Impact of Iron Contamination on CZ-Silicon Solar Cells

We have systematically investigated the influence of iron on the performance of industrial Czochralski monocrystalline silicon solar cells. The study focused on the behavior of interstitial iron as a recombination center at three stages of the industrial solar cell fabrication sequence: (i) on as-cut wafer level, (ii) after phosphorous diffusion and (iii) after completed solar cell process. The investigated wafer groups were carefully chosen to represent different ingots with various electronic properties (lifetime and purity levels). The electronic properties of the wafers were investigated by minority carrier lifetime measurements in order to determine the effective lifetime and the interstitial iron content. In one of the ingots, neutron activation analysis was used to determine the total impurity concentration of the silicon raw material, revealing many different contaminants. In this ingot, iron was found to have the highest impurity concentration. Combining the results of both methods, we found that 0.2 % of the total iron atoms present in the sample are in interstitial form. Complete solar cells were made from all wafer groups and analyzed by means of current-voltage characteristics. The surprising finding is that, in spite of high iron contamination, very high solar cell conversion efficiencies could be achieved. Considerable amounts of interstitial iron were detected not only in as cut wafers but also after phosphorous diffusion and even in the readily processed solar cells, revealing the gettering steps to be not as efficient as expected.