Sunday, March 4, 2012

Solar Cell Under the Scope Using EBIC!

If you’re a curious person, much like me, you will definitely appreciate this electric wonder! Using a scanning electron microscope (SEM), a current can be applied to a solar cell allowing viewers to gaze upon the mesmorizing distribution of charge flowing on the cell’s surface. How you may ask? Simple! We use a SEM technique called electron-beam induced current (EBIC).
How EBIC Microscopy Works
When the electron beam in a SEM strikes the surface of a semi-conductor, it generates electron hole pairs (in our case, the semi-conductor is simply the solar cell). The p-n junction field inside the semi-conductor causes the electron-hole pairs to separate, drifting the electrons to the n-side and holes to the p-side. A current amplifier is introduced to connect the p- and n-sides. This connection allows the electrons and holes to begin flowing, inducing a current on the surface of the solar cell. Finally, the output of the current amplifier is utilized as the imaging signal for the SEM and observation of the charge distribution is possible.
The NOVA Center Setup
The purpose of the experiment was to determine the efficiency of the charge distribution on the surface by observing any topographical defects. With the assistance of Martin Klein (owner of Ellcie) and engineer Peter Marienhoff, Dr. Edward Kintzel of the Nondestructive Analysis Center executed the experimental setup of EBIC. The image below shows the Solar Amplification System (SASy) that they built and the solar cell to be investigated.
Image above shows Solar Amplification System (SASy), Solar Cell, and Faraday Cup.

A Faraday cup was also constructed to characterize the electron beam. Faraday cups are metal conductive cups designed to catch the electrons from our beam and then use the resulting current to determine the number of electrons initially striking the cup. The SEM image below was taken to demonstrate the unique deformation of the drilled metal casing holding the faraday cup.
SEM Image of Faraday Cup

The following images show the illuminated surface of the solar cell when EBIC is in action! Each consecutive image is increasing magnification. The initial image shows areas of defects in the surface, revealed by the darker regions where charge is not flowing. Potential causes or these defects may be related to dirt on the surface or scratches made during the manufacturing process. Eliminating these regions would enhance the efficiency of charge distribution.
Dark regions signify defects in surface where charge is not distributing

Paralled silver strips aligned solar cell appear dark next to the illuminated charge!

Magnified image of the charge distribution!

Magnified image of charge distribution!
Notice the intricate pathes in which the charge travels! 

Besides high resolution imaging of the surface, we also did a chemical analysis using Energy Dispersive Spectroscopy (EDS) on the solar cell. This SEM technique provides the ability to not only recognize elements present, but also create mappings of their locations and their percent composition. Below shows two different regions examined: a silver strip (conductive region) and a region of silicon (non-conductive region).
Shown left: Percent composition of elements present.
Shown right: An elemental mapping of only the element Silver (shown in magenta)



Shown far left: Percent composition of elements present
Shown middle and far right: Elemental mappings only Silicon and then Titanium