Semiconductor Materials for Intermediate Band Solar Cells
October 19, 2004GCEP Solar Energy Workshop
Stanford, CA
W. WalukiewiczElectronic Materials ProgramMaterials Sciences Division
Lawrence Berkeley National Laboratory
This work was supported by the Director's Innovation Initiative,National Reconnaissance Office and by the Office of Science, U.S. Department of Energy under Contract No. DE-AC03-76SF00098.
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Outline
New materials for multijunction solar cells: GaxIn1-xNIntermediate (impurity) band solar cell materials
Intermediate band solar cell conceptHighly mismatched alloys (HMAs)Non-equilibrium synthesis of HMAsII-Ox-VI1-x HMAs as intermediate band materials
Challenges and prospects
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Solar CellsUltimate Efficiency Limits
Intrinsic efficiency limit for a solar cell using a single semiconducting material is 31%.
Light with energy below the bandgap of the semiconductor will not be absorbedThe excess photon energy above the bandgap is lost in the form of heat. Single crystal GaAs cell: 25.1% AM1.5, 1x
Multijunction (MJ) tandem cellMaximum thermodynamically achievable efficiencies are increased to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps
AM
1.5
sol
ar fl
ux(1
021 p
hoto
ns/s
ec/m
2 /µm
)
1 2 3 4Energy (eV)
1
2
3
4
5
Eg1 > Eg2 > Eg3
Cell 1 (Eg1)
Cell 2 (Eg2)
Cell 3 (Eg3)
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Multijunction Solar Cells
State-of-the art 3-junction GaInP/Ga(In)As/Ge solar cell: 36 % efficient
05
10152025303540
1980 1985 1990 1995 2000 2005Year
Effic
ienc
y (%
)
MJ (World)
Conc (World)
MJ (Japan)
Conc (Japan)
M. Yamaguchi et. al. – Space Power Workshop 2003
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Direct bandgap tuning range of In1-xGaxNPotential material for MJ cells
The direct energy gap of In1-xGaxN covers most of the solar spectrum Multijunction solar cell based on this single ternary could be very efficient
LBNL/Cornell work: J. Wu et al. APL 80, 3967 (2002)
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InGaN is radiation hardelectron, proton, and alpha irradiation
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In1-xGaxN alloys as solar materials
Significant progress in achieving p-type dopingExceptional radiation hardness established
Surface electron accumulation in In-rich alloys
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Intermediate band solar cells
Several other groups analyzed different aspects of the multiband solar cell conceptP. Wurfel Sol. Energy Mater. Sol. Cells 29, 403 (1993), M. A. Green, Prog. Photov. :Res. Appl., 9, 137, (2001)
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Multijunction vs. Multiband
junction1
junction2
junction3
I
Multi-band• Single junction (no lattice-mismatch)• N bands ⇒ N·(N-1)/2 gaps
⇒ N·(N-1)/2 absorptions• Add one band ⇒ add N absorptions
Multi-junction• Single gap (two bands) each junction• N junctions ⇒ N absorptions• Efficiency~30-40%
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Intermediate Band Solar Cell
L. Cuadra, et. al., Thin Solid Films, 451-452, 593 (2004)
CB
IB-materialVB
EFC
EFI IB(1)
(2)
(3) EG
qV
EFV
p n
EFC, EFV, EFIquasi-Fermi levels for the electrons in respective bands
CB – conduction bandVB – valence bandIB – intermediate band
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Theoretical efficiency of Intermediate band solar cells
0
Ei
Eg
Ec
ivµ
ciµcvµ
VB
IB
CB
qV
Intermediate Band Solar Cells can be very efficientMax. efficiency for a 3-band cell=63%Max. efficiency for a 4-band cell=72%In theory, better performance than any other ideal structure of similar complexity
But NO multi-band materials realized to dateLuque et. al. PRL, 78, 5014 (1997)
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But how to make the intermediate band(s)?
Impurity bandsPorous materialsSuperlatticesQuantum dotsNo successful demonstrations
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Well-Matched semiconductor alloysIII-V, and II-VI semiconductors
Electronegativity ElectronegativityXAs=2.18; XP=2.19 XSe=2.55; XS=2.58
Atomic radius Atomic radiusRAs=0.13nm; RP=0.12nm RSe=0.12.; RS=0.11
Relatively easy to synthesize in the whole composition range
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
0 0.2 0.4 0.6 0.8 1
Ene
rgy
(eV)
Composition, y
GaAs1-y
Py
EΓ
EX
GaAs GaP
2.6
2.8
3
3.2
3.4
3.6
0 0.2 0.4 0.6 0.8 1
Ban
d G
ap E
nerg
y (e
V)
Composition, y
ZnSe1-y
Sy
ZnSe ZnS
EΓ
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Highly Mismatched Alloys: III-N-Vs
ElectronegativityXN = 3.0XP = 2.2XAs = 2.2XSb = 2.05
Atomic radiusRN = 0.075 nmRP = 0.123 nm RAs = 0.133 nm RSb = 0.153 nm
Nitrogen in III-V compounds introduces a localized N
level close to the conduction band edge
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Unique HMA effectBand Anticrossing
295 KGaAs1-xNx
Photon Energy (eV)0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
PR S
igna
l (ar
b.un
its)
E_ E_+∆0 E+
E+E_
E_
E_+∆0
E0+∆0E0
x=0
x=0.008
x=0.015
x=0.02
E+
E1
E1
E1
E_+∆0
E1
1.2
1.4
1.6
1.8
2.0
2.2
-15 -10 -5 0 5 10 15k (106 cm-1)
E+ (k)
EL
Ec(k)
E_(k)
GaAs0.995
N0.005
@ 295K
Fundamental band gap is reduced and a new optical transition is formed…
GaAs1-xNx
( ) ( )[ ] ( )[ ]⎭⎬⎫
⎩⎨⎧ ⋅+−±+=± xCEkEEkEkE NM
LCLC 22 421
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Bandgap tuning with HMAsGaAs1-xNx
N localized level situated above GaAs conduction band edge
Anticrossing pushes CB edge down
Effect is large4% N reduces gap by 0.4 eV
0.9
1
1.1
1.2
1.3
1.4
1.5
0 0.01 0.02 0.03 0.04 0.05
Uesugi, et. al.Keyes, et. al.Malikova, et. al.Bhat, et. al.BAC theory
Nitrogen fraction, x
GaNxAs
1-x @ 295K
( ) ( )[ ] ( )[ ]⎭⎬⎫
⎩⎨⎧ ⋅+−±+=± xCEkEEkEkE NM
LCLC 22 421
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Example: N ion implanted GaAs•Ion implantation of diluting species•Pulsed-laser melting (PLM): liquid phase epitaxy at submicrosecondtime scales
Outcome
•Growth of epitaxial, single crystal •Supersaturation of implanted species•Suppression of secondary phases Homogenized excimer laser pulse
(λ=308 nm, 30 ns FWHM, ~0.2-0.8 J/cm2)
N ions
GaAs
GaAsGaNxAs1 -x
How to synthesize an HMA?Implantation + pulsed laser melting
Finished Sample
GaAsion induced damage
0 1000 2000 30000.014
0.018
0.022
0.026
melt duration typ. 200 - 500 ns
TRR data revealsliquid phase
ion induced damageLiquid
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PLM epilayer qualityIII-N-Vs
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How to optimize an intermediate band material?II-O-VI HMAs
Oxygen in II-VI compounds has the requisite electronegativity and atomic radius difference
XO = 3.44; RO = 0.073 nmXS = 2.58; RS = 0.11nmXSe = 2.55; RSe = 0.12 nmXTe = 2.1; RTe = 0.14
Oxygen level in ZnTe is 0.24 eV below the CB edge
Can this be used to form an intermediate band?
SynthesisVery low solid solubility limits of O in II-VI compoundsNonequilibrium synthesis required
-2.0
0.0
2.0
5.2 5.6 6.0 6.4 6.8
VBCB
Ener
gy (e
V)
Lattice parameter (Å)
EFS
EO
CdTe
CdTe
MnTe
MnTe
ZnTe
ZnTeZnSe
ZnSe
MgTe
MgTe
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PLM synthesis of II-O-VIsX-ray diffraction
As-grown and as-implanted samples show diffraction peaks of the ZnTe only
O implanted ZnTe/GaAsfollowed by PLM shows a layer of ZnOTe with lattice parameter 0.60 nm
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Is there an intermediate band?Zn1-yMnyOxTe1-x
K. M. Yu et. al., Phys. Rev. Lett., 91, 246403 (2003)
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Predicted PV operationZn1-yMnyOxTe1-x
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Photovoltaic action
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How efficient can they be?Multi-band ZnMnOTe alloys
1.6
1.8
2.0
2.2
2.4
2.6
2.8
0.00 0.05 0.10 0.15 0.20
Zn0.88
Mn0.12
OxTe
1-x
E_E
+
0 0.01 0.02 0.03 0.04
ener
gy (e
V)
CLM
2x (eV2)
EM
=2.32eV
EO=2.06eV
O mole fraction, x
E_
E+
30
35
40
45
50
55
60
2.1
2.2
2.3
2.4
2.5
2.6
2.7
0 0.01 0.02 0.03
max
imum
effi
cien
cy (%
)
optimal operation voltage (V
)
oxygen mole fraction
Zn0.88
Mn0.12
OxTe
1-x
EO=2.06eV
EM=2.32eV
C=3.5eV
The location and the width of the intermediate band in ZnMnOxTe1-x is determined by the O content, xCan be used to maximize the solar cell efficiency
Calculations based on the detailed balance model predict maximum efficiency of more than 55% in alloys with 2% of O
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Intermediate band semiconductors Challenges an prospects
Synthesis of suitable materials with scalable epitaxial techniques (MBE growth of ZnOxSe1-x achieved)
N-type doping of intermediate band with group VII donors (Cl, Br)Control of surface properties of the PLM synthesized materialsOther highly mismatched alloys: GaPyNxAs1-x-y
FundamentalsNature of the intermediate band: localized vs. extendedCarrier relaxation processes
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Collaborators
J. Wu, K. M. Yu, W. Shan, J. W. Ager, J. Beeman, E. E. Haller, M. Scarpulla, O. Dubon, and J. Denlinger
Lawrence Berkeley National Laboratory, University of California at Berkeley
W. Schaff and H. Lu, Cornell UniversityA. Ramdas and I. Miotkowski, Purdue UniversityP. Becla, Massachusetts Institute of Technology