© 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
© IMEC 2011
SEM
G. Brammertz, PV/Explore 3
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.
© IMEC 2011
SEM
G. Brammertz, PV/Explore 4
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.
© IMEC 2011
SEM-EDX
G. Brammertz, PV/Explore 5
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
G. Brammertz, PV/Explore 6
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
IV-measurements solar cells
G. Brammertz, PV/Explore 7
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
IV-measurements solar cells
G. Brammertz, PV/Explore 8
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.
© IMEC 2011
EQE-measurements solar cells
G. Brammertz, PV/Explore 9
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.
© IMEC 2011
Minority carrier lifetime solar cells
G. Brammertz, PV/Explore 10
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
G. Brammertz, PV/Explore 11
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)
Experimental dataSecond order fit
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)
Experimental dataSecond order fit
50 100 150 200 2500.145
0.15
0.155
0.16
0.165
VAC
(mV)
C/A
(F
/cm
2)
Experimental dataSecond order fit
Cell A Cell B Cell C Cell D
© IMEC 2011
CV-measurements solar cells
G. Brammertz, PV/Explore 12
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.
© IMEC 2011
CV-measurements solar cells
G. Brammertz, PV/Explore 13
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
CV-measurements solar cells
G. Brammertz, PV/Explore 14
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
G. Brammertz, PV/Explore 15
• 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
G. Brammertz, PV/Explore 16
• 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.
© IMEC 2011
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