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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 Supply•Limits to Photovoltaic Efficiency•Limits to Photovoltaic Efficiency•PV Technology Comparison: Si, Thin Films, Concentrators, Nanostructures•Multijunction PV: Path to Ultrahigh Efficiency•Nanostructures 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 Can’t 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 margin•750 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 layerEF
Energy
EV
Back Metal Contact
Vbi
05/08/2008
hν > EGElectron 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:
OCSCVJ VOC
Voltage
JL
VM
Efficiency:
JSC
JLJM
incident
M
PP
=η
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 King3/15/09
H. 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 sunlightabsorption
recombinationrate
3/15/09H. Atwater Caltech
Open Circuit Voltage Offset from Bandgap: Photon EntropyΔF = ΔH – TΔS
2.0 800VocEg from EQE
Voc of solar cells with wide range of bandgaps and
qVoc = Eg – T kln Ω = Eg - kT ln 46,200 = Eg – 10.7 kT
1.5
oc (
V)
600
700
W/(m
2 . eV
))
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 ⋅n
m)
hν < EG 0.4
0.6
0.8
1.0
ower
Den
sity
(W/m
hν ≈ EG hν > EG 500 1000 1500 2000 25000.0
0.2
Po
Wavelength (nm)
1 0
1.2
1.4
1.6
m2 ⋅n
m)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 E3
I ffi i t0.50 0.55 0.60 0.65 0.70 0.75 0.800.90
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
η = 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 E1
E2
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
ΔE (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-matched
EQE, 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 cm2
aperture area
Voc = 3.089 V Jsc = 3.377 A/cm2
FF = 88.24%Vmp = 2.749 V
Voc = 3.089 V Jsc = 3.377 A/cm2
FF = 88.24%Vmp = 2.749 V
• 39.0% record efficiency,236 suns 25°C
Efficiency = 39.0% ± 2.3%
236 suns (23.6 W/cm2) intensity0.2691 cm2 aperture area25 ± 1°C, AM1.5D, low-AOD spectrum
Efficiency = 39.0% ± 2.3%
236 suns (23.6 W/cm2) intensity0.2691 cm2 aperture area25 ± 1°C, 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
GaInPhν = 1.9eVGaAshν = 1.42eVInGaAsPhν = 1.05eV
InGaAshν = 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 Å
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 Å 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 Å
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-2
250oC, 10 minSurfaces– Pressure
2. Bond formation:– Smooth particle-
,
1.
– Smooth, particle-free surfaces
– Surface activation3. Thermal processing:
25 nm
device sub
<100>
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 (0.3μm; 5x1018cm-3)
n+ InGaAsTi/Pt/Au
n InP Window (0.05μm; 1-2x1018cm-3)(1x1019cm-3; 0.05μm)
I G A B (3 1 1017 3)
n+ InGaAs Emitter (0.3μm; 5x1018cm-3)
n+ InGaAsTi/Pt/Au
n InP Window (0.05μm; 1-2x1018cm-3)(1x1019cm-3; 0.05μm)
I G A B (3 1 1017 3)
n+ InGaAs Emitter (0.3μm; 5x1018cm-3)
n+ InGaAsTi/Pt/Au
n InP Window (0.05μm; 1-2x1018cm-3)(1x1019cm-3; 0.05μm)
Performance Equivalent to State of the Art InGaAs/InP
-0.4
-0.3
0.2
Cur
rent
(mA
)
n+ InP Buffer / Spreading Layer (1μm; 1x1019cm-3)
Ti/Pt/AuInGaAs Contact Layer
InGaAs Tunnel Junction
p InGaAs Base (3μm; 1x1017cm-3)
p+ InP BSF (0.05μm; 1-2x1018cm-3)
InP Etch Stop Layer (0.1μm; 1x1018cm-3)
n+ InP Buffer / Spreading Layer (1μm; 1x1019cm-3)
Ti/Pt/AuInGaAs Contact Layer
n+ InP Buffer / Spreading Layer (1μm; 1x1019cm-3)
Ti/Pt/AuInGaAs Contact Layer
InGaAs Tunnel Junction
p InGaAs Base (3μm; 1x1017cm-3)
p+ InP BSF (0.05μm; 1-2x1018cm-3)
InP Etch Stop Layer (0.1μm; 1x1018cm-3)InGaAs Tunnel Junction
p InGaAs Base (3μm; 1x1017cm-3)
p+ InP BSF (0.05μm; 1-2x1018cm-3)
InP Etch Stop Layer (0.1μm; 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 Cost•Template Fabrication Cost ~ Epi Cost
3/15/09H. Atwater Caltech
Zn3P2 - An Earth Abundant Semiconductor
Zn + (P4)n
500˚C
650˚C
200˚C
850˚C
Zn + P4
650 C 850 C
Zn3P2
•Energy Gaps – Direct
Zn3P2 (tetragonal)
cmEnergy Gaps Direct
and Indirect nearly aligned @ ~1.3 eV
1 cm diameter•Zinc 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
ΔEc = 0.2 eV
VOC ~ 0.8 V
ΔEv = 2.3 eV
Si Wire Array Solar Cellsħω1/α 1/
t
n-type
~
ħω1/α 1/α
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 η > 15%, Voc > 500 mV achievable withLn = 1 µm 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 μm
• 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 µm diameter, 50 µm long wires we see:
Jsc ≈ 20 mA/cm2, FF = 58 %, η≈ 5.7 %
-0 02
0.00
0.02
J (A
/c
dark
light
(diffused B emitter, 5 Ω 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 μm/s)• High growth rates (up to μm/s)
• Inexpensive gas phase precursors
• Atmospheric pressure growth possible 1 μm
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
<111>
30 μm
5 nm
100% vertically aligned, 75 μm 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 μmGrowth 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 μμmm
Rwire= 230 kΩ → N = 1016 cm-3
LpL+
E
L-
δp ( ) 11m
+=± ELL kTqL
pp
L ff ~ 10 µm
xA. L. Fahrenbuch and R. H. Bube.
“Fundamentals of Solar Cells”, pp. 83Lp,eff 10 µm
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
( ) ( ) ( ) ( )( )( ) ( )( ) ( )
λληλξλ θAQA radexttot =(direct)R
Aq A0 LG A ( )q
( )( )
( )⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
⎭⎬⎫
⎩⎨⎧
⎟⎠⎞
⎜⎝⎛−−
+
+= ∫
∫θ
θλα
θθ
θλπ
πθ dL
dA
cosexp1
cos1
cos12/
02
2
( )( ) ( )( ) ( )λλλξ 011 ARQext −−+
33
on
GaAs (a)q ( ) ⎟⎠
⎜⎝
⎭⎩ ⎠⎝+∫ θθ dcos100
(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-λ (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 ×= −σ
3-18 cm103cm105.3
×≈
×=
ρ
σ -14 cm10≈×= ρσα
I
m1cm10 41 μα =≈= −−L
CdSe QD Layer10 nm
I0
I (P l“thick” Ag film
CdSe quantum dot layer
1.5
2.0 Fit
mis
sion e-Dα/2
L=α-1=1.2μm50-100x 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-capable•Close to Limiting PV Efficiency for Single Junction Cells•Silicon PV being overtaken by thin films as leading technology•Silicon PV being overtaken by thin films as leading technology•Multijunction 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 cs•Earth-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