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

In this study, aluminum oxide (AlO) films were prepared by a spatial atomic layer deposition using deionized water and trimethylaluminum, followed by oxygen (O), 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 (Q) and interface defect density (D), respectively, using capacitance-voltage measurement. The results show that O annealing gives a high Q of - 3.9 × 10 cm, whereas FG annealing leads to excellent Si interface hydrogenation with a low D of 3.7 × 10 eV cm. 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 AlO film annealed sequentially in oxygen and then in forming gas exhibits a high Q (2.4 × 10 cm) and a low D (3.1 × 10 eV cm), yielding the best minority carrier lifetime of 1097 μs. The SiN/AlO 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.