Light‐induced degradation (LID) is a well‐known problem faced by p‐type Czochralski (Cz) monocrystalline silicon (mono‐Si) wafer solar cells. In mono‐Si material, the physical mechanism has been traced to the formation of recombination active boron‐oxygen (B–O) complexes, which can be permanently deactivated through a regeneration process. In recent years, LID has also been identified to be a significant problem for multicrystalline silicon (multi‐Si) wafer solar cells, but the exact physical mechanism is still unknown. In this work, we study the effect of LID in two different solar cell structures, aluminium back‐surface‐field (Al‐BSF) and aluminium local back‐surface‐field (Al‐LBSF or PERC (passivated emitter and rear cell)) multi‐Si solar cells. The large‐area (156 mm × 156 mm) multi‐Si solar cells are light soaked under constant 1‐sun illumination at elevated temperatures of 90 °C. Our study shows that, in general, PERC multi‐Si solar cells degrade faster and to a greater extent than Al‐BSF multi‐Si solar cells. The total degradation and regeneration can occur within ∼320 hours for PERC cells and within ∼200 hours for Al‐BSF cells, which is much faster than the timescales previously reported for PERC cells. An important finding of this work is that Al‐BSF solar cells can also achieve almost complete regeneration, which has not been reported before. The maximum degradation in Al‐BSF cells is shown to reduce from 2% (relative) to an average of 1.5% (relative) with heavier phosphorus diffusion.
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.
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