Industrial application of uncapped Al2O3 and firing-through Al-BSF in open rear passivated solar cells

Current bottlenecks for industrial application of Al₂O₃ deposited by Atomic Layer Deposition (ALD) on open rear passivated solar cells are low growth rate, firing stability of thin and uncapped layers, and Firing-through BSF formation during co-firing. Long term stability results on the performance of a high throughput ALD prototype, the Levitrack, are presented. Excellent passivation properties have been obtained after firing: for 12 nm thick films deposited on p-Fz (1.8 Ω·cm) a T_eff of 2.2 ms (Δn=3×10¹⁵ cm^-3) was obtained that remained constant over a period of 20 weeks. Furthermore, we report on the passivation quality of the firing-through BSF and Al₂O₃ coating as function of wafer peak temperatures between 790 and 845°C. Bifacial cells covered for 50% Al contacts resulted in local BSF thicknesses of 5 to 7 μm and FF of 78% and 77% for the best mc and mono crystalline cell respectively. For this study 12 nm uncapped aluminum oxide layers are deposited on the rear of p-type mc and mono bifacial cells. LBIC measurements confirm literature reports that uncapped Al₂O₃ passivation in general is heavily dependent on the wafer peak temperatures during firing in the investigated range.