Anodic oxide rear contact schemes for silicon solar cells

Abstract

Rear-passivated solar cells demonstrate higher efficiency due to reduced rear surface recombination; however surface passivation and patterning methods used need to be robust, capable of high throughput and readily incorporated in production lines. This thesis examined possible cost-effective improvements in rear local contact formation for silicon solar cells that could be used in manufacturing, with a special focus on the formation of anodic oxide dielectric layers and patterning of these layers. An initial study explored the effects of patterning by laser and chemical etching on the properties of local back surface fields (LBSF) regions formed by aluminium alloying. Chemical etching resulted in 2 - 3 µm thicker LBSF regions and reduced Kirkendall void formation. This understanding inspired the development of a new inkjet etching patterning technique for patterning anodic aluminium oxide (AAO), a proposed new low cost dielectric for silicon solar cells. Point opening diameters of 20 – 30 µm and LBSF regions that were up to 7 µm deep were realised in 600 nm AAO layer. However, properties associated with the clip anodisation process used to form the AAO layer limited the reliability of this patterning process. A novel light-induced anodisation (LIA) technique was developed to address the limitations of clip anodisation. Anodic silicon dioxide (SiO2) layers formed using LIA achieved comparable effective minority carrier lifetimes to thermal SiO2 with a low density of interface states (Dit) of 6 × 10^11 eV-1 cm-2 and leakage currents that were six orders of magnitude lower than thermal SiO2. The Dit of AAO layers formed by LIA of aluminium were measured to be as low as 1 × 10^10 cm-2 eV-1, a value which is lower than any reported aluminium oxide layer deposited by other deposition techniques. Cells fabricated with SiO2/AAO rear passivation layers achieved open circuit voltages of 660 mV. This value is comparable to that achieved by cells passivated with the industrial standard of aluminium oxide/silicon nitride, therefore highlighting the potential of this new low-cost AAO passivation layer and the novel LIA process to reduce the manufacturing cost of rear-passivated silicon solar cells.