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.

Besides the well-known local sub-grain boundaries (SGBs) defects, monolike Si ingots grown by Directional Solidification present distributed background cellular dislocation structures. In the present work, the influence of stress level, time under stress, and doping by O and Ge, on the formation of dislocation cells in monolike silicon, is analysed. This is achieved by performing a comparative study of the dislocation structures respectively obtained during crystallisation of pilot scale monolike ingots on Czochralski (CZ) and monolike seeds, during annealing of Float Zone (FZ), CZ, and 1 x 10(20) at/cm(3) Ge-doped CZ (GCZ) samples, and during 4-point bending of FZ and GCZ samples at 1300 degrees C under resolved stresses of 0.3, 0.7 and 1.9 MPa during 1-20 h. Synchrotron X-ray White-beam Topography and Rocking Curve Imaging (RCI) are applied to visualize the dislocation arrangements and to quantify the spatial distribution of the associated lattice distortions. Annealed samples and samples bent under 0.3 MPa present dislocation structures corresponding to transient creep stages where dislocations generated from surface defects are propagating and multiplying in the bulk. The addition of the hardening element Ge is found to block the propagation of dislocations from these surface sources during the annealing test, and to retard dislocation multiplication during bending under 0.3 MPa. On the opposite, cellular structures corresponding to the final stationary creep stage are obtained both in the non-molten seeds and grown part of monolike ingots and in samples bent under 0.7 and 1.9 MPa. A comparative discussion is made of the dynamics of formation of these final dislocation structures during deformation at high temperature and monolike growth. (C) 2018 Elsevier B.V. All rights reserved.

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.

Monolike silicon wafers can achieve solar cells efficiencies close to those of CZ silicon. However, this performance is affected by the presence of 2D structural defects, especially sub-grain boundaries zones that expand in the upper part of the ingots. In the present work, the relations between the structure of different types of 2D defects, previously characterized by EBSD and synchrotron based X-ray topography, and their electrical activities are analyzed. The defects are generated independently by misorienting the seeds by various tilt angles Δθ relative to the growth direction <100>. LBIC and PL imaging are used to quantify the surface recombination velocity (SRV) of isolated defects at different heights of the ingot. The relative effects of the differences in structure of these defects, position in ingot, and cell processing treatment are exemplified and 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.

The growth structure of photovoltaic multicrystalline silicon formed by directional solidification presents a high fraction of Σ3 and higher order twins. Previous studies proposed that these complex structures are formed by a succession of 2D nucleation events of Σ3 twins on {111} growth facets at the triple line formed by their intersection with the crucible wall, another crystal, or the surface. In this work, we report the reproducible formation of multiple twinned domains inside solar grade Si single crystals grown by directional solidification above a (111) seed. These domains start on a Σ3 twin nucleated inside the crystal bulk, and systematically develop into similar twinned structures characterized by a ternary arrangement of grains in Σ3, Σ9, and Σ27 relationship. The mechanism of formation of the initial twin nucleus is discussed, and a scenario is proposed for the processes of subsequent multiple twinning. The growth competition between twin grains is shown to promote the appearance of incoherent twin boundaries, and dislocations near grain boundaries and in the twin grains themselves. The electrical activity of Σ-boundaries is measured, and the correlation between the structure of the defects and the resulting detrimental electrical activity is then discussed.

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.

Rocking curve imaging (projection and section X-ray topography) has been used to study the generation and propagation of defects at the junctions between and above the seed crystals in mono-like silicon ingots. The images of different kinds of defects such as precipitates, dislocations and twins in the integrated intensity, full width at half-maximum and peak position maps resulting from the experiment have been studied. The qualitative and quantitative information that can be extracted from these maps, in particular the contrast of the images of the various defects, is discussed. These defects have a detrimental effect on solar cell efficiency and their detailed investigation allows clues to be obtained in order to improve the growth process. This work shows that synchrotron X-ray diffraction imaging techniques, because of their high angular resolution (<10−4°) and large field of view (several mm2), constitute a powerful tool for investigating the initial stages of growth of directionally solidified mono-like silicon.

The present work studies the generation and propagation of sub-grain boundaries and dislocations in mono-like silicon ingots grown on monocrystalline seeds with a very small relative misorientation between them (<0.06° around the x, y and z axes). Special emphasis is put on the region close to the area between the seeds at the bottom of the crucible, which appears to be crucial in determining the crystalline quality of the final ingot. For this investigation, X-ray rocking curve imaging (RCI) in transmission geometry, a directly quantitative version of monochromatic beam Bragg diffraction imaging (“topography”) has been used. This technique has been developed at the European Synchrotron Radiation Facility (ESRF), beamline BM05 and allows us to visualize the spatial distribution of the lattice distortion of a single crystal. It was found that the solidified ingot takes the crystallographic orientation of the seeds without creating any distorted area at the interface. However, dislocation bunches having a strong screw component are generated between the seeds and propagate along the growth direction. Sub-grain boundaries above the top of the seeds, mainly composed of edge dislocations, were also observed. These have a detrimental influence on the minority carrier lifetime of the ingot, since, as is known, sub-grain boundaries show active recombination. In this work taking advantage of the high angular resolution and the sensitivity of the technique it was shown that a relative misorientation between the seeds (δθx, δθy, δθz) as small as 0.02° can produce cascades of dislocations that propagate and multiply higher up in the ingot becoming electrically active.

Photovoltaic silicon ingots are currently grown in SiO2 crucibles coated with a porous silicon nitride layer which acts as an interface-releasing agent between silicon and the crucible. The present investigation focuses on the initial stages of Si–crucible interactions, involving infiltration of the porous coating by molten silicon. In this study, performed using the sessile drop technique in flowing argon, three types of infiltration are considered: infiltration under the drop, which is representative of infiltration occurring at the crucible/bulk silicon interface; infiltration at the coating surface in front of the nominal triple line, leading to the formation of a silicon-rich film; and infiltration under this film. The experimental results, obtained by varying the infiltration duration, the temperature of the coating heat treatment prior to the infiltration experiment and the argon flow, are interpreted with the help of an analytical model, taking into account diffusive transport in the infiltrated coating, in the bulk liquid and in the flowing gas.

The generation of structural defects in directionally solidified mono-like silicon on a pavement of seeds has been investigated by synchrotron X-ray imaging, micro-FTIR mapping and electronic techniques. In particular, we analyse the region where the liquid Si penetrates between two seeds and we correlate the segregation and precipitation of impurities with the generation of cascades of dislocations during crystal growth. The solidified silicon grows epitaxially on the seeds without creating any distortion at the interface; however, due to the relative misorientation between the two seeds a highly and inhomogeneously distorted sub-grain boundary is created. Locally distorted zones, in particular linked to precipitates, are detected along and near the sub-grain boundary. The precipitates mainly consist of Si, C, N and O. Dislocations generated in these distorted zones propagate away from the sub-grain boundary towards the un-melted portions of the seeds, but they are blocked by barriers of precipitates formed at the positions of the initial seed surfaces. Higher in the ingot, bunches of dislocations propagate and multiply in the bulk.

Photovoltaic (PV) cell performance is dictated by the material of the cell, its quality and purity, the type, quantity, size and distribution of defects, as well as surface treatments, deposited layers and contacts. A synchrotron offers unique opportunities for a variety of complementary X-ray techniques, given the brilliance, spectrum, energy tunability and potential for (sub-) micron-sized beams. Material properties are revealed within in the bulk and at surfaces and interfaces. X-ray Diffraction Imaging (X-ray Topography), Rocking Curve Imaging and Section Topography reveal defects such as dislocations, inclusions, misorientations and strain in the bulk and at surfaces. Simultaneous measurement of micro-X-Ray Fluorescence (μ-XRF) and micro-X-ray Beam Induced Current (μ-XBIC) gives direct correlation between impurities and PV performance. Together with techniques such as microscopy and Light Beam Induced Current (LBIC) measurements, the correlation between structural properties and photovoltaic performance can be deduced, as well as the relative influence of parameters such as defect type, size, spatial distribution and density (e.g [1]). Measurements may be applied at different stages of solar cell processing in order to follow the evolution of the material and its properties through the manufacturing process. Various grades of silicon are under study, including electronic and metallurgical grades in mono-crystalline, multi-crystalline and mono-like forms. This paper aims to introduce synchrotron imaging to non-specialists, giving example results on selected solar photovoltaic silicon samples.

ABSTRACTWork on silicon crystal quality improvement and defect control has been carried out on lab‐scale seeded growth ingots allowing wafers with controlled grain orientations. Both <111> and <100> monocrystalline‐like ingots were produced using a combination of quartz rod dipping and a modulated conductive heat extraction system, made in‐house, in a directional solidification system. Two mono‐like wafer morphology types have been produced. Their structural and electrical properties are presented in detail. Copyright © 2011 John Wiley & Sons, Ltd.