Enhanced Si Passivation and PERC Solar Cell Efficiency by Atomic Layer Deposited Aluminum Oxide with Two-step Post Annealing.

In this study, aluminum oxide (Al2 O3 ) films were prepared by a spatial atomic layer deposition using deionized water and trimethylaluminum, followed by oxygen (O2 ), forming gas (FG), or two-step annealing. Minority carrier lifetime of the samples was measured by Sinton WCT-120. Field-effect passivation and chemical passivation were evaluated by fixed oxide charge (Qf ) and interface defect density (Dit ), respectively, using capacitance-voltage measurement. The results show that O2 annealing gives a high Qf of - 3.9 × 1012  cm-2 , whereas FG annealing leads to excellent Si interface hydrogenation with a low Dit of 3.7 × 1011  eV-1  cm-2 . Based on the consideration of the best field-effect passivation brought by oxygen annealing and the best chemical passivation brought by forming gas, the two-step annealing process was optimized. It is verified that the Al2 O3 film annealed sequentially in oxygen and then in forming gas exhibits a high Qf (2.4 × 1012  cm-2 ) and a low Dit (3.1 × 1011  eV-1  cm-2 ), yielding the best minority carrier lifetime of 1097 μs. The SiNx /Al2 O3 passivation stack with two-step annealing has a lifetime of 2072 μs, close to the intrinsic lifetime limit. Finally, the passivated emitter and rear cell conversion efficiency was improved from 21.61% by using an industry annealing process to 21.97% by using the two-step annealing process.