This document is designed to assess the effect of light induced degradation at elevated temperatures (LETID) by application of electrical current at higher temperatures. LETID is activated by excess carriers caused either through illumination or injection of electrical current at temperatures above 50 °C. In fielded conditions, LETID develops over the course of several months up to years as opposed to days for Boron Oxygen (B-O) degradation. The degradation phase is followed by an even slower recovery phase that develops over a significantly longer time scale. In this document, only the current injection approach for the detection of LETID is addressed.
This document does not address the B-O and lron Boron (Fe-B) related degradation phenomena, which already occur at room temperatures under the presence of light and on much faster time scales. B-O defects may influence the results, and this document attempts to separate them by application of certain procedures. However, it is noted that the separation may not be perfect. Fe-B effects are excluded by introduction of waiting times before power determination. The proposed test procedure can reveal sample sensitivity to LETID degradation mechanisms, but it does not provide an exact measure of field observable degradation. The magnitude and time scale of degradation seen in the field depends on climate and the module technology.
In this document LETID testing is done via current injection as this is a simple method in terms of the test equipment required. This allows for better control over conditions compared to application of light and thus ensures comparability between different labs. Application of light with equivalent conditions (i.e. same temperature and excess carrier density/ injection level) will yield comparable results.
The stress conditions in this document differ in injection level from the test method for cells described in IEC TS 63202-4. Whereas IEC TS 63202-4 aims at providing a quick quality check for a known cell that can be used in outgoing or incoming inspection of a specific product, the procedures in this document aim at providing a general product independent test method for LETID detection in a module that can be applied without prior knowledge about the cells inside a module and will yield reproducible results.
