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N-type solar grade silicon for efficient p+n solar cells (NESSI)


A cell process technology on thin n-type wafers is developed including several possible innovations, for high conversion efficiency at low cost. The cell process is tested and optimised for yield in an industrial line. The target is an efficiency of 15.5% in an industrial process. The project is aimed at rapid industrial implementation. To obtain high conversion efficiency for the thin wafers, focus is on advanced process sequences for surface passivation. The feasibility of exploitation is judged in economic feasibility studies and life cycle analyses for a 30MWp/year cell line.
New evidences have been obtained about the existence of an internal gettering process of impurities at extended defects that occurs during the solar cell production process. These studies were published in some papers, as reported in the annexes, and a complete analysis of the results obtained will be published in a future cumulative paper. Thanks to the project two important instruments were installed in our laboratory. The already working photoluminescence bench was implemented with a new monochromator. The photoluminescence technique was extensively used in the material characterization. During the second project year the Electron Beam Induced Current (EBIC) tool was introduced in our scanning electron microscope (SEM). This technique was used in the last part of the project for the study of the electrical properties of extended defects in the as-grown material and in wafers after the solar cell process. From the academic point of view, the Nessi project allowed the introduction of young students in the study of material for photovoltaic application. Three ungraduated students have done their thesis on this topic. They carried out part of the ingot characterization having the opportunity to study the evolution of the material properties from the growth stage till to the cell. This is a good opportunity for students in order to have a look to an immediate application of the theoretical studies carried out during regular courses.