Shunt analysis in solar cells - electro-optical classification and high resolution defect diagnostics

The investigation on solar cells defects plays an important role in the optimization of processes and solar cell efficiencies, since based on the information about the causes and mechanisms of these defects will help to develop solutions for avoiding these defects. Contaminations or structural damages can limit and reduce the solar cell efficiency. The focus of this work is to present a methodology of a classification and target preparation for small defect analysis. As a high-resolution electro-optical method Dark Lock-In Thermography (DLIT) was applied for the localization and voltage-dependent classification of electrical defects (e.g. shunts) on solar cells. The local dark current-voltage (I-V) properties are measured after separation of distinct single defects on solar cells fragments (∼1 cm2). Different types of shunts are distinguished based on their I-V characteristics. A detailed root cause analysis was done on the microscopic scale using reverse biased electroluminescence (ReBEL) combined with an optical microscopy (μReBEL). After defect localization on a sub-μm scale, target preparation has been performed by means of Focused Ion Beam (FIB) techniques. Process-induced structural defect properties are revealed by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) with Nanospot Energy Diffractive X-ray (EDX) analysis. In the present work, we will present the methodology of classification, preparation and identification of local shunts. The analysis will show that this procedure of defect diagnostic based on electro-optical classification parameters and target preparation is a suitable method to detect contamination sources and to optimize production processes.