Interface Characterization to aid in the Development of alternative … · 2016-09-26 · University of Würzburg, KIT, AVANCIS GmbH, UNLV, Advanced Light Source, HZB, Brandenburgische

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)





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





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


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


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)


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





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


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


• 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!

[email protected], [email protected]

Documents

Interface Characterization to aid in the Development of alternative … · 2016-09-26 · University of Würzburg, KIT, AVANCIS GmbH, UNLV, Advanced Light Source, HZB, Brandenburgische