interface characterization to aid in the development of alternative … · 2016-09-26 ·...
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Clemens HeskeInstitute for Photon Science and Synchrotron Radiation
Institute for Chemical Technology and Polymer ChemistryKarlsruhe Institute of Technology
Department of Chemistry and Biochemistry,University of Nevada, Las Vegas
[email protected], [email protected]
Interface Characterization to aid in the Development of alternative Buffer Layers
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Kyle George
ClemensHeske
The Group at UNLVMike Weir
Moni Blum
Key partners: W. Yang, J.D. Denlinger, Advanced Light Source, Berkeley LabTeam members: Weinhardt-group at KIT, Bär-group at HZB, Reinert-group at U WüAnd former group members: S. Pookpanratana, S. Krause, I. Tran, Y. Zhang, R. Felix,M. Folse, G. Gajjala, S. Sudarshanam, J. White, A. Ranasinghe, A. Luinetti, ...
TimoHofmann
Kim Horsley
MarcusBär
Lothar Weinhardt
Sarah Alexander
MichelleMezher
Ryan Bugni
Yu Liu
Marc Haeming, Samantha Rosenberg, Chase Aldridge, Dirk Hauschild (KIT)
Doug Duncan
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• Electron and soft X-ray spectroscopies
• Quick review: CdS/Cu(In,Ga)(S,Se)2
• Chemical structure of annealed InxSy/CuIn(S,Se)2
• Electronic structure of Zn(O,S)/Cu(In,Ga)Se2
Outline
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EConduction band
Photoelectron-Spectroscopy (PES)
X-Ray EmissionSpectroscopy ( )XES
e-
e-
e-
Valence band
Core level
h ’’
h
h ’
UV/Soft X-ray/Electron Spectroscopies
Auger Electron Spectroscopy (AES)
e
X-ray Absorption Spectroscopy (XAS)
hPhotoelectron Spectroscopy(PES, XPS, UPS)
e-e
Inverse Photo-emission (IPES)
UV-Visible Absorption Spectroscopy (UV-Vis)
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Applying soft x-ray/electron spectroscopies to appliedquestions
• Experimental approach needs to be custom-tailored to the actual question
• Sometimes, cutting-edge and/or unconventional approaches needed
• Need expertise (know what you are doing)
• In-situ!
scienceblogs.com/zooillogix
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Surface and Interface Analysis at UNLV
Sample preparation and distribution
Scanning ProbeMicroscope
Gloveboxes
High dyn. rangeXPS, UPS, Auger, IPES
High-res XPS, UPS, Auger
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Beamline 8.0 – Advanced Light Source – Lawrence Berkeley National Lab
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X-ray EmissionIn-situ cell
Photoemission
U Würzburg, UNLV,HZ Berlin, KIT
SALSA: Solid And Liquid Spectroscopic Analysis
RSI 80, 123102 (2009)
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• Electron and soft X-ray spectroscopies
• Quick review: CdS/Cu(In,Ga)(S,Se)2
• Chemical structure of annealed InxSy/CuIn(S,Se)2
• Electronic structure of Zn(O,S)/Cu(In,Ga)Se2
Outline
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XES of various sulfur compounds
Peak identification:
(1): S 3s S 2p sulfide
(2): Cd 4d S 2p S-Cd bonds
(3): S 3s S 2p S-O bondsIn 5s S 2p S-In bonds
(4): Cu 3d S 2p S-Cu bonds
(5): S 3d S 2p S-O bonds
Local environment of sulfur atoms can be identified ! 145 150 155 160 165
(5)
(4)(3)(2)
(1)
XES
CuIn(S,Se)2
CdSO4
Cu2S
CuS
CdSN
orm
aliz
ed In
tens
ity
Emission Energy (eV)phys.stat.sol. (a) 187, 13 (2001)
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Intermixing at the CdS/CISe interface (XES)
• Chemical bond between Cd and S
• Cd-S bond is absent for thin overlayer
→ diffusion of S into the CIGS film
APL 74, 1451 (1999)
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• detectable Se signal for thick CdS layers
Se segregation
• detectable In signal for (less) thick layers
In segregation
APL 74, 1451 (1999)
Intermixing at the CdS/CISe interface (PES)
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Intermixing: Summary
Cu(In,Ga)Se2
CdS
Mo
Na-lime glass
ZnO
Cu(In,Ga)Se2
CuInSxSe2-x
CdvInwS1-ySey
CdS1-zSez
APL 74, 1451 (1999)
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144 148 152 156 160 144 148 152 156 160
(b)
12.5 min
8 min
4 min
2 min
1 min
0.5 min
0 min(x 140)
(x 90)
(x 65)
(x 30)
(x 8)
(x 2)
Nor
m. I
nten
sity
Emission Energy [eV]
Se M2,3 & S L2,3
hexc = 200 eV
CdS Ref.
(x 1)
(a)
CuInS2
4 min Diff
2 min Diff
1 min Diff
0.5 min Diff
1 min data
Cu2S
Ga2S3
(x 17)
(x 2.4)
(x 2.7)
Emission Energy [eV]
(x 4.5)
In2S3
What grows at the interface? (step 1)
• 1999: 12 hours (total) for “5 nm” spectrum• 2009: 10 minutes for “1 min” spectrum
APL 97, 074101 (2010)
Cu(In,Ga)Se2
CdS
Mo
Na-lime glass
ZnO
Cu(In,Ga)Se2
CuInSxSe2-x
CdvInwS1-ySey
CdS1-zSez
Allows to see Se M2,3!
• Then: Siemens• now: NREL (17.6%)• Subtract Se M2,3 andCdS S L2,3 contribution
• Residual looks like In2S3 or Ga2S3
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• Spectral separation allows to draw a “depth profile”• Additional sulfide species is localized at the interface
0 1 2 3 4 5 6 7 8 9 10 11 12 13
0.0
0.2
0.4
0.6
0.8
1.0
64.4
27.0
19.1
16.0
9.6
Effective CdS Thickness [Å]
CIGSe
Difference
Frac
tion
CdS CBD time [min]
CdS0
What grows at the interface? (step 2)
APL 97, 074101 (2010)
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Sulfur gradient-driven Se diffusionat the CdS/CuIn(S,Se)2 solar cell interface (step 1)
• Mg K XPS spectra of AVANCIS (a-c) and NREL (d) absorbers
• Different S/Se ratios (derived from fits) are given on right ordinate
168 166 164 162 160 158 156
Ga LMM
d)
c)
b)
0
Ga 3s
0
x3
3
0.25
a)
Se 3p
x3
x3
Nor
mal
ized
Inte
nsity
Binding Energy (eV)
x3
S 2p
S/Se-ratio
APL 96, 182102 (2010)
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The CdS/CIGSSe junction
CuInSe surface2 Thin CdS
on CuInSe2
CdS surface(on CuInSe )2
1.4 (± 0.15) eV2.2 (± 0.15) eV
CBMCBM
VBM
VBM
EFEF
CBO = 0.0 (± 0.2) eV
VBO = 0.8 (± 0.2) eV
CdS/CISe: APL 79, 4482 (2001)
CuIn(S,Se) surface2CdS/CuIn(S,Se)
heterojunction2 thick CdS/CuIn(S,Se)
surface2
CBO = 0.0 (±0.15) eV
VBO = 1.0 (±0.15) eV
1.4 (±0.15) eV2.4 (±0.15) eV
CBM CBM
VBM
VBM
EF
CdS/CISSe: EuPVSEC17 (2001), p.1261
0.46 (±0.1) eV0.86 (±0.1) eV
VBO = -1.06 (±0.15) eV
CBO=-0.45 (±0.15) eV
2.47 (±0.15) eV
1.76 (±0.15) eVEF
interfaceCdS/Cu(In,Ga)S2
surfaceCu(In,Ga)S2
surface
CdS/CIGS: APL 86, 062108 (2005) • Good devices have a flat conduction band offset
• S-Se intermixing, which can be controlled by S content in CIGSSe
• CdS/CIGS: cliff in the conduction band
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• Electron and soft X-ray spectroscopies
• Quick review: CdS/Cu(In,Ga)(S,Se)2
• Chemical structure of annealed InxSy/CuIn(S,Se)2
• Electronic structure of Zn(O,S)/Cu(In,Ga)Se2
Outline
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Annealing-Induced Effects on the Chemical Structure of the In2S3/CuIn(S,Se)2 Interface
D. Hauschild et al., JPC C 119, 10412 (2015)
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Annealing-Induced Effects on the Chemical Structure of the In2S3/CuIn(S,Se)2 Interface
D. Hauschild et al., JPC C 119, 10412 (2015)
University of Würzburg, KIT, AVANCIS GmbH, UNLV, Advanced Light Source, HZB, Brandenburgische Technische Universität Cottbus-Senftenberg, ANKA
As-grown:• abrupt interface
After heat treatment(200 °C) to simulate subsequent process steps:• strong copper diffusion
into the In2S3 layer• strong sodium diffusion
into the In2S3 layer
1076 1072 934 932 930 56 54 52
Inte
nsity
(arb
. u.)
Na 1s
Binding Energy (eV)
Cu 2p3/2 Se 3d In2S3/
80 nm
80 nmannealed
12.5 nm5.6 nm3.8 nm
1.8 nm
0.5 nm
CISSe
CISSe
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0 10 20 30 40 50 60 70 800
10
20
30
40
50
60
Com
posi
tion
(%)
nom. Buffer Layer Thickness (nm)
In S Cu Se exp. decay
800
10
20
30
40
50
60
Cu
annealed
In5
S8
D. Hauschild et al., JPC C 119, 10412 (2015)
As-grown:• formation of a sulfur-
poor (indium-rich) In2S3surface
After heat treatment (200 °C) to simulate subsequent process steps:• copper diffusion into the
In2S3 layer;Cu concentration: Cu1
• In concentration near In5• S concentration near S8
formation of a copper−indium−sulfide phase
Quantification: Chemical Structure of the In2S3/CuIn(S,Se)2 Interface
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152 154 156 158 160 162 164 166 168
e) 80 nm annealed In2S3 / CISSe
d) In2S3
Inte
nsity
(arb
. u.)
Emission Energy (eV)
a) CISSe
c) 80 nm
XES h= 200 eVS L2,3
In2S3 / CISSe
(2)
b) 10 nm
(1)
D. Hauschild et al., JPC C 119, 10412 (2015)
As-grown:• formation of an In2S3 surface• sulfur atoms in both In2S3 and
CuIn(S,Se)2 chemical environments
After heat treatment (200 °C) to simulate subsequent process steps: formation of a copper−indium−sulfide phase
154 156 158 160 162
In2S3/CISSe
Emission Energy (eV)
0.40 x CISSe0.60 x In2S3
Residual
annealed 80 nm Sum b)
Inte
nsity
(arb
. u.)
XES S L2,3 h= 200 eV
10 nm In2S3/CISSe
0.55 x CISSe
0.45 x In2S3
Sum
Residual
a)Analysis:
S environment at the In2S3/CuIn(S,Se)2
Interface
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• Electron and soft X-ray spectroscopies
• Quick review: CdS/Cu(In,Ga)(S,Se)2
• Chemical structure of annealed InxSy/CuIn(S,Se)2
• Electronic structure of Zn(O,S)/Cu(In,Ga)Se2
Outline
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UPS and IPES of the Zn(O,S)/CIGSe interface
-8 -6 -4 -2 0 2 4 6
(CBM)
± 0.18 eV
± 0.18 eV
-2.30 eV
0.45 eV
-1.05 eV
IPESUPS - He I
Zn(O,S)
CIGSe
2.75 eV
1.55 eVNor
mal
ized
Inte
nsity
Binding Energy rel. EF (eV)
EF
0.50 eV
(VBM)
Mezher et al., Progress in Photovoltaics: Research and Applications, 2016, In Print
• UPS and IPES spectra of bare absorber (bottom) and thickest Zn(O,S)/CIGSe sample (top)
• Error bars are ±0.10 and ±0.15 eV for the VBM and CBM determination, respectively
• VBM and CBM are determined bylinear extrapolation of the leading edge
• This is not the full picture! Must takeinterface-induced band bending intoaccount
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XPS: Core-Level Peak Positions
Core Level CIGSe BE (eV) Thin 5 min Zn(O,S) BE (eV) Shift
Se 3d 54.33 54.30 0.03In 3d5/2 444.78 444.68 0.10Cu 2p3/2 932.56 932.52 0.04
Core Level Thin 5 min Zn(O,S) BE (eV)
Thick 22.5 min Zn(O,S) BE (eV)
Shift
S 2p3/2 161.91 162.12 0.21O 1s (Zn(OH)2) 532.06 532.25 0.19
O 1s (ZnO) 530.89 531.08 0.19Zn 2p3/2 1022.33 1022.41 0.08
• Core level peak positions of the bare absorber, 5 min, and 22.5 min Zn(O,S)/CIGSe sample
• Relative shifts shows there is band bending as the interface forms
Mezher et al., Progress in Photovoltaics: Research and Applications, 2016, In Print
0.06 eV
0.20 eV
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Eg: 1.55 0.18 eV Eg: 2.75 0.18 eVEg: 2.75 0.18 eV
0.15 eV
0.10 eV 0.15 eV
0.10 eV
0.15 eV 0.20 eV
VBO:
InterfaceZn(O,S) Surface
1.05 eV
0.50 eV
2.30 eV
EF
0.45 eV 0.09 eV
1.11 eV
CIGSe Surface
0.06 eV
0.20 eV
CBO:
XPS, UPS, IPES: Interface Band Alignment
Mezher et al., Progress in Photovoltaics: Research and Applications, 2016, In Print
• Small interface-induced band bending
• Very small conduction band offset (CBO)
• Small spike (essentially flat) conduction band alignment – similar to high-efficiency CdS/CIGSe devices
• Sizable valence band offset (VBO) – hole barrier!
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Summary
• Soft x-ray and electron spectroscopies allow the investigation of surfaces and interfaces in a unique way:
• Atom-specific and chemically sensitive• Chemical properties (intermixing, impurities, ...)• Electronic structure (gaps, offsets, ...)
• Can help in optimizing manufacturing processes and industrial products
• Particularly suited for thin film PV materials, and especially CIGSSe!