Surface Passivation of Crystalline Silicon by Sputtered Aluminium Oxide
Abstract
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.