Characterization Of CdTe Thin Films For Photovoltaic Applications

Prashant Kumar Singh

University of Rochester
Material Science Department

OPT407: Practical Electron Microscopy
Spring 2008

Final Project


Introduction

  1. Introduction
  2. Effects of Cadmium Chloride Heat Treatment
Characterization Techniques
  1. Sample Preparation
  2. Atomic Force Microscopy
  3. Secondary Electron Microscopy
  4. Elemental Mapping
  5. ImageJ Analysis
  6. Energy Dispersive Spectroscopy
  7. Colorization
  8. Electron Flight Simulation

Conclusions and Acknowledgements

  1. Conclusion
  2. Acknowledgements
  3. References
  4. Comments


 

Introduction

 1. CdTe Photovoltaics

CdTe photovoltaic describes a photovoltaic technology that uses CdTe thin films designed to absorb light and convert it to electricity. A simple heterojunction design involves use of p-type CdTe matched with n-type cadmium sulfide (CdS). Cadmium telluride PV is the first and only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness for a significant portion of the PV market.[1]

The goal of the continued research in this field is to achieve low cost mass production while retaining and improving the efficiency of the solar cells. The key parameters which affect the efficiency of a solar cell include the optical and electrical properties of the various layers. The interface between CdTe- CdS and the roughness of these layers plays an important role in determining the fill factor of the cells. CdTe grown by various techniques like physical vapor deposition and sol gel have shown high surface roughness with pinholes after NP treatment. A turning point for thin-film CdTe performance was the application of a post deposition air-heat treatment of CdTe/CdS structures with cadmium chloride vapors.

 

2. Effects of Cadmium Chloride Heat Treatment

In a typical heat treatment, the as grown CdTe films on glass/ITO/CdS substrate is exposed to Cadmium chloride/oxygen vapor mixture by maintaining a constant vapor pressure of Cadmium chloride over the surface of the film.The treatment improves the overall performance of the solar cells and appears to improve the yield and uniformity of devices[2]. It also enables fabrication of pinhole free ultrathin CdTe/CdS structures resulting in lower fabrication costs.[3]




 

Characterization Techniques

 

1. Sample Preparation

The as prepared samples were sputter coated with gold in order to make them sufficiently conducting for electron microscopy. Typically a deposition time of 60 seconds and a current of 15 amperes was used for depositing gold.Atomic force microscopy was performed on the samples without any sample preparation. Necessary precautions were taken during sizing of the samples for both AFM and electron microscopy.

 

2.Atomic Force Microscopy

 

Figure 1:Atomic force micrograph of CdTe showing the rough surface of the as prepared sample.

 

3. Secondary Electron Microscopy

    

Figure 2:SEM micrograph of a)untreated CdTe showing striations,b)Treated CdTe without striation, c) untreated CdTe/CdS interface and d) treated CdTe/CdS interface.

 

4.Elemental Mapping

    

Figure 3: Shows distribution of a)Cd,b)Te,c)Au and d)S at the CdTe/CdS interface.

 

5.ImageJ Analysis

     

Figure 4:Shows the area analysis of a) Untreated CdTe and b) Treated CdTe.

 

6.Energy Dispersive Spectroscopy

  

Figure 5:Shows EDS spectrum of CdTe/CdS interface at a)CdS layer and b)CdTe.

 

7.Colorization

  

Figure 5:Atomic force micrgraph of CdTe shwing the rough surface of the as prepared sample.

 

8.Electron Flight Simulation

    

Figure 6:Shows electron flight simulation for CdTe/CdS/ITO layers with a total thickness of 1700nm at an accelerating voltage of 15kV a) without tilt and b) 10 degree tilt.



 

CONCLUSION AND ACKNOWLEDGEMENTS

 

1. Conclusion

The increase in the efficiency of the CdTe/CdS solar cells after cadmium chloride treatment can be attributed to the morphological changes brought about by the treatment. The increase in grain size along with smoothening of the interface results in a decrease in the series resistance resulting in higher efficiency.

 

2. Acknowledgements

Sincerest thanks to Professor Ching W. Tang and his group for their constant support and for the availability of the samples used in the characterization.I would like to thank Brian McIntyre,course instructor, for his ingenious ideas on the project and for being an integral part of my problem solving.Thanks to Wei Xia for his efforts dedicated on numerous sample preparations.

 

3. References

1. ZWEIBEL, K. (1999). “ISSUES IN THIN FILM PV MANUFACTURING COST REDUCTION.” SOLAR ENERGY MATERIALS AND SOLAR CELLS, 59

2.M. RAMI, E. BENAMAR, M. FAHOUME, F. CHRAIBI, AND A. ENNAOUI(2000)” EFFECT OF HEAT TREATMENT WITH CADMIUM CHLORIDE ON THE ELECTRODEPOSITED CADMIUM TELLURIDE/CADMIUM SULPHIDE HETEROJUNCTION”, CONDENSED MATTER,VOL.3.

3.ZHIZHONG BAI, LEI WAN, ZERONG HOU(2010)” EFFECT OF CdCL2 ANNEALING TREATMENT ON CDS THIN FILMS AND CdTe/CdS THIN FILMS", PSS,VOL.8.


 

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