Device Modeling

20

Eff (%)

20

Eff (%)

Si Planar Cell Si Radial Junction Cell

1010

20decreasing surface recombination velocity

00

LnCell thickness Ln=RCell thickness

1 D carrier transport modeling indicates that high efficiencies can be1-D carrier transport modeling indicates that high efficiencies can be maintained for low diffusion length materials

Modeling indicates > 15%, Voc > 500 mV achievable withLn = 1 m in Si, provided junction recombination is not limiting

B. M. Kayes et al., J. Appl. Phys. 97, 114302 (2005)

Prototype Si Wire Array Devices

Reactive Ion Etching used to define wires, to deconvolute

Device geometry effects

from20 m

material quality issues, and device fabrication difficulties

0.08

device size ~0.1 cm2, Voc = 505 mV, 2

0.04

0.06

cm2 )

With 5 m diameter, 50 m long wires we see:

Jsc 20 mA/cm2, FF = 58 %, 5.7 %

-0 02

0.00

0.02

J (A

/c

dark

light

(diffused B emitter, 5 cm n-Si(100) base)High V achievable in Si wire array cells

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

-0.02

V (V)B. M. Kayes et al., Proc. of 33rd IEEE PVSC (2008)

High Voc achievable in Si wire array cells

Vapor-Liquid-Solid (VLS) Growth

Silicon freezes out

Single crystal wiresSingle crystal wires

Growth direction controlled by substrate orientation

High growth rates (up to m/s) High growth rates (up to m/s)

Inexpensive gas phase precursors

Atmospheric pressure growth possible 1 m

R. S. Wagner and W. C. Ellis, App. Phys. Lett. 4, 89 (1964)

Wide range of diameters possible

Large Area (> 1 cm2) Si Wire Arrays

5 nm

30 m

5 nm

100% vertically aligned, 75 m length microwire arrays over areas > 1 cm2

B. M. Kayes, M. A. Filler et al., App. Phys. Lett. 91, 103110 (2007)

Si Wire Array Cell Milestones

30 mGrowth of vertically-aligned, patterned Si wire arraysover large (>1 cm2) areas, using Au, Cu, and Nicatalyst metals

Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Lewis, N. S.;Atwater, H. A. Applied Physics Letters 2007, 91, (10), 103110-3.

catalyst metals.

Demonstration of Si wire arrayphotoelectrochemical cell

Maiolo, J. R. I.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S.Journal of the American Chemical Society 2007 129 (41) 12346-12347Journal of the American Chemical Society 2007, 129, (41), 12346-12347.

Goodey, A. P.; et. al. J. Am. Chem. Soc. 2007, 129, 12344-12345.

Si Wire Array Cell Milestones

Removal of wires fromgrowth substrate as a

M A Fill K E Pl t l Ad M t (2008)

growth substrate as aflexible, polymer-embedded, ordered wirearray

Recycling of patterned

M. A. Filler, K. E. Plass, et al. Adv. Mater. (2008)array

Spurgeon J M ; Plass K E ; Kayes B M ;

growth substrate and repeated re-growth of

wire arraysSpurgeon, J. M.; Plass, K. E.; Kayes, B. M.;Brunschwig, B. S.; Atwater, H. A.; Lewis, N. S.Applied Physics Letters 2008, 93, (3), 032112-3.

Diffusion Length from Scanning Photocurrent of Single Wiresof Single Wires

Ni-catalyzed NW

20 20 mm

Rwire= 230 k N = 1016 cm-3

LpL+E

L-p ( ) 11 m+= ELL kTqLp p

L ff ~ 10 m

xA. L. Fahrenbuch and R. H. Bube.

Fundamentals of Solar Cells, pp. 83Lp,eff 10 m

Absorption concentration: Si wires act as waveguideSi wires act as waveguide array

l b i f i

14.4% area

Triangular60

80

M (%

)

Percent solar absorption vs. area fraction

3.0

Square (Ni)Penrose

40

60

rpti

on F

OM

1 0

2.0

Quasi-randomDodecagonal

0

20

0 5 10 15 20

Ab

sor 1.0

0 5 10 15 20

Packing Fraction (%)

Two Plasmonic PV conceptsNanoparticle Scatterer/ Di l t i W id

Backside SPP WaveguideDielectric Waveguide Waveguide

1

0 6

0.8

1

n fr

actio

n (-

)

GaAs/Ag

GaAs/Al

Si cells

0.4

0.6

ond.

abs

orpt

ion

Si/Ag

Si/Al

0

0.2

200 400 600 800 1000 1200

Sem

ico Si/Al

H. R. Stuart and D. G. Hall, APL 69, 2327, 1996 S. Pillai et al, APL 88, 161102, 2006

Wavelength (nm)V.E. Ferry, et.al. Nano Letters, 8, 4391-4397 (2008)

Optically thin plasmonic GaAs solar cellAg, Al Nanoparticles

Porous Al2O3Template

0

Energy

p-Al0.8Ga0.2As(2 09 eV)

Wind

Metalevaporation

100P

(2.09 eV)p-GaAs

(1.42 eV)

dow

Abs o

20

Positio

n / n

mn-GaAs(1.42 eV)

orber layer00

n-Al0.8Ga0.2As(2.09 eV)E

C

EV

EF

BSF

rs

Thin GaAs Solar Cell

300

CV F

F

K.Nakayama, K.Tanabe, and HAAAppl. Phys. Lett. 93, 121904 (2008)

Particle scattering and absorption effects on spectral responsep p

(scattered)(di )

Total absorptionx : coverage x Qext 1-x Qext( ) ( ) ( ) ( )

( )( ) ( )( ) ( ) AQA radexttot =

(direct)RAq A0 L

G A ( )q( )

( )( )

+

+=

dL

dA

cosexp1

cos1

cos12/

02

2

( )( ) ( )( ) ( ) 011 ARQext +

33

on

GaAs (a)q ( ) + dcos10 0

(Calculated absorption) 60nm_x = 39 %110nm x = 30%

1

2

omalized

EQE

1

2

alized

absorptio 110nm_x 30%

150nm_x= 25 %

0

300 400 500 600 700 800 900 1000

No

0

300 400 500 600 700 800 900 1000

Norm

Wavelength(nm)Wavelength(nm)

K.Nakayama, K.Tanabe, and HAA Appl. Phys. Lett. 93, 121904 (2008)

Surface Plasmon Incoupling at Sub- (100 nm)Grooves

SPP Mode

500 nm Si

ki Si Slab Modes

Ag

Angular Dependence of Absorption Enhancement

Wavelength Dependence Single vs. Multiple Grooves

V.E. Ferry, et.al. Nano Letters, 8, 4391-4397 (2008)

SPP-Induced Quantum Dot Excitonic Absorption215 cm1053 =

3-18 cm103cm105.3

=

-14 cm10=

I

m1cm10 41 == L

CdSe QD Layer10 nm

I0

I (P lthick Ag film

CdSe quantum dot layer

1.5

2.0 Fit

mis

sion e-D/2

L=-1=1.2m50-100x reduction in optical thickness

IT(P,l)

1.0

zed

Tran

sm in optical thickness

Pacifici, et al

0 2000 4000 60000.0

0.5

Nor

mal

i et.al., Nature Photonics July 2007

0 2000 4000 6000

Slit-Groove Distance (nm)

Ergodic Light Trapping in Thin SheetsE. Yablonovitch, JOSA 72, 899, (1982)

Assumptions:

geometrical ray optics

optically thin sheetoptically thin sheet

random texture

Detailed balance between absorption and emission cones

Results:

light in medium will be randomized in direction

I di 2 2( ) ti t i t it th i id t li ht (f Si 50 )In medium, 2n2(x) times greater intensity than incident light (for Si, ~50x)

Coupling to Slab Guided Modes Beats the Ergodic Limit

A tiAssumptions:Dipole excites modes of a Si slab

Ag back-side metalg

AR Coating

TE di l

3 um Si

TE, dipole

TE dipole

TM dipole

Summary

Photovoltaics Resource is TW-capableClose to Limiting PV Efficiency for Single Junction CellsSilicon PV being overtaken by thin films as leading technologySilicon PV being overtaken by thin films as leading technologyMultijunction PV a viable path to >50% Wire Array PV scaleable large area technology

For Terawatt PV:Reduce Material Thickness plasmonicseduce ate a c ess p as o csEarth-Abundant Materials!

61

AcknowledgmentsAcknowledgments

Richard King - Spectrolab

Caltech Students/Postdocs:Richard King Spectrolab

Christiana Honsberg U. DelawareDave Carlson BP SolarDi k S S

Brendan KayesJames ZahlerDick Swanson Sunpower

Jurgen Werner U. StuttgartMartha Symko-Davies - NREL

James ZahlerMelissa ArcherKatsu TanabeMike Kel enbergMartin Green - UNSW

Nate Lewis - CaltechMike KelzenbergVivian FerryDomenico PacificiProvided ideas slides inspirationProvided ideas, slides, inspiration...

3/15/09 H. Atwater Caltech