Methodology for Measuring Trace Metal Surface Contamination on Pv Silicon Substrates

Abstract

The production of high efficiency PV cells requires a strict control on metal contamination levels. Metals like Cu and Fe are well known to cause lifetime degradation in silicon. Analytical methodologies for determination of surface metal contamination have been well established for mirror polished, also referred to as chemical mechanical polished (CMP) silicon substrates [1]. It will be explained why these methods cannot be merely applied to photovoltaic (PV) wafers because of the large roughness and specific topography of the surface. Here we extend and adapt these analytical methodologies for surface metal contamination analysis on PV substrates also referred to as PV wafers. We demonstrate how vapour phase decomposition droplet collection (VPD-DC) in combination with total reflection X-ray fluorescence spectroscopy (TXRF) or inductively coupled plasma mass spectrometry (ICP-MS) has been modified to accommodate analysis on a non-polished PV silicon wafer surface. In addition we demonstrate how metal contamination on as-cut PV wafers has been determined by liquid surface etching (LSE), i.e. an oxidizing and slow etching solution used to dissolve metal contaminants, followed by TXRF analysis. Measurements of as-cut wafers shows the presence of Fe, Cu and Zn in very high concentrations (up to more than 1x1013at/cm2), residuals of the brass-coated steel wires used during wire sawing [10].