Improving solar cell performance through surface modification of silicon

Abstract

This project sets out to improve the efficiency of thin crystalline silicon solar cells by enhancing the photoexcitation through light harvesting, and by developing a novel surface passivation technique via the covalent attachment of organic molecules to the surface. To aid the characterisation of these novel structures, we have developed a new measurement technique for surface recombination using the Kelvin prove.

A passivation method through the attachment of alkyl monolayers to the silicon surface has been developed. These layers were shown to have good passivation properties whilst retaining excellent resilience to oxidation. The passivation effect was determined to be caused by the generation of surface charge, as measured by the Kelvin probe. Further functionalisation of the organic monolayers was undertaken to attach fluorescent chromophores. A novel method for measurement of the surface recombination velocity was developed utilising the Kelvin probe. Changing the incident photon flux, and thus the photogenerated current, allows measurement of the surface recombination current through the change in surface photovoltage. This dependence can then be used to extract the value of the surface recombination velocity. Furthermore, we have shown that this method can be developed further into a mapping technique for surface recombination lifetime, of potentially significant industrial interest.

The effect of the silicon surface charge on the passivation observed has been investigated through the attachment of charged monolayers. It was found that through the attachment of a positive charge, the observed recombination lifetime in n-type silicon decreased whilst a negative charge (through the attachment of carboxylic acid groups to the surface) was found to improve the surface passivation. The carboxylic acid functional groups were charged through immersion in triethylamine (base) and returned to the neutral, starting state through immersion in acetic acid. We have found that the recombination lifetime decreases linearly with decreasing charge-surface distance. This technique allows an in-depth study of surface passivation to be carried out by separating the two principal causes for passivation – the removal of surface states and charge attachment to the surface.

Fluorescent chromophores were attached to the silicon surface by two different techniques – through the reaction of an alcohol-terminated monolayer with an acyl chloride porphyrin and by palladium-catalysed cross-coupling of an allyl-terminated surface with a cyanine dye. The fluorescence quenching was investigated at various chromophore-silicon distances by varying the length of the alkyl chain spacer. We find that the fluorescence lifetime decreases with decreasing chromophore-silicon distance, and follows a logarithmic trend. Further work is required, however, to combine sensitisation with surface passivation as incorporation of a sensitisation layer by the palladium cross coupling of an allyl-terminated surface results in metal contamination to the surface, reducing recombination lifetime.

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