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.
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 µm
o Cadmium Sulfide - 100nm
o Indium Tin Oxide - 100nm
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
Figure 1. SEM image showing the surface of Molybdenum Oxide
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.
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.
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.
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.
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.
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 CdC₂. 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 CdCl₂, 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.
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.
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.
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.
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.
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.
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.