Publications

In order to estimate the quality of the crystallisation process of multi-crystalline Silicon ingots, we have developed a new optical method for the determination of grain sizes on as-cut wafers that were taken from different height positions of the ingot. This fast method is based on image analysis and allows in a simple way to get numerical data which represent the crystallographic quality of silicon wafers. The obtained data were used to calculate a grain size distribution over the analysed wafer area as well as an average grain size by using two different algorithms. It was thus possible to represent the average grain size as a function of ingot height, which indicates ingot regions of lower crystalline quality, for example regions of equiaxed growth of very small grains (“grit”).

This technique was used for the characterisation of different multi-crystalline Silicon ingots, using Silicon feedstock of different quality (electronic, upgraded metallurgical or solar grade), and also to monitor the impact of modifications of the crystallisation process parameters on the grain sizes distribution. It was found that this method is able to give a first qualitative impression of the multi-crystalline ingots and wafers which in the end are used to produce solar cells.

Solar Grade Silicon obtained by purification of metallurgical grade Silicon becomes an important source of Silicon feedstock for the crystalline Silicon based PV industry. This paper presents a new process and furnace for the crystallization of multi-crystalline Silicon ingot, using purified metallurgical grade Silicon as feedstock. In particular, the influence of the remaining dopant concentrations, such as Boron and Phosphorus, in purified metallurgical Silicon on the electrical characteristics of the obtained ingots, wafers and solar cells are discussed.

Indications have been found that compensation of n-type and p-type dopants can lead to an improved minority carrier diffusion length, improving the overall efficiencies of solar cells: Efficiencies of 14 % have been obtained on ingots that were grown from feedstock with relatively high concentrations of Boron (2.5x1017 cm-3) and Phosphorus (3.5x1017 cm-3) respectively. Although the feedstock is n-type due to the higher concentration of Phosphorus, the ingot showed a p-type polarity with a resistivity of 0.5 Ohm-cm over 75% of its height starting from the bottom, which is due to a more effective segregation of Phosphorus.

This also leads to an accumulation of Phosphorus atoms in the top region of the ingot, which turns n-type again and a transition region of high compensation.

Keywords: Multi-Crystalline, Compensation, Solar grade Silicon

The crystallization of purified metallurgical Silicon often leads to multi crystalline ingots which present regions of strong compensation and an inversion of the polarity type. These effects result from the presence of different dopant atoms, donors and acceptors, in this type of Silicon and their different segregation behavior during the crystallization process.

The most commonly found dopant atoms in Silicon, Boron and Phosphorous, have relatively high segregation coefficients with an important difference in their absolute value. As a result, suitable resistivities in the 0.5 to 1.0 Ωcm range are obtained in an important part of the ingot, but at a relatively high compensation ratio.

This paper discusses these compensation effects, as observed on upgraded metallurgical Silicon from the PHOTOSIL project and using a new crystallization process and furnace developed by CYBERSTAR and APOLLONSOLAR.

A new, innovative crystallisation process and furnace for the growth of multi-crystalline Silicon ingots is presented. Important key features of the furnace and process include a newly designed, thermally anisotropic quartz crucible and a very uniform inductive heating and cooling system. This thermal configuration allows for a perfect lateral and vertical temperature control during crystallisation so that high temperature gradients can be obtained, which are especially beneficial for the segregation of remaining impurities in lower quality silicon.

Solar cells have been processed on wafers from multi-crystalline Silicon ingots that were  crystallized with this new process and furnace, using doped electronic grade silicon and purified metallurgical silicon.

The resulting efficiencies were 14.3% in case of the purified metallurgical Silicon and 15.0% in case of the doped electronic grade Silicon.

This paper presents an overview of significant crystallisation results obtained with purified metallurgical grade silicon in the framework of the French Photosil project.

Especially we show that in case of a high Boron concentration in the feedstock (>2.1017 cm-3), the higher the compensation level is, the higher the solar cells efficiency will be. Several ingots were crystallised with different concentrations of boron and phosphorus and the best solar cell efficiency (15.2%) was obtained with the highest compensated ingot.

Moreover we show that this performance improvement is due to an increase of carrier lifetime which largely counterbalances the decrease of carrier mobilities, likely caused by scattering effect of ionized dopants.

However, due to the different segregation coefficients of the major dopant atoms, Boron and Phosphorus, compensated multi-c Silicon ingots often show n-type regions, decreasing the overall material yield. Based on these findings, we suggest a novel concept of doping engineering, allowing a control of the compensation level through the entire ingot height, by introducing a well defined mix of dopant atoms (B, P and Ga) to the silicon before crystallisation. This can lead at the same time to a higher electrical performance and a higher material yield of the crystallised Silicon. As a further perspective the use of lower grade and less expensive Silicon with a high electrical performance and material yield can be expected.

This study is devoted to the variations of the carrier lifetime and minority carrier diffusion length with the compensation level in solar-grade crystalline silicon. Especially we show, by using the Shockley-Read-Hall statistics, that an increase in the compensation level reduces the recombination strength of doping species and of some metal impurities.

These theoretical results are confirmed by the chemical and electrical characterizations of strongly compensated multicrystalline silicon wafers and solar cells, from silicon purified by the metallurgical route.

These results are of paramount importance since an accurate control of the compensation level can lead to strong improvements in silicon solar cells efficiencies. Nevertheless, possible limits of too high compensation levels are also evoked.

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 highly purified metallurgical silicon via a modified PHOTOSIL process.

Solar cells have been processed on 12.5x12.5 cm² wafers from both ingots using industrial type standard screen printed processes at the CEA-INES. Solar cells from the PHOTOSIL ingot were fabricated with an industrial process optimized for SoG silicon directly purified from MG 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. The fabricated cells showed 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.

This article presents the status the PHOTOSIL project, which includes partners from industry, R&D institutes and equipment manufacturers. The objectives of this project are the production of solar grade (SoG) silicon at costs <15€/kg and of multi-crystalline ingots at costs <35€/kg, starting with metallurgical silicon and using a combination of innovative up-grading and purification techniques. On the basis of encouraging results on laboratory level, the PHOTOSIL consortium has obtained the funding for the construction of an industrial scale pilot line. This line will become fully operational in October 2006 and it will serve to demonstrate the industrial viability of the PHOTOSIL technology by up scaling the different laboratory scale processes to an industrial level. In a first stage, the pilot line operates with batch sizes of 60kg which will be doubled to 120 kg in a second stage, arriving at a nominal capacity of 200 tons per year.

The PHOTOSIL process includes metallurgical and plasma purification techniques, giving rise to a complete vertical integration from the metallurgical Silicon production to the fabrication of exploitable multi-crystalline Silicon ingots for the PV industry, of either p- or n-tpye. At present, a resistivity level of 0.3 – 0.5 Wcm has been reached after the combined metallurgical and plasma purification. Metal impurity concentrations have been reduced to 20 ppm for Fe, 15 ppm for Al and <2 ppm for Ti after the metallurgical purification by segregation.

Keywords: Silicon, Metallurgical Grade, Multicrystalline

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A plasma-refining technique is applied to upgraded metallurgical grade silicon (UMG) to produce solar grade silicon for multi-c silicon ingots at direct costs lower than 15€/kg. Using oxygen and hydrogen as reactive gases injected in the plasma, boron is removed from the material mainly in form of BOH and BO. The boron volatilization time has been reduced to 50 min compared to previous processes, by increasing the temperature of the silicon bath. At the same time, the Al, Ca, C, O concentrations are strongly reduced. From a first batch of purified UMG Silicon, mutli-crystalline ingots (12 kg), wafers (125x125 mm²) and solar cells have been produced for an evaluation of this intermediate material. The obtained solar cells gave efficiencies of up to 11.7 %. Process development towards an up-scaled pilot equipment is on the way to further increase the purification efficiency.

Keywords: Silicon, Metallurgical-Grade, PV Materials

ABSTRACT: Apollon Solar’s NICE (New Industrial Solar Cell Encapsulation) module technology is an innovative technology to encapsulate and electrically interconnect solar cells without neither soldering nor encapsulant. An organic edge sealing delimits the internal volume in which an underpressure is created, allowing to establish the electrical contact between solar cells and interconnectors by pressure. This technology represents a completely new approach for the solar cell encapsulation: besides industrial and cost advantages, a better long term module performance stability and an improved lifetime are expected.

The IEC certification of this technology represents a major step in its development, in order to prove the product robustness. This article present the results of the first certification process according to IEC of NICE modules manufactured with our prototype production line designed by Vincent Industrie and installed at INES. The certification is carried out by the TÜV Rheinland in Cologne, and shows a very high performance stability, with a maximal power degradation of 2 % (while IEC norms require a degradation lower than 5%).

In addition to the IEC tests, we also present results of several largely extended mechanical and ageing tests carried out at INES and TÜV Rheinland. We notably obtained remarkable results, with a power degradation after 1000 thermal cycling lower than 2%.

ABSTRACT: APOLLON SOLAR’s NICE (New Industrial Solar Cell Encapsulation) technology aims at drastically reducing the manufacturing costs of PV modules, while at the same time increasing the total module lifetime. It makes use of a sealing technology which is well established in the insulating glass industry to replace the state-of-the-art lamination technology for PV modules.

An additional feature is the soldering free electrical series connection of the solar cell busbars with the metal interconnectors, thanks to an under-pressure inside the module. Thanks to these features, the NICE process is completely inline and easy to automate.

Modules with 36 silicon solar cells have been produced with the NICE technology and evaluated, including by tests according to the IEC 61215 standard. Although the power degradation of the tested modules remained largely in the acceptable range, the evaluation revealed two areas onto which additional work was necessary to increase the overall performance and reliability of the NICE modules: (i) mechanical aspects concerning the stability of cells and migration of cells and interconnectors during thermo-cycling tests, (ii) performance losses due to a lack of optical continuity between front glass and solar cell. This work reports on solutions to overcome both performance limiting factors.

Keywords: Module Manufacturing, Cost Reduction, Encapsulation

APOLLONSOLAR’s NICE (New Industrial Solar Cell Encapsulation) technology makes use of an air and humidity tight sealing technology known from the insulating glass industry and based on the application of an organic sealing material from the family of poly-isobutylene (PIB).

Important features of the NICE technology are the absence of EVA lamination and the use of an underpressure inside the module to provide the electrical series connexion between solar cell contact lines and metal interconnectors, thus suppressing the soldering of metal interconnectors to the solar cell busbars, which is the most widely used technology today. Other important features include a specially developed metal foil that is used as for the metal rear surface and new external connector which is integrated in the edge-sealing of the module. From a production point of view, the NICE module technology presents a largely simplify module assembling technology since it allows for a complete inline operation which can be fully automated.

This paper presents first results obtained with the new NICE pilot production line, realised by VINCENT INDUSTRIES. The pilot line incorporates all necessary automated production stations without the automatic loading and sorting.

Keywords: Modules, Encapsulation, Sealing quality

ABSTRACT: APOLLON SOLAR’s NICE (New Industrial Solar Cell Encapsulation) technology aims at drastically reducing the manufacturing costs of PV modules, while at the same time increasing the total module lifetime. It makes use of a sealing technology which is well established in the insulating glass industry to replace the state-of-the-art lamination technology for PV modules.

An additional feature is the soldering free electrical series connection of the solar cell busbars with the metal interconnectors, thanks to an under-pressure inside the module. Thanks to these features, the NICE process is completely inline and easy to automate.

Modules with 36 silicon solar cells have been produced with the NICE technology and evaluated, including by tests according to the IEC 61215 standard. Although the power degradation of the tested modules remained largely in the acceptable range, the evaluation revealed two areas onto which additional work was necessary to increase the overall performance and reliability of the NICE modules: (i) mechanical aspects concerning the stability of cells and migration of cells and interconnectors during thermo-cycling tests, (ii) performance losses due to a lack of optical continuity between front glass and solar cell. This work reports on solutions to overcome both performance limiting factors.

Keywords: Module Manufacturing, Cost Reduction, Encapsulation

The direct production costs of state-of-the-art PV modules, produced with solar cells fabricated on wafers, represent between 30% and 40% of the total costs per Watt peak of the entire PV production chain, including ingot production, wafer cutting, cell production and module assembly. The NICE technology aims at reducing these costs by more than 50%.

It combine an air- and humidity tight sealing technique, which is already known and proven from the insulating glass industry, with a solar cell interconnection that makes use of an underpressure between the front and back sheet which delimitate the module. The underpressure inside the module assures low resistivity contacts between cells and metal interconnectors. The entire NICE process takes place at room temperature avoiding all heat related risks of cell degradation. Compared to the state-of-the-art technology the solder connection between the cells and metal connectors is completely avoided, as well as the batch type lamination process, largely facilitating the automation of the NICE module fabrication process.

A number of test modules with 36, 16 and 6 125x125 mm² silicon solar cells have been produced with the NICE technology and tested according to the IEC 61215 standard, resulting in no degradation of the module power.

Keywords: Module Manufacturing, Cost Reduction, Encapsulation