Cadmium Telluride/Cadmium sulfide
photovoltaic cell
in the Scanning Electron Microscope
University of Rochester
Department of Chemical Engineering
OPT407: Scanning Electron Microscopy
Spring
2012
Final
Project
Preparation of Samples and Imaging
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 µm
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
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.
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.