Investigation of kesterite solar cells on molybdenum foil substrates

Ford, Bethan (2022) Investigation of kesterite solar cells on molybdenum foil substrates. Doctoral thesis, Northumbria University.

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Abstract

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is an attractive photovoltaic absorber material, demonstrating high theoretical power conversion efficiency. CZTSSe is comprised of low toxicity, low cost and earth abundant elements, has a high absorption coefficient and a tuneable direct energy band gap. In this work, Cu2ZnSnS4 (CZTS) nanoparticle inks were fabricated using hot injection of metallic precursors in surfactant, deposited on flexible Mo foil substrates and processed into to thin film CZTSSe solar cell devices. The work focuses on studying flexible CZTSSe devices for wide scale applications; in particular, striving towards comparable efficiencies to those on rigid substrates, investigation into the behaviour of flexible devices when bent and the recovery of flexible CZTSSe over time.

In order to achieve high efficiency devices on flexible substrates, an alkali doping source is required. When fabricating on rigid soda lime glass (SLG) an intrinsic Na source is present in the substrate, reported to promote grain growth and reduce recombination centres in the absorbers. Introducing a slide of SLG during the high temperature selenisation step of the device fabrication on Mo foil substrates results in an absolute efficiency improvement of 1%. Using this doping method, CZTSSe solar devices were fabricated on Mo foil demonstrating a power conversion efficiency (PCE) of 4.5%. Flexible CZTSSe devices show no PCE degradation when bent to a radius of curvature (ROC) of 64 mm. This is a representative ROC for new applications which include curved solar cells. Another factor, which is important for new PV application, is device degradation over time. CZTSSe devices exhibit time-based device degradation. By introducing an annealing step of 150C for 10 minutes in Ar, the initial device performance is recovered.

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