By Douglas Malchow, Business Development Manager for Industrial Products Sensors Unlimited, Inc., part of Goodrich Corporation
Contactless machine-vision inspection using photoluminescence (PL) imaging with shortwave infrared (SWIR) cameras can help solar cell producers improve both efficiency and quality of their photovoltaic products. Inspection of silicon bulk ingots, sliced wafers, processed layers, and complete photovoltaic cells is possible with SWIR imaging. The PL emissions occur at the wavelengths associated with the semiconductor bandgap, which are visible to high-sensitivity, uncooled, indium gallium arsenide (InGaAs) cameras, even while imaging at video frame rates. This inspection technique illuminates the objects of interest with high optical power at one wavelength, and photons are absorbed in the bulk material. Some energy is lost as heat in the interaction with the molecular structure and the remaining energy causes a photon to be re-emitted at a longer wavelength. The uniformity and intensity of the resulting glow is sensitive to many parameters of the material, and of subsequent processing steps. In many cases, properties of the material, such as minority carrier lifetime, can be mapped from the PL image, and these maps will directly relate to how well the final product will perform as a solar cell.