Scanning electron microscope (SEM) pictures are useful for examining the fine structure of solar cells. Even in large area commercial devices, for example, an SEM photograph can show the depth of the rear surface aluminium alloyed layer. Many of the cell features are of the order of microns and so not possible to view with an optical microscope. An additional advantage of an electron microscope is its higher depth of field. With an electron microscope it is possible to have the whole device in focus at once whereas in an optical microscope at high magnification only parts of the device will be in focus at any time.
Electron Beam induced current (EBIC) is useful for showing the electrically active areas of the device. To create an EBIC image an electron beam is swept across the sample and the output current of the device measured. Those electrons that are collected by the junction show up as bright regions while electrons uncollected remain dark. It is common to superimpose the EBIC images on SEM picture to show more clearly show the location of the active areas.
In the figure below the cell has been cleaved to take a picture of the cell edge. The cleaved surface is unpassivated so most carriers will recombine in a very short distance and only those created right at the junction edge will be collected. Hence the junction shows up as a very bright line.
EBIC is also used in material characterisation. Altering the energy of the beam changes the depth and volume at of electrical excitation and this can be used to characterise defects. By sweeping the beam across the surface of the device it is possible to identify the electrically inactive areas such as grain boundaries. A numerical analysis of the induced current gives a measure of the diffusion length 1.
- 1. , “Simulating Electron-Beam-Induced Current Profiles Across p-n Junctions”, 16h European Solar Energy Conference. pp. 1590-1593, 2000.
- 2. , “Dependence of aluminium alloying on solar cell processing conditions”, 13th Workshop on Crystalline Silicon Solar Cell Materials and Processes. 2003.
- 3. , “Aluminium Back Surface Field in Buried Contact Solar Cells”, University of New South Wales, 2000.