Silicon microwires as solar cells

Abstract

In this work it has been shown that microwire radial junction solar cells can be realized from silicon core glass fibers made via the molten core fiber drawing method starting with low quality silicon. Through a segregation based purification method the original 3N purity starting material turned into 5N purity silicon micro-fibers that were found to solidify with millimetre-sized grains. The segregation process itself has been investigated through solidification studies on silicon micro-flakes and the purification process has been found to be in agreement with non-equilibrium segregation models. The silicon fibers have been characterized with a wide range of techniques, including Hall measurements, electron microscopy and electron beam induced current. The latter technique showed a ≈ 40 µm bulk minority carrier diffusion length in the fibers, and together with adequate doping densities and surface quality the fibers were determined to be of sufficient quality for use as solar cells. Working prototype solar cells were produced both with a vertical geometry as well as with a horizontal geometry and showed in both cases efficiencies of approximately 3.5%. The limitation of efficiency for both geometries is believed to be non-optimized processing, with the quality of the material itself suggesting that much higher efficiencies are possible. In both solar cell geometries avenues for achieving high light absorption per unit of silicon were addressed and evidence that up to ≈ 90 % of the incident sunlight can be absorbed by employing only ≈ 10 % as much silicon as for a wafer based cell. Due to dimensions and flexibility of the fibers, they offer the possibility of realizing flexible and/or partially transparent crystalline silicon solar cells.