Properties of al2o3 films deposited by atomic layer deposition for photovoltaic applications

Abstract

With the gradual decrease in silicon solar cell thickness, the overall efficiency has become more limited by the surface passivation. It has been recognized that the current dominant p-type silicon solar substrates will be replaced by the n-type materials. This thesis focuses on the properties of ALD aluminium-oxide layers, which is a promising dielectric material for the high efficiency n-type solar cells. Firstly, the impact of laterally non-uniform carrier lifetime on the determination of the lifetime from photoconductance-based measurements, based on the self-consistent method was investigated. An overestimation of the mean lifetime was simulated and experimentally confirmed, with the magnitude of the error mainly dependent on the distribution of the effective lifetime across the area sensed by the photoconductance coil. We investigated the passivation of ALD aluminium-oxide layers on both undiffused and boron diffused (111) and (100) oriented surfaces. It was found that the additional surface boron diffusion can reduce recombination at the alumina/Si interface. Aluminium-oxide passivated (100) surfaces displayed better passivation than (111) surfaces, which is due the finding that aluminium-oxide films on (100) orientated silicon form a higher negative fixed charge density than films on (111) samples, while there is not obvious difference on the interface state density. Aluminium-oxide/silicon-nitride stacks utilising an ultrathin aluminium-oxide layer were also studied concentrating on the correlation between the silicon nitride composition and its capping performance in terms of passivation and thermal stability. Excellent passivation was obtained by 1-nm-alumina/silicon-nitride stacks. The passivation performance of such stacks depends critically on both the alumina thickness and the silicon-nitride composition. It was found that to achieve low recombination factor with 1-nm-alumina/silicon-nitride stacks, the silicon nitride hydrogen concentration was required to be low, less than 8e21 per cubic cm. Both the alumina/Si interface and charge density of 1-nm-alumina/silicon nitride stacks can be impacted by the silicon-nitride capping layer. The outstanding passivation quality of 1-nm-alumina/silicon-nitride stacks is due to a favourable combination of both chemical and electrostatic passivation. Finally, the effect of humidity on aluminium-oxide passivated silicon samples was investigated. It was found that samples exposed to a saturated humidity ambient show a much higher degradation rate than those exposed to 85% relative humidity (RH). Moreover, the electrical resistance of aluminium-oxide film also decreased significantly following saturated humidity exposure. PECVD silicon nitride capping layers are effective at protecting aluminium-oxide films from damp-heat. Two degradation regimes are proposed to explain the degradation: (i) initial reversible degradation at shorter time exposures, which is ascribed to the loss of field effect passivation; (ii) severe degradation after longer term exposure, which is believed to be due to a substantial loss of chemical passivation with the generation of new species from the reaction of aluminium-oxide and water. The second regime was only observed for saturated humidity conditions. Show more