21.4% efficient SMART mono-Si PERC solar cells with adapted firing process for LeTID mitigation

Abstract

We have developed a fast firing oven (FFO) firing profile that mitigates light and elevated temperature induced degradation (LeTID) in boron doped passivated emitter and rear cells (PERC), made from high-performance multicrystalline silicon (hp mc) and mono-cast material. During LeTID testing, the highest degradation in relative efficiency in the samples fired with this profile is only -2%rel < Δη <-1%rel depending on the cell, compared to -5%rel < Δη < -6%rel in samples fired with the standard profile. To show that the benefit of this technique can be transferred to modules, the influence of the temperature profile during lamination was investigated. The effect of LeTID mitigation persisted after the simulated lamination process. This new firing profile did not significantly influence the initial efficiencies of the cells that have been fired with it compared to samples fired with a standard firing profile (-0.2%abs < Δη < +0.1%abs). Since the new FFO profile is only a slight alteration to a standard profile, it can be easily integrated into existing production lines. The altered firing profile therefore seems to be a promising way to handle the LeTID challenge and ensure highest cell efficiencies throughout the whole lifecycle of modules made from mono-cast, hp mc, and probably other types of silicon solar cells in the field. The mono-cast wafers used in this work were grown with the seed manipulation for artificially controlled defects technique (SMART). It solves the challenge of grain boundaries growing inward from the crucible wall by introducing dislocation clusters near the wall that prevent this growth. We show that the efficiency of cells that were made with this technique (21.4%) is similar to that of magnetically grown Czochralski (mCz) cells (21.6%) which were otherwise produced identically. A third group of hp mc cells which have also been identically produced except for an acidic texture instead of random pyramids, however only achieved 19.5% efficiency. Therefore, using a standard PERC process, SMART mono material is clearly superior to hp mc material and is similar to mCz silicon. We also show that mCz cells do not show LeTID degradation.