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Enhanced rear‐side reflection and firing‐stable surface passivation of silicon solar cells with capping polymer films

Low refractive index polymer materials have been investigated with a view to form the back surface mirror of advanced silicon solar cells. SiOx:H or AlOy SiOx:H polymer films were spun on top of an ultra‐thin (<10 nm) atomic‐layer‐deposited (ALD) Al2O3 layer, itself deposited on low‐resistivity (1 Ω cm) p‐type crystalline silicon wafers. These double‐layer stacks were compared to both ALD Al2O3 single layers and ALD Al2O3/plasma‐enhanced chemical vapour deposited (PECVD) SiNx stacks, in terms of surface passivation, firing stability and rear‐side reflection. Very low surface recombination velocity (SRV) values approaching 3 cm/s were achieved with ALD Al2O3 layers in the 4–8 nm range. Whilst the surface passivation of the single ALD Al2O3 layer is maintained after a standard firing step typical of screen printing metallisation, a harsher firing regime revealed an enhanced thermal stability of the ALD Al2O3/SiOx:H and ALD Al2O3/AlOy SiOx:H stacks. Using simple two‐dimensional optical modelling of rear‐side reflection it is shown that the low refractive index exhibited by SiOx:H and AlOy SiOx:H results in superior optical performance as compared to PECVD SiNx, with gains in photogenerated current of ∼0.125 mA/cm2 at a capping thickness of 100 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)