Communications: NREL PowerPoint Presentation Template with Light Background

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CONDUCTIVE ATOMIC FORCE MICROSCOPY APPLIED TO
CdTe/CdS SOLAR CELLS H.R. Moutinho, R.G. Dhere,
C.-S. Jiang and M.M. Al-Jassim
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OBJECTIVES

• Demonstrate the applicability of C-AFM to study
  CdTe/CdS solar cells, investigating and
  identifying limitations, problems, and advantages
  of this technique.

• Study the effects of the standard etching
  processes on the electrical properties of
  CdTe/CdS devices.

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INTRODUCTION

• C-AFM uses the probe of an AFM to contact the
  sample surface, and measures the current through
  a sample for a given applied potential.

• Main advantage High spatial resolution (down to
  10 nm, but highly dependent on the sample).

• Provides current images simultaneously with
  topographic images.

• Provides I-V curves.

• Specially suitable to find micro shunting in
  solar cells.

• Contact between tip and sample may be a problem.

• Can be destructive.

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EXPERIMENTAL PROCEDURE

• Sample Structure glass\SnO2\CdS\CdTe.

• Deposition Standard NREL CSS CdTe and CBD CdS.

• Treatment CdCl2 vapor 400C/5 min.

• Etching 0.5 bromine in methanol solution (BM
  etch), for 2 seconds, or phosphoric acidnitric
  aciddeionized water (88135) solution (NP
  etch), for 30 to 45 seconds.

• C-AFM Digital Instruments DI 3100, using
  doped-diamond coated tips.

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Effect of the force exerted by the tip on the
sample surface.
Ftip 1.2x10-7 3.4x10-7 N
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Ftip 1.2x10-7 3.4x10-7 N
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Decrease in current with time.
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Decrease in current with time.
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Forward
Reverse
I-V curve for an as-deposited CdTe/CdS solar cell.
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As-deposited

• No significant effects of topography.

• Different grains have different currents
  (electric conductivity).

• No grain boundary effects.

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CT273cl.013
Vdc 4 V
CdCl2 treated

• Different grains have different currents.

• No grain boundary effects.

• No significant effects of CdCl2 treatment.

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CT174cl-br.011
Vdc 8 V
After bromine/ Methanol etch

• Different grains have different currents.

• Strong grain boundary effects.

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CT174cl-br.011
Vdc 8 V
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Vdc 5 V
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Vdc -5 V
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After NP etch

• Magnitude of the current increases significantly.

• Loss of spatial resolution.

• Current image is not uniform.

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• I-V curve ohmic behavior.

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CT252cl-nps-b
0.6
0 mV
0.4
-300 mV
0.2
-665 mV
I (mA)
-500 mV
0
-0.2
0
0.5
0
0.5
0
0.5
0
0.5
0
0.5
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Distance (mm)
Linescans for sample illuminated with white light
and biased under different voltages
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CT340b.001
After NP etch
X-C-AFM
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CT340d.005
After bromine/methanol etch
X-C-AFM
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CT340b.007
After NP etch
X-C-AFM
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CONCLUSIONS

• We demonstrated the application of C-AFM to the
  study of CdTe/CdS solar cells.

• As-deposited Grains with different currents,
  indicating differences in electric conductivity.

• CdCl2 treatment No significant differences.

• BM etch Relative increase in current at grain
  boundaries, due to Te-rich layer at these regions.

• NP etch Increase in current over the entire CdTe
  surface (Te-rich surface layer), change in the
  electric contact between the tip and the sample
  surface from rectifying to ohmic, and
  photocurrent.

• X-C-AFM higher penetration for the np etch and
  capability of the technique to detect shunts in
  the device.

• For our treatment conditions, NP etch is more
  suitable for use as back contact in CdTe/CdS
  solar cells.

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FUTURE WORK
Surface and Cross-Section

• Study the effect of changing the dipping time
  for both, NP and bromine solutions, to optimize
  the etching process.

Cross-Section

• Study the effect of using different back
  contacts (Au-Cu and ZnTe).

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CT250cl-br2s.009 CT250cl-br2s.013
Linescan
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B129ad
I-V curves for different tip forces.
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CT273cl-np
Strong white light
Room light
No (intentional) illumination
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Vdc 1 V
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Vdc -3 V
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