Impact of the Firing Temperature Profile on Light Induced Degradation of Multicrystalline Silicon

Light- and elevated temperature-induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc-Si wafers is analysed. Five profiles with measured high peak temperatures 800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N*t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp-up and/or ramp-down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed.