19% efficiency heterojunction solar cells on Cz wafers from non-blended Upgraded Metallurgical Silicon

Abstract

Highly purified n-type UMG (“Upgraded Metallurgical”) Silicon is a material with a strong potential for high efficiency low cost solar cells. Compared to p-type Silicon, n-type Silicon is in general less susceptible to lifetime degradation due to residual metal impurities or to light induced degradation due to the Boron-Oxygen complex. In this work a 15kg 6 inch mono-c Cz Silicon ingot has been grown from 100% highly purified UMG Silicon obtained with the PHOTOSIL process. In this feedstock the Boron and Phosphorus concentrations measured by GDMS were found to be 0.3 ppmw and 2 ppmw, respectively. The resulting ingot is n-type, fully mono c1 rystalline and has a resistivity range from 0.2 to 1 ohm.cm. Other impurities, especially metals, were not detectable with the analysis techniques applied (GDMS, ICP-OES). The ingot was cut into 125×125 mm2 pseudo square wafers of 180 micron thickness. A first series of solar cells were processed on these wafers using an industrial hetero-junction process by Roth & Rau. The best solar cell from a batch of 14 had an energy conversion efficiency of 19.0% (compared to an average: 18.6%) under standard testing conditions with a very high Voc of 725mV.. An independent confirmation of these results is pending.