Silicon wafers for solar cells used in outer-space are cleaned safely and efficiently

Citation

(2005), "Silicon wafers for solar cells used in outer-space are cleaned safely and efficiently", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 3. https://doi.org/10.1108/aeat.2005.12777cad.001

Silicon wafers for solar cells used in outer-space are cleaned safely and efficiently

Keywords: Silicon, Cleaning

Silicon substrate wafers for the manufacture of solar cells used in outerspace are now exclusively cleaned with DBE Dibasic Esters at RWE Space Solar Power GmbH of Heilbronn, Germany. The company replaced reproduction-toxic dimethyl formamide (DMF) with the non- classified and readily biodegradable solvent from INVISTA (formerly DuPont Textiles & Interiors) during the course of its ISO 14001 eco- certification. A further benefit claimed of DBE is that it costs less than DMF – while additional earnings can also be generated by selling the used solvent to a recycling company.

During the manufacture of solar cells, the silicon substrate wafers undergo several process steps such as coating, masking, exposure and removal of the masking layer. Residual masking material is washed off in a multistage rinsing operation (cascade rinsing) using DBE. It is reported that due to its high dissolving power, this product removes all masking film residue both quickly and efficiently. The cleaning step is necessary to ensure the specified surface purity of the silicon wafers for use in the solar cells.

Compared with the previously used DMF cleaning solution, DBE is markedly safer to handle since it poses no health hazards (and hence, does not require classification). RWE Space Solar Power sells the used DBE to a recycling company. The disposal of DMF, on the other hand, was associated with significant costs.

Solar cells for the use in outerspace must withstand temperatures in the range -150°C to +150°C, apart from being lightweight and highly efficient. A proven substrate for this application is high- purity monocrystalline silicon. Solar cells made with this material attain an efficiency of about 17 per cent. By comparison, terrestrial solar cells based on polycrystalline silicon (for photovoltaic systems) or its amorphous counterpart (used, for example, in watches or pocket calculators) achieve 14 per cent at best. The solar cells for outerspace, which are manufactured from monocrystalline silicon are 130 urn thin and have a surface area of 7cm×4cm:

Apart from silicon wafer cleaning, DBE is suitable for other applications such as cleaning diodes or as a substitute for the N-methyl pyrrolidone (NMP) often employed in the production of semiconductor chips.

Details available from: Samuel Banner and Co. Ltd. Tel: +44 (0)1928 597000; Fax: +44 (0)1928 597002; E-mail: chall@samuelbanner.co.uk