Development and improvement of methods for reducing contamination of silicon-kerf from wafer slicing

During wafer slicing, it is shown that carbon contamination can be limited by using organic additive in low concentration. In this way, a reduction of a factor two or three of carbon level in silicon-kerf could be obtained by changing the coolant added to water, with a concentration ten times lower than usual. A second proposed evolution concerns the beam composition, which is usually filled with aluminum-based compounds. New polyester resin beams filled with silicon powder were successfully tested: they did not affect the cutting performance and allows to decrease aluminum concentration in silicon-kerf to a few tens of ppm instead of hundreds. With this evolution of the beams, the main residual contaminant becomes nickel. It was shown that chemical treatment reduced metals in a ratio of three and that after an additional thermal treatment, the carbon level decreased by a factor of six, to reach about zero point two percent. As a conclusion, thanks to cutting liquid and beam composition change, a three-N purity of raw silicon-kerf has been reached at the exit of wafer slicing, without modifying the cutting process. Moreover, additional soft chemical treatment, followed by thermal treatment can reduce carbon concentration and increase silicon-kerf purity to four-N. An improvement of a factor one hundred compared to classical industrial silicon-kerf

Development and improvement of methods for reducing contamination of silicon-kerf from wafer slicing

International audienceThe work, carried out within the framework of the European project ICARUS, concerns the quality improvement of silicon powder resulting from wafer slicing, in photovoltaic industry. Wafering commonly uses diamond-wire sawing and causes the loss of about 35% of silicon which could be valued as raw material. This silicon kerf is contaminated by light elements , in particular oxygen and carbon, and metals in the amounts of several percent for the firsts and hundreds or even thousands of ppm for the seconds, with preponderant elements such as aluminium and nickel. They come from cutting liquid, wire, brick holder and spontaneous oxidation of silicon in water. Therefore, the purity of raw silicon-kerf from wafering is 2N (99%) at best.The aim of the present study is to reach at least 4N (99.99%) silicon-kerf purity with an approach combining a slight adaptation of the cutting process, with an emphasis on industrially acceptable solutions, and light chemical and thermal treatments. Firstly, it is shown that the use of a coolant with low Carbon Oxygen Demand (COD) allows reducing carbon content by half, compared with usual liquid, to approach 1% C by weight. Secondly, alternative beams made of polyester resin filled with silicon powder allow to drastically decrease aluminium content. Then, a hydrochloric acid treatment, followed with a heating step at 500°C under argon flow, allows dividing carbon concentration by six and that of metals by almost three. The final silicon purity effectively reaches 4N, excluding oxygen and carbon. This quality could be further improved by segregation processes to get solar grade silicon

Development and improvement of methods for reducing contamination of silicon-kerf from wafer slicing

International audienceThe work, carried out within the framework of the European project ICARUS, concerns the quality improvement of silicon powder resulting from wafer slicing, in photovoltaic industry. Wafering commonly uses diamond-wire sawing and causes the loss of about 35% of silicon which could be valued as raw material. This silicon kerf is contaminated by light elements , in particular oxygen and carbon, and metals in the amounts of several percent for the firsts and hundreds or even thousands of ppm for the seconds, with preponderant elements such as aluminium and nickel. They come from cutting liquid, wire, brick holder and spontaneous oxidation of silicon in water. Therefore, the purity of raw silicon-kerf from wafering is 2N (99%) at best.The aim of the present study is to reach at least 4N (99.99%) silicon-kerf purity with an approach combining a slight adaptation of the cutting process, with an emphasis on industrially acceptable solutions, and light chemical and thermal treatments. Firstly, it is shown that the use of a coolant with low Carbon Oxygen Demand (COD) allows reducing carbon content by half, compared with usual liquid, to approach 1% C by weight. Secondly, alternative beams made of polyester resin filled with silicon powder allow to drastically decrease aluminium content. Then, a hydrochloric acid treatment, followed with a heating step at 500°C under argon flow, allows dividing carbon concentration by six and that of metals by almost three. The final silicon purity effectively reaches 4N, excluding oxygen and carbon. This quality could be further improved by segregation processes to get solar grade silicon

Standardized cross-linking determination methods applied to POE encapsulants in lamination recipe emphasizing

International audienceEthylene vinyl acetate is the most common encapsulation material in photovoltaic panels. Due to gradual engineering, it ensures to meet performance requirement of standard cells, low-cost and well understood cross-linking behaviour, both physically and chemically. Nowadays polyolefin elastomers (POE) have been entering the PV industry requirements by advanced cells concepts and/or novel degradation phenomena noticed on bifacial modules. POE exhibit several advantages based on its intrinsic high volume resistivity, low permeation, processability and most importantly, the absence of harmful by-products (such as acetic acid) generated upon humidity exposure[1, 2]. However, this new family of materials may behave differently from EVA during crosslinking, thus it is necessary to verify and adapt standard measurement methods. Therefore, the main objective of this study is to investigate the cross-linking behaviour of POEs with the final goal of exploring the process window of the lamination. The characterization methods like differential scanning calorimetry (DSC) and Soxhlet extraction have been used to determine crosslinking rate and chemical structure of several encapsulants. Similar to EVAs, cross-linking rate of POEs measured by Soxhlet extraction increases with lamination duration until reaching a plateau. The indirect cross-linking rate measurement by DSC analysis is usually favoured through its simple, fast implementation, absence of toxic chemicals when compared to Soxhlet extraction. Remarkable correlations between the two techniques were obtained for a commercially available POE, allowing the extension of the IEC standard to new encapsulants. Nevertheless, in the case of highly engineered materials, clear deviations are recorded, highlighting validity limits of direct correlation between Soxhlet and DSC methods

Standardized cross-linking determination methods applied to POE encapsulants in lamination recipe emphasizing

International audienceEthylene vinyl acetate is the most common encapsulation material in photovoltaic panels. Due to gradual engineering, it ensures to meet performance requirement of standard cells, low-cost and well understood cross-linking behaviour, both physically and chemically. Nowadays polyolefin elastomers (POE) have been entering the PV industry requirements by advanced cells concepts and/or novel degradation phenomena noticed on bifacial modules. POE exhibit several advantages based on its intrinsic high volume resistivity, low permeation, processability and most importantly, the absence of harmful by-products (such as acetic acid) generated upon humidity exposure[1, 2]. However, this new family of materials may behave differently from EVA during crosslinking, thus it is necessary to verify and adapt standard measurement methods. Therefore, the main objective of this study is to investigate the cross-linking behaviour of POEs with the final goal of exploring the process window of the lamination. The characterization methods like differential scanning calorimetry (DSC) and Soxhlet extraction have been used to determine crosslinking rate and chemical structure of several encapsulants. Similar to EVAs, cross-linking rate of POEs measured by Soxhlet extraction increases with lamination duration until reaching a plateau. The indirect cross-linking rate measurement by DSC analysis is usually favoured through its simple, fast implementation, absence of toxic chemicals when compared to Soxhlet extraction. Remarkable correlations between the two techniques were obtained for a commercially available POE, allowing the extension of the IEC standard to new encapsulants. Nevertheless, in the case of highly engineered materials, clear deviations are recorded, highlighting validity limits of direct correlation between Soxhlet and DSC methods