Fabrication of CZTS Solar Cells Using Electrodeposition Techniques

Abstract

Copper zinc tin sulphide (CZTS) is a p-type semiconductor that can be used as the light absorbing layer in thin-film heterojunction solar cells, with the specific advantage of being comprised only of non-toxic, earth abundant elements. There are many methods through which CZTS can by synthesised, one of which is electrodeposition, which is an industrially scalable process used extensively in the steel industry. This thesis details a study of the electrodeposition of stacked elemental layers and their subsequent sulphurisation in the manufacture of CZTS. A range of electrodeposition parameters are tested for each elemental layer, each of which is characterised through a range of techniques, including scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), which enables the development of optimised conditions. It was found that the deposition of copper favoured potentiostatic deposition, with a smooth granular structure being deposited onto molybdenum at -0.98V vs Hg|HgO from a sodium hydroxide based electrolyte, while tin required galvanostatic deposition from a methanesulfonic acid electrolyte in order to return consistent results. This was optimised to an initial high current density period of -20 mA cm-2 for 1.2 s to nucleate grains, falling to -5 mA cm-2 to minimise hydrogen evolution thereafter. Trial of numerous electrolyte formulae found that an acid-sulphate electrolyte gave the most promising results, with galvanostatic deposition at -50 mA cm-2 being found to be suitable. Optimised stacked elemental layer precursors are then progressed to the annealing and sulphurisation stage for conversion into CZTS. One key area of study is the inclusion of a pre-alloying annealing step prior to sulphurisation, and its effect on the morphology of the CZTS films and subsequent solar cell device performance. Pre-alloyed metallic films are extensively characterised by means of X-ray photoelectron spectroscopy (XPS) depth profiling, X-ray diffractometry (XRD) and EDS elemental mapping as part of an optimisation process, and Raman spectroscopy is used in conjunction with XRD and EDS in the analysis of CZTS films sulphurised in a rapid thermal processing (RTP) furnace. A pre-alloying step at 300 °C for 10 minutes was found to be sufficient for the deposited elements to fully intermix. It was discovered that not only does the inclusion of an optimised pre-alloying step improve the morphology of the CZTS films and the subsequent solar cell performance, but the integration of a pre-alloying stage with the sulphurisation in a single furnace operation does not lead to any evidence of disadvantage when compared with pre-alloying and sulphurisation processes conducted separately. In fact, 8 out of 45 cells with an integrated pre-alloying process achieved 0.1% efficiency or greater, compared to 5 out of 45 for those that underwent a separate pre-alloying process, and 0 out of 45 for those that received no pre-alloying process. This positive result for the integration of the pre-alloy offers simplification of the manufacturing process for a potential future scaled-up CZTS solar cell device.