Source and effects of sodium in solution-processed kesterite solar cells

Based on solution-processed kesterite-type Cu2ZnSn(S1-x,Se-x)(4) solar cells, a detailed investigation of the source and effects of sodium in the absorber will be presented. In contrast to most investigations in literature so far, the main process for sodium supply during selenization does not seem to be the diffusion out of the sodium-containing glass substrate but a transfer from the environment into the absorber. Studies on the influence of sodium on the morphological properties of the absorbers let us assume that the formation of big grains is not necessarily coupled to the presence of sodium during selenization. However, a beneficial effect on the electrical properties can mainly be seen in the open-circuit voltage (V-oc) and the fill factor (FF) which should be related to an increase in doping concentration (N-A).