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. As an indirect bandgap semiconductor, most of the recombination in silicon 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. Electroluminsecence 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 1.
Detectors
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 optimised for sensitivity in the near-infrared and cool to reduce thermal noise. As with digital cameras, there are detectors with multiple mega-pixels resolutions of 2048 × 4096 pixels enabling high resolution images of entire modules. A significant drawback of a silicon detector is that they have a poor response beyond 1000 nm due to the low absorption coefficient of silicon. An alternative detector is arrays of InGaAs photodiodes. It has a much better response over the 1000 to 1300 nm wavelength range enabling much faster data acquisition but with significantly higher cost. Resolution tends to be in the sub-megapixel range with 640 × 512 pixels common.
Quantum efficiency of silicon CCD detectors and InGaAs photodiode array. The low cost and resolution of the silicon CCD makes up for the poor response over the 1000 -1200 nm region of interest.
Images
The key advantage as noted above is the ability of electroluminescence imaging an entire solar cell or module in a relatively short space of time. The light output increases with the local voltage so that regions with poor contact show up as dark.
Electroluminscence 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 with visual inspection.
- 1. , “Photographic surveying of minority carrier diffusion length in polycrystalline silicon solar cells by electroluminescence”, Applied Physics Letters, vol. 86, p. 262108, 2005.