Surface and contact passivation for silicon solar cells: exploring new approaches

This thesis explored new approaches for engineering surface and contact passivation layers for silicon solar cells. Of particular interest, was the development and characterisation of new room temperature and solution-processed approaches for the deposition of passivating and carrier-selective contacting layers on crystalline silicon (c-Si) solar cells. The availability of lower-cost processes for forming these layers which are compatible with large-scale manufacturing may enable more rapid uptake of new cell designs employing passivated contacts. An initial re-assessment of the current dielectric deposition techniques revealed an important experimental artefact arising from a commonly-used plasma reactor. The artefact was shown to impact the optical (~1.3% relative loss in photocurrent) and improve the electrical [up to ~16 cm/s reduction in surface recombination velocity (Seff_UL)] properties of the deposited dielectric layer. These differences in film quality can complicate the transfer of technology from laboratory to factory and motivated the development of alternative room temperature, solution processed methods for the growth of surface and contact passivating layers. Ultra-thin tunnelling SiOx layers, formed by the new process of field-induced anodisation (FIA), which intrinsically ensures uniform oxide growth, were demonstrated to have positive charge densities of ~1.5×10^12 cm^-2 and Seff_UL of ~3.5 cm/s on n-type silicon when capped with SiNx and after a forming gas anneal. This surface passivation exceeds that demonstrated with chemically-grown SiOx under the same conditions. The improvement was attributed to the lower fraction of silicon-rich sub-oxides detected in the FIA SiOx compared to chemical SiOx, which resulted in a wider distribution of interface defect states and a smaller minority carrier capture cross section. The formation of hole-selective contact layers through spin-coating hydrogen metal bronze solutions on c-Si was also demonstrated. The solution-processed MoOx layers had similar interfacial properties to thermally-evaporated MoOx layers, evidenced by their comparable contact resistivity (~43 mΩ cm^2) and Seff_UL (~ 190 cm/s) on p-type silicon surfaces. Provided the uniformity of the spin-coated films on hydrophobic silicon surfaces can be addressed, this approach offers the potential to be used in large-scale manufacturing as it has advantages in terms of room temperature processing, simplicity in formation and reduced cost