The influencing factors and formation mechanism of the dark ring of monocrystal silicon cells

The formation mechanism of p-type monocrystalline silicon solar cells´ dark rings in the electroluminescence images was researched by Fourier transform infrared spectroscopy, minority carrier lifetime mapping, photoluminescence mapping. It is shown that the minority carrier lifetime sharply decreases during the oversaturated interstitial oxygen precipitation. Because the center oxygen concentration and vacancy density are higher than the edge, the center is easier to form the oxygen precipitates. Therefore, the lifetime drops more at the center than the edge which results in the dark rings and solar cell efficiency losses. Except for the oxygen and carbon concentration of the wafer, the dark rings are also related to the thermal process in the cell processing. Furthermore, when the oxygen and carbon concentration are lower than a certain value, the dark rings would be disappeared.