Abstract
In this study, aluminum oxide (Al2O3) 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 Al2O3 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/Al2O3 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.
Highlights
Passivated emitter and rear cells (PERCs) have emerged as a promising technology for both high efficiency and competitive cost in recent years
The lifetime greatly improves after the annealing process as a consequence of chemical passivation and field effect passivation brought
Comparing O2 annealing with forming gas (FG) annealing, the former yields a thicker silicon oxide (SiOx) interfacial layer and the higher Fixed oxide charge (Qf) density of − 3.9 × 1012 cm−2, indicating a superior field effect passivation
Summary
Passivated emitter and rear cells (PERCs) have emerged as a promising technology for both high efficiency and competitive cost in recent years. The most difference between the PERC and the traditional full-aluminum back surface field silicon solar cell is rear passivation of wafers. Considerable efforts have been made in order to improve wafer surface passivation. Minority carrier lifetimes of 0.8–8 ms have been reported for p-type floating zone wafers passivated by vacuum [1–4] or spatial atomic layer deposition (ALD) aluminum oxide (Al2O3) [5–7]. Hsu et al Nanoscale Research Letters (2019) 14:139 annealing. The annealing temperature is typically below 500 °C, beyond which dehydrogenation occurs. Other annealing processes for further improving passivation quality are rarely reported
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