Correlation between impurities, defects and cell performance in semicrystalline silicon [solar cells]

An in-depth analysis of Solarex controlled directional solidification (CDS) semicrystalline silicon has been performed, and correlations between the efficiency, impurities, and defects present in the material have been made. Comparisons were made between solar cell performance and variations in interstitial oxygen, substitutional carbon, grain size, etch pit density, and trap location as a function of position in the ingot. The oxygen concentration was found to decrease with increasing distance from the bottom of the ingot, while the carbon concentration as well as average grain size was found to increase. The best cell performance was obtained on wafers with minimum oxygen and maximum carbon (top). No correlation was found between etch pit density and cell performance. Deep-level transient spectrometry (DLTS) and JVT measurements revealed that samples with higher oxygen content (bottom) gave lower cell performance due to a large number of distributed states, possibly due to extended defects like oxygen precipitates. Low-oxygen samples (top) showed predominately discrete states, improved cell performance, and a doping-dependent average trap density