HCl gas gettering for crystalline silicon thin film solar cells

Crystalline silicon thin film (cSiTF) solar cells could be an attractive alternative for standard silicon solar cells. Only a small amount of the expensive high purity silicon is needed for the epitaxial deposition on a low-cost silicon substrate made from e.g. metallurgical grade (MG) or upgraded metallurgical grade (UMG) silicon. The resulting product is called epitaxial wafer equivalent (EpiWE) because it can be processed in a standard wafer cell production. MG-Si and UMG-Si still contain a huge amount of metallic impurities. These impurities have to be removed by gettering methods in order to prevent diffusion into the highly pure active silicon layer during the high-temperature deposition step. A promising gettering technique which is investigated at the Fraunhofer ISE is HCl gas gettering, a cheap and fast one-step gettering method. In this work we introduce a simplified model to simulate HCl gas gettering. We apply HCl gas gettering to UMG-Si wafers and analyse the content of metallic impurities before and after gettering by common analytical methods like Inductively Coupled Plasma with Optical Emission Spectrometry (ICP-OES) and Instrumental Neutron Activation Analysis (INAA). The gettering efficiency is calculated by the analysis results. Additionally, we show results of EpiWE solar cells which were made from UMG wafers with and without gettering step to evaluate the improvement of the electrical properties by gettering. HCl gas gettering shows great potential in reducing metal impurity levels at the surface as well as in the bulk of Si wafers. It is an advantageous method since it can be easily included into the EpiWE cell concept. After gettering of the substrate, the back surface field, the base, and the emitter can be grown epitaxially and in-situ by Chemical Vapor Deposition (CVD) on top. Standard steps like texturing, surface passivation, metallization and anti-reflection coating (ARC) can be added to finish the wafer equivalent to a solar cell.