sustainable pv technologies for future mass deployment ...€¦ · and light soaking) front...

Sustainable PV technologies for future mass deployment:

Kesterites

Dr. Edgardo Saucedo

Catalonia Institute for Energy Research (IREC)

Sant Adrìà de Besòs (Barcelona), Spain

IX Barcelona Global Energy Challenges – June 19th 2014

2

Founded in 2008, and located in

Barcelona, Spain:

“…[objective of] creating a more

sustainable future for energy usage

and consumption…”

Divided into six main areas:

• Advanced materials for energy

• Offshore wind energy

• Electrical engineering

• Lighting

• Bioenergy and biofuels

• Thermal energy and building

Catalonia Institute of Energy Research

- Solar energy materials and systems

- Functional nanomaterials

- Materials and catalysts

- Nanoionics and fuel cells

- Energy storage and harvesting

systems

3

Solar Energy Materials and Systems Lab

Group leader: Prof. Alejandro Pérez-Rodríguez

Head of processes lab: Dr. Edgardo Saucedo

Head of characterization lab: Dr. Victor Izquierdo-Roca

Coordination of three European FP-7 projects – KESTCELLS,

SCALENANO, INDUCIS – and involvement in numerous other national

and international projects through public and private ventures

4

Solar Energy Materials and Systems Lab

Preparation of entire solar cell structure,

from the back contact to the completed

device

Main research lines include:

New materials and concepts for high

efficiency PV devices, focus on

chalcopyrite and kesterite absorbers

Advanced characterization processes,

specifically with Raman scattering

spectroscopy

Low cost processes for industrially

viable chalcogenide based

technologies, such as spray pyrolysis

and electrodeposition

5

Introduction:

• First, second, and third generation PV

• Thin film device structure

• Cu2ZnSn(S,Se)4 review and properties

Secondary phase formation and identification

and main Kesterite Raman modes

Front interface

Back interface

Conclusions

Outline

6

First generation PV: mono- and polycrystalline silicon (c-Si) - Highest cell (25%) and module efficiencies (15%)

- Expensive materials (monocrystalline silicon in particular)

- Indirect bandgap of 1.1 eV >150 μm active layer

Three generations of PV

Second generation PV: thin film (Cu(In,Ga)Se2, CdTe, a-Si:H) - Relatively high cell and module efficiencies, lower than first gen: CIGS

(20.9%), CdTe (20.6%) and a-Si-H (12%)

- Direct bandgap materials <5 μm absorber layer

- Reduced material cost and weight, useable with flexible substrates, facilitates

building integration

Third generation PV: organic, tandem, intraband, photon up/down

converter, quantum dots, etc. - Still under research, some ideas not practically proven

- Expensive

7

Three main thin film absorbers:

• Cu(In,Ga)Se2

• CdTe

• a-Si:H

Thin film PV device structure

CdS buffer

ZnO-i and ZnO:Al front contact

CZTS absorber

Mo back contact

Substrate

In-Ga scarce elements, usage of H2Se

Te scarce element, Cd toxicity

Long-term stability, conversion efficiency

Cu2ZnSn(S,Se)4 (CZTS) are formed

by earth abundant and low toxic

elements

Use similar technologies than those

developed for Cu(In,Ga)Se2:

• Back contact layer – Mo

• Buffer layer – CdS

• Front contact layer – In2O3:Sn,

ZnO:Al, SnO2:F

Sustainable materials based on earth abundant elements: KESTERITES

8

Noted as a PV active material in 1988, first device in 1996 (0.7%). Very

limited research till 2010.

In 2010 breakthrough result published by IBM, a 9.6% device

Gaining interest as a mid- to long-term alternative to CIGSe because it

is composed of earth abundant elements

Current record efficiency of 12.6% (2013)

Some of the main challenges include secondary phase formation and

identification, elemental loss during annealing (Zn, Sn-S/Se), front and

back interface optimization

Cu2ZnSn(S,Se)4

9

I-III-(II-IV)-VI compounds based on ZnS structure

Non-equilibrium and defect structures:

• CIGSe – OVC, Cu-Au

• CZTSSe – disordered kesterite, stannite, wurtzite

Properties – structure

[1] S Schorr, Sol Eng Mat Sol Cells 95, 2011, pp. 1482

Blue – Cu

Orange – Zn

Red – Sn

Yellow – S/Se

ZnS

sphalerite

CuInSe2

chalcopyrite

Cu2ZnSnSe4

kesterite

Zn(II)

Cu(I) In(III)

Cu(I) Zn(II) Sn(IV)

10

Properties – optical

[1] J He, et al. J Alloy and Comp 511, 2012, pp. 129

CZTSe

1.0 eV

CZTS

1.5 eV

Absorption coefficient ~104-105

above bandgap Absorbing layers of <2 μm

Band-gap Ideal range for sun light

absorption

11

But we have a problem: to much elements...

Band structure

and point

defects Morphology

Crystal structure

Secondary phases

Contact Materials

SOLAR CELLS PROPERTIES

12

Introduction:






Front interface

Back interface

Conclusions

Outline

13

Non-stoichiometric compositions used for both

CIGSe and CZTSSe:

• CZTSSe: Cu-poor and Zn-rich (high

efficiency devices)

Formation of binary sulfides/selenides

Cu-S/Se can form locally, even in Cu-poor

conditions

Secondary phase formation

Absorber Eff. (%) Cu/(Zn+Sn) Zn/Sn Method Reference

Cu2ZnSn(S,Se)4 12.0 --- --- Hybrid sol-part. M Winkler, et al, Energ & Envir Sci 2013

Cu2ZnSn(S,Se)4 9.7 0.80 1.20 Hybrid sol-part. T Todorov, et al, Adv Energ Mat 2010

Cu2ZnSnS4 6.7 0.85 1.25 Co-sputter H Katagiri , et al, Appl Phys Exp 2008

Cu2ZnSnSe4 9.2 0.86 1.15 Sputtering I Repins, et al, Sol Energ Mat Sol Cells 2012

[1] C Platzer-Björkman C, et al. Sol Eng Mat Sol Cells 98, 2012, pp. 110

14

Secondary phase identification

14

Processing under Zn-excess conditions leads to the formation of ZnS/Se

in addition to CZTS/Se, though other phases can form depending on the

processing conditions

Sq

rt. In

t. (

arb

. u

nits)

Cu2ZnSnSe

4

25 30 35 40 45 50 55 60

2

Cu2SnSe

3

ZnSe

ZnS is a high bandgap (3.7 eV) and resistive

phase, potentially deteriorating cell

performance

KCN-based etches are often used with CZTS,

but they are ineffective at removing ZnS/Se

Distinction between some phases in XRD

difficult, namely for CZTS/Se, ZnS/Se, and

CTS/Se

15

Secondary phase identification: resonant Raman scatering

100 200 300

204

250

237

182

Inte

nsity (

arb

.un

its)

Raman shift (cm-1)

173

196

CTSe

ZnSe

CZTSe

X-ray diffraction

100 200 300 500 750

747

ZnSe

748

501

498

ZnSe

250 ZnSe

CZTSe

ZnSe

250

173

196

Inte

nsity (

arb

.un

its)

Raman shift (cm-1)

Sq

rt. In

t. (

arb

. u

nits)

Cu2ZnSnSe

4

25 30 35 40 45 50 55 60

2

Cu2SnSe

3

ZnSe

Raman with λexc = 514 nm λexc = 458 nm

2nd and 3rd order

ZnSe peaks

Common laser wavelengths (nm)

Bandgaps of Cu-Zn-Sn-S-Se phases (nm)

16

Main Kesterites Raman modes

16 16

Simultaneous fitting of spectra allowed identification of 18 peaks, that are attributed to

the 27 optical modes theoretically expected for this crystalline structure [1].

Detection of 5 peaks not observed

previously, but theoretically predicted.

Well resolved peak at 302.1 cm-1, that

according to simulation data, has been

identified as the third A symmetry mode

from the CZTS kesterite phase

The peak at 347.3 cm-1 usually attributed

as ZnS peak, in this case is identified as

CZTS mode due to the absence of

second order peak of ZnS.

Polarization measurements enabled

determination of the symmetry of the

modes.

[1] M. Dimitrievska, Appl. Phys. Lett., vol. 104, no. 2, p. 021901, Jan. 2014.

17

Introduction:






Front interface

Back interface

Conclusions

Outline

18

CdS most common buffer

layer, alternatives: ZnS,

In2S3

Band alignment at junction

can be a cliff or spike:

Front interface: CdS

[1] M Bär, et al. J Elec Spec Relat Phen, in press 2012

[2] M Bär, et al. Appl Phys Lett 99, 2011, pp. 112103

• Spike increases barrier to charge carriers

• Cliff increases interface recombination,

and reduces VOC

CIGSe has 0-0.4 eV offset (spike) with CdS

Conflicting theoretical and experimental results

on the type of alignment, which may vary

between CZTS and CZTSe

Secondary phases?

Cliff alignment

19

Elimination of JV

distortions: red kink

and cross over (soft

thermal annealing

and light soaking)

Front interface: CdS

M. Neuschitzer, Y. Sánchez et al., ‚‘‘Optimization of CdS Buffer Layer for High Performance CZTSe Solar Cells and the Effects of Light Soaking:

Elimination of Cross Over and Red Kink‘‘, 2014, submitted.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-20

0

20

40

60

80

100before white light illumination

CZTSe/CdS1/i-ZnO+ITO

Jsc

= 31.6 mA/cm2

Voc

= 387 mV

FF = 52.3%

eff. = 6.4%

Rshunt

= 154 cm2

Rs = 0.89 cm

2

dark curve before

longpass filter 550 nm

1 sun illumination

dark curve after

cu

rre

nt

de

ns

ity

[m

A/c

m2]

voltage [V]

red kink

crossover

(a)

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8-40

-20

0

20

40

60

80

100(b)before white light illumination

CZTSe/CdS2/

i-ZnO+ITO

Jsc

= 31.9 mA/cm2

Voc

= 386 mV

FF = 61.8%

eff. = 7.6%

Rshunt

= 313 cm2

Rs = 0.46 cm

2

dark curve before


1 sun illumination

dark curve after 1 sun

cu

rre

nt

de

ns

ity

[m

A/c

m2]

voltage [V]

crossover

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5-40

-20

0

20

40

60

80

100

120

140

160(d)after light soaking


Jsc

= 31.9 mA/cm2

Voc

= 377 mV

FF = 57.9%

eff. = 7.0%

Rshunt

= 225 cm2

Rs = 0.40 cm

2

dark & illuminated curve


cu

rre

nt

de

ns

ity

[m

A/c

m2]

voltage [V]

crossover

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5-40

-20

0

20

40

60

80

100

120

140

160(e)after light soaking

dark & illuminated curve



Jsc

= 32.4 mA/cm2

Voc

= 392 mV

FF = 64.4%

eff. = 8.2%

Rshunt

= 302cm2

Rs = 0.32 cm

2

cu

rre

nt

de

ns

ity

[m

A/c

m2]

voltage [V]

Using different precursors for the CdS allows

obtaining improved JV curves, increasing

devices efficiency.

20

Front interface: ZnS etching

320 340 360

20 20

λexc = 325 nm

HCl etching to remove ZnS from CZTS films

[1] A Fairbrother, et al. J Amer Chem Soc 134, 2012, pp. 8018

21

Front interface: ZnSe etching

21 21

Oxidizing based agent etching to remove ZnSe from CZTSe films

KMnO4/H2SO4 Na2S

CZTSe

ZnSe

Se0

CZTSe

• SEM confirms small

aggregates (ZnSe) dissolve

with KMnO4/H2SO4 leaving

porous aggregates( Se0) to

further be removed with

Na2S.

• Jsc increases due to ZnSe

removal , higher EQE for etched

samples in the ZnSe absorption

region

• Voc drastically increased, up to

100 mV, EQE higher in the p-n

junction region (600-1000nm)

• η increased due to

improvement of the p-n junction.

Chemical passivation!!

[1] M. López-Marino et al., Chemistry A European Journal, 2013, 19, 14814 – 14822

22

Front interface: Sn(S,Se) etching

22 22

Simple chemical etching based in (NH4)2S to remove Sn(S,Se) from

CZTSSe films

[1] H. Xie et al., ACS Materials and Interfaces, 2014, submitted.

Two types of Sn(S,Se) overgrowths: those related with the processes parameters

(type 1) and those condensed from the annealing atmosphere (type 2)

Nevertheless the etching is effective in removing both of them

SnSe0.9S0.1

23

Front interface: Sn(S,Se) etching

23 23

Simple chemical etching based in (NH4)2S to remove Sn(S,Se) from

CZTSSe films

[1] H. Xie et al., ACS Materials and Interfaces, 2014, submitted.

J-V: large improvement of the J-V curves after etching (Jsc, Voc, FF)

EQE: 500-900nm there are significant increases in external quantum efficiency

EQE(-1V)/EQE(0V): better carrier collection efficiency after etching. Increase in the

short wavelength range for both samples (low electron lifetime, or low field strength at

the p-n interface), relatively well solved in the case of the best cell.

24

Introduction:






Front interface

Back interface

Conclusions

Outline

25

Mo is the conventional back contact material for both CIGSe and

CZTSSe

For CIGSe it is stable and forms ohmic contact (with MoS/Se2)

Evidence of CZTSe decomposition, which has been previously shown for

CZTS, and predicted for CZTSe

More stable materials for CZTSSe?

Barriers or interfacial layers?

Back interface

100 150 200 250 300

CuxSe

ZnSe

MoSe2

MoSe2

MoSe2

Inte

nsity (

arb

. u

nits)

Raman shift (cm-1)

Substrate

[1] S Lopez-Marino, et al. J Mat Chem A 1, 2013, pp. 8338

λexc = 532 nm

26

Zinc oxide layer can present decomposition by formation of a thin ZnSe

layer during annealing, and also significantly improve the interface quality

Back interface – ZnO barrier

No ZnO 10 nm ZnO

[1] S Lopez-Marino, et al. J Mat Chem A 1, 2013, pp. 8338

100 150 200 250 300

CuxSe

ZnSe

MoSe2

MoSe2

MoSe2

w/o ZnO

Inte

nsity (

arb

. u

nits)

Raman shift (cm-1)

10 nm ZnO

layer

Substrate

λexc = 532 nm

27

Titanium nitride acts as a diffusion barrier to reduce formation of MoSe2

layer at high annealing temperatures and pressures

Back interface – TiN barrier

20 nm TiN

T = 570 ºC

pSe = 0.2 bar

η = 3.0% η = 8.9%

[1] B Shin, et al. Appl Phys Lett 101, 2012, pp. 053903

28

To date CZTSSe-based device structure and processes have been

mostly based on CIGSe

After a rapid increase in efficiency, unique problems are being

identified, indicating that the optimized conditions for CIGSe are not

ideal for CZTSSe

In addition to an incomplete knowledge of the basic material

properties, other aspects of CZTSSe related to processing need to be

understood, including the formation of secondary phases and their

influence on device properties, and instabilities at both the front and back

contact regions

The major challenge of Kesterites is their advantage: earth abundant

but to much elements

Summary

29

This work has been carried out with the collaboration of my colleagues in

the Solar Energy Materials and Systems Group at IREC: Prof. A. Pérez-

Rodríguez, Dr. V. Izquierdo-Roca, Dr. Marcel Placidi, Dr. Diouldé Sylla, Y.

Sánchez, Dr. A. Fairbrother, Dr. X. Fontané, M. Espíndola-Rodríguez, S.

López-Marino, C. Insignares, M. Dimitrievska, M. Neuschitzer, H. Xie

This research was supported by the Framework 7 program under the

project KESTCELLS (FP7-PEOPLE-2012-ITN-316488)

I would like to thanks the MINECO for the Ramón y Cajal fellow (RYC

2011-09212)

Acknowledgements

Patrons:

With financial support from:

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