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Reducing light and elevated temperature induced degradation (LeTID) is important for the industrial success of PERC (passivated emitter and rear cell) solar cells fabricated on high-performance multicrystalline silicon (multi-Si) wafers. Although there has been a lot of progress in understanding the degradation kinetics, the defect(s) responsible for LeTID in multi-Si wafers at elevated temperatures (%80 C, 1 Sun) has yet to be identified. In this study, the authors look at the possibility of using phosphorus diffusion gettering (PDG) for reducing LeTID in multi-Si wafers and solar cells. By measuring light induced defect concentrations in multi-Si wafers before and after LeTID, the authors observe that PDG can substantially reduce the average defect concentration. Trace element analysis using inductively coupled plasma mass spectrometry reveals that multi-Si wafers from the edge of the ingot contain a high concentration of Cu, Ni and Ti in grains that degrade more than neighboring grains. To explore PDG for reducing LeTID in multi-Si PERC solar cells, the authors fabricate cells with two different emitter profiles. Etching back a heavily diffused emitter to obtain a high sheet resistance is observed to improve the LeTID performance of the solar cells, an effect that is very likely related to a reduced impurity concentration within the wafer.

Light and elevated temperature induced degradation (LeTID) of the effective charge carrier lifetime significantly lowers the efficiency of multicrystalline silicon (mc-Si) solar cells and is a major challenge currently faced by the silicon photovoltaic industry. Optimization of the temperature profile used in the rapid thermal anneal (RTA) step of the metallization line has been found to significantly reduce LeTID of mc-Si solar cells. Hence, the authors experimentally study the impacts of varying the RTA process parameters on the LeTID behavior of mc-Si lifetime samples. It is shown that a low peak temperature, slow ramp-up rate (slow belt speed) reduce LeTID in mc-Si lifetime samples. Also, subsequent dark anneal at a moderate temperature (%300-550 C) further reduces LeTID. Samples already subjected to LeTID conditions also benefit from the post-degradation dark anneal. The recovered effective carrier lifetime of the degraded samples dark annealed at 550 C is even higher than the post-firing effective carrier lifetime value. The defect state achieved post-dark anneal at 550 C is stable for the studied period of %60 h, unlike the well-known case of low-temperature dark anneals (<200 C) where samples degrade again when subjected to LeTID conditions. The authors believe that the optimized conditions identified in this work can be applied to mass-produced mc-Si solar cells.