Large performance improvement in Cu2ZnSnSe4 based solar cells by surface engineering with a nanometric Ge layer

This work presents a radically new approach based on the application of very small Ge quantities on the CZTSe surface (from 1 nm to 25 nm thick Ge layers), allowing for a liquid assisted improved crystallization due to the formation of a Ge-Se (Se-rich) liquid phase. This modification improves the charge transport properties at this interface and consequently the device´s voltage and electro-optical properties in general. Using TEM and TOF-SIMS we demonstrate that Ge is barely incorporated into the absorber; nevertheless we observe a drastic increase of the VOC of the solar cells (from 405 mV for the reference to 470 mV for the best Ge modified one). This in turn has a large impact on the performance, increasing it from 7.0% (reference) to 10.1% (Ge modified), which sets a new record efficiency for a Ge containing kesterite and a VOC among the highest obtained for Se-based kesterite solar cells. First characterizations indicate that this is related to an improved grain growth assisted via Ge-Se liquid phases, the minimization of Sn-reduced species and the formation of Ge-O nano-clusters. Our approach not only allows to go towards high efficiency concepts and to contribute to solve the voltage deficit problems in kesterites, but also opens new perspectives for the possible band-gap engineering of kesterite devices with very low Ge concentrations.