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) 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) 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 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/SiO_x:H and ALD Al₂O₃/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/cm² at a capping thickness of 100 nm. (