Kesterite workshop 2012 Luxembourg

© IMEC 2011

IMEC ENERGY

Correlation between electrical and physical properties of Cu2ZnSnSe4 solar cells fabricated by selenization of sputtered metal layers

G. Brammertz, M. Buffière, Y. Mevel, Y. Ren, A.E. Zaghi, N. Lenaers, Y. Mols, R. Moors, C. Koeble, J. Vleugels, M. Meuris, J. Poortmans

© IMEC 2011

1. Sequential DC-sputtering of Sn, Zn and Cu on 5x5 cm2 Mo on soda lime glass substrates (Flamac):▸ Sample A: 110 nm Cu / 150 nm Zn / 170 nm Sn▸ Sample B: 110 nm Cu / 110 nm Zn / 170 nm Sn▸ Sample C: 110 nm Cu / 150 nm Zn / 125 nm Sn▸ Sample D: 55 nm Cu / 115 nm Zn / 255 nm Sn / 110 nm Cu / 110 nm Zn

2. Anneal in vacuum - 15 min - 450C - continuous flow of 20 sccm of pure H2Se directed onto the sample surface (imec).

3. KCN etch for secondary phase removal (HZB).

4. Chemical bath deposition of 50 nm of CdS (HZB).

5. AC-sputtering of 120 nm of intrinsic ZnO and 250 nm of highly Al-doped ZnO (HZB).

6. Evaporation of 50 nm Ni - 1 µm Al finger contact pattern through a shadow mask (imec).

7. Lateral isolation of 1x1 cm2 cells by needle scribing (imec).

Process flow Cu2ZnSnSe4 solar cells

G. Brammertz, PV/Explore 2

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SEM


Typical top-view SEM image of absorber after selenization

• Polycrystalline material with typical grain sizes of 1 μm.• The smaller grains are typically binary or ternary phases.

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SEM


Cross section SEM images of finished solar cells

• Typical grain size for samples A, B, D around 1 µm, for sample C smaller.• Holes are visible at CZTSe/Mo interface.

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SEM-EDX


Cross-section EDX of finished solar cells

• Zn and Sn strongly evaporate during selenization, Cu/(Zn+Sn) increases considerably.• Sample C is very Zn rich after selenization, Zn/Sn = 1.46).• All samples are very Cu-poor, Cu/(Zn+Sn) ~ 0.8. • All samples are more or less stoeichiometric with respect to Se content, Se/(Cu+Zn+Sn) ~

1.

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5Cu/(Zn+Sn)

after sputtering

Zn/Snafter

sputtering

Cu/(Zn+Sn) after

selenization

Zn/Sn after selenization

Se/(Cu+Zn+Sn)after

selenization

Sample ASn

170 nmZn

150 nmCu

110 nm/ / 0.57 1.57 0.77 1.26 1.05

Sample BSn

170 nmZn

110 nmCu

110 nm/ / 0.7 1.14 0.85 1.21 1.00

Sample CSn

125 nmZn

150 nmCu

110 nm/ / 0.64 2.1 0.79 1.46 1.01

Sample DZn

110 nmCu

110 nmSn

255 nmZn

115 nmCu

55 nm0.57 1.57 0.79 1.14 1.13

© IMEC 2011

IV-measurements solar cells


AM1.5G-illuminated IV of fully processed CZTSe solar cells

• Cells A and B show efficiencies of the order of 6.3%.

• As expected, sample C with small crystals shows lowest efficiency.

• Voc of cell C is 407 mV.

• Jsc of cell B is 36.1 mA/cm2.

• Series resistance is lowest for sample A and shunt resistance is highest for sample B.

• Strong cross-over between light IV and dark IV visible in all cases.

Efficiency (total area)

VOC JSC

Fill Factor

Rseries Rshunt

% mV mA/cm2 % cm2 cm2

Cell A 6.3 390 31.3 52 1.1 70Cell B 6.2 360 36.1 48 3.7 400Cell C 0.7 233 9.1 34 9.4 54Cell D 4.4 407 22.1 49 3.4 76

© IMEC 2011



Illuminated IV of cell B as a function of temperature

• Extrapolation of the temperature dependence of Voc to 0K gives a value for the activation energy of the dominant recombination process which is lower than the bandgap of the absorber.

• Possible reasons:• Ec < 0 (cliff-like heterojunction).• Fermi level pinning at the absorber/emitter interface (high interface recombination).• Non-homogeneity of the absorber bandgap.

Ea = 810 meV

© IMEC 2011



Illuminated IV of cell B as a function of temperature

• The series resistance of all measured cells increases in vacuum (in the cryostat) to values above 40 cm2, which makes measurements at low temperature difficult. The reason for this increase is probably bad adhesion between the CZTSe and the Mo.

• The series resistance decreases exponentially with temperature, pointing towards the existence of a back contact barrier of the order of 100 meV or of even decreased adhesion due to tension at lower temperatures.

*

* Barkhouse et al., Prog. Photovolt: Res. Appl. 2012; 20:6–11.

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EQE-measurements solar cells


External quantum efficiency

• The external quantum efficiency stays high up to very low photon energies, no drop at higher wavelengths. Minority carrier lifetime seems to be good.

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Minority carrier lifetime solar cells


Time-resolved photoluminescence

• Cells A and D have minority carrier lifetimes of about 7 ns.• Cell B has a minority carrier lifetime of about 4 ns.• Cell C has a minority carrier lifetime of less than 0.2 ns.

1

10

100

1000

0 10 20 30 40 50

Coun

ts (

#)

Time (ns)

Cell A

Cell B

Cell C

Cell D

© IMEC 2011

CV-measurements solar cells


Doping determination from capacitance response by Drive Level Capacitance Profiling (DLCP) and from Mott-Schottky plots (M-S).

-2 -1 00.02

0.04

0.06

0.08

0.1

0.12

Voltage (V)

C/A

(F

/cm

2)

-2 -1 00

0.05

0.1

0.15

0.2

0.25

Voltage (V)

C/A

(F

/cm

2)

-2 -1 00

0.02

0.04

0.06

0.08

0.1

0.12

Voltage (V)

C/A

(F

/cm

2)

-2 -1 00.16

0.165

0.17

0.175

0.18

0.185

0.19

Voltage (V)

C/A

(F

/cm

2)

0 100 200 3000.03

0.035

0.04

0.045

0.05

0.055

0.06

VAC

(mV)

C/A

(F

/cm

2)

Experimental dataSecond order fit

0 100 200 3000.03

0.035

0.04

0.045

0.05

0.055

0.06

VAC

(mV)

C/A

(F

/cm

2)


0 100 200 3000.008

0.009

0.01

0.011

0.012

0.013

0.014

0.015

VAC

(mV)

C/A

(F

/cm

2)


50 100 150 200 2500.145

0.15

0.155

0.16

0.165

VAC

(mV)

C/A

(F

/cm

2)


Cell A Cell B Cell C Cell D

© IMEC 2011



Doping determination from capacitance response by Drive Level Capacitance Profiling (dashed lines) and from Mott-Schottky plots (solid lines).

• Doping of cells A and B is of the order of 1016 cm-3, for cell D it is slightly lower. Cell C has high doping of the order of 5 1018 cm-3.

• There is a difference between the data from DLCP and Mott-Schottky plot, due to deep level defects which add to the capacitance in the case of the Mott-Schottky plot.

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Relationship Doping-Zn/Sn ratio

• There seems to be an exponential relationship between the doping and the Zn/Sn ratio measured in the CZTSe*.

• Zn and/or Sn atoms in the lattice seem to influence or seem to be directly involved in the dominant intrinsic doping defect in CZTSe (ZnSn antisite defect good candidate for acceptor-like defect).

* Brammertz et al., Thin Solid Films, DOI: 10.1016/j.tsf.2012.10.037 (2012).

0.001

0.010

0.100

1.000

10.000

100.000

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Dop

ing

(1017

cm-3

)

Zn/Sn ratio

© IMEC 2011



Relationship main deep level defect-Zn/Sn ratio

• There also seems to be an exponential relationship between the main deep level defect in CZTSe, the doping and the Zn/Sn ratio*.

• More evidence about the defect structure in CZTSe and its influence on electrical and optical behavior will be published soon (submitted to APL, Nov. 2012):

* Brammertz et al., submitted to Appl. Phys. Lett. (2012).

0.001

0.010

0.100

1.000

10.000

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

wdC

/dw

max

(F/

cm2 )

Zn/Sn ratio

© IMEC 2011

Conclusions


• We have fabricated CZTSe solar cells with a total area efficiency of 6.3 % by selenization of sequentially sputtered Sn-Zn-Cu multilayers.

• Physical and electrical characterization of the cells show:

• Strong evaporation of Zn and Sn during selenization. • Doping in the absorber of the order of 1016 cm-3. • Dominant recombination centers with an activation energy of about 810

meV leading to reduced Voc of the order of 400 meV in the best case.• Good short circuit currents of 36.1 mA/cm2 in the best case.• Strongly increased series resistance at low pressure due to weak

adhesion between CZTSe and Mo back contact.

• The doping density and main deep level defect density in the CZTSe absorber both seem to depend exponentially on the Zn/Sn ratio.

© IMEC 2011

Acknowledgments


• Tom De Geyter, Greetje Godiers and Guido Huyberechts from Flamac in Gent for sputtering of the metal layers.

• AGC for providing substrates.

• The Flemish ‘Strategisch Initiatief Materialen’ (SIM) SoPPoM program for their collaboration.

• Hamamatsu Photonics for providing the C12132 near infrared compact fluorescence lifetime measurement system.

Kesterite workshop 2012 Luxembourg

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