In this study we try to indentify relation between carrier lifetime, resistivity and two mains impurities concentration in a p-type upgrade metallurgical multicrystalline (UMG) silicon ingot.Thanks to this relation, we could prevent the Light Induced Degradation (LID) phenomenon and the SiC particles formation which are, respectively, at the origin of Voc losses and shunts in solar cells.So these 2 parameters are important for photovoltaic panels' efficiency.Lifetime measurements are achieved by means of the Microwave Photoconductivity Decay "μw-PCD" technique, and concentration measurements are determined by FTIR.We demonstrate that resistivity variations depend on oxygen's concentration but carbon analyses must be continued.

For all fabrication processes of the photovoltaic (PV) industry based on silicon, grain boundaries, dislocations, and impurity contamination control during solidification remains a major challenge to improve the electrical properties. In particular, carbon (C) is a major deleterious impurity for solar cells. The combination of X-ray radiography and Bragg diffraction imaging (topography) achieved in situ during silicon solidification allowed us to characterise the dynamics of the growth mechanisms involved in the formation of the grain structure and of defects, related to the presence of C. Ex situ techniques were used to characterise the grain structure and for a more precise analysis of the defects and their associated distortion fields. In the presence of C, it is shown that the resulting grain structure is constituted by a higher proportion of high-order and incoherent twin boundaries compared with the case of pure samples. Crystal distortion is characterised at the grain scale level and at a lower scale, both in situ and after cooling-down. The highest distortion at the grain scale corresponds to the position of the high order twin boundaries and is accentuated during cooling-down following solidification. Locally distorted regions and sub-grains are distributed all over the samples. They are observed in situ during the solidification from various seeds containing C (mono-crystals, industrial ribbons and multi-crystalline samples) and are retrieved after solidification. A model implying the presence of SiC precipitates at the solid-liquid interface is proposed to explain the formation during solidification of these sub-grains and of the associated local distortion.

To study dislocation dynamics in a model sample, an intrinsic float zone (FZ) Si wafer is chosen as seed to initiate directional solidification. During the temperature ramp, a Von Laue diffraction spot is recorded by a camera. It provides time‐resolved information on the evolution of silicon crystalline quality and on the defect nucleation locations and dynamics. With increasing temperature, dislocations are observed to propagate through the seed starting mainly from the sample edges. The effective thermomechanical local stress is estimated as low as (1.2 ± 0.4) × 10 5 Pa in a thermal gradient of (1.8 ± 0.2) × 10 2 K m −1 . The latter is sufficient to allow dislocation nucleation and motion at temperatures beyond (1523 ± 9) K. Dislocation velocity increases with temperature and reaches a maximum velocity of (3.0 ± 0.5) × 10 −4 m s −1 close to the silicon melting point. From the dislocation velocity measurements as a function of temperature, an activation energy of (3.1 ± 0.6) eV is estimated and this value is discussed, along with dynamical interactions between defects and dislocations.

To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.

All silicon (Si) ingot fabrication processes share challenges to control grain structure, defect, and impurity contamination during the solidification step to improve the material properties. The final grain structure and inherent structural defects issued from the solidification step are responsible for the photovoltaic (PV) properties for a large part, all the more as they are often associated with impurity distribution. Impurities play a major role as they not only can modify the development of the grain structure formation but can also interact with structural defects creating the regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes are selected and analyzed as‐grown or observed by X‐ray imaging during resolidification from as‐grown seeds. The growth features and relative crystallographic orientation of neighbor grains are characterized. Moreover, minority carrier lifetime measurements are performed and correlated with the growth features. The complementarity of the different techniques improves the understanding of phenomena at stake during the formation of grains and twins, the effect of impurities, and their impact on photovoltaic properties. The results show the significant influence of light and metallic impurities, such as copper, on the grain structure and on the electrical properties.

It has been recently shown (Mazur et al) (1-7) that a simple way to improve the photocurrent of a silicon-based solar cell is to irradiate the silicon surface with a series of femtosecond laser pulses, in the presence of a sulfur containing gas. This improves the formation of micro- spikes on the silicon surface that strongly reduces the reflectivity of the illuminated surface for the incident solar light (Black Silicon). We have prepared photovoltaic structures with different nano-texturization obtained by means of a femtosecond laser, without the use of corrosive gas (under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase of about 30 % in the laser textured zones.

•The undercooling of the silicon solid-liquid interface is measured.•Growth laws linking the growth rate to the undercooling are determined.•Undercooling of the solid-liquid front increases linearly with growth rate.•A similar growth law is observed for pure silicon and silicon containing C and O.•Effect of impurities on the crystalline structure is highlighted.

The paper is devoted to the identification of the metallic impurities in silicon wafers by using Temperature Dependent Lifetime Spectroscopy (TDLS). We consider the variation of all recombination mechanisms, intrinsic and extrinsic, to follow the variation of lifetime with the temperature. The extrinsic recombination mechanism is based on the standard Shockley-Read-Hall theory (SRH) [1], [2] and we simulated the variation of SRH lifetime for two impurities: gold and iron. The simulation results show that their SRH lifetime variations with the temperature are opposite and that the presence of a peak is characteristic of the impurity studied. Experimental measurements are displayed showing the identification of gold impurity by means of Phase-Shift TDLS (PS-TDLS) measurement. Thanks to these results, we demonstrate that PS-TDLS is an efficient method to identify gold and iron impurities at concentrations as low as 1.1010cm-3 for a doping level of 1.1015cm-3.

The thermal donor (TD) generation in dopant-rich compensated Czochralski silicon was studied by pulling an ingot from a feedstock containing large amounts of donors and acceptors. In a wafer located in the vicinity of the change of conductivity type, thermal donors were formed and the evolution of their concentration was similar to that in noncompensated lowly boron-doped silicon. Thus, simultaneous high densities of both boron and phosphorus do not have a significant impact on the TD formation. This brings the experimental evidence that for a given oxygen concentration and annealing temperature, the TD formation is controlled by the electron density.

The present study focuses on the kinetics of the boron-oxygen defect activation in solar cells fabricated from two Czochralski silicon ingots having different compositional properties (particularly various carbon and germanium contents). We show first that the difference between the experimental and the expected time constants associated with the degradation of the cells performances under illumination due to the boron-oxygen activation is higher in the last solidified part of the ingots. Secondly at such locations a pronounced slowing down of the long-term kinetics is observed. Possible explanations are proposed and discussed among which the segregation of some impurities which could interact with oxygen or oxygen dimers. Particularly a clear correlation is highlighted between the carbon content and the amplitude of this slowing down. Eventually this work shows that the experimental conditions generally used for evaluating the amplitude of the light-induced degradation effects have to be reconsidered.

In this study, based on p-type strongly compensated electronic grade monocrystalline Cz silicon, one tried to find if self interstitials have a direct or an indirect role in light and dark induced degradation (LID and DID respectively). Many studies have been carried out on LID phenomenon due to the formation of boron-oxygen complexes under light exposure. Some of them compare as-cut wafers and wafers which have been treated by phosphorus diffusion. They show that the LID phenomenon is reduced when samples have been phosphorus diffused. This effect has been explained by self interstitial injection in the bulk during the phosphorus diffusion, but the role of these self-interstitials on LID phenomenon is not yet well defined [1] [2] [3]. In this paper, investigations on the degradation kinetic are made in as-cut wafers, in phosphorus diffused wafers and in wafers which were annealed in the same conditions used for the phosphorus diffusion. Comparisons of the results allow us to pinpoint the role of self-interstitials on LID phenomenon. It seems that the presence of a high self interstitial concentration reduce the formation of BsO2i complexes responsible for the first faster degradation.

The light-induced degradation (LID) due to the boron-oxygen (B-O) complexes is particularly harmful for solar cells made from the boron-doped p-type Czochralski silicon (Cz-Si). Many studies focused on this phenomenon and some processes have been proposed to reduce it. Recently, Cz-Si voluntarily doped with germanium (Ge) was used to inhibit the effect of the B-O complexes activation. We studied here both conventional Cz-Si (CZ) and Ge-doped Cz-Si (GCZ), and compared their sensitivity to the LID. The first result was that solar cells produced with GCZ had the same performances than solar cells made with CZ despite the impurities like iron unintentionally introduced in GCZ by the Ge powder. Then, we found that the degradation under illumination of the efficiency was lower in GCZ only for the solar cells produced from the last solidified part of the ingot (where the Ge content is the highest). These differences were correlated with the amount of interstitial oxygen (O-i) which was found to be lower for the GCZ wafer, possibly due to the formation of Ge-O-i related defects and/or to Ge-enhanced oxygen precipitation effects. At the cell level, we also compared the kinetics of the LID in both materials and confirmed that the degradation was slower in GCZ. Finally, we showed that the mechanism of light-induced regeneration (LIR) also occurs in GCZ solar cells, with similar kinetics with respect to conventional Cz-Si cells. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Light-induced defect generation seriously reduces the minority-carrier lifetime of crystalline silicon (c-Si) wafers which causes a decrease in solar cell efficiency. In this paper we investigate the impact of boron-oxygen complexes and iron impurities on the light induced minority-carrier lifetime degradation in c-Si, comparing electronic grade and upgraded metallurgical grade materials. For the later, the characteristic of the decay process is shown to be composed of a fast initial decay and a subsequent slow asymptotic decay. We conclude that the dissociation of iron-boron pairs must be taken into account to explain the light-induced lifetime reduction.