SiliconBEACON – Unlimited, Resource-Saving Solar Cells Based on Metallurgical-Grade Silicon

Characterization Based on Time-Modulated Luminescence

Duration: June 2007 - June 2013
Contracting Authority/ Sponsors: Fraunhofer Zukunftsstiftung
© Fraunhofer ISE
Fig. 1: Multicrystalline silicon wafer in our experimental setup – timemodulated luminescence is used for carrier lifetime measurements of wafers and solar cells.
© Fraunhofer ISE
Fig. 2: Effective charge carrier lifetime τeff of a silicon wafer, determined as a function of the excess charge carrier density Δn by application of a procedure developed at Fraunhofer ISE on the basis of dynamic photoluminescence.

Time-modulated luminescence is a procedure to determine charge carrier lifetimes in silicon wafers and solar cells. It is very robust against the artefacts which often appear in lifetime measurements and can be applied in material science applications in many different ways.

In time-modulated (quasi-steady-state) luminescence, the charge carrier lifetime of an optically or electrically excited semiconductor substrate is determined from the phase shift between the time-dependent profile for exciting free charge carriers and their radiative recombination. Although this phase shift is generally not identical with the charge carrier lifetime, the actual injection-dependent lifetime can be determined from this phase shift with an iteration procedure developed at Fraunhofer ISE. Further, it was demonstrated that the phase shift of a quasi-steady-state, time-modulated experiment corresponds to a so-called differential lifetime. The underlying theory enables the actual lifetime to be determined analytically from the measurable differential lifetime. This means a drastic reduction in the experimental effort needed for all differential lifetime measurements.

The following applications of time-modulated luminescence were newly developed:

  • A procedure to determine the effective lifetime over a very wide injection range (Fig. 2). The measurement at very low injection density is highly relevant for diverse applications in materials science.
  • procedures to determine the net doping of silicon substrates
  • lifetime determination in solar cells applying dynamic electroluminescence