# Presentation: 348234

## CONDUCTIVE ATOMIC FORCE MICROSCOPY APPLIED TO CdTe/CdS SOLAR CELLS

- H.R. Moutinho, R.G. Dhere, C.-S. Jiang and M.M. Al-Jassim

## 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.

## 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.

## Slide 4

## Slide 5

## EXPERIMENTAL PROCEDURE

- Sample Structure: glass\SnO2\CdS\CdTe.

- Treatment: CdCl2 vapor 400°C/5 min.

- Etching: 0.5% bromine in methanol solution (BM etch), for 2 seconds, or phosphoric acid:nitric acid:deionized water (88:1:35) solution (NP etch), for 30 to 45 seconds.

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

- Deposition: Standard NREL CSS CdTe and CBD CdS.

## Ftip = 1.2x10-7 – 3.4x10-7 N

- Ftip = 1.2x10-7 – 3.4x10-7 N

- Effect of the force exerted by the tip on the sample surface.

## Ftip = 1.2x10-7 – 3.4x10-7 N

- Ftip = 1.2x10-7 – 3.4x10-7 N

## Decrease in current with time.

## Decrease in current with time.

## I-V curve for an as-deposited CdTe/CdS solar cell.

- Forward

- Reverse

##  Different grains have different currents (electric conductivity).

-  No significant effects of topography.

-  No grain boundary effects.

- As-deposited

## Vdc = 4 V

- CT273cl.013

- Vdc = 4 V

-  Different grains have different currents.

-  No grain boundary effects.

-  No significant effects of CdCl2 treatment.

- CdCl2 treated

## Vdc = 8 V

- Vdc = 8 V

-  Different grains have different currents.

-  Strong grain boundary effects.

- After bromine/
- Methanol etch

## Vdc = 8 V

- CT174cl-br.011

- Vdc = 8 V

## Vdc = 5 V

- Vdc = 5 V

## Vdc = -5 V

- Vdc = -5 V

##  Magnitude of the current increases significantly.

-  Loss of spatial resolution.

-  Current image is not uniform.

- After NP etch

##  I-V curve: ohmic behavior.

-  I-V curve: ohmic behavior.

## Linescans for sample illuminated with white light and biased under different voltages

**I (********A)**

**0.4**

**0.2**

**0.6**

**-0.2**

**0**

**-300  mV**

**-500  mV**

**0 mV**

**0**

**0.5**

**0**

**0.5**

**0**

**0.5**

**0**

**0.5**

**0**

**0.5**

**1**

**Distance (********m)**

**-665 mV**

- CT252cl-nps-b

## X-C-AFM

- After NP etch

- CT340b.001

## After bromine/methanol etch

- X-C-AFM

- CT340d.005

## X-C-AFM

- After NP etch

- CT340b.007

## CONCLUSIONS

**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.

## 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).

## Linescan

- CT250cl-br2s.009
- CT250cl-br2s.013

## I-V curves for different tip forces.

## No (intentional) illumination

- Room light

- Strong white light

- CT273cl-np

## Vdc = 1 V

- Vdc = 1 V

## Vdc = -3 V

- Vdc = -3 V

## Slide 31