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. SiO x :H or AlO y SiO x :H polymer films were spun on top of an ultra‐thin (<10 nm) atomic‐layer‐deposited (ALD) Al 2 O 3 layer, itself deposited on low‐resistivity (1 Ω cm) p‐type crystalline silicon wafers. These double‐layer stacks were compared to both ALD Al 2 O 3 single layers and ALD Al 2 O 3 /plasma‐enhanced chemical vapour deposited (PECVD) SiN x 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 Al 2 O 3 layers in the 4–8 nm range. Whilst the surface passivation of the single ALD Al 2 O 3 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 Al 2 O 3 /SiO x :H and ALD Al 2 O 3 /AlO y SiO x :H stacks. Using simple two‐dimensional optical modelling of rear‐side reflection it is shown that the low refractive index exhibited by SiO x :H and AlO y SiO x :H results in superior optical performance as compared to PECVD SiN x , with gains in photogenerated current of ∼0.125 mA/cm 2 at a capping thickness of 100 nm. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)