A fully solution-processed high performance Cu2ZnSn(S,Se)4 (CZTSSe, kesterite) device has been demonstrated. It is based on the rational engineering of elemental spatial distributions in the bulk and particularly near the surface of the film from nanocrystal precursors. The nanocrystals are synthesized through a modified colloidal approach, with excellent solubility over a large compositional window, followed by a selenization process to form the absorber. The X-ray photoluminescence (XPS) depth profiling indicates an undesirable Sn-rich surface of the selenized film. An excessive Zn species was quantitatively introduced through nanocrystals precursor to correct the element distribution, and accordingly a positive correlation between the spatial composition in the bulk/surface film and the resulting device parameter is established. The enhanced device performance is associated with the reduced interfacial recombination. With a Zn content 1.6 times more than the stoichiometry; the optimized device, which is fabricated by employing a full solution process from the absorber to the transparent top electrode, demonstrates a performance of 8.6%. This composition-control approach through stoichiometric adjustments of nanocrystal precursors, and the developed correlation between the spatial composition and device performance may also benefit other multielement-based photovoltaics.
Keywords: Cu2ZnSn(S,Se)4; nanocrystals; photovoltaics; solution process; spatial composition.