In-situ CVD processes for crystalline silicon thin-film solar cells
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.