High performance solar cells made from 100% UMG silicon obtained via the PHOTOSIL process

Abstract

The presented work is part of the French PHOTOSIL project which deals with the purification of metallurgical grade (MG) silicon to obtain Solar Grade (SoG) silicon by a combination of innovative refinement/up-grading techniques such as segregation and plasma purification. The main objectives of this project are production costs <;15€/kg, a photovoltaic performance of >15% solar cell efficiencies, and material yields >85% after crystallization. In this paper we present the latest results obtained with a intensely purified metallurgical silicon via a modified PHOTOSIL process. Chemical analysis by Glow Discharge Mass Spectroscopy (GDMS) on this purified Silicon revealed a boron concentration below 0.5ppmw after the plasma treatment and a phosphorus concentration close to 1ppmw after the metallurgical purification by segregation. The total amount of metallic impurity concentrations has been reduced below 2ppmw (Fe, Al, etc.,) thanks to the successive segregation steps. A multicrystalline silicon ingot from 100% of this purified metallurgical silicon was crystallized using an innovative crystallization method. It was p-type on 80% of its height and exhibited a resistivity range between 1-10 ohm.cm, due to the segregation of the remaining phosphorus. For reference purposes, a second ingot was crystallized in identical conditions using intrinsic EG silicon, that was intentionally boron-doped to 1 ohm.cm. Solar cells have been processed on 12.5 × 12.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> wafers from both ingots using industrial type standard screen printed processes at the CEA-INES. The solar cell process that has been applied to the PHOTOSIL wafers has been specially optimized for purified metallurgical grade Silicon. In case of the EG ingot the average efficiency was 16.3% with a maximum of 17%. In case of the ingot from PHOTOSIL silicon, solar cells from the p-type region have reached an average efficiency of 15.7 % including a best cell with 16.2%. In addition, a 6" Cz ingot was crystallized from the same purified silicon feedstock. This ingot turned out to be entirely mono-crystalline which confirms the very low impurity content of the Silicon after purification. Solar cells were fabricated on 12.5 × 12.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pseudo-square wafers and a high average efficiency of 17,4% was reached with a maximum efficiency of 17,6%, which is one of the highest efficiency reported so far if not the highest on purified metallurgical silicon. These results clearly demonstrate the potential of the metallurgical silicon route for application in PV and the possibility to reach high efficiencies.