Project: Recycling of silicon for solar cells by crystal pulling from the melt

Solar grade silicon is one of the major factors that contribute to the costs and energy requirements for production of solar cells. Due to the large growth of the market solar grade silicon is one of the limiting factors for further growth of the industry. _x000D__x000D_In the silicon ingot casting process the impurities generated in the process, and from the feedstock is segregated to the top of the ingots and also the material that has been in direct contact with the crucible will be contaminated and cut off before the ingot is processed to wafers. In addition, aborted runs, section of runs with low quality will be scrapped. Broken wafers represents a considerable amount of silicon._x000D__x000D_Recycling of silicon scrap from the ingot and wafer producers will be important for cost reduction and reduction in energy payback time. _x000D__x000D_SiPro is a company which is recycling material for wafer producers. SiPro is refining their silicon scrap and returning it as useful feedstock. The present technology removes particles and surface contamination from the silicon. However, new technology that can also remove dissolved elements is necessary to further increase the recycling ratios._x000D__x000D_The new process is based on the Czochralski (CZ) process for pulling of single crystals from a crucible with liquid silicon. In this project, the CZ process will be optimized for scrap recycling. Even tough the standard CZ process gives feedstock comparable to virgin solar grade feedstock, it can be optimized to reduce energy and cost especially for refining where there is no demand to the crystal structure of the finished ingot._x000D__x000D_The CO goal of the project is to develop a refining process for the company SiPro for recycling of solar grade silicon. _x000D__x000D_ This will be reached by the following sub goals:_x000D__x000D_i) Further optimize the existing process for removal of surface contamination from PV silicon scrap_x000D_ii) Develop a low cost, low energy consuming refining process based on the CZ pulling process._x000D_iii) Prove the technology and approach the European market by refining silicon supplied by potential customers. Developing cost models and LCA for the value change including the refining step._x000D_iv) Develop solar cell processes that are suitable for the new material. Define standards for recycled materials and intermediate silicon scrap material._x000D__x000D_Industrial scale experiments will be performed to develop the new process, prove the concept and qualify the product at potential customers. Modelling, characterisation and analysis will together with feedback from solar cell production be used to optimize the process._x000D__x000D_The project will be carried out with SiPro as the project leader. SINTEF will develop the new recycling process together with SiPro. ECN will develop the solar cell process that is necessary to define the required purity of the recycled material and prove this for potential customers.

Acronym RESIM (Reference Number: 6588)
Duration 01/08/2011 - 31/01/2013
Project Topic Sipro is recycling silicon scrap for the PV industry and returning this as feedstock. The project aims to develop a new process for refining of silicon that will make it possible to remove dissolved impurities. This will to increase the recycling ratios in the industry for less cost.
Project Results
(after finalisation)
1) Solar cells with less than 1% relative efficiency loss, compared to cells from conventional wafers, can be produced from 'recycled' p-type monocrystalline wafers following the Si Pro (CO participant) recycling route. Implementation of this route in industry would mean about 9% cost reduction (Euro/Wp) and 10% energy consumption reduction for a PV module, compared to the standard route in which polysilicon is used as feedstock. The 'recycled' wafers were made using a standard ingot manufacturing process in which the feedstock was replaced by scrap offcut material from multicrystalline silicon ingots. The offcuts were cleaned with a proprietary cleaning process developed by Si Pro AS. _x000D__x000D_2) Important insights on the role of oxygen incorporation during growth of a monocrystalline ingot and its effect on the efficiency of solar cells were gained._x000D__x000D_3) ECN developed (together with a different project) a process recipe for n-type multicrystalline solar cells with a record conversion efficiency of 17.3% and a good quality (high minority carrier lifetime) n-type multicrystalline ingot was grown from n-type Si pot scrap from n-type monocrystalline Si ingots.
Network Eurostars
Call Eurostars Cut-Off 6

Project partner

Number Name Role Country
3 Si Pro Holding AS Coordinator Norway
3 Stiftelsen SINTEF Partner Norway
3 Stichting Energieonderzoek Centrum Nederland Partner Netherlands