Anodic Aluminium Oxide for Localized Contacting and Passivation in Silicon Solar Cells

Abstract

Surface passivation and metallisation patterning have been the main drivers for continuous cell efficiency improvement in silicon photovoltaics (PV) over the past three decades. This thesis explored the use of anodic aluminium oxide (AAO) for silicon solar cells, with the aim of developing a cost-effective surface passivation scheme that can both reduce surface recombination and enable self-patterned, localised metallisation. Localised metallisation was demonstrated onp-type silicon using AAO as a self-patterning template. Screen-printed solar cells with AAO-mediated metal contacts achieved a Fill Factor (FF)of ~ 79% with ~15% localised contact fraction. However, the device voltage was found to be impacted by the metallisation process. Loss analyses highlighted the critical requirement for field-effect passivation to ensure that minority carriers are shielded from the metal-silicon contact regions. Anodic aluminium oxide films formed using constant voltage anodisation can have an effective stored charge density (Qeff) up to 2.5 1012cm-2. While this makes the layers well-suited to reducing recombination at n-type surfaces, much higher emitter saturation current density (J0e)values resulted for p+ surfaces. This finding led to the development of pulsed anodisation to manipulate the stored charge magnitude and polarity in AAO. Pulsed anodisation uses a current source with a periodically changing polarity. It is shown that the magnitude and polarity of the stored charge in AAO/SiO2stacks can be manipulated from –5.2 × 1011to 2.5 × 1012q/cm2. Furthermore, it is demonstrated that J0efor p+ silicon surfaces can be reduced from 210 to 100 fA/cm2when the Qeffis reduced from 1.8 1012to −2.5 1011cm-2. Finally, it is shown that AAO can reduce recombination in the bulk of the silicon wafer, presumably due to the release of hydrogen from the AAO films when the test structures are annealed. These results highlight the potential for pulsed anodisation to optimise stored charge in anodic oxides to accommodate passivation of surfaces having different dopant polarities and concentrations; at the same time it may provide a source of hydrogen that can be used to reduce recombination at the bulk of silicon.