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

In this study, aluminum oxide (Al2O3) films were prepared by a spatial atomic layer deposition using deionized water and trimethylaluminum, followed by oxygen (O-2), 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(f)) and interface defect density (D-it), respectively, using capacitance-voltage measurement. The results show that O-2 annealing gives a high Q(f) of -3.9 x 10(12)cm(-2), whereas FG annealing leads to excellent Si interface hydrogenation with a low D-it of 3.7 x 10(11)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 Al2O3 film annealed sequentially in oxygen and then in forming gas exhibits a high Q(f) (2.4 x 10(12)cm(-2)) and a low D-it (3.1 x 10(11)eV(-1)cm(-2)), yielding the best minority carrier lifetime of 1097s. The SiNx/Al2O3 passivation stack with two-step annealing has a lifetime of 2072s, 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.