Identification of defects causing performance degradation of high temperature n-type Czochralski silicon bifacial solar cells

Abstract

Four industrial-scale n-type Czochralski silicon crystals were grown with different impurity contents, i.e. metallics, phosphorus and oxygen. Horizontal slices were obtained from the top and middle of the crystals and were characterized in terms of lifetime and both defect and impurity distribution. The slabs characterization was performed both under as-grown conditions and after 2-step oxidation. Solar cells were fabricated from neighboring wafers with n-PASHA process and the overall efficiency determined.The crystal grown under low oxygen incorporation conditions has shown higher average as-grown lifetimes compared to the other crystals. The addition of lower phosphorus concentration to the melt resulted in an improvement of the lifetime of the crystal while the use of low grade feedstock has mainly affected the quality at the middle of the corresponding crystal. However, the major difference between top and middle heights was still the interstitial oxygen concentration for all crystals under investigation. The distribution of the interstitial oxygen concentration has not shown significant fluctuations through the samples radial direction. In addition, significant void fluctuations were found through the slabs radial direction, and demonstrate possible variations in the vacancy incorporation at the solidification interface during crystal growth. After heat treatment, large fluctuations of the lifetime were found through the samples radial direction, and are mainly associated to the local fluctuations of the oxygen precipitate density. However, these variations were suppressed in the crystal of lower oxygen incorporation. Solar cells performance degradation was mainly correlated with oxygen precipitation during their fabrication. It was also demonstrated that their efficiency is largely affected by the formation of detrimental striations of high periodicity. Their formation was thus associated to fluctuations of the vacancy concentration due to transient growth and their interaction with oxygen during crystal cooling. These oxygen nuclei may grow during solar cells processing and become even more detrimental to their performance.