Resistivity topography: a grain boundary characterisation method

Abstract

Solar cells made from multicrystalline silicon are the ideal basis for photovoltaic systems. The solar cell efficiencies are still limited by the crystal defects (dislocations, grain boundaries) and their electrical activity. As a fast and efficient assessment of the electrical activity of specific grain boundaries high resolution resistivity maps will be used, measured on a set of wafers as cut coming from the same block. Resistivity maps proved to be a precise and yet simple method to characterise grain boundary activity. While measurements in the bulk of the crystal are symmetric because of the isotropy of the material, measurements crossing rain boundaries are depending on the relative orientation of the boundary with respect to the measurement geometry. The large number of measurement points of a high resolution resistivity map allows a statistical treatment of the data to evaluate one quantitative value for the grain boundary electrical activity. Solar cells from those have been processed. In the regions with a higher electrical activity at grain boundaries, the open circuit voltage of the solar cell decreased. A specific evaluation of the electrical activity of grain boundaries on the basis of the thermionic model allowed the fine tuning of production parameters leading to a homogeneous quality of cast mc-Silicon wafers bearing the potential of approaching highest efficiencies in industrial production processes.