Electroluminescence (EL) imaging takes advantages of the radiadive inter-band recombination of excited charge carriers in solar cells. For electroluminescence investigation the module is operated as a light emitting diode and the emitted radiation due to recombination effects can be detected with a sensitive Si-CCDs camera. For electroluminescence images, the solar cells are supplied, via their metal contacts, with a defined external excitation current while the camera takes an image of the emitted photons at wavelength > 850 nm. Since EL is a low light source, a dark environment is necessary in order to decrease the background noise during the measure. Damaged areas of a solar module appear dark or shine less than good areas. Electroluminescence imaging can be used to detect a multitude of defects in crystalline silicon solar cells, furthermore EL technique provide images with very high resolution that enable to resolve details lesser in size than IR-images that should be hardly perceptible to the eye, such as:

  • Micro cracks.
  • Bad finger contacts.
  • Electrical shunts.
  • Broken contacts.
  • Fragments in broken cells.
  • Electrically separated cell areas.
  • Grain boundaries.
  • Crystallization faults in cell material.

The picture here below shows different defects in a polycrystalline silicon module, such as:

  • A long crack over 10 cells (1).
  • A smaller crack over 3 cells (2).
  • Inactive cell parts (3).
  • Impurity inside the cells (4).
  • Cell part isolated from the bus bar by micro cracks (5).
  • Mark of the sinteroven’s grate (6).