Cadmium Telluride/Cadmium sulfide photovoltaic cell
in the Scanning Electron Microscope

Aanand Thiyagarajan

University of Rochester
Department of Chemical Engineering

OPT407: Scanning Electron Microscopy
Spring 2012
Final Project


Introduction

 

Structure

 

Preparation of Samples and Imaging

 

Results and Discussion

 

Acknowledgements

 



 

Introduction

 Cadmium Telluride (CdTe) is a common semiconductor material used in the fabrication of inorganic solar cells. The ideal band gap of the material (around 1.5 electron volts, corresponding to high absorption in the solar spectrum) makes the choice of this material very attractive in photovoltaic applications.



 

Structure

 

The traditional structure of a conventional CdTe solar cell (from bottom to top) is as follows :

 

o   Glass - Superstrate

o   Indium Tin Oxide (ITO) - Back contact

o   Cadmium Sulfide - Thin layer

o   Cadmium Telluride - Thick layer

o   Molybdenum Oxide - Front contact

o   Nickel - Substrate

 

The device under consideration has the following structure :

 

o   Molybdenum substrate - 1mm

o   Molybdenum Oxide - varied from 40-300 nm (4 samples) but usually 250nm

o   Cadmium Telluride - 8 m

o   Cadmium Sulfide - 100nm

o   Indium Tin Oxide - 100nm



 

Preparation of samples and Imaging

 

The CdTe layer was deposited using Closed Space Sublimation (CSS). The CdS layer was prepared by Chemical Bath Deposition (CBD). One set of samples containing only Molybdenum Oxide (with different thicknesses - 40, 100, 200 and 300 nm) were prepared. The next set of samples had a layer of CdTe. Pristine CdTe samples treated with Cadmium Chloride were prepared.A third set of samples included a layer of both the CdTe and CdS. Two different methods of CdS preparation were used - the CIGS recipe and the Chu recipe. A final set of samples had both the layers of CdTe and CdS under different conditions. The first was a pristine set. The second was a set of samples annealed in air at 400 degrees Celsius for 30 minutes. The third was a set of samples annealed in vacuum at 400 degrees Celsius for 30 minutes. A fourth set had the samples annealed in Cadmium Chloride at 400 degrees Celsius for 30 minutes.

 

The imaging was performed using mainly the InLens and the SE2 detectors, while occasionally using the Backscatter detector. The most common magnifications used were 5000x and 10000x. Some images were taken in the AFM and electron flight simulation was also performed.

 

The various techniques used for imaging the surfaces were:

 

         Secondary Electron Microscopy (SEM)

         Atomic Force Microscopy (AFM)

         Bach Scattered Electron imaging (BSE)

         X-ray spectrum

         X-ray mapping

         Electron Flight Simulation

         Image Colorization


 

Results and Discussion

 

MolyO2-100nm5k

 

Figure 1. SEM image showing the surface of Molybdenum Oxide

 

 

Moly 2DMoly pristineMolyannealed

 

Figure 2. AFM images showing the surface of Molybdenum Oxide

 

 

It can be seen from Figures 1 and 2 that the Molybdenum Oxide layer is a relatively smooth surface with only a few irregularities in the structure. The image at left shows a pristine Molybdenum Oxide layer; the image at the center shows a 3D profile of the layer; the image at right shows the Molybdenum Oxide layer annealed in Oxygen.

 

 

pristineCdTe250xbpristineCdTe1.5kx

 

Fig. 3. Back scattered images of CdTe on Molybdenum Oxide

 

 

The above figure shows the Back scattered images at two different magnifications. Cadmium and Tellurium, having higher atomic numbers than Molybdenum and Oxygen, appear brighter when imaged using the Back Scatter detector.

 

 

SensHeight CdTe pristineCdTe VCL 3D

 

Fig. 4. 3D profiles of CdTe as seen in the AFM

 

 

Images of the CdTe layers are shown in Figure 3. These surfaces are more rough when compared to the Molybdenum Oxide. The image on the left is a pristine CdTe layer, while the one at right shows CdTe annealed with Cadmium Chloride.

 

 

P-CdTe100nm5kinlensCdTe-VCL100nm5k

 

Fig. 5. Comparison of grain structures of pristine CdTe and CdTe treated with CdCl2

 

 

From Fig. 5. it can be seen that while the grain structures of the pristine CdTe are relatively smooth, the layers of the CdTe treated with Cadmium Chloride appear to have some smaller granular structures embedded in the main structure.

 

 

CdTe-CIGS300nm10kinlens

 

Fig. 6. CdTe after deposition of CdS

 

 

After the CdS deposition, even smaller structures are visible on the CdTe, as can be seen from Fig.6. These are the smaller particles of CdS sticking on to the surface of the larger CdTe grains.When annealing is done under different conditions, after the deposition of CdS, more differences can be made out.

 

 

CdS1-10kaCdS2-10kinlens4

CdS3-10kcCdS4-10ka

 

Fig. 7. Image showing CdTe samples at different stages of annealing and treatment

 

 

The image at the top left is a non- annealed sample of CdTe/CdS. The one at the top right is annealed in air. The bottom left images shows a sample annealed in vacuum. The one at the bottom right shows a sample treated with CdC. Annealing in vacuum does not seem to make much difference to the appearance of the layer of CdTe/CdS. But when the sample is treated with CdCl, the grains coalesce together, forming a structure that is highly conductive.

Treatment using Cadmium Chloride shows the grains to have come together, with lesser grain boundaries and defects. This is considered to be ideal as this facilitates the grain growth by reducing the number of defects and grain boundaries and thus improves the conductivity of the material by reducing the recombination of charge carriers that occur at these defect sites.

 

 

edax11

 

Fig. 8. X-ray spectrum of Molybdenum Oxide layer

 

In this image it can be seen that Molybdenum and Oxygen are the two dominant elements, as is expected.

 

 

edaxcdte

 

Fig. 10. X-ray spectrum of CdTe layer

 

 

In this X-ray spectrum of CdTe, a few impurities apart from Cd and Te, such as Ti, Cr and Co can be noticed. It may also be an error by the detector in identifying the elements.

 

 

CdS4

 

Fig. 11. X-ray spectrum of CdS layer

 

 

In this X-ray image of the CdS layer a little Oxygen is also visible, apart from Cd and S.

 

 

Colorized

 

Fig. 12. A colorized image of the CdTe/CdS layer

 

One of the images was colorized using Adobe Photoshop to make the image look more attractive and also to bring out the contrast better.

 

bulkEFS2

 

Fig. 13. Image showing the trajectory of the electron beam

 

An Electron Flight Simulation of a 10keV electron beam incident on a bulk sample of Cadmium Telluride is seen. The image shows the interaction volume of the electrons (the amount of material that is affected by the beam) and also the depth of penetration of the beam.


Acknowledgements

 I thank Brian McIntyre for the time spent and the ideas and suggestions given. I would also like to thank Prof. Ching Tang of the department of Chemical Engineering for permission to carry out this project and also his graduate student Sunny Wu who helped in preparing the samples for imaging.

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