Firing temperature profile impact on light induced degradation in multicrystalline silicon

Light- and elevated temperature-induced degradation (LeTID) in multicrystalline silicon can reduce the efficiency of solar cells significantly. We analyse the influence of the firing temperature profile on the degradation behaviour of neighbouring mc-Si wafers, varying peak temperatures above 800°C (measured) as well as heating and cooling ramps. The degradation intensity is determined by the normalized defect concentrations Nt* using spatially resolved and lifetime calibrated photoluminescence images. Wafers which were fired in a standard industrial fast firing furnace with steep ramps suffer from significant LeTID whereas samples that were subjected to the same or even higher peak temperatures but with slower heating and cooling rates hardly degrade. A spatially resolved analysis of Nt* over the whole wafer area shows that at the beginning of the experiment, the degradation is restricted to low-lifetime areas around dislocation clusters. After several hours, a very strong degradation is observed also in initially good grains. The possible roles of metallic impurities and hydrogen in-diffusion are discussed.