Quality control of as-cut multicrystalline silicon wafers using photoluminescence imaging for solar cell production

Measuring the lifetime of excess charge carriers gives the opportunity to access the electric quality of the material. However, on as-cut wafers before production this quantity is strongly limited by the surface of the material. On a batch of solar cells we show that the open circuit voltage of the finished cells only scales with the lifetime, measured on as-cut wafers, if the material quality is very low. The difference between moderate and high material quality cannot be resolved. Using photoluminescence imaging the lifetime can be acquired with high spatial resolution. We show that by analyzing crystallization-related features in the images, certain defects can be identified: Such features are defects of crystal growth (e.g. dislocations) and areas of reduced lifetime that form at the edges of the crystallization crucible or near the top or bottom of a brick. Those features can be detected before production and we show their influence on cell parameters. By recognizing and rating these features, a more accurate quality control for wafers can be introduced.