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Optical reflection and transmission formulae for thin films - PDF

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					Semiconductor Materials for Intermediate Band Solar Cells
October 19, 2004 GCEP Solar Energy Workshop Stanford, CA

W. Walukiewicz
Electronic Materials Program Materials 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.

Outline
New materials for multijunction solar cells: GaxIn1-xN Intermediate (impurity) band solar cell materials
Intermediate band solar cell concept Highly mismatched alloys (HMAs) Non-equilibrium synthesis of HMAs II-Ox-VI1-x HMAs as intermediate band materials

Challenges and prospects

GCEP workshop 10/19

Solar Cells
Ultimate Efficiency Limits
5 AM 1.5 solar flux 2 photons/sec/m /µm) (10 1
21

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 absorbed The excess photon energy above the bandgap is lost in the form of heat. Single crystal GaAs cell: 25.1% AM1.5, 1x

4

3

2

1

2 Energy (eV)

3

4

Multijunction (MJ) tandem cell
Maximum thermodynamically achievable efficiencies are increased to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps

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

40 35 30 Conc (World) 25 20 15 10 M J (World) 5 0 1980 1985

Efficiency (%)

Conc (Japan)

M J (Japan)

1990 1995 Year

2000

2005
GCEP workshop 10/19

M. Yamaguchi et. al. – Space Power Workshop 2003

Direct bandgap tuning range of In1-xGaxN
Potential 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 hard
electron, proton, and alpha irradiation

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In1-xGaxN alloys as solar materials
Significant progress in achieving p-type doping Exceptional 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 concept P. 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

junction3

I

junction2 junction1

• • •

Multi-junction Single gap (two bands) each junction N junctions ⇒ N absorptions Efficiency~30-40%

• • •

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
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Intermediate Band Solar Cell
L. Cuadra, et. al., Thin Solid Films, 451-452, 593 (2004) CB

EFC EFI
EFV

(2)

IB
(1) (3)

qV

EG
p

VB

IB-material

n

CB – conduction band VB – valence band IB – intermediate band

EFC, EFV, EFI
quasi-Fermi levels for the electrons in respective bands

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Theoretical efficiency of Intermediate band solar cells
CB
Eg Ec E
i

µ cv µ ci

qV
IB
µ iv

0

VB
Intermediate Band Solar Cells can be very efficient
Max. 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 date
Luque et. al. PRL, 78, 5014 (1997)
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But how to make the intermediate band(s)?

Impurity bands Porous materials Superlattices Quantum dots
No successful demonstrations

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Well-Matched semiconductor alloys
III-V, and II-VI semiconductors
2.8 2.6 2.4 Energy (eV) 2.2 2 1.8
3.6

GaAs P

1-y y

3.4 Band Gap Energy (eV)

ZnSe S

1-y y

3.2

E

X

3

E
2.8

Γ

E
1.6 1.4 0 GaAs

Γ

2.6

0.2

0.4

0.6

0.8

Composition, y

1 GaP

0 ZnSe

0.2

0.4

0.6

0.8

Composition, y

1 ZnS

Electronegativity XAs=2.18; XP=2.19 Atomic radius RAs=0.13nm; RP=0.12nm

Electronegativity XSe=2.55; XS=2.58 Atomic radius RSe=0.12.; RS=0.11

Relatively easy to synthesize in the whole composition range
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Highly Mismatched Alloys: III-N-Vs
Electronegativity
XN = 3.0 XP = 2.2 XAs = 2.2 XSb = 2.05

Atomic radius
RN = 0.075 nm RP = 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 effect
Band Anticrossing
GaAs1-xNx
2.2
E0 E0+∆0 x=0
E1

2.0
E_ E_+∆0 E+ E1

1.8 EL 1.6 E (k)
c

PR Signal (arb.units)

E+ (k)

x=0.008
E_+∆0 E_

E+

E1

x=0.015
E_ E_+∆0 E+ E1

1.4 GaAs0.995 N0.005 @ 295K 1.2 -15 -10 -5 0 k (10 cm )
6 -1

E (k)
_

5

10

15

x=0.02 295 K GaAs1-xNx
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

E ± (k ) =

1⎧ C L ⎨ E (k ) + E ± 2⎩

[

] [E

C

2 (k ) − E L ] 2 + 4C NM ⋅ x ⎫ ⎬

⎭

Photon Energy (eV)

Fundamental band gap is reduced and a new optical transition is formed…
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Bandgap tuning with HMAs
GaAs1-xNx
1.5

1.4

GaN As
x

1-x

@ 295K

1.3

N localized level situated above GaAs conduction band edge
Anticrossing pushes CB edge down

1.2

1.1

1

Uesugi, et. al. Keyes, et. al. Malikova, et. al. Bhat, et. al. BAC theory 0 0.01 0.02 0.03 0.04 0.05

Effect is large
4% N reduces gap by 0.4 eV

0.9

Nitrogen fraction, x

E ± (k ) =

1⎧ C L ⎨ E (k ) + E ± 2⎩

[

] [E

C

2 (k ) − E L ] 2 + 4C NM ⋅ x ⎫ ⎬

⎭
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How to synthesize an HMA?
Implantation + pulsed laser melting
• Ion implantation of diluting species • Pulsed-laser melting (PLM): liquid phase epitaxy at submicrosecond time scales Outcome • Growth of epitaxial, single crystal • Supersaturation of implanted species • Suppression of secondary phases
melt duration typ. 200 - 500 ns
0.026

Example: N ion implanted GaAs
N ions

ion induced damage GaAs

Homogenized excimer laser pulse
(λ=308 nm, 30 ns FWHM, ~0.2-0.8 J/cm2)

ion induced damage Liquid

GaAs

0.022

TRR data reveals liquid phase

GaNxAs1 -x GaAs Finished Sample

0.018

0.014 0 1000 2000 3000

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PLM epilayer quality
III-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
E
O

2.0
ZnSe

MgTe ZnTe MnTe

Energy (eV)

CdTe

0.0
VB CB

E

XO = 3.44; XS = 2.58; XSe = 2.55; XTe = 2.1;

RO = 0.073 nm RS = 0.11nm RSe = 0.12 nm RTe = 0.14

FS

CdTe ZnTe ZnSe MnTe MgTe

Oxygen level in ZnTe is 0.24 eV below the CB edge
Can this be used to form an intermediate band?

Synthesis
6.8

-2.0 5.2

5.6

6.0

6.4

Lattice parameter (Å)

Very low solid solubility limits of O in II-VI compounds Nonequilibrium synthesis required

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PLM synthesis of II-O-VIs
X-ray diffraction
As-grown and asimplanted samples show diffraction peaks of the ZnTe only O implanted ZnTe/GaAs followed 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 operation
Zn1-yMnyOxTe1-x

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Photovoltaic action

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How efficient can they be?
Multi-band ZnMnOTe alloys
O mole fraction, x
2.8 2.6 2.4 0 0.01 0.02 0.03 E
+

0.04

60 55 50 45 Zn 40 35
0.88

2.7 2.6 2.5 2.4 Mn
0.12

Zn

0.88

Mn

0.12

O Te
x

1-x

optimal operation voltage (V)

E =2.32eV
M

2.2

maximum efficiency (%)

energy (eV)

O Te
x

E =2.06eV
2.0 1.8 1.6 0.00
O

1-x

E =2.06eV
O

2.3 2.2 2.1

E =2.32eV
M

E_ E
+

E_
30 0

C=3.5eV

0.01

0.02

0.03

0.05

0.10
2 C x LM

0.15
2

0.20

oxygen mole fraction

(eV )
Calculations based on the detailed balance model predict maximum efficiency of more than 55% in alloys with 2% of O

The location and the width of the intermediate band in ZnMnOxTe1-x is determined by the O content, x Can be used to maximize the solar cell efficiency

GCEP workshop 10/19

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 materials Other highly mismatched alloys: GaPyNxAs1-x-y Fundamentals
Nature of the intermediate band: localized vs. extended Carrier 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 University A. Ramdas and I. Miotkowski, Purdue University P. Becla, Massachusetts Institute of Technology

GCEP workshop 10/19


				
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