Classification of Recombination-Active Defects in Multicrystalline Solar Cells Made from Upgraded Metallurgical Grade (UMG) Silicon

In this contribution a classification of recombination active defects in multicrystalline silicon solar cells made from electronic grade (eg) and upgraded metallurgical grade (umg) silicon feedstock is introduced. On a macroscopic scale the classification is performed by using forward and reversed biased electroluminescence imaging (EL / ReBEL) and imaging of sub-band defect luminescence (ELsub). The luminescence behavior due to structural defects already present in the wafer can be divided into two groups based on their recombination and prebreakdown behavior. As a first step towards a more detailed analysis of the cause for these differences, the classification was also performed on microscopic scale. For this ReBEL and ELsub was performed under an optical microscope (µReBEL/µELsub) and EL was replaced by Electron Beam Induced Current (EBIC). The defect types observed on a macroscopic scale could also be observed on a microscopic scale; however, a third defect type had to be introduced. Finally we propose a qualitative model for the different classified types of recombination active defect structures that can explain the observed recombination and prebreakdown behavior.