Ultralow Interface State Density Achieved by Light-Induced Anodization of Aluminum on Silicon Solar Cell Surfaces

A light-induced anodization (LIA) method, which uses the light-induced current of a silicon solar cell to anodize aluminum and form an anodic aluminum oxide (AAO) layer that can reduce recombination at a p-type surface of the cell, is reported. This method can result in anodic oxides with uniform properties over the surfaces of industrially sized silicon wafers since the current flows through the wafer rather than in the surface aluminum layer, as it does when the aluminum is directly electrically contacted. Uniform AAO layers can be formed within 4 min on p-type silicon surfaces by anodizing 300-nm-thick aluminum layers, and when formed over a thin interfacial silicon dioxide layer, the interface state density at the p-type silicon wafer interface can be reduced to values as low as 1 × 10¹⁰ cm^-2·eV^-1 and a fixed charge density of 2 × 10¹¹ cm^-2. Photoconductance carrier lifetime measurements indicate that these AAO dielectric stack layers are capable of passivating silicon surfaces to a level similar to that provided by silicon nitride layers, which are deposited using plasma-enhanced chemical vapor deposition, although at potentially a much-reduced cost. The high-quality surface passivation, rapid room temperature processing, and large-area uniformity of LIA make it a promising method for producing passivation materials that can further improve efficiency and potentially reduce the cost of photovoltaic solar cells and other p-n junction devices.