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.

GCEP workshop 10/19

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