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.
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.
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.
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 |