Investigation of ppb-level surface contamination of n-type silicon solar cells

Abstract Contamination of silicon semiconductor cells strongly influences their working properties, such as minority carrier lifetime. During manufacture, i.e. cutting off the silicon ingot, the entry of metal contaminants into the silicon needs to be avoided since it decreases the performance of solar cells. To study this effect, controlled contaminations were performed by immersing silicon wafers in dilute HCl-based (pH = 1.3) solutions, containing precisely defined amounts of metal concentrations of 30, 100, 300, and 1000 ppb. The two most common and most damaging transient metals, the iron and copper, were tested. Determination of surface contaminations was carried out by two different methods: Total Reflection X-ray Fluorescence (TXRF) measurements and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The minority carrier lifetime was checked by photoluminescence (PL) measurements. The LA-ICP-MS method was proved to be an effective arrangement for direct determination of ppb-level surface impurities and their penetration depths. The results received by TXRF and LA-ICP-MS were compared. The advantage of LA-ICP-MS was that in addition to a simple contamination level determination it could be used to acquire the total surface map of contamination.