Inhibiting MoS2 formation by introducing a ZnO intermediate layer for Cu2ZnSnS4 solar cells

• A 10 nm ZnO intermediate layer was introduced between Cu2ZnSnS4/Mo layer.
• The ZnO layer can effectively limit the MoS2 formation at back contact.
• The ZnO layer can facilitate the reaction between S and metallic stacking layers.
• The ZnO intermediate layer enhances solar cell efficiency from 2.1% up to 3.3%.

A 10 nm ZnO layer have been employed as an intermediate layer between absorber and back contact in Cu2ZnSnS4 thin film solar cells prepared by sulfurization of the metallic stacking layers. By introducing this layer, the device conversion efficiency increases from 2.1% up to 3.3%, open-circuit voltage increases from 476 mV to 657 mV while short-circuit current decreases from 14.52 mA/cm2 to 10.72 mA/cm2. The ZnO layer can significantly suppress the reaction of Mo back contact with S and facilitate the reaction between S and metallic stacking layers. It results in the MoS2 thickness decrease from 300 nm to 80 nm and less unreacted elemental Sn left in the CZTS absorber after sulfurization annealing as shown in transmission electron microscopy images. However, due to the Sn loss through a path of SnS at high annealing temperature, the reacted and eliminated elemental Sn results in a higher density of voids formed at the bottom of sulfurized stacking layers. The improvement in the cell efficiency and open-circuit voltage could be related to both reduced MoS2 thickness and removal of shunting path generated by elemental Sn. However, the voids formation at back contact deteriorates the carrier transportation and decreases short-circuit current.