A wise old proverb says, “Quality has to be produced, not approved.” What can we learn from this wisdom for the production of crystalline photovoltaic modules? It tells us that we have to use electroluminescence tests during production and not only as a final test of the modules.
Let us firstly briefly examine what we can see with an electroluminescence test. Electroluminescence relies on the same principle as a light emitting diode (LED). Current is fed into a solar cell (essentially a large diode), and radiative recombination of carriers causes light emission. Most of the recombination in silicon, which is an indirect bandgap semiconductor, occurs via defects or Auger recombination. The amount of band-to-band recombination producing radiative emission is relatively low. However, there is a small amount of radiative recombination that happens even in silicon, and this signal can be sensed using an external detector. The technique requires electrical contact and so can only be used once the metallization has been applied and the cell is substantially complete. Electroluminescence provides a wealth of data about the area-related uniformity of solar cells and modules. It is non-destructive and relatively fast, with measurement times of 1 s possible.
The luminescence signal of silicon peaks at 1150 nm, corresponding to the energy of the bandgap.
Electroluminescence has become increasingly popular with the advent of low cost silicon CCD arrays. They are similar to the ones used for digital cameras, but optimized for sensitivity in the near-infrared and cool to reduce thermal noise.
The key advantage, as noted above, is the ability of electroluminescence to image an entire solar cell or module in a relatively short time. The light output increases with the local voltage, such that regions with poor contact show up as dark.
Electroluminescence image of a monocrystalline silicon wafer. The intensity of the light given off is proportional to the voltage, so poorly contacted and inactive regions show up as dark areas. The microcrack and printing problem are not detectable through visual inspection.
Due to further cost reduction, photovoltaic cells are becoming thinner and thinner. This makes them much more sensitive not only to mechanical stress, but also to temperature influences during the production process. To assure high quality modules in the end, we have to monitor all relevant production processes using electroluminescence images. This gives us the ability to feedback control the production processes and react immediately in case of deviations. Using electroluminescence during the production of photovoltaic cells and modules assures high quality and controlled production processes and makes it possible to produce thinner cells and, as a result, further reduce cost.