Industrially relevant Al2O3 deposition techniques for the surface passivation of Si solar cells

Abstract

We present independently confirmed efficiencies of 21.4% for PERC cells with plasma-assisted atom-ic-layer-deposited (plasma ALD) Al2O3 rear passivation and 20.7% for cells with thermal ALD-Al2O3. Additionally, we evaluate three different industrially relevant techniques for the deposition of surface-passivating Al2O3 layers on 1-cm p-type silicon wafers, namely high-rate spatial ALD (spatial ALD), plasma-enhanced chemical vapour deposi-tion (PECVD) and reactive sputtering. Using spatial ALD and PECVD, surface recombination velocities (SRVs) be-low 10 cm/s are obtained. Sputtered Al2O3 layers still provide an SRV of 35 – 70 cm/s. Despite their lower passiva-tion quality compared to the Al2O3 films deposited by spatial ALD and by PECVD, we demonstrate that the sputtered Al2O3 layers are still suitable for the fabrication of 20.1% efficient PERC cells. After firing at ~800°C in a conveyor-belt furnace the SRV provided by the Al2O3 films deposited by spatial ALD is still below 20 cm/s, indicating an ex-cellent firing stability. Both PECVD and sputtered Al2O3 passivation layers degrade to SRVs larger than 100 cm/s af-ter firing. Hence, the firing stability of PECVD and – in particular – sputtered Al2O3 needs further optimisation