Multicrystalline silicon material: Effects of classical and rapid thermal processes

Abstract

For photovoltaic applications silicon is still the predominant material. Besides monocrystalline Czochralski wafers (Cz-Si), multicrystalline sheets (mc-Si) play an important role in terrestrial power applications (almost 50%). Large mc-Si ingots (up to 250 kg) are now produced in large scale by the industry using various directional solidification methods in appropriate crucibles (or molds). However, if the crystallographic properties are now quite satisfactory (columnar structure with large grains of more than 1 cm2, dislocations and intragrains defects), multicrystalline silicon contains larger quantities of impurities than single crystalline silicon which can have detrimental effects on the bulk minority carrier diffusion length (Ln,p). These impurities, including metals as well as high concentrations of carbon and/or oxygen, can degrade the photovoltaic properties of solar cells. Thermal treatments such as gettering, performed in a classical or rapid thermal furnace, studied separately or in conjunction with the doping steps can limit or avoid the degradation of the bulk diffusion length, but its efficiency is strongly dependent on the presence of these impurities in Si.