Multicrystalline silicon (mc-Si) is silicon material with multiple grains of crystals with different orientation and shape. Mc-Si is often referred to synonymously as polycrystalline silicon, however, mc-Si usually refers to silicon material with a grain or crystal size with larger than 1 mm. Mc-Si is produced by directional solidification in a quartz crucible. Solar cells fabricated with mc-Si silicon are the most common type of the solar cells, with approximately 60 % market share [1]. Mc-Si material has the advantage of being cheaper than silicon material fabricated using the Czochralski process. However, due to the presence of defects in the material—such as grain boundaries and metallic impurities such as iron—mc-Si solar cells typically have a lower conversion efficiency than their monocrystalline silicon counterparts. Furthermore, grain boundaries can also reduce the solar cell performance via the creation of multiple shunt paths.
During the cooling process impurities (most commonly iron) diffuse from the crucible to the ingot creating a so-called “red zone” region of highly concentrated impurities. Furthermore, non-uniformity of impurity distribution and cooling rates, the wafers from the outer region of the ingot typically have lower quality compared to the rest of the ingot. As a result, the wafers from the edges are often not used, reducing the total wafer yield from each ingot.
In addition to the higher impurity concentration, during the growth of mc-Si ingots many defects such as grain boundaries, dislocations, inclusions, and oxides, can be created. To overcome these issues, high-performance multicrystalline silicon (HP-multi Si) was developed [2]. In the HP-multi Si process, seeds are used in combination with careful control of the melt cooling rate to ensure a uniform material with relatively small grains. HP-multi Si has a lower dislocation density compared to conventional mc-Si. This is due to the grain boundaries mitigating the stress which results from the difference in mass density of the silicon melt and the solid. This controlled cooling results in fewer dislocation clusters. In addition to other defects such grain boundaries or iron contamination, it is particularly challenging to neutralise the recombination activity arising from dislocations in the wafer material.
The multicrystalline silicon casting method is shown schematically in the animation below.
[1] – International Technology Roadmap for Photovoltaics (ITRPV) 10th Edition, 2019