Effect of Ingot Variability on Performance of Silicon Heterojunction Solar Cells

Silicon ingots grown by the Czochralski method have natural distributions in their bulk material specifications such as bulk resistivity and metal impurities. Since high efficiency solar cell technologies, like silicon heterojunction cells, have superior surface passivation, bulk material is the dominant factor in cell performance. Device performance can be impacted by variations in bulk along the growth-axis of the ingot. In this paper, we sample wafers from every part of the ingot and measure bulk resistivity, before and after thermal annealing. We then passivate those samples to determine bulk lifetimes. We also prepare heterojunction structures and measure the minority carrier lifetime and pseudo efficiency of the wafers. It is seen that the wafers at the top of the ingot have higher values of resistivity after annealing which indicates the presence of thermal donors. This behavior is consistent with wafers of the ingot from another manufacturer. The lifetimes of wafers decrease as we move from top to bottom of the ingot pointing to the presence of metal impurities in the tail end of the ingot. However, these variations only lead to approximately 1% variation in cell efficiency, with possibly lower effect of bulk variation of ingot in thinner cells.