Controlling the spacing of silicon MW arrays for radial junction solar cells via microsphere lithography

Abstract

Silicon microwires (MWs) have been widely investigated for realizing the high-efficiency silicon solar cells thanks to the outstanding light absorption property and ease of the formation of the radial junction. To achieve the high-efficiency MW based radial junction solar cells, it is necessary to control the structure of MW arrays such as diameter and length of the MWs and spacing between MWs. The structure of MW arrays is generally determined by the process conditions of the optical lithography, which is the method of forming the pattern. However, the patterning process based on optical lithography reaches the fundamental diffraction limit of about 200 nm. Also, the optical lithography process uses photoresist that is harmful to the human body and requires expensive equipment and a specific place, such as a clean room. In this presentation, we introduced a novel fabrication process of MW arrays through microsphere lithography. In general, the microsphere lithography is the technique to form hexagonally packed ordered arrays of micrometer-sized latex or silica spheres as lithography masks. The spacing between the spheres can be controlled simply by etching the uniformly arranged the microsphere arrays. Also, microsphere lithography has the advantages of low cost, large scale fabrication, and high throughput performance. We formed a monolayer of polystyrene beads using microsphere lithography on a silicon wafer and controlled the spacing between the beads through the oxygen plasma etching up to 100 nm. As a result, we successfully fabricated MW arrays with a dense spacing of 100 nm using polystyrene beads monolayer as an etching mask. The fabricated MWs via the microsphere lithography process exhibited an excellent light absorption of 97% at the entire wavelength region (300-1100 nm) even without anti-reflection film layer. The MW arrays with dense spacing enable the use of MWs that are shorter than that of MWs with micro-scale spacing due to the reducing of a reflective surface. Also, the reduction of the MW length has the effect of reducing the surface recombination of the carriers separated by the radial junction of the MW. Therefore, we expect the proposed microsphere lithography would be the essential process for the formation of optimized MW arrays for the high-efficiency radial junction solar cells.