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CZTSSe thin-films and solar cells: effects of Cu-Zn order-disorder transition Germain Rey Now at UNSW Formerly at University of Luxembourg SPREE Seminar, Sydney, 22 th November 2018 g.rey@unsw.edu.au Laboratory for Photovoltaics 1

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Laboratory for Photovoltaics germain.rey@uni.lu 1

g.rey@unsw.edu.au

CZTSSe thin-films and solar cells: effects

  • f Cu-Zn order-disorder transition

Germain Rey

Now at UNSW Formerly at University of Luxembourg

SPREE Seminar, Sydney, 22th November 2018

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Laboratory for Photovoltaics germain.rey@uni.lu 2

g.rey@unsw.edu.au

  • CZTS : Cu2ZnSnS4

– Eg = 1.5 eV

  • CZTSe : Cu2ZnSnSe4

– Eg = 1.0 eV

  • CZTSSe : Cu2ZnSn(S,Se)4

– Eg = 1.0-1.5 eV

Introduction

  • Large absorption coefficient for hν > Eg
  • Non-toxic and abundant metals

[1] N. Terada et al. Thin Solid Films 582 (2015) 166

[1]

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g.rey@unsw.edu.au

Introduction

  • Cell structure

[1] D. B. Mitzi, et al. Phil. Trans. R. Soc. A 371 (2013) 20110432

12.6% (2013)

  • Efficiency

CZTSSe Mo TCO Substrate Ni Al Buffer [1]

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Laboratory for Photovoltaics germain.rey@uni.lu 4

g.rey@unsw.edu.au

Introduction

  • Efficiency limitation

[1] L. Grenet, et al. Appl. Ener. Mat. 1 (2018) 2103

[1] [1]

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g.rey@unsw.edu.au

Introduction

  • Voc and Band-tails

[1] S. De Wolf et al. J. Phys. Chem. Lett., 5 (2014) 1035 [2] S. Siebentritt et al. Sol. Ener. Mat. & Sol. Cells, 158 (2016) 126

[1] [2]

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g.rey@unsw.edu.au

  • Ordered Kesterite

Introduction

  • Disordered Kesterite

[CuZn+ZnCu]

  • Structural observation of disorder in Cu/Zn planes:

– Neutron diffraction [1], NMR [2], anomalous XRD [3]

  • Theoretical prediction:

– Low formation energy of [CuZn+ZnCu] [4]

[1] S. Schorr SEM&SC 95 (2011) 1482 [2] L. Choubrac et al. PCCP 15 (2013) 10722 [3] A. Lafond et al. Acta Cryst. B 70 (2014) 390 [4] S. Chen et al. Adv. Mater. 25 (2013) 1522

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g.rey@unsw.edu.au

Introduction

  • Cu-Zn Disorder:

– Increase in unit cell volume – Rise Valence band

[1] S. Schorr SEM&SC 95 (2011) 1482 [2] S. Chen et al. Adv. Mater. 25 (2013) 1522

[2] [1]

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g.rey@unsw.edu.au

CZTSe Band gap and Cu-Zn (dis)order

ΔEg = 110 meV Reversibility and continuity => order-disorder transition T & R quenching annealing

Change dwell temperature

  • G. Rey et al. Applied Physics Letters, 105 (2014) 112106
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g.rey@unsw.edu.au

Theoretical description: Vineyard’s model[1]

Zn Cu S(e)

  • Long range order parameter:

– Perfect order S=1 – Complete disorder S=0

  • Theoretical description: Vineyard’s model [1]

– Motion equation for direct exchange:

𝑇 = 𝑄𝐷𝑣

𝐷𝑣 − 𝐺 𝐷𝑣

1 − 𝐺

𝐷𝑣

𝑒𝑇 𝑒𝑢 = 1 2 𝐿𝑃 1 − 𝑇 2 − 𝐿𝐸 1 + 𝑇 2

with 𝐿𝑃/𝐸 = 4𝑔 exp

−𝑉 𝑙𝐶𝑈 exp ±3𝑊𝑇 𝑙𝐶𝑈

[1] G. Vineyard Phys. Rev. 102 (1956) 981

  • G. Rey et al. Applied Physics Letters, 105 (2014) 112106
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Laboratory for Photovoltaics germain.rey@uni.lu 10

g.rey@unsw.edu.au

CZTSe Band gap and Cu-Zn (dis)order

  • Comparison band gap and order parameter

Critical temperature = 200°C for CZTSe Eg can be used as an order parameter 𝑈

𝑑 = 200 °𝐷

𝑔 = 1 𝑈𝐼𝑨 𝑉/𝑙𝐶 = 15000 𝐿

Linear relationship

  • G. Rey et al. Applied Physics Letters, 105 (2014) 112106
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g.rey@unsw.edu.au

Cu-Zn (dis)order probed by Raman

Modification of Raman spectrum : phonon confinement + change in symmetry (Ord K: I4, Dis K: I42m)

[1] T. Gurel et al. Phys. Rev. B 84 (2011) 205201

  • G. Rey et al. Applied Physics Letters, 105 (2014) 112106
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Laboratory for Photovoltaics germain.rey@uni.lu 12

g.rey@unsw.edu.au

Cu-Zn (dis)order probed by Raman

  • Evolution of Raman spectrum during ordering at 100°C

Reversibility and continuity => order-disorder transition

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Laboratory for Photovoltaics germain.rey@uni.lu 13

g.rey@unsw.edu.au

CZTSe thin film and Cu-Zn (dis)order

  • Ordering increases the band gap by ~ 10%
  • Tc for the order-disorder transition in CTZSe 200°C
  • The band gap can be used as a secondary order parameter
  • Raman spectrum reflects the changes induced by the order-

disorder transition :

– symmetry – coherence length

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Laboratory for Photovoltaics germain.rey@uni.lu 14

g.rey@unsw.edu.au

Cu-Zn (dis)order effect on device

  • Sample preparation:

– Coevaporation at 470°C

  • ORD DIS post-treatment

In-situ Ex-situ

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g.rey@unsw.edu.au

Cu-Zn (dis)order effect on device

3-28d 1h quenching ORD DIS Std 1h ORD+DIS

  • ORD and DIS postdeposition treatment
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Cu-Zn (dis)order effect on device

  • ORD and DIS postdeposition treatment
  • G. Rey et al. Sol. Ener. Mat. & Sol. Cells, 151 (2016) 131
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g.rey@unsw.edu.au

Cu-Zn (dis)order effect on device

Constant Voc deficit with order: ↑ Ord r ↑ Eg ↑ Voc : ↑ Voc/Eg

  • G. Rey et al. Sol. Ener. Mat. & Sol. Cells, 151 (2016) 131
  • Voc deficit
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g.rey@unsw.edu.au

Cu-Zn (dis)order effect on device

  • Effect on QE and Jsc

nd (cm-3) by CVp Dis+Ord 1015 Dis 1016

Ex-situ Ord or Dis -> no or limited effect on Jsc (change in doping) In-situ Ord -> ↑ Jsc (7 mA/cm2) d to ↑ coll ctio at lo g λ

nd (cm-3) by CVp In-Ord 3x1016

  • G. Rey et al. Sol. Ener. Mat. & Sol. Cells, 151 (2016) 131
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  • Carrier collection length is increased by the ordering treatment
  • Ordering increases the band gap and Voc
  • Ordering does not affect Voc deficit

Voc (mV) Jsc (mA/cm2) Eff (%) Std 299 35.5* 5.6* In-Ord 331 42.1* 8.1* +32 +6.6* +2.5*

* measured with an halogen lamp

  • G. Rey et al. Sol. Ener. Mat. & Sol. Cells, 151 (2016) 131

Cu-Zn (dis)order effect on device

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g.rey@unsw.edu.au

Nature of kesterite tails

  • Fluctuating electrostatic

potential

  • Fluctuating band-gap energy

[1] B. Shklovskij & A. Efros, Electronic Properties of Doped Semiconductors (1984) Springer-V.

𝛿𝑓𝑚 ∝ 𝑂𝐽

2

𝑞 + ∆𝑞 + ∆𝑜

1/3

Electrostatic potential fluctuation can be screened [1]

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g.rey@unsw.edu.au

  • Temperature and excitation dependent PL of CZTSSe (~9% eff.)

Nature of kesterite tails

Strong blue shift w. excitation & red shift w. temperature => fluctuating band-edges

15 meV/dec

Iexc = 4.5x103 W.m-2

Increased Excitation Increased Temperature TI TT

  • G. Rey et al. Solar Energy Material and Solar Cells 179 (2018) 142
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TI + TT

EL & BG BG

  • Behaviour of fluctuation depth with excitation

Nature of kesterite tails

Limited decrease in γ => Band-gap fluctuation is the main mechanism of band-tail formation: 2/3 Band-gap fluctuations + 1/3 Electrostatic fluctuations

  • G. Rey et al. Solar Energy Material and Solar Cells 179 (2018) 142
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Cu-Zn (dis)order effect on band-tail

  • Measurement of tail absorption using PL [1] +TMM [2]

[1] E. Daub & P. Würfel Phys. Rev. Lett. 74 (1995) 1020 [2] G. Rey et al. Phys. Rev. Appl. 9 (2018) 064008

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  • CZTSSe Absorption vs Cu-Zn (dis)order

Cu-Zn (dis)order effect on band-tail

Cu-Zn (dis)order parameter: Eg Band-tail parameters: Eu and σ

  • G. Rey et al. Solar Energy Material and Solar Cells 179 (2018) 142
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Cu-Zn (dis)order effect on band-tail

Cu/Zn ordering by thermal postdeposition treatment does not reduce Eu and σ => No reduction of the PL red-shift vs. Eg [1] => No improvement of Voc deficit [1][2] CZTSSe CZTSe

[1] G. Rey et al. Solar Energy Material and Solar Cells 151 (2016) 131 - 138 [2] S. Bourdais et al. Advanced Energy Materials 6 (2016)

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  • Avoiding disorder by alloying, example from literature:

Cu-Zn (dis)order effect on band-tail

[1] C. Yan et al. Energy Letter 2 (2017) 930 [2] T. Gershon et al. Advanced Energy Materials 6 (2016)

(Cu,Ag)ZnSnSe4 [2] Cu(Zn,Cd)SnS4 [1]

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g.rey@unsw.edu.au

  • Potential candidate:

Cu-Zn (dis)order effect on band-tail

[2CuZn + SnZn] would be a good candidate to explain the large band- tailing in kesterite

[1] S. Chen et al. Advanced Materials 25 (2013) 1522-1539

[1] [1]

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g.rey@unsw.edu.au

Cu-Zn (dis)order effect on band-tail

  • Nature of the kesterite band-tail:

– 2/3 band-gap fluctuations – 1/3 electrostatic fluctuations

  • Cu-Zn ordering by post-deposition annealing has no effect on

Eu or σ.

  • [CuZn+ZnCu] is not the direct main cause of band-tailing,

instead we propose [2CuZn+SnZn] as potential candidate.

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Laboratory for Photovoltaics germain.rey@uni.lu 29

g.rey@unsw.edu.au

Thank you for your attention

  • Acknowledgments:

– Susanne Siebentritt and Laboratory for PV team – G. Dennler, G. Larramona & S. Bourdais – M. Guennou – R. Carius & M. Nuys

  • Funding:
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Disorder and Band-Tail

  • Absorption spectrum of kesterite:

𝐹𝑕 =1.184 eV and Eu = 25 meV and σ = 74 meV

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g.rey@unsw.edu.au

Disorder and Band-Tail

  • Measuring α from PL:
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Band gap and (dis)order

  • Composition EDX 20kV

No significant change in composition after annealing

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g.rey@unsw.edu.au

Cu-Zn (dis)order effect on band-tail

  • Voc and Band-tail

[1] S. De Wolf et al. J. Phys. Chem. Lett., 5 (2014) 1035

[1]

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g.rey@unsw.edu.au

  • CZTSSe & CZTSe Band-tail vs Cu-Zn (dis)order

Band-tailing & Cu-Zn (dis)order

Cu/Zn ordering by thermal treatment does not reduce Eu and σ CZTSSe CZTSe

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g.rey@unsw.edu.au

  • CZTSSe & CZTSe Band-tail vs Cu-Zn (dis)order

Band-tailing & Cu-Zn (dis)order

Cu-Zn (dis)order is not the main cause of potential fluctuation

[1] J. J. S. Scragg et al. Physica Status Solidi (b) 253 (2015) 247–254 [2] P. Zawadzki et al. Physical Review Applied 3 (2015) 034007

[1] [2]

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K value

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g.rey@unsw.edu.au

Shift

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g.rey@unsw.edu.au

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PDS

Acknowledgment: R. Carius and M. Nuys from Forschungszentrum Jülich GmbH Institut für Energie und Klimaforschung, 52425 Jülich, Germany.

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g.rey@unsw.edu.au

Disorder and Band-Tail

  • Cu-Zn disorder and tail in kesterite:

No change of tail parameters with Cu-Zn disorder

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g.rey@unsw.edu.au

Theoretical description

  • Vineyard model

Cu Zn Cu Cu Cu Zn Zn Zn Cu Zn Cu Cu Cu Zn Zn Cu

Cu-Zn Zn-Cu Energy Energy U U

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g.rey@unsw.edu.au

Voc deficit

Ea matches PLmax except for ORD+DIS Recombination occur mainly in the bulk

[1] R. Scheer & H.-W. Schock, Chalcogenide Photovoltaics Wiley-VCH

𝑊

𝑝𝑑 = 𝐹𝑏

𝑟 + 𝐵𝑙𝑈 𝑟 ln −𝐾𝑡𝑑𝜃 𝐾00 [1]

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g.rey@unsw.edu.au

Admittance spectroscopy and CV

↑ord r ↓p Order changes the carrier freeze out

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g.rey@unsw.edu.au

(Dis)order effect on device

  • Process:

CZTSe by co-evaporation Disordering: 1h@250 or 300°C + Quenching

Dis

  • SC

Ord

  • SC

Std- SC Mo/ SLG Ord+Dis

  • SC

Dis+Ord

  • SC

Ordering: 72@100 Ordering: 72h@100°C Disordering: 1h@250°C + Quenching

Dis

  • SC

Ord

  • SC Ord+Dis
  • SC

Dis+Ord

  • SC

Ordering: 72@100 Ordering: 72h@100°C Solar cell finishing: CdS + TCO + grid Ordering: 72h@100°C

In-situ Ex-situ

Std In-ORD

Cu/(Zn+Sn)=0.86 Zn/Sn=0.96 Cu/(Zn+Sn)=0.82 Zn/Sn=1.10 Cu/(Zn+Sn)=0.81 Zn/Sn=1.16

In-Ord

  • SC
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g.rey@unsw.edu.au

Introduction

  • Voc: the kesterite chalenge
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Introduction and review

  • Structural observation of disorder in Cu/Zn planes:

– By neutron diffraction CZTS(e) [1] – By NMR CZTS [2] – By x-ray resonant diffraction CZTS [3]

  • Theoretical prediction:

– Low formation energy

  • f [CuZn+ZnCu]

CZTS(e) [4]

[1] S. Schorr SEM&SC 95 (2011) 1482 [2] L. Choubrac et al. PCCP 15 (2013) 10722 [3] A. Lafond et al. Acta Cryst. B 70 (2014) 390 [4] S. Chen et al. Adv. Mater. 25 (2013) 1522

[4]

A P Q M N P G Formation Energy (eV)

1.5 1.0 0.5

[CuZn+ZnCu]

CZTSe

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Zn Cu Sn S(e) 2d 2c 2a 2b

Introduction and review

  • No Cu(2a)/Zn(2d) exchange :

– Position 2a occupied by Cu only (ND) CZTS(e) [1] – No Zn on 2a (NMR) CZTS [2]

  • No disorder in the Cu/Sn planes:

– Position 2a occupied by Cu only (ND) CZTS(e) [1] – Plane randomisation energy (ab initio) CZTS[3]:

  • Cu/Sn plane = 78 meV/at
  • Cu/Zn plane = 9 meV/at

[1] S. Schorr SEM&SC 95 (2011) 1482 [2] L. Choubrac et al. PCCP 15 (2013) 10722 [3] S. Chen et al. APL 94 (2009) 041903

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Theoretical description

  • Motion equation for direct exchange
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Band gap and (dis)order

  • Sample preparation:

– Coevaporation at 490°C Composition: Cu/(Zn+Sn) = 0.86 Zn/Sn = 0.96

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Band gap and (dis)order

  • Annealing:

– Two zones tubular oven – Vacuum (2.10-3 mbar)

  • Quenching:

– Transfert to cold zone – N2 gaz flow

  • Band gap measurement by spectrophotometry:

– Non coherent free standing film

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  • Cu-Zn (dis)order and band-gap

Band-tailing & Cu-Zn (dis)order

Cu-Zn (dis)order could be a potential source of band-gap fluctuations [1]

[1] G. Rey et al. Applied Physics Letters 105 (2014) 112106 [2] D. Többens et al. Physica Status Solidi (b) 253 (2016) 1890-1897

[2]

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(dis)order effect on device

  • EQE and PL for CZTSe on Mo/SLG

80 meV 105 meV 160 meV 134 meV

Decrease in band gap with decrease in order

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Temperature and excitation dependent PL

  • Peak position evolution:

Blue-shift w. excitation, red-shift w. temperature (low T) Heavily doped & heavily compensated SC

  • Yu, P. W.

Excitation dependent emission in Mg , Be , Cd , and Zn implanted GaAs Journal of Applied Physics, 1977, 48, 5043-5051

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Temperature and excitation dependent PL

  • Peak position evolution w. excitation:

Non-mobile carrier at low T

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Temperature and excitation dependent PL

  • Peak position evolution w. temperature (low T):

Mobility ↑ with T

  • Gokmen, T.; Gunawan, O.; Todorov, T. K. & Mitzi, D. B.

Band tailing and efficiency limitation in kesterite solar cells Applied Physics Letters, 2013, 103, 103506

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Temperature and excitation dependent PL

  • Peak position evolution w. temperature (low T):

Effect of non-radiative r combi atio ↑ with T

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Temperature and excitation dependent PL

  • Radiative transition vs. temperature:
  • Spindler, C.; Regesch, D. & Siebentritt, S. Revisiting radiative deep-level transitions in CuGaSe2 by

photoluminescence Applied Physics Letters, 2016, 109, 032105

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Temperature and excitation dependent PL

  • Radiative transition vs. temperature:
  • Levcenko, S.; Just, J.; Redinger, A.; Larramona, G.; Bourdais, S.; Dennler, G.; Jacob, A. & Unold, T. Deep

Defects in Cu$_2$Sn(S, Se)$_4$ Solar Cells with Varying Se Content Phys. Rev. Applied, 2016, 5, 024004

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Temperature and excitation dependent PL

  • Nature of the fluctuation:

γbg γel γmax