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

We report on the degradation and recovery of surface passivation of fired poly-Si/SiO_x passivating contacts with hydrogen containing Al₂O₃ during annealing in the dark and under illumination. Upon firing to a peak temperature of 670°C, the iV_oc for symmetric test structures with n⁺, p⁺, and intrinsic poly-Si/SiO_x contacts decreases due to a loss of surface passivation. Upon further annealing over the temperature range of 200–350°C in the dark, depending on the type of doping, the surface passivation either shows further degradation followed by recovery, or direct recovery to the initial iV_oc. Annealing at higher temperatures and/or higher illumination intensities accelerates the kinetics for both degradation and recovery processes. We show that the degradation and recovery processes are thermally activated and proceed identically in subsequent firing and annealing steps showing their cyclic nature. We present a series reaction model to explain the kinetics of degradation and recovery processes for n⁺ and intrinsic poly-Si/SiO_x contacts. By fitting the model's rate expressions to the data, the determined effective activation energy barriers for degradation and recovery for n⁺ poly-Si/SiO_x contacts in the dark are 1.24 and 1.51 eV, which are lowered under 7.5 Suns illumination to 0.76 and 1.15 eV, respectively.