How small amounts of Ge modify the formation pathways and crystallization of kesterites

<p>The inclusion of Ge into the synthesis of Cu2ZnSn(S,Se)4 absorbers for kesterite solar cells has been proven to<br> be a very efficient way to boost the device efficiency in a couple of recent publications. This highlights the<br> importance to elucidate the mechanisms by which Ge improves the kesterite solar cells properties to such a<br> large extent. In this contribution, we first show how controlling the position and thickness of a very thin<br> (10&ndash;15 nm) layer of Ge greatly influences the crystallization of kesterite thin films prepared in a sequential<br> process. Typically, Cu2ZnSnSe4 (CZTSe) films form in a bi-layer structure with large grains in the upper region<br> and small grains at the back. By introducing Ge nanolayers below our precursors, we observe that large<br> CZTSe grains extending over the whole absorber thickness are formed. Additionally, we observe that Ge<br> induces fundamental changes in the formation mechanism of the kesterite absorber. In a detailed analysis of<br> the phase evolution with and without Ge, we combine the results of X-ray fluorescence, X-ray diffraction<br> and Raman spectroscopy to demonstrate how the Ge influences the preferred reaction scheme during the<br> selenization. We reveal that the presence of Ge causes a large change in the in-depth elemental distribution,<br> induces a stabilizing Cu&ndash;Sn intermixing, and thus prevents drastic compositional fluctuations during the<br> annealing process. This finally leads to a change from a tri-molecular towards, mainly, a bi-molecular CZTSe<br> formation mechanism. Kesterite thin films with surprisingly large crystals of several microns in diameter can<br> be fabricated using this approach. The results are related to the increase in device performance, where power<br> conversion efficiencies of up to 11.8% were obtained. Finally, the consequences of the disclosed crystallization<br> pathways and the extension to other chalcogenide technologies are discussed.</p>