Correlative characterization of co-evaporated Cu2ZnSnSe4 thin-films - CORRKEST

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Recently, kesterite structured Cu2ZnSn(S,Se)4 compound semiconductors have been intensively investigated as absorber materials for thin-film solar cells. The rapidly growing interest in these compounds can be ascribed to the low cost and abundance of the constituting raw materials as well as their beneficial opto-electronic properties (direct band gap and high absorption coefficients), which are similar to those of the established chalcopyrites. However, one of the main limitations of kesterite solar cells is their large loss in open circuit voltage, compared to the band gap, where the reasons for this phenomenon are unclear. The electronic structure of kesterite solar cells is most likely strongly influenced by secondary phases. Secondary phases can be easily formed during fabrication and processing of kesterite films, as kesterite compounds have very narrow existence regions in phase diagrams. However, a clear link between the occurrence of secondary phases and the electronic structure of kesterite absorbers has not yet been established. This is due to the fact that characterization of secondary phases is highly challenging. In this proposed project, we plan to perform correlative characterization on Cu2ZnSnSe4 films, prepared by co-evaporation. The analytical techniques, which will be applied, will include atom probe tomography, electron microscopy, photo- and cathodoluminescence as well as electrical characterization. The main objective of this project is to understand the correlation between the electronic structure of kesterite thin-films (band gap, defects, recombination paths), the bulk composition and the microstructure (secondary phases, order-disorder transition). By establishing fundamental composition-structure-property relationships, we aim to develop a material design concept for kesterites that will support a knowledge driven development of solar cells.