Messung von Minoritätsladungsträger-Diffusionslängen in Silicium-Solarzellen mit Lumineszenzmethoden

Abstract

Spatially resolved luminescence images of silicon solar cells and wafers reveal information on quantities which affect the efficiency of solar cells. These quantities comprise recombination properties such as minority charge carrier diffusion length and surface recombination velocities as well as local series- and parallel resistance and optical properties of interfaces.This work aims at improving the interpretation of luminescence measurements on PV-Silicon with respect to minority charge carrier diffusion length. Both electrical excitation of solar cells (electroluminescence) and optical excitation of silicon wafers (photoluminescence) are investigated with an emphasis on the determination of a spatially resolved excess charge carrier depth distribution through optical filtering.To begin with, a method to determine electron diffusion lengths in electrically excited Silicon solar cells based on optical filtering was tested (Würfel et al., J. Appl. Phys. 101, 123110). The successful implementation of this method required some considerations that have not been mentioned in literature so far - such as a certain class of lateral inhomogeneity of optical filters and taking into account the exact sample temperature. It turned out that the extension of this method to Photoluminescence on wafers is complicated by several challenges. A new technique to determine minority charge carrier diffusion lengths from single photoluminescence images was implemented, which is in good agreement with reference techniques. Building on this approach as well as on the work of Würfel et al., a spatially resolved separation of recombination properties between defects related to bulk material on the one hand and defects related to surfaces on the other hand was developed and successfully tested on both solar cells and wafers.Finally, a new technique to estimate low minority charge carrier diffusion lengths in saw damaged silicon wafers (as-cut) was implemented.