How to assess the electrical quality of silicon material
Presentation held at 33rd European Photovoltaic Solar Energy Conference and Exhibition, EU PVSEC 2017, 25 - 29 September 2017, Amsterdam
Assessing the electrical quality of silicon material is of utmost importance for comparing different materials and improving the production process. The aim is to provide the wafer and cell manufacturers with a value that allows an immediate judgement of the efficiency the material can achieve after cell processing. However, up to now no standardized procedure has been defined which measurement values should be reported. The SEMI PV13-0211 for measuring the minority carrier lifetime at a fixed carrier density via an eddy-current sensor is the closest to a standard available, but only suitable for homogeneous material. For inhomogeneous material with laterally varying recombination properties (mc-Si and also sometimes Cz-Si) other measurement techniques such as photoluminescence imaging (PLI) have to be used, but here carrier density is not fixed anymore. To complicate things the carrier lifetime depends on metastable defect state, processing state and base doping density. Here, we present a procedure that considers all these relevant factors and show in detail which information should be reported and why. In the end, only two values are of most importance: the diffusion length at a fixed carrier density close to maximum power point (MPP) conditions and the resistivity of the material.