Light Beam Induced Current (LBIC)

Light Beam Induced Current (LBIC) is a primarily in the photovoltaic sector well established method for the spatial resolved measurement of recombination active defects in ready-progressed solar cells.

The proceeding is based on the measurement of the local short circuit current Isc in the cell, which is produced through appropriate excitation. For the measurement the solar cell is contacted and locally irradiated with laser light. Advantages are the simple buildup and the high resolution, disadvantage is the exigency of contacting the sample, for which reason only ready-progressed cells can be examined. Predictions about lifetime, mobility or defect details are possible.

From the measured short circuit current the external quantum efficiency can be determined via:

where P is the laser power and l the wavelength. The external quantum efficiency depends on the properties of the cell volume but also on the reflection properties of the surface. If the reflection can be neglected the following mechanism determines the EQE:

  • recombination of minority carriers in the volume

  • recombination of minority carriers at the surfaces at the front and the back

  • shunts in the solar cell

The internal quantum efficiency IQE includes the reflection of the light at the surface:

In order to determine the diffusion length, it is necessary to measure IQE for at least 4 different wavelengths, which should differ in the penetration depth. The slope of 1/IQE vs. 1/a is the reciprocal of the effective diffusion length.

Fig. 1: internal quantum efficiency of a solar cell measured with 4 different wavelength