Effects of metal impurities at the edges of cast Si ingot on crystal quality and solar cell performance

Abstract

In this study, the effects of metal impurity diffusion at the edges of cast crystalline Si on crystal quality and solar cell performance were investigated and analyzed. The metal impurity diffusion resulted in a decrease of the minority carrier lifetime within Si crystal, low luminous intensity in photoluminescence (PL) images for wafers, high density leakage points in electroluminescent (EL) images and the decrease of photoelectric conversion efficiency(η) for solar cells. The study reveals that the region of low minority carrier lifetime (i.e., red-zone) in Si wafers can be narrowed down by using annealing treatment, in a temperature range of 500–600 °C. However, the annealing treatment on Si wafers had little effect on the efficiency of solar cells. A novel slicing direction for Si wafers, along to the crystal orientation (ACO), allowed the fabrication of Si wafers with non-red zone or with complete red-zone from the same side Si block. The η of the solar cells based on Si wafers with non-red zone was 1.7 % higher than that of the solar cells based on Si wafers with complete red zone. In addition to that, while being sliced from the same side Si block, the mean η of cells, based on wafers from the ACO slicing method was calculated to be 0.11 % higher than that of cells, which based on wafers from the traditional perpendicular crystal orientation (PCO) slicing method.