The effect of Na doping on the electronic properties of CuIn1 xGaxSe2 thin films A local probe study

We investigated the effect Na doping on the electronic properties of polycrystalline CuIn0.7Ga0.3Se2 thin films using various scanning microscopy techniques. The doping, achieved by depositing the films onto a NaF precursor layer, is not homogeneous over the surface and appeared to cluster in islands. Our results show a downward band bending in these Na rich areas with respect to adjacent Na poor regions. No band bending was found on control samples prepared without the NaF underlayer, for which also no traces of Na were detected. Moreover, our tunneling spectroscopy results show indications for low density of states at the grain boundaries only in the Na doped samples. It thus appears that the frequently measured downward band bending at grain boundaries in CuIn1 xGaxSe2 films of x lt; 0.5 and the previously measured reduced density of states there is at least partially related to the large amount of Na present there, manifesting its impact on the operation of CuIn1 xGaxSe2 solar cell