Photoluminescence Imaging
Luminescence imaging is a fast, spatially-resolved method for performing minority carrier lifetime measurements of silicon wafers and solar cells [1]. By detecting photons released by the relaxation of conduction band electrons to the valence band (known as radiative recombination), the carrier lifetime across a silicon wafer can be imaged. This luminescence signal can be generated by external illumination (photoluminescence, PL) or forward biasing of a solar cell (electroluminescence, EL).
The luminescence intensity in low-level injection can be described according to the expression [2]:
where B is the radiative recombination coefficient, is the doping concentration of the silicon wafer, is the excess carrier density generated during illumination/forward biasing and is a calibration constant to convert relative PL signals into a quantitative measure of the injected carrier density. The intensity of a luminescence image is proportional to the separation of the quasi-Fermi level of holes and electrons [3]. In low-level injection conditions, the quasi-Fermi level of the majority charge carrier is pinned by the background doping level and the quasi-Fermi level of the minority carriers is equal to the excess carrier density. Consequently, for silicon wafers with a uniform background doping density brighter regions in a photoluminescence image indicate regions of higher effective minority carrier lifetime. Calibration of these images with for example photoconductance measurements can allow the conversion of luminescence intensity to quantitative images of silicon the effective minority carrier lifetime or implied open-circuit voltage. In Figure 1 a photoluminescence image of a multicrystalline silicon solar cell is shown, clearly illustrating that even though solar cell may appear uniform optically they can be highly non-uniform from an electrical point of view.
Figure 1: Photoluminescence image of a multicrystalline silicon solar cell. Bright areas indicate high effective minority carrier lifetimes (or local separations of the quasi-Fermi levels for holes and electrons) while dark areas indicate low lifetimes. These low lifetime areas can be related to crystallographic defects such as dislocations or grain boundaries or high defect-related recombination due to poor bulk lifetime or surface passivation.
Luminescence imaging is a powerful tool that allows several different issues in silicon wafers or solar cells to be identified, including:
- Defects in silicon, such as SRH (Shockley-Read-Hall) recombination centres, grain boundaries and dislocations;
- Shunt and series resistance issues with solar cell contacts;
- Identify areas of lower diffusion lengths;
- Iron contamination.
In the video below we show a photoluminescence measurement taken at UNSW Sydney.
References:
- Trupke, T., R.A. Bardos, M.C. Schubert, and W. Warta, Photoluminescence imaging of silicon wafers. Applied Physics Letters, 2006. 89: p. 44107.
- Trupke, T., R.A. Bardos, and M.D. Abbott, Self-consistent calibration of photoluminescence and photoconductance lifetime measurements. Applied Physics Letters, 2005. 87: p. 184102.
- Wurfel, P., THE CHEMICAL-POTENTIAL OF RADIATION. Journal of Physics C-Solid State Physics, 1982. 15(18): p. 3967-3985.