Comparison of ICP-AlOx and ALD-Al2O3 Layers for the Rear Surface Passivation of C-Si Solar Cells

Abstract

The deposition rate of the standard (i.e. sequential) atomic layer deposition (ALD) process is very low compared to the plasma-enhanced chemical vapour deposition (PECVD) process. Therefore, as a short- and medium-term perspective, PECVD aluminium oxide (AlOx) films might be better suited for the implementation into industrial-type solar cells than ALD-Al2O3 films. In this paper, we report results achieved with a newly developed PECVD deposition process for AlOx using an inductively coupled plasma (ICP). We compare the results to high-quality ALD-Al2O3 films. We examine a stack consisting of a thin AlOx passivation layer and a PECVD silicon nitride (SiNy) capping layer. Surface recombination velocities below 9 cm/s were measured on low-resistivity (1.4 Ωcm) p-type crystalline silicon wafers passivated either by ICP-PECVD-AlOx films or by ALD-Al2O3 films after annealing at 425̊C. Both passivation schemes provide an excellent thermal stability during firing at 910̊C with SRVs below 12 cm/s for both, Al2O3/SiNy stacks and single Al2O3 layers. A fixed negative charge of −4×1012 cm−2 is measured for ICP-AlOx and ALD-Al2O3, whereas the interface state density is higher for the ICP-AlOx layer with values of 11.0×1011 eV−1cm−2 compared to 1.3×1011 eV−1cm−2 for ALD-Al2O3. Implemented into large-area screen-printed PERC solar cells, an independently confirmed efficiency of 20.1% for ICP-AlOx and an efficiency of 19.6% for ALD-Al2O3 are achieved.