Electrical characterisation of crystalline silicon thin-film material

Abstract

Epitaxial wafer equivalents combine the high efficiencies of conventional wafer-based silicon solar cells with the low production costs of thin-film solar cells. However, many electrical properties of this material are still unknown.One important part of this work comprised the development and establishment of measurement and analysis methods which enable the assessment of the electrical quality of crystalline silicon thin-film (cSiTF) material. Lifetime measurement methods that are already well-established for the characterisation of standard wafer material, such as the microwave-detected photoconductance decay (MWPCD) method, photoluminescence (PL) or quasi-steady-state photoconductance (QSSPC) measurements, have been adapted or modified to suit the needs of cSiTF material.Another important aspect of this thesis was the assessment of the determined minority carrier lifetime and the estimation of its impact on the performance of the final thin-film solar cell. Therefore, a series of solar cells have been processed, analysed and their cell results compared to the lifetimes measured on the starting material. By applying a standard cell process and using highly-doped Czochralski substrates, cell efficiencies of around 15% at an epitaxial base thickness of 15 µm have been achieved, approaching 16.5% for 40 µm thick epitaxial layers. Furthermore, if the same epitaxial layers are deposited on (electrically active) 1 Ωcm p-type float zone substrates, efficiencies up to 18.0% have been archieved.