Passivation effect of water vapour on thin film polycrystalline Si solar cells

In this work, we investigated a passivation of both surface and bulk of polycrystalline silicon films by water vapour. To assess the passivation effect we used Suns-V-OC method to measure the open-circuit voltage V-OC of polycrystalline silicon thin film solar cells. The treatment conditions, i.e. temperature, steam pressure, duration, were systematically varied to optimise the passivation process. According to our results, there is a parameter interaction because the same passivation effect can be achieved at different treatment conditions. A sufficiently high temperature (350-450 degrees C) is necessary for a successful silicon passivation. The impact of this parameter cannot be replaced either by an elevated steam pressure or a prolonged exposure time. Nevertheless, the passivation effect of steam can be strengthened by an elevated steam pressure. Different gases were tested beside water vapour, e.g. H-2, H-2 + H2O, O-2 + H2O, air, but none of them resulted in higher V-OC than pure steam (360 mV from starting 220 mV). Results from Fourier transform infrared spectroscopy indicate that water vapour passivation is rather oxidation while hydrogen plays a significant supporting role in the process. We conclude that the water vapour passivation is able to passivate defects in the whole silicon volume. However, its passivation effect is not strong enough to become an adequate alternative to the plasma hydrogenation with the best result of V-OC similar to 497 mV. On the other hand, it provides advantage of simplicity (no vacuum system and deionised water steam as the only input). Recombination activity of defects in polycrystalline Si can be suppressed by their saturation for instance in hydrogen plasma (Si-H) or in water vapour (Si-O-Si). [GRAPHICS] (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim