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
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
• 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
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)
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
Surface and Interface Analysis at UNLV
Sample preparation and distribution
Scanning ProbeMicroscope
Gloveboxes
High dyn. rangeXPS, UPS, Auger, IPES
High-res XPS, UPS, Auger
Beamline 8.0 – Advanced Light Source – Lawrence Berkeley National Lab
X-ray EmissionIn-situ cell
Photoemission
U Würzburg, UNLV,HZ Berlin, KIT
SALSA: Solid And Liquid Spectroscopic Analysis
RSI 80, 123102 (2009)
• 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
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)
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)
• 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)
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)
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
• 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)
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)
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
• 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
Annealing-Induced Effects on the Chemical Structure of the In2S3/CuIn(S,Se)2 Interface
D. Hauschild et al., JPC C 119, 10412 (2015)
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
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
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
• 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
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
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
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!
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!