This paper aims at evaluating the impact of boron concentration in PHOTOSIL Si feedstock on the performance of pand n-type Cz mono-crystalline solar cells. By comparing solar cells made with 2 different batches of PHOTOSIL Si with different B concentration we demonstrate that reducing it from 0.3 ppmw to 0.12 ppmw leads to a small improvement of efficiency and light-induced degradation in p-type Si solar cells and to a strong improvement of efficiency in n-type bifacial solar cells. Average efficiencies of up to 18.1% in p-type solar cells and 18.5% in n-type solar cells made with 100% PHOTOSIL Si are obtained, demonstrating the very high quality of this material. Surprisingly weak light-induced degradation is measured to happen, particularly in n-type Si solar cells. The potential reasons for such a low light-induced degradation are briefly discussed.

In order to have a market perspective as UMG Silicon for the PV industry, PHOTOSIL Silicon needs to meet quality criteria in terms of PV cell and module performance as well as production cost objectives. To address these points a 2 generation industrial refining equipment has been made operational, with a total annual capacity of 100 MT, allowing for an industrial evaluation and qualification of PHOTOSIL Silicon. Results of two external evaluation tests of PHOTOSIL feedstock are presented in this article: (i) Using an industrial pilot line from Multi-c Silicon ingots to solar cells and (ii) by an industrial PV producer using state of the art multi-c ingot, wafering and cell processing equipment. In all cases 100% PHOTOSIL Silicon was used as feedstock for the ingot crystallization. The average cells efficiency obtained by these two routes is superior to 16% and the homogeneous material quality both on ingot and wafers is comparable to using Polysilicon as feedstock. The feedback of the industrial customer was positive, and 5 tons of PHOTOSIL Silicon has been ordered. All critical aspects regarding UMG Silicon have been carefully investigated and taken under control, like risk of SiC, SiN inclusions, resistivity control, etc. For a final validation of the quality of PHOTOSIL Silicon, modules have been made from multi-c Si PHOTOSIL Silicon and a total capacity of 100kWp has been installed in southern France in 2012 for monitoring and power rating. The energy production has been compared to standard modules with the same efficiency; the difference of energy production over a whole year is below 2%. Under high irradiation, the performance of both module types is similar, but under low irradiation the performance of the PHOTOSIL modules is 5% lower. No specific degradation has been observed, which confirms the potential for an industrial PHOTOSIL silicon production.

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 c 1 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 mm 2 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.

Highly purified n-type UMG (“Upgraded Metallurgical”) Silicon shows 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-crystalline and has a resistivity 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 125x125 mm pseudo square wafers of 180 micron thickness. A first series of solar cells were processed on these wafers using an industrial hetero-junction process of Roth & Rau. The best solar cell had an energy conversion efficiency of 19.0% (average: 18.6%) under standard testing conditions with a very high Voc of 725mV. According to the knowledge of authors this is the highest efficiency ever reported on industrial type solar cells fabricated on 100% UMG Silicon.

This article gives an up-date on the progress of the French PHOTOSIL project from a technical and an economical point of view. During the last 5 years, the French PHOTOSIL consortium formed by FerroPEM, CEAINES, CNRS SiMAP and APOLLON SOLAR has been working on a combination of new, innovative up-grading and purification techniques for MG Silicon on an industrial pilot level, to arrive at UMG Silicon that is compatible with the purity and economical requirements of the PV industry. The objectives of this project are production costs 15% solar cell efficiencies on multi-crystalline wafers from ingots made of 100% UMG Silicon and a material yield of >85% after crystallisation.

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 cm2 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 cm2 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.