Refining of metallurgical silicon for crystalline solar cells

International audienceA plasma-retining 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 volatili- Zation 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 Íirst batch of puritied UMG Silicon, multi-crystalline ingots (l2kg), wafers (125X125mm2) and solar cells have been produced for an evaluation of this intermediate material. The obtained solar cells gave efticiencies of up to ll.7%. Process development towards an up-scaled pilot equipment is on the Way to further increase the puritication efticiency

Effet Getter externe par diffusion de phosphore dans des plaquettes de silicium multicristallin. Additivite de l'hydrogenation

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 83312 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Temperature dependent lifetime spectroscopy (TDLS) for the identification of metallic impurities

6th International Conference on Crystalline Silicon Photovoltaics (SiliconPV), CEA INES, Chambery, FRANCE, MAR 07-09, 2016International audienceThe 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 1.10(10)cm(-3) for a doping level of 1.10(15)cm(-3). (C) 2016 The Authors. Published by Elsevier Ltd

Electrical and photovoltaic properties through a large multicrystalline Si ingot

Conference on Photovoltaic Cell and Module Technologies, San Diego, CA, AUG 27-28, 2007International audienceLarge multicrystalline cast silicon ingots (> 3 10 kg) are cost effective in the photovoltaic industry and attenuate the feedstock shortage. The bulk lifetime tau(n) and diffusion length L-n of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity, concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom, which is solidified first, is contaminated by the contact with, the crucible. It was found that c, and L. are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central part of the ingot L. is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection coating layer. The variations along the ingot height of the conversion efficiency eta and of tau(n) in raw wafers are well correlated, that can predict the values of eta, allowing an in-line sorting of the wafers, before solar cells are made. If tau(n) is smaller than 1 mu s, as observed at the extremities of the-ingot, eta will be limited to 10% only; if tau(n). is higher than 2.5 As eta achieve 15% at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely or partly p or n type, suggesting that a purification step tawards the dopants is required

Behaviour of Light Induced Defect Generation and Carrier Lifetime Degradation in Solar Grade Silicon

14th International Conference on Defects-Recognition, Imaging and Physics in Semiconductors, Miyazaki, JAPAN, SEP 25-29, 2011International audienceLight-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 latter, 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

Role of Impurities in Silicon Solidification and Electrical Properties Studied by Complementary in Situ and Ex Situ Methods

International audienceAll 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 interact as well with structural defects creating regions of deleterious minority carrier lifetime recombination. Samples containing different levels of impurities and solidified with different processes were selected and analyzed as-grown or observed by X-ray imaging during re-solidification from as-grown seeds. The growth features and relative crystallographic orientation of neighbor grains were characterized. Moreover, minority carrier lifetime measurements were performed and correlated to the growth features. The complementarity of the different techniques allows improving 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.

In situ and real-time investigation of the solidification of silicon by X-ray imaging

Aiming to the production of low cost and high efficiency solar cells based on silicon material, all processes either innovative or conventional face challenges linked to formation and development of the grain structure, distortion of the crystal and crystalline defects during the solidification step. Our contribution consists in studying these key and fundamental solidification mechanisms by conducting in situ and time-resolved investigations. Two imaging techniques based on X-ray synchrotron radiation are combined during solidification: X-ray radiography and Bragg diffraction (topography). X-ray radiography brings information on the morphology and kinetics of the solid/liquid (S/L) interface. X-ray Bragg diffraction (topography) gives complementary information about misorientations, structural defect formation and the global and local level of distortion of the crystal. The dynamics of twinning, grain competition, sub-grain formation and of the related crystal distortions have been studied in silicon with this method as well as the effect of the impurities on grain nucleation and defect formation

Refining of metallurgical silicon for crystalline solar cells

International audienceA plasma-retining 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 volatili- Zation 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 Íirst batch of puritied UMG Silicon, multi-crystalline ingots (l2kg), wafers (125X125mm2) and solar cells have been produced for an evaluation of this intermediate material. The obtained solar cells gave efticiencies of up to ll.7%. Process development towards an up-scaled pilot equipment is on the Way to further increase the puritication efticiency