The aim of this paper is to present in-situ and cost-effective processes for crystalline silicon thin-film solar cells grown by high-temperature chemical vapour deposition on low-cost silicon substrates. The central approach is the epitaxial wafer-equivalent (EpiWE) cell structure, consisting of an epitaxial layer deposited on a low-cost silicon substrate. This EpiWE is then processed using a standard wafer solar cell process. Novel in-situ chemical vapour etching (CVE) and deposition (CVD) processes extend this concept to a nearly completed solar cell, the `EpiCell'. These processes enable the use of substrates produced from slightly purified metallurgical silicon and result in efficiencies potentially as high as for standard multicrystalline wafers. The results show that the saw damage can easily be removed by etching the samples at high temperatures with HCl gas. Gettering of highly contaminated substrates reduces the amount of impurities by up to 80%. This is the first time that the gettering effect of HCl gas diluted in hydrogen for PV application is demonstrated. By porosification of the substrates and texturing the front side, the incident light is confined in the deposited layer. The epitaxial growth allows the fabrication of high efficiency emitters resulting so far in solar cell efficiencies up to 15.2% on Cz substrates (92 cm2). A front side texture by HCl etching shows a diffuse fraction of nearly 90% of the spectral reflectance. All these processes can be transferred to the newly designed ProConCVD, our high-throughput in-line CVD tool
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 t he 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
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