Inhibiting the absorber/Mo-back contact decomposition reaction in Cu2ZnSnSe4 solar cells: the role of a ZnO intermediate nanolayer

This work reports a process based on the use of an ultrathin (10 nm) ZnO intermediate layer for the improvement of the absorber/back contact interface region in Cu2ZnSnSe4 (CZTSe) kesterite solar cells. Raman microprobe measurements performed directly on the substrate surface after mechanical removal of the absorber layer indicate the occurrence of a decomposition reaction of Cu2ZnSnSe4 in contactwith the Mo substrate. This leads to a significant degradation of the quality of the absorber/back contact interface, with the formation of a high density of voids. The presence of an intermediate ZnO layer on the Mo coated substrates inhibits the decomposition reaction, because it prevents interaction between the CZTSe and Mo layers during the annealing process. This leads to a significant improvement in the interface morphology as observed by detailed cross-section scanning electronmicroscopy. It also correlates with the observed increase of the device conversion efficiency from 2.5% up to 6.0%. The improvement in the optoelectronic characteristics of the cells could be related to a significant decrease of the device series resistance due to the formation of a smoother interface with low density of voids, resulting from the effective inhibition of the CZTSe decomposition reaction at the Mo back contact layer.