Search

Shielded hydrogen passivation (SHP) is a recently developed technique for introducing atomic hydrogen into materials and it offers significant advantages over the other hydrogenation techniques. Hydrogen de-activation of boron followed by electrochemical CV profiling was used to demonstrate that substantial quantities of atomic hydrogen can permeate though palladium/silver alloy foils which are 10 µm thick. It is thought that such thickness will be sufficient to withstand pressures of up to 1 atmosphere allowing passivation in an in-line process. Further, it is shown that poisoning of the foil by using sulphur increases the flux of atomic hydrogen released. SHP delivers extremely good passivation of SiO2-Si interfaces, as demonstrated by using thermally oxidised 1 Ω-cm, n-type silicon where the lifetime, at 1015 cm−3 injection level, was found to increase from 12 to 1.05 ms after SHP processing. Upon application of corona charge, the lifetime further increased to 6.3 ms, equivalent to SRV ≤ 0.17 cm s−1. [ABSTRACT FROM AUTHOR]

The silicon photovoltaic industry is currently shifting towards lightly doped emitters. These have electrical properties that benefit solar cells, compared to the traditional heavily doped emitters. This move brings new challenges, as gettering efficiencies of impurities are lowered as the doping reduces. This is particularly problematic in multicrystalline silicon (mc-Si) since cell performance is typically boosted by the effective gettering of such impurities. In prior work, we proposed the novel gettering technique, saw damage gettering (SDG), which improved effective carrier lifetime of standard performance mc-Si red zone material. In this work, we expand the study of SDG to various types of industrially relevant mc-Si: upgraded metallurgical grade (UMG), high performance bottom red zone (HPRZ), and diamond sawn high performance (DHP). The optimal condition for SDG is found to be an annealing temperature of 850 °C. With this condition it was demonstrated that the effective carrier lifetime can be increased in all silicon types upon SDG. The largest increase was observed for HPRZ material by a factor of 10, and the largest final effective lifetime post SDG was that of UMG, with τeff = 61.3 µs. SDG is a potentially viable gettering method to work in conjunction with lightly doped emitters in removing the impurities of mc-silicon feedstock and thus, improving the efficiency of the cells made therefrom. [ABSTRACT FROM AUTHOR]

A key requirement in the recent development of highly efficient silicon solar cells is the outstanding passivation of their surfaces. In this work, plasma enhanced chemical vapour deposition of a triple layer dielectric consisting of amorphous silicon, silicon oxide and silicon nitride, charged extrinsically using corona, has been used to demonstrate extremely low surface recombination. Assuming Richter's parametrisation for bulk lifetime, an effective surface recombination velocity Seff = 0.1 cm/s at Δ n = 1015 cm-3 has been obtained for planar, float zone, n -type, 1 Ω cm silicon. This equates to a saturation current density J0s = 0.3 fA/cm2, and a 1-sun implied open-circuit voltage of 738 mV. These surface recombination parameters are among the lowest reported for 1 Ω cm c-Si. A combination of impedance spectroscopy and corona-lifetime measurements shows that the outstanding chemical passivation is due to the small hole capture cross section for states at the interface between the Si and a-Si layer which are hydrogenated during nitride deposition. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The recombination of electric charge carriers at the surface of semiconductors is a major limiting factor in the efficiency of optoelectronic devices, in particular, solar cells. The reduction of such recombination, commonly referred to as surface passivation, is achieved by the combined effect of a reduction in the trap states present at the surface via a chemical component, and the reduction in the charge carriers available for a recombination process, via a field effect component. Here, we propose a technique to field effect passivate silicon surfaces using the electric field effect provided by alkali ions present in a capping oxide. This technique is shown to reduce surface recombination in a controlled manner, and to be highly stable. Surface recombination velocities in the range of 6-15 cm/s are demonstrated for 1 Ω cm n-type float zone silicon using this technique, and they are observed to be constant for over 300 days, without the use of any additional surface chemical treatment. A model of trapping-mediated ionic injection is used to describe the system, and activation energies of 1.8-2 eV are deduced for de-trapping of sodium and potassium alkali ionic species. [ABSTRACT FROM AUTHOR]

The origins of an increase in the series resistance of PERC multicrystalline silicon solar cells due to post-firing thermal processes are investigated. This effect has been shown to be capable of reducing the fill factor of finished cells by up to 20% ABS , severely degrading their performance. It is observed that electric currents applied either during or after these thermal processes can greatly alter the series resistance, either causing it to increase by more than an order of magnitude or suppressing the effect entirely. It is demonstrated that this behavior is in good agreement with the expected interactions of hydrogen with dopants and electric fields within silicon wafers. It is therefore speculated that at least part of the observed increase in resistance is due to the motion of hydrogen within the cell itself. [ABSTRACT FROM AUTHOR]

In this article we investigate the observation of increased contact resistance in both PERC and TOPCon solar cells linked to hydrogen dynamics at the interface. We study the changes in series resistance (R S) as a result of applied forward and reverse bias in the temperature range from 350 °C–400 °C. In PERC cells, we use a modified geometry to isolate the root cause of the increased R S by isolating the current path through the different parts of the cell. We show that contact resistance in PERC cells occurs between the Ag contact and the n + silicon region at the front surface. We also report the first observation of increased contact resistance in industrial n-type TOPCon solar cells, likely linked to H dynamics. For both PERC and TOPCon cells, we show that the temperature of the measurement has a profound impact on the amount of contact resistance. However, the response of the two cell architectures under varied biasing conditions is not identical. TLM measurements reveal that in TOPCon cells the increased R S is caused by the Ag contact to the n + polysilicon region, which, unlike in PERC cells, corresponds to the rear surface, away from the p-n junction. Recent results have shown that severe surface-related degradation in TOPCon solar cells can be mitigated by annealing treatments at temperatures similar to those explored herein. Therefore, identifying contact resistance in TOPCon cells may have a profound impact on further studies exploring degradation mitigation pathways in TOPCon cells. • Hydrogen induced contact resistance (HICR) is observed in TOPCon solar cells. • This contact resistance occurs at the Ag/n + Si interface in both PERC and TOPCon. • The response to applied bias in PERC and TOPCon is different. • These insights are important for further TOPCon degradation studies. [ABSTRACT FROM AUTHOR]