Lifetime Imaging on Silicon Bricks Using the Ratio of Photoluminescence Images With Different Excitation Wavelengths

The efficiency of silicon solar cells is eventually limited by the bulk lifetime of minority carriers in the wafer. The characterization of the bulk quality, measured on silicon bricks early in the production process, provides an immediate indication of the efficiency potential of the material, the feedstock purity, and the quality of the crystallization process. Previously, the intensity ratio of two photoluminescence images that are acquired with different spectral filters (i.e., the two-filter method) has been used to measure the bulk lifetime of silicon bricks. In this paper, we report on a similar approach, which uses the ratio of two photoluminescence images each acquired with a different illumination wavelength (915 and 1064 nm); one of which is within the photoluminescence emission band of silicon. We demonstrate with modeling that this two-laser method is more robust against uncertainties that affect the spectrum of photoluminescence emission and its detection, while the two-filter method is more robust against uncertainties in the carrier density profile and the illumination system properties. Experimentally, both the two-laser and two-filter methods show good agreement across a monocrystalline and multicrystalline silicon brick. However, challenges using the two-laser method arose when a brick with strongly injection-dependent lifetime was measured.