Impact of compensation on solar grade silicon for photovoltaics

• Main Author: Modanese, Chiara
• Format: Doctoral Thesis
• Language: English
• Published: Norges teknisk-naturvitenskapelige universitet, Institutt for materialteknologi 2012
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-16244; http://nbn-resolving.de/urn:isbn:978-82-471-3369-9 (printed ver.); http://nbn-resolving.de/urn:isbn:978-82-471-3370-5 (electronic ver.)

The steep increase in the market price for silicon feedstock for solar cells experienced at the beginning of the last decade, coupled with a developing industry, opened the possibility for materials produced from different sources and delivered at lower production cost. Compensated solar grade silicon for solar cell provided these opportunities. The motivation behind the present work was, therefore, to study the effect of such a material on the bulk and cell properties, in respect to the commonly used feedstock produced via the Siemens process. The materials presented in this work were produced at Elkem Solar AS, who has provided materials with decreasing dopant concentration over the project time. The main focus was to investigate the manner in which materials cast from compensated solar grade feedstock behave compared to materials cast from reference electronic grade feedstock. Different properties were measured, both in the bulk and on solar cells. Resistivity was observed to increase along the growth direction due to the different segregation profiles of the main doping species, i.e. boron and phosphorus. Dissolved oxygen concentration was observed not to depend on feedstock, whereas higher dissolved carbon concentration was measured in the compensated materials. Minority carrier lifetime is reduced, although the performance of the materials after solar cell processing is comparable to the electronic grade reference materials. Majority carrier mobility is strongly reduced at temperatures below 150K, whereas the reduction at high temperatures (above 250 K) is much lower. The possibility to use international standards to convert the measured resistivity in net doping density for compensated materials was investigated. The aim was to provide an easy and direct method to assess these materials. Through the comparison of the net doping density calculated from the measured resistivity and the net doping density measured by glow discharge mass spectrometry, the accuracy of the standard was evaluated for the compensated materials. Silicon nitride crucibles are an alternative to the widely used silica crucibles for directional solidification of mc-Si ingots. Their main advantages are reusability for repeated castings and the opportunity to eliminate the crucible as a source of oxygen contamination of the ingot. In this work, several ingots cast in such crucibles were studied and compared to ingots cast in silica crucibles. Advantages and limitations related to the use of such crucibles are discussed, with focus on the required changes to the solidification process. Chromium is an impurity known to be detrimental for the material quality of silicon for solar cells. Its effect is of particular interest for less pure materials such as those studied in this work, where the concentration of Cr can be expected to be greater than in electronic grade materials. The impact of Cr on the properties of the compensated solar grade silicon materials was studied through the comparison of ingots with and without deliberate chromium doping. It was found that, although minority carrier lifetime is significantly impaired by the addition, a strong improvement occurred through adequate solar cell processing, which included a phosphorus diffusion step.