At present and in the foreseeable future the dominating PV-technology is crystalline silicon, in form of multi-crystalline (mc) and mono- or quasimono crystalline material, as shown by several market surveys. Mono-crystalline silicon is at the moment mainly grown by crystal pulling (Czochralski) and multi-crystalline silicon mainly by directional solidification of large crystal ingots. As the market share of multi-crystalline silicon (47%) is significantly exceeding the market share of mono-crystalline silicon (35%), the directional solidification process can be considered as the best compromise between production costs ($) and achievable solar power (Wp) among the different crystalline silicon PV-technologies. The quasimono technology by directional solidification seems to have some potential for material improvement at low costs but still has several challenges to overcome before entering the industrial production scale.

Today crystalline silicon ingots with a weight of 240kg up to 1 000kg are industrially produced in silicon nitride coated silica crucibles with a square base of up to 1 x 1 m² within a few days. It has been shown that both the impurity contents (metals, carbon, oxygen etc.) and the microstructure (grain boundaries, dislocations) of the silicon material can significantly influence the production yield and the resulting solar cell efficiency. Thus, if one wants to improve these crystallization processes with respect to higher material quality (i.e. higher cell efficiency) and lower production cost, it is first necessary to clarify which crystal properties are relevant for the solar cell. Therefore, it will be briefly summarized which crystal defects in silicon crystals have a strong influence on the performance of solar cells. Then, the formation of some crystal defects is correlated to the process conditions of crystallization and examples are given for the optimization of the crystallization processes by the aid of thermal modeling and lab scale experiments. Finally, a conclusion will be given, how the crystallization of crystalline silicon might be carried out in the future.

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1. Behaviour of Particle Depots in Molten Silicon Under Terrestrial And Microgravity Conditions
2. Influencing the crystallization behaviour of multi crystalline silicon in a R&D furnace
3. Interaction of Foreign phase particles with moving solid-liquid interface during directional solidification of silicon for photovoltaics