N. Eustathopoulos’s research while affiliated with Grenoble Alpes University and other places

Oxygen, an usual impurity of photovoltaic silicon, is known to be detrimental for the electrical performances of this material. In this investigation, solidification experiments are performed in coated silica crucibles in order to study the influence of crucible size, crucible coating, argon flow and crucible material on silicon contamination by oxygen. In order to interpret the results, an analytical, semi-quantitative model is established taking into account two oxygen flows: the contamination flow at the crucible/silicon interface and the evacuation flow at the silicon free surface.

Graphite crucibles are potentially interesting for the directional solidification processing of photovoltaic silicon, because, contrarily to standard silica crucibles, they can be used many times. In the present work, two types of graphite crucibles are studied: i) graphite directly coated with the Si3N4 powder classically used as a releasing layer for standard silica crucibles, and ii) graphite protected by a dense SiC layer before deposition of the releasing coating. In both cases, spontaneous detachment of the silicon ingot is observed to occur during cooling. Results are given and discussed concerning the impurities introduced in the silicon during the melting-solidification cycle, as well as their consequences on the photo-electrical characteristics of silicon. These results are compared with those obtained using the standard silica crucible. It is shown that re-useable SiC protected graphite crucibles lead to the same performances as standard silica. Reproducible results are found after four runs with the same crucible, and no degradation of the crucible is observed.

The crystallization of silicon for photovoltaic applications is currently performed by directional solidification in amorphous silica crucibles. In order to avoid sticking, silica crucibles are coated with a layer of silicon nitride which acts as an interface releasing agent between the silicon and the crucible. Due to silica softening and subsequent transformations during the melting-solidification cycle, Si3N4-coated silica crucibles can be used only one time. A more interesting solution would be to have a graphite crucible which could be used several times in view of its high mechanical performances at elevated temperatures. The goal of this study is to determine in which way the Si3N4 coating can also be used for graphite crucibles. The study is conducted by means of the sessile drop technique and microstructure characterizations carried out by optical microscopy. For comparison purposes, experiments are also performed for the standard configuration of Si3N4-coated silica.

Photovoltaic silicon ingots are currently grown in silica crucibles coated with a porous silicon nitride layer which acts as an interface releasing agent between the silicon and the crucible. The interactions between Si and the Si3N4 coating determine the infiltration and sticking phenomena occurring at the interface and also affect the pollution of Si by the components of the coating. In this investigation the interfacial interactions and microstructure are studied in crystallization experiments performed in crucibles involving high silicon masses (tens of kg) and long contact time between the silicon and the coated silica (tens of hours). It is shown that for long times, a dramatic change in the nature of the coating/Si interface takes place, with the formation of a self-crucible which prevents the direct contact between the silicon and the coating. The stability of the self-crucible is modeled taking into account the capillary and hydrostatic pressures. The influence of the self-crucible on different practical aspects of the photovoltaic silicon crystallization process is discussed.

Processing of photovoltaic silicon by solidification is currently carried out under argon flow in silica crucibles coated with an oxidized silicon nitride powder. A series of experiments was performed to study the reactions between coating components under argon flow by varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of gaseous reaction species from the inside of the porous coating to the flowing argon. The conclusions drawn are used to discuss different practical aspects of the photovoltaic silicon crystallization process.

Photovoltaic silicon is currently grown in silica crucibles coated with an oxidized silicon nitride powder, which acts as an interface releasing agent between the silicon and the crucible. A series of experiments was performed to study the reactions between coating components under high vacuum, varying the temperature, the holding time and the oxygen content in the coating. The results are discussed with the help of a simple analytical model taking into account the diffusive transport of reaction species from the inside of the porous coating to its surface and then their evaporation into the vapour phase.

Processing of photovoltaic quality silicon (PV Si) starting from metallurgical Si involves contact between liquid silicon and the refractory materials used as crucibles for melting and crystallisation. The interactions (i.e. wetting, infiltration and sticking) between silicon and two types of crucible used in the course of PV Si processing, namely graphite and coated silica, are described and interpreted. A model of the wetting and infiltration phenomena, coupling thermodynamics and kinetics, is used to analyse the influence of the main parameters of the material (microstructure, chemistry) and the process (temperature and atmosphere). Examples are given of the ways to use the understanding of elementary processes at silicon/crucible interfaces in order to select crucible material and determine the desired process parameters.

The wettability of ceramic and metallic solids by liquid metals is discussed with emphasis on the effect of interfacial reactions on the spreading kinetics and the final degree of wetting. Two types of reaction are considered, simple dissolution of the solid in the liquid and dissolution followed by formation of a new compound at the interface. The review includes results obtained during the last 15 years mainly by the Grenoble group.