Optimization of Belt Furnace Anneal to Reduce Light and Elevated Temperature Induced Degradation of Effective Carrier Lifetime of P‐Type Multicrystalline Silicon Wafers

Abstract

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.

Progress in the understanding of light‐ and elevated temperature‐induced degradation in silicon solar cells: A review

“…126 Annealing in the dark has also been shown to suppress LeTID. 41,73,88 Similar to the approach in the belt-furnace, dark annealing has been shown to result in a drop in device FF for longer durations and higher temperatures. 122 To the avoid the drop in device FF, Sen et al 127 proposed a possible alternative method whereby the anneal is performed prior to metal contact firing.…”

“…The maximum degradation extent as a function of peak firing temperature for (A) SiN x only passivated symmetrical test structures and (B) structures with at least one side AlO x or Al 2 O 3 passivated with SiN x capping as extracted from previous studies. 12,14,34,[36][37][38][39][40][41] The corresponding legends highlights the various test structures and conditions used. Lower temperatures and unfired wafers have been excluded as they typically show negligible degradation.…”

“…Several postprocessing methods have been introduced to suppress LeTID, all of which involve annealing of the finished solar cell either under illumination, 14,15,17,20 with current injection 2,15,121 or in darkness, 41,73,88,122 demonstrating that there are likely two paths to mitigating the LeTID defect, through the injection of carriers at elevated temperatures and through a purely thermal route. Annealing under laser illumination to accelerate the degradation and regeneration process was proposed by both Payne et al and Krauß et al, in an approach similar to that proposed for rapid mitigation of the BO defect in the past.…”

“…Sharma et al 41 Optimisation of firing conditions with reductions in peak firing temperature and modification of belt speed. Use of post metallization dark annealing at moderate temperatures (300 C-550 C) Sharma et al 130 Incorporation of postfiring forming gas annealing (FGA)…”

LeTID mitigation via an adapted firing process in p‐type PERC cells from SMART cast‐monocrystalline, Czochralski and high‐performance multicrystalline silicon

“…Even though no results were published to back this claim, the idea inspired further research. So far, it has been shown that greatly reducing the cooling rate or slowing down the whole firing process by lowering the belt speed reduces LeTID in hpm lifetime samples 18,19 . Only reducing the slope of the cooling ramp has been shown to work on lifetime samples out of conventional mono‐cast material, when reducing the cooling rate to 20 K/s 20 or even up to 50 K/s in p‐type Cz lifetime samples 21 .…”