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

In this study, aluminum oxide (Al(2)O(3)) 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 × 10(12) cm(−2), whereas FG annealing leads to excellent Si interface hydrogenation with a low D(it) of 3.7 × 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 Al(2)O(3) film annealed sequentially in oxygen and then in forming gas exhibits a high Q(f) (2.4 × 10(12) cm(−2)) and a low D(it) (3.1 × 10(11) eV(−1) cm(−2)), yielding the best minority carrier lifetime of 1097 μs. The SiN(x)/Al(2)O(3) 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.