Pei Hsuan Lu

Anodic Aluminum Oxide Passivation For Silicon Solar Cells

The requirement to form localized rear metal contact regions for higher silicon solar cell efficiencies places demand on patterning techniques in terms of the small size of the openings and the ability to perform the patterning at commercial wafer processing rates. We suggest here the possibility of using a self-patterning approach which offers the potential of enhanced surface passivation and nanoscale patterning achieved using a single electrochemical anodization process. It is shown that when nanoporous anodic aluminum oxide (AAO) layers are formed by anodizing an aluminum layer over an intervening SiO2 or SiN x dielectric layer, the implied open-circuit voltages of p-type silicon test structures can be increased by an average of 40 and 47 mV, respectively. Capacitance-voltage measurements show that these passivating AAO dielectric stack layers store positive charges, which differs from what is observed for Al2O3 layers deposited by plasma-enhanced chemical vapor deposition or atomic layer deposition. Furthermore, we show that the magnitude of the stored charge in the dielectric stacks depends on the anodization conditions, highlighting the possibility of controlling the charge storage properties of these layers for specific cell requirements. Although the passivating properties of the anodized aluminum layer appear to be strongly influenced by charge effects, it is also possible that hydrogenation effects may play a role as has been previously observed for other electrochemical processes, such as metal plating.

Field-induced anodization for silicon solar cell passivation

Anodic aluminium oxide formed on silicon wafers can passivate phosphorus-doped silicon surfaces. We describe here a new method for forming these layers which involves forward-biasing a solar cell such that the n-type surface becomes anodic. Aluminium, deposited on the n-type silicon surface over a thin tunnel oxide, can be anodized to form a porous layer. However, unlike anodization processes where the aluminium is physically contacted, increases in effective minority carrier lifetime were only observed after a subsequent anneal at 400 °C in an industrial belt furnance. Anodic aluminium oxide layers formed over n-type silicon surfaces may find applications as rear surface passivation layers for n-type cells. Furthermore, the process can be adapted to the anodization of silicon and thus provide a low temperature method of forming thin silicon dioxide layers which are commonly-used in conjunction with other passivation layers.

Anodic aluminium oxide rear passivated laser-doped selective-emitter solar cells

This paper presents a passivated emitter and rear cell (PERC) design featuring an anodic aluminium oxide (AAO) - thin SiO2 stack layer for rear surface passivation. Rear contacts were formed by laser-doping lines through the nano-porous AAO layer and then sintering aluminium through the lines to make electrical contact. Open circuit voltages as high as 660 mV were achieved which is comparable to voltages achieved from cells with plasma enhanced chemical vapor deposited Al2O3 and a-SiNx:H rear stack layers. High series resistance and a poor SiNx antireflection coating limited the fill factor and short circuit current, respectively.

Stored charge properties of anodic aluminium oxide on silicon substrate

Anodic aluminium oxide (AAO) has been demonstrated to electronically passivate silicon surfaces, with one of the important contributors to the passivation being the electric field created by the stored charges in the AAO. This paper reports the stored charge properties of AAO calculated from MOS (Metal-Oxide-Semiconductor) capacitance-voltage (C-V) measurements and contactless corona C-V measurements. Results from both methods suggested that AAO stores a high density of positive charges, which are influenced by the properties of the intervening dielectric layers. Test structures were fired in an industrial belt furnace and the impact of firing on fixed charge density, interface state density and passivation were ascertained.

Formation of metal-metal oxide patterns using masked light-induced anodization

A selective patterning technique using light-induced anodization (LIA) of a masked metal surface is reported. The method can be used to form metal patterns under masked regions whilst the unmasked regions are anodized to create an anodic metal oxide dielectric layer. The through-wafer current flow in LIA allows the anodization process to be modelled as an array of resistors in series with the solar cell. In this paper, such a model was used to confirm that residual metal remains adjacent to the mask thereby limiting the resolution of the selective anodization process. It is shown that thinner metal layers and larger anodic currents can result in higher pattern resolution. Coupled with the use of a higher resolution masking method with smaller/thinner masked regions, it should therefore be possible to form higher resolution metal-dielectric patterns that can be used for seed layer metal applications.