Limitations of a simplified dangling bond recombination model for a-Si:H

The validity of a widely used simple closed-form expression for the recombination associated with dangling bonds in hydrogenated amorphous silicon (a-Si:H) is linked to the relative position of the distribution of the dangling bond states with respect to the quasi-Fermi levels for trapped electrons and holes. However, these quasi-Fermi levels for traps have not been derived before. In this work, we derive the four relevant quasi-Fermi levels for traps associated with dangling bonds in a-Si:H and clarify the limitations of the simple model

Surface Passivation of Crystalline Silicon by Sputtered Aluminium Oxide

Efficient and inexpensive solar cells are necessary for photovoltaics to be widely adopted for mainstream electricity generation. For this to occur, the recombination losses of charge carriers (i.e. electrons or holes) must be minimised using a surface passivation technique suitable for manufacturing. In the literature, it has been shown that the aluminium oxide films are negatively charged dielectrics that provide excellent surface passivation of silicon solar cells. Meanwhile, sputtering has been shown to be an inexpensive thin film deposition method that is suitable for manufacturing. This thesis work aims to combine the excellent passivation properties of aluminium oxide with the manufacturing advantages offered by sputtering. We show - for the first time - that sputtering is capable of depositing negatively charged aluminium oxide films that provide very good surface passivation of crystalline silicon. Effective surface recombination velocities of 24.6 cm/s and 9 cm/s are achieved on 0.8 Ohm.cm p-type crystalline silicon and 1 Ohm.cm n-type crystalline silicon respectively, with charges in the range of -1E11 to -1E13 per square centimetre. We specify the sputtering requirements and processing conditions required for achieving these results, showing the effect of the various deposition and annealing parameters. After investigating the physical characteristics of the sputtered aluminium oxide films using thin film measurement techniques such as Rutherford Backscattering Spectrometry and Secondary Ion Mass Spectroscopy, we conclude that the current levels of surface passivation attained using aluminium oxide films appear to be closely related to the interfacial layer and the presence of hydrogen. In some cases the level of surface passivation is most likely limited by the incorporation of unwanted impurities. We determine the composition and bonding of aluminium oxide films, discussing their significance to the various hypotheses concerning the origin of the negative charge. Finally, we demonstrate that sputtered aluminium oxide can be applied to solar cells by fabricating passivated emitter and rear cells with efficiencies as high as 20%. The results of this thesis provide the foundation for the sputtered aluminium oxide technology and its application to industrial solar cells

Role of hydrogen in the surface passivation of crystalline silicon by sputtered aluminum oxide

Aluminum oxide films can provide excellent surface passivation on both p-type and n-type surfaces of silicon wafers and solar cells. Even though radio frequency magnetron sputtering is capable of depositing aluminum oxide with concentrations of negative charges comparable to some of the other deposition methods, the surface passivation has not been as good. In this paper, we compare the composition and bonding of aluminum oxide deposited by thermal atomic layer deposition and sputtering, and find that the interfacial silicon oxide layer and hydrogen concentration can explain the differences in the surface passivation

Effective surface passivation of crystalline silicon by rf sputtered aluminum oxide

In recent years, excellent surface passivation has been achieved on both p-type and n-type surfaces of silicon wafers and solar cells using aluminum oxide deposited by plasmaassisted atomic layer deposition. However, alternative deposition methods may offer practical advantages for large-scale manufacturing of solar cells. In this letter we show that radiofrequency magnetron sputtering is capable of depositing negatively-charged aluminum oxide and achieving good surface passivation both on p-type and n-type silicon wafers. We thus establish that sputtered aluminum oxide is a very promising method for the surface passivation of high efficiency solar cells

Passivation of highly boron doped silicon surfaces by sputtered AlOx and PECVD SiN, a comparison

We show that boron-diffused emitters can be passivated with AlOx deposited using RF sputtering of an Al target. The surface passivation achieved so far is inferior to that obtained using an optimised PECDV SiN process that includes a chemically grown SiO2 interfacial layer. Nevertheless, the levels of passivation obtained, expressed by emitter recombination current densities of JoE=228-349 fA/cm2 for sheet resistances of 88-210 Ω/□, are already consistent with solar cell with efficiencies in the 20% range

Influence of oxygen on the sputtering of aluminum oxide for the surface passivation of crystallinen silicon

While sputtering has been shown to be capable of depositing aluminum oxide suitable for surface passivation, the mechanisms for this are yet to be firmly established and its potential realized. In this paper, we investigate the relationships between the oxygen in the sputtering process to the resulting composition of the deposited film and the surface passivation obtained. We find that surface passivation is not strongly dependent on the bulk composition of the film. Instead the results indicate that the interfacial silicon oxide layer that forms after annealing between the aluminum oxide film and the silicon is a much more important factor; it is this combined structure of aluminum oxide, silicon oxide and silicon that is crucial for obtaining negative charges and excellent surface passivation

Effect of a post-deposition anneal on AL2O3/SI interface properties

While Al2O3 has been proven to provide an excellent level of surface passivation on all sorts of p-type doped silicon surfaces, the passivation mechanism of this layer and especially the influence of the post-deposition anneal on the Al2O3/Si interface properties is not yet completely understood. A great increase in the surface passivation is observed after a post-deposition anneal, i.e. a postdeposition anneal is mandatory to activate the surface passivation. Thus, the influence of this anneal on the interface properties, density of negative fixed charges Qf and density of interface traps Dit, will be investigated and correlated to the measured minority carrier lifetime. In the case of plasma enhanced ALD, Qf is already high in the as-deposited state and the annealing process only has a minor effect on Qf (Q f increases by 20-50 %, depending on the annealing temperature). The Dit however is strongly reduced by the post-deposition anneal, decreasing by two orders of magnitude. This large reduction in Dit is a prerequisite for benefiting from the strong field effect induced by the high density of negative charges of the Al2O3