No abstract

We used time-resolved terahertz spectroscopy to study ultrafast photoconductivity of polycrystalline thin-film silicon solar cells. We selected a series of samples, which exhibited variable conversion efficiencies due to hydrogen plasma passivation under various technological conditions. The decay of the transient terahertz conductivity shows two components: the fast one is related to the charge recombination at interfaces, while the slow nanosecond one is attributed to the trapping of photocarriers by defects localized at grain boundaries or at dislocations in the polycrystalline p− layer of the structure. We observed a clear correlation between the open-circuit voltage and the nanosecond-scale decay time of the transient terahertz conductivity of the solar cells. Thus, the terahertz spectroscopy appears to be a useful contactless tool for inspecting the local photoconductivity of solar cells including, in particular, various nanostructured schemes.

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‐VOC method to measure the open‐circuit voltage VOC 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 °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. H2, H2 + H2O, O2 + H2O, air, but none of them resulted in higher VOC 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 VOC ∼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).

Thin film polycrystalline silicon (poly-Si) solar cells were annealed in water vapour at pressures below atmospheric pressure. PN junction of the sample was contacted by measuring probes directly in the pressure chamber filled with steam during passivation. Suns-VOC method and a Lock-in detector were used to monitor an effect of water vapour to VOC of the solar cell during whole passivation process (in-situ). Tested temperature of the sample (55°C – 110°C) was constant during the procedure. Open-circuit voltage of a solar cell at these temperatures is lower than at room temperature. Nevertheless, voltage response of the solar cell to the light flash used during Suns-VOC measurements was good observable. Temperature dependences for multicrystalline wafer-based and polycrystalline thin film solar cells were measured and compared. While no significant improvement of thin film poly-Si solar cell parameters by annealing in water vapour at under-atmospheric pressures was observed up to now, in-situ observation proved required sensitivity to changing VOC at elevated temperatures during the process.