Chemistry and Dynamics of Ge in Kesterite: Toward Band-Gap-Graded Absorbers

The selenization of metallic Cu-Zn-Sn-Ge precursors is a promising route for the fabrication of low-cost and efficient kesterite thin-film solar cells. Nowadays, efficiencies of kesterite solar cells are still below 13%. For Cu(In,Ga)Se-2 solar cells, the formation of compositional gradients along the depth of the absorber layer has been demonstrated to be a key requirement for producing thin-film solar cells with conversion efficiencies above the 22% level. No clear understanding has been reached so far about how to produce these gradients in an efficient manner for kesterite compounds, but among the possible candidates, Ge arises as one of the most promising ones. In the present work, we evaluate the potential of incorporating Ge in Cu2ZnSnSe4 to produce compositional gradients in kesterites. Synchrotron-based in situ energy dispersive X-ray diffraction and X-ray fluorescence have been used to study the selenization of Cu-Zn-Sn-Ge metallic precursors. We propose a reaction mechanism for the incorporation of Ge atoms into the kesterite lattice after the formation of Cu2ZnSnSe4. Electron microscopy reveals that the annealing process leads to Cu2Zn(Sn,Ge)Se-4 absorber layers with an increase of Ge content toward the back contact with independence of the original location of Ge in the precursor layer. The effect of the Ge gradient on the optoelectronic properties of the absorber layer has been evaluated with room-temperature cathodoluminescence. The implications of the results for the development of kesterite solar cells are discussed, with the aim of encouraging new synthesis routes for compositionally graded absorbers.