Electrical and photovoltaic properties through a large multicrystalline Si ingot

Abstract

Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate the feedstock shortage. The bulk lifetime τn and diffusion length Ln of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with the crucible. It was found that τn and Ln are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central part of the ingot Ln is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations along the ingot height of the conversion efficiency η and of τn in raw wafers are well correlated, that can predict the values of η, allowing an in-line sorting of the wafers, before solar cells are made. If τn is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τn is higher than 2.5 μs η achieve 15 % at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely or partly p or n type, suggesting that a purification step tawards the dopants is required.