# Using X-ray induced Auger electron spectroscopy transitions to explore the surface reactivity of CIGS: In and Ga degradation kinetics at different probing depths

Anna Gagliardi, Neal Fairley, Solène Béchu

► To cite this version:

Anna Gagliardi, Neal Fairley, Solène Béchu. Using X-ray induced Auger electron spectroscopy tran-
sitions to explore the surface reactivity of CIGS: In and Ga degradation kinetics at different probing
depths. ECASIA 2024, Jun 2024, Goteborg, Sweden. hal-04907705

HAL Id: hal-04907705

https://hal.science/hal-04907705v1

Submitted on 23 Jan 2025

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# Using X-ray induced Auger electron spectroscopy transitions to explore the surface reactivity of CIGS: In and Ga degradation kinetics at different probing depths

Anna Gagliardi,<fnref n="1" /> Neal Fairley,<fnref n="2" /> Solène Béchu<fnref n="1" />

<footnote n="1">¹ Institut Lavoisier de Versailles (ILV), Versailles Saint-Quentin-en-Yvelines University, CNRS, UMR 8180, Versailles, France.</footnote>

<footnote n="2">² Casa Software Ltd, Bay House, 5 Grosvenor Terrace, Teignmouth, Devon TQ14 8NE, United Kingdom.</footnote>
# CuIn<sub>0.7</sub>Ga<sub>0.3</sub>Se<sub>2</sub>: a solar device material

<figure><img src="image_2.png" /></figure>

* Cu(In<sub>x</sub>Ga<sub>1-x</sub>)Se<sub>2</sub> (CIGS): solar absorber with efficiencies up to 23.6%

* **Long-term stability of the cell:** main factors responsible for degradation

* Relative humidity

* Temperature

* Illumination

* Gazeous environment

M. Theelen, F. Daume, Sol. Energy., 2016, 133, 586–627

<figure><img src="image_3.png" /></figure>

Degradation of
CIGS material itself?

<figure><img src="image_4.png" /></figure>

Influence of RH for CIGS surface reactivity

Surface
modifications?

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

Kinetics of degradation?

$$ \frac{Ga_{ox}}{GaCIGS} \& \frac{In_{ox}}{InCIGS} $$
# CuIn<sub>0.7</sub>Ga<sub>0.3</sub>Se<sub>2</sub>: a solar device material

<figure><img src="image_8.png" /></figure>

* Cu(In<sub>x</sub>Ga<sub>1-x</sub>)Se<sub>2</sub> (CIGS): solar absorber with efficiencies up to 23.6%

* **Long-term stability of the cell:** main factors responsible for degradation

* Relative humidity

* Temperature

* Illumination

* Gazeous environment

M. Theelen, F. Daume, Sol. Energy., 2016, 133, 586–627

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Influence of RH for CIGS surface reactivity

Surface modifications?

<figure><img src="image_11.png" /></figure>

Aged surface (≈2000 h)

<figure><img src="image_12.png" /></figure>

Binding Energy (eV)

Binding Energy (eV)

<figure><img src="image_13.png" /></figure>

Kinetics of degradation?

$$ \frac{Ga_{ox}}{GaCIGS} \& \frac{In_{ox}}{InCIGS} $$
# Why using X-AES transitions? The case of CuIn<sub>0.7</sub>Ga<sub>0.3</sub>Se<sub>2</sub>

<figure><img src="image_14.png" /></figure>

Evolution of In and Ga under aging?

<figure><img src="image_15.png" /></figure>

most decomposed
photpeaks

photpeaks with the
same probing depth

<figure><img src="image_16.png" /></figure>

S. Béchu et al., Appl. Surf. Sci., **2022**, 576, 151898

Atomic level

Binding Energy
Kinetic Energy
Depth probed

In M<sub>4,5</sub>N<sub>4,5</sub>N<sub>4,5</sub>
1105 - 1068
381 - 417
≈ 3 nm

In 3s
826
660.6
≈ 4 nm

In 3p
702 - 664
822.6
≈ 5 nm

In 3d
451 - 443
1035.6 - 1043.6
≈ 6 nm

In 4s
122
1364.6
≈ 7 nm

Similar kinetics

In 4d
16
1470.6
≈ 8 nm

Different kinetics due to the
different probing depths

Ga L<sub>3</sub>M<sub>4,5</sub>M<sub>4,5</sub>
430 - 410
1058 - 1075
≈ 6 nm

Ga 3s
158
1328.6
≈ 7 nm

Ga 3p
107 - 103
1379.6 - 1383.6
≈ 8 nm

Ga 3d
18
1468.6
≈ 8 nm

Ga 2s
1298
188.6
≈ 2 nm

Ga 2p
1143 - 1116
343.6 - 370.6
≈ 3 nm

<figure><img src="image_17.png" /></figure>

Extreme surface (≈2 - 3 nm)

<figure><img src="image_18.png" /></figure>

Sub surface (≈8 nm)

B.E and K.E of In and Ga elements for an Al Kα source
Probing depths are calculated with the TPP2M formula for CIGS
# The X-AES transitions: phase identifications and quantification

<figure><img src="image_19.png" /><figcaption>Wagner plot and modified auger parameter</figcaption></figure>

Ga $2p_{3/2}$ – Ga LMM Wagner plot

<figure><img src="image_20.png" /><figcaption>Quantifications with X-AES lines</figcaption></figure>

Advanced analysis of copper X-ray
photoelectron spectra

Mark C. Biesinger*

<figure><img src="image_21.png" /></figure>

Surf. Interface Anal. **2017**, *49*, 1325–1334

SURFACE and
INTERFACE
ANALYSIS
# Non-linear versus linear least square decomposition

## 2 methods to decompose the X-AES transitions

### Non-linear least square decomposition

Arbitrary use of Gaussian Lorentzian curves to simulate
one chemical environment

<figure><img src="image_22.png" /></figure>

Ga L<sub>3</sub>M<sub>4,5</sub>M<sub>4,5</sub> non-linear least square decomposition for CIGS

<figure><img src="image_23.png" /></figure>

Possibility of slight adjustments for the positions, the FWHM values

<figure><img src="image_24.png" /></figure>

For a mix of chemical environments, necessity of many
arbitrary peaks → possible error

J. J. Moré, *Numer. Anal.*, **1978**, 630, 105

### Linear least square decomposition

Implementation of a spectral shape

<figure><img src="image_25.png" /></figure>

Ga L<sub>3</sub>M<sub>4,5</sub>M<sub>4,5</sub> linear least square decomposition for CIGS

<figure><img src="image_26.png" /></figure>

Easy to implement with different chemical environments

<figure><img src="image_27.png" /></figure>

No FWHM or position adjustments
for a series of fits

G. H. Golub and C. Reinsch, *Numer. Math.*, **1970**, 14, 403
# NLLS decomposition of CIGS X-AES transitions

## Modifications induced by the aging

<figure><img src="image_28.png" /></figure>

Growth of In₂O₃ and Ga₂O₃ phases in addition to the CIGS one
Identified with the modified Auger parameter

In M₄,₅N₄,₅N₄,₅ X-AES line

<figure><img src="image_29.png" /></figure>

P.E. 20 eV; step size: 0.1 eV; dwell time: 100 ms
Bckg: U2 Tougaard – resolution 0.52 eV

<figure><img src="image_30.png" /></figure>

<table><tr><td></td><td>PP</td></tr><tr><td>Position</td><td>407.7 ± 0.1</td></tr><tr><td>FWHM</td><td>2.10 ± 0.05</td></tr><tr><td>Area</td><td>/</td></tr></table>

<table><tr><td>Peak 6</td><td>Peak 7</td><td>Peak 8</td><td>Peak 9</td></tr><tr><td>PP-7.4</td><td>PP-11.5</td><td>PP-16.5</td><td>PP-19.5</td></tr><tr><td>PP*1.11</td><td>PP*2.34</td><td>PP*2.04</td><td>PP*2.45</td></tr><tr><td>PP*0.91</td><td>PP*0.59</td><td>PP*0.46</td><td>PP*0.34</td></tr></table>

9 arbitrary peaks to decompose the
In M₄,₅N₄,₅N₄,₅ X-AES transition of CIGS
→ 5 for the Ga L₃M₄,₅M₄,₅ X-AES transition of CIGS
# NLLS decomposition of CIGS X-AES transitions

## Modifications induced by the aging

<figure><img src="image_31.png" /></figure>

Growth of In₂O₃ and Ga₂O₃ phases in addition to the CIGS one
Identified with the modified Auger parameter

<figure><img src="image_32.png" /></figure>

<figure><img src="image_33.png" /></figure>

<table><tr><td></td><td>PP</td></tr><tr><td>Position</td><td>404.5 ± 0.1</td></tr><tr><td>FWHM</td><td>2.85 ± 0.05</td></tr><tr><td>Area</td><td>/</td></tr></table>

<table><tr><td>Peak 5</td><td>Peak 6</td></tr><tr><td>PP-7.3</td><td>PP+10.7</td></tr><tr><td>PP*1.13</td><td>PP*1.81</td></tr><tr><td>PP*1.34</td><td>PP*0.58</td></tr></table>

Thermo Nexsa
P.E. 20 eV; step size: 0.1 eV; dwell time: 100 ms
Bckg: U2 Tougaard - resolution 0.52 eV

6 arbitrary peaks to decompose the
In M₄,₅N₄,₅N₄,₅ X-AES transition of In₂O₃
→ 5 for the Ga L₃M₄,₅M₄,₅ X-AES transition of Ga₂O₃
# LLS decomposition of CIGS X-AES transitions

LLS

## Modifications induced by the aging

<figure><img src="image_34.png" /><figcaption>Growth of In₂O₃ and Ga₂O₃ phases in addition to the CIGS one
Identified with the modified Auger parameter</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>Use of the experimental spectral shape to define the chemical environments</figcaption></figure>

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

Thermo Nexsa
P.E. 20 eV; step size: 0.1 eV; dwell time: 100 ms
Bckg: U2 Tougaard - resolution 0.52 eV
# Evolution of the In M<sub>4,5</sub>N<sub>4,5</sub>N<sub>4,5</sub> X-AES transitions over air aging

LLS

<figure><img src="image_38.png" /></figure>

500 hours of
air exposure
@ 60% RH

<figure><img src="image_39.png" /></figure>

1300 hours of
air exposure
@ 60% RH

<figure><img src="image_40.png" /></figure>

NLLS

<figure><img src="image_41.png" /></figure>

500 hours of
air exposure
@ 60% RH

<figure><img src="image_42.png" /></figure>

1300 hours of
air exposure
@ 60% RH

<figure><img src="image_43.png" /></figure>
# Evolution of the Ga L₃M₄,₅M₄,₅ X-AES transitions over air aging

LLS

<figure><img src="image_44.png" /></figure>

500 hours of
air exposure
@ 60% RH

<figure><img src="image_45.png" /></figure>

1300 hours of
air exposure
@ 60% RH

<figure><img src="image_46.png" /></figure>

NLLS

<figure><img src="image_47.png" /></figure>

500 hours of
air exposure
@ 60% RH

<figure><img src="image_48.png" /></figure>

1300 hours of
air exposure
@ 60% RH

<figure><img src="image_49.png" /></figure>
# LLS and NLLS decompositions comparison

<figure><img src="image_50.png" /></figure>

3% of relative error max.

<figure><img src="image_51.png" /></figure>

1% of relative error max.

Linear least squares decomposition and non-linear least squares decomposition provide similar results for X-AES transitions.

## NLLS Evolution of FWHM parameter

<figure><img src="image_52.png" /></figure>

<figure><img src="image_53.png" /></figure>

Only small FWHM variations observed during the aging
Application to CIGS aging

<figure><img src="image_54.png" /></figure>

<figure><img src="image_55.png" /></figure>

<figure><img src="image_56.png" /></figure>

1800 hours of
air exposure
@ 60% RH

<figure><img src="image_57.png" /></figure>
Application to CIGS aging

<figure><img src="image_58.png" /></figure>

<figure><img src="image_59.png" /></figure>

<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /></figure>

<figure><img src="image_62.png" /></figure>
Application to CIGS aging

<figure><img src="image_63.png" /></figure>

<figure><img src="image_64.png" /></figure>

<figure><img src="image_65.png" /></figure>

1800 hours of
air exposure
@ 60% RH

<figure><img src="image_66.png" /></figure>

<figure><img src="image_67.png" /></figure>

<figure><img src="image_68.png" /></figure>

1800 hours of
air exposure
@ 60% RH

<figure><img src="image_69.png" /></figure>
# Conclusions

Decomposition of X-AES lines → 2 methods with advantages and disadvantages giving similar results

LLS

* Easy to use, with only one shape per chemical environment

<figure><img src="image_70.png" /></figure>

* No possibility to provide any adjustments with aging (FWHM values...)

<figure><img src="image_71.png" /></figure>

NLLS

* Possibility to perform evolution along with air aging (FWHM values, positions...)

<figure><img src="image_72.png" /></figure>

* Up to 15 arbitrary peaks to simulate two chemical environments

<figure><img src="image_73.png" /></figure>

Application to CIGS aging → access to a depth range probed for 2 elements within the same scale

* 3 different depths probed for Ga and In: 3, 6, and 8 nm using XPS photopeaks and X-AES lines

* Progressive oxidation from the extreme surface to the sub-surface, with a higher level of oxidation on the extreme surface

<figure><img src="image_74.png" /></figure>
<figure><img src="image_75.png" /><figcaption>Neal Fairley</figcaption></figure>

<figure><img src="image_76.png" /></figure>

CasaXPS

<figure><img src="image_77.png" /><figcaption>Anna Gagliardi</figcaption></figure>

<figure><img src="image_78.png" /></figure>

ILV

Institut Lavoisier
de Versailles

THANK YOU

Want to know more about the degradation of the sub surface of CIGS?

Don't miss Anna Gagliardi's talk

This afternoon @ 2.50 p.m.

11 NRG

S.B. and A.G. thank the French Agence Nationale de la Recherche for funding under the contract ANR-22-CE50-0008-01

<figure><img src="image_79.png" /></figure>

ILV

Institut Lavoisier
de Versailles