Charakterisierung von multikristallinem Solarsilicium mittels Anwendungen der Neutronenaktivierungsanalyse
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
License
Abstract
This work presents a part of the so called "Solarproject" of University of Mainz and Fraunhofer Institute of Solar Energy Systems (ISE). The main topic of this project is to optimize manufactoring methods of silicon for photovoltaic, to produce cheaper silicon by constant efficiency of the solar cell. This efficiency is directly linked to the concentration of impurities and dopants in the semiconductor. Impurities like the 3d-transition metals have negative effects on the efficiency, whereas dopants like boron (p-type) and phosphorus (n-type) are necessary for the p-n-junction in the solar cell. At Fraunhofer ISE the directional solidification is used and investigated as a part of the production process of solar silicon. This method has the advantage, that the 3d-transition metals accumulate in the liquid stage during solidification and the dopants can be added additionaly. Unfortunately there is also a diffusion of the impurities from the crucible system and the top of the ingot into the silicon during solidification and cooling. Due to that a precise analysis of the distribution of 3d-transition metals and dopants is necessary to improve the manufacturing method.
Since the instrumental neutron activation analysis (INAA) is well established in the detection of trace elements, it was chosen for the analysis of the 3d-transition metals. The aim of these analysis was to determine the distribution of especially cobalt, iron, chromiun and nickel in the silicon ingot at sub ppb-level. With altogether three long-time irradiations at the high-flux reactors BR2 in Mol and FRM-II in Munich concentration profiles in the horizontal and vertical cuts of four silicon ingots could be analysed. The determination of iron and chromium was possible in several samples, while cobalt could be detected in all samples. A comparison between simulations from Fraunhofer ISE and experiments shows good agreements for the cobalt concentrations. With a dedicated experiment the back diffusion of chromiun into silicon could be investigated. A comprehensive study of the diffusion of 3d-transition metals into silicon during sample preparation and irradiation was performed additionaly.
Phosphorus, as a pure beta-emitter, can not be detected by traditionally used gamma-spectroscopy. Therefore, a method for the analysis of phosphorus in silicon had to be developed. A set-up, consisting of a beta- and a gamma-detector was constructed, to measure beta-spectra by beta-gamma-anticoincidence. This method allows the determination of phosphorus instrumentally by supressing beta-particles of other radionuclides. With preliminary tests, carried out at TRIGA Mainz, the limit of detection and the supression of the mainly disturbing elements was investigated. The results show, that this method is suitable for the analysis of phosphorus in solarsilicon. First quantifications of phosphorus in silicon samples from directional solidification were successful and could be verified by data from Fraunhofer ISE.
With this two applications of INAA and the PGAA for the boron analysis, it is possible to determine most of the important elements in solarsilicon production.