Development of Cu2ZnSnSe4 and Cu2SnS3 based absorbers by PVD processes

Marquez Prieto, Jose (2016) Development of Cu2ZnSnSe4 and Cu2SnS3 based absorbers by PVD processes. Doctoral thesis, Northumbria University.

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Abstract

Kesterite thin film solar cells are one of the most promising technologies for the future thin film PV market. The term “kesterite” refers to the crystal structure that the Cu2ZnSn(S,Se)4 compound adopts.

This thesis discusses the study of the formation of the pure selenide of the kesterite compound Cu2ZnSnSe4 (CZTSe) as an absorber layer. The layers were produced by a 2-stage physical vapour deposition (PVD) of Cu-Zn-Sn precursor films by sputtering followed by a reactive conversion step in the presence of Se. Solar cells have been fabricated with the absorbers produced.

The research explored the evolution of phases during the formation of CZTSe and the influence of the absorber composition on its optical and microstructural properties. In addition, the work involved: optimisation of the CZTSe synthesis process, studying the influence of the Se source, the role of temperature of the conversion process, the role of ramping rate and the ambient pressure, and the role of these for maximising device performance.

From the study of the evolution of phases it was concluded that CZTSe can be formed from Cu-Zn-Sn precursors over a wide range of temperatures (380-550 oC). The formation of the ternary compound Cu2SnSe3 (CTSe) from Cu-Sn precursors using the same synthesis approach was also demonstrated. Whilst this material was considered unsuitable as a solar PV absorber layer due to its low bandgap, the pure sulphide ternary phase Cu2SnS3 (CTS) was considered more suitable and was synthesised using a single step co-evaporation PVD method. A device with an efficiency of 1.8% demonstrated the possibility of using this earth abundant compound for thin film PV.

A combination of X-ray diffraction and Raman spectroscopy studies demonstrated that CZTSe films with very Cu-poor and Zn-rich compositions led to a high population of the beneficial VCu + ZnCu defect clusters, and CZTSe phase domains with a less disordered kesterite type structure. This led to devices with efficiencies over 8% and VOC values greater than those of the current world record CZTSe solar cells. The research of this thesis provides a combination of practical and fundamental knowledge that could become a key towards minimising the efficiency gap between kesterites and their commercialised chalcogenide predecessors: CdTe and Cu(In,Ga)Se2.

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