Degradation and Accelerated Recovery of Surface Passivation in n+ Poly-Si/SiOx Passivating Contacts for TOPCon Solar Cells

We studied the surface degradation and recovery of fired poly-Si/SiOx passivating contacts during subsequent dark and illuminated annealing. We report on an industrially-viable path for accelerated recovery of surface passivation. The degradation is influenced by the type of doping in the poly-Si. Phosphorus doped n+ poly-Si/SiOx contacts show degradation followed by recovery, undoped poly-Si/SiOx contacts only show recovery during annealing. Boron doped p+ poly-Si/SiOx contacts show neither degradation nor improvement with annealing. Both degradation and recovery are thermally-activated processes and are completely reversible and cyclic in nature. The activation energy of degradation and recovery in dark for n+ poly-Si/SiOx contacts are 1.27 and 1.33 eV respectively. Dark annealing at elevated temperatures is effective for complete recovery but takes a long time (~30 min at 350 degrees C) due to higher activation energies. Annealing under 7.5 Suns of illumination lowers the activation energy for degradation and recovery to 0.88 and 0.90 eV, respectively. Using this data, we have developed an industrially viable post-firing treatment for accelerated recovery of TOPCon cells by annealing them at elevated temperatures and under intense illumination for a few minutes.