Silicon for photovoltaic applications

Silicon is used in photovoltaics (PV) as the starting material for monocrystalline and multicrystalline wafers as well as for thin film silicon modules. More than 90% of the annual solar cell production is based on crystalline silicon wafers. Therefore, silicon is the most important material for PV today. The challenge which the PV-industry is currently facing is to decrease the manufacturing costs per Wp annually by 5%. Since approximately 70% of the costs for solar cells are caused by wafer costs, there are two main avenues to achieve the cost reduction. One is the development of cheap solar grade Si feedstock material, the other is the development of a cheap ingot manufacturing process for multicrystalline silicon wafers. Therefore, one aim of the PV-industry is to produce sufficiently pure solar grade silicon at low manufacturing costs. Three different routes for the production of solar grade Si are currently considered. Processes like decomposition of trichlorosilane by means of a fluidized bed reactor or decomposition and melting by means of a tube reactor are under development. Monosilane decomposition by means of a free space reactor is also in progress. Every process should be able to achieve the criteria for solar grade Si. Especially, the decomposition of monosilane by means of a free space reactor will be able to meet this challenge as will be explained in more detail. Within the last 10 years multicrystalline (mc) silicon ingots for PV with weights of 150, 240 and 300 kg have been developed and are produced today in the standard production process. The state of the art growth rate of these ingots is 0.5–1.5 cm/h. The two main targets of the PV-industry today are first to increase the ingot weight and second to accelerate the growth of multicrystalline silicon ingots. First, results of the preparation of very large ingots with an ingot weight of 400 kg or ingots grown with growth rates higher than 2 cm/h will be presented.