Experimental Demonstration on Suppression of LeTID in P-type Multi-Crystalline PERC Silicon Solar Cells by Biased Annealing Process

Abstract

P-type multi-crystalline PERC silicon solar cells are susceptible to strong light- and elevated temperature-induced degradation (LeTID). This effect can cause more than significant power loss over several years of module operation in the field. Various approaches, such as illuminated annealing and modified thermal processes, have been investigated to suppress LeTID. An undesirable side effect of these processes is an increased contact resistance at the front screen-printed metal fingers, and therefore a lower fill factor (FF), which is the ratio of maximum obtainable power to the product of the open-circuit voltage (V_OC) and short-circuit current (I_SC) of a solar cell device, and the subsequentially its energy conversion efficiency. Previous work has indicated that this may be caused by the transport and accumulation of hydrogen at the metal contacts and that this behaviour can be limited by applying a reverse bias across the cell during a thermal anneal in the dark. In this work, we have conducted a similar biased annealing process to investigate the effect of different cooling conditions on PERC cell performance and developed a biased annealing treatment that not only improves the efficiency but also maintains the stability during the light soak test. Preliminary results of transferring this process into 6-inch PERC cells using a full-scale industrial tool are also presented.