Effect of intentional Na-incorporation on the structural, optical and electrical properties of CuInS2:Na thin films for solar energy conversion

Na-doped CuInS2 material is obtained by the incorporation of the sodium of an atomic percentage in amounts of 0, 0.1, 0.5, 1, 2 and 3% with a stoechiometric mixture of the elements copper, indium and sulfur taken in the stoechiometric proportions. We have investigated in this work the effect of intentional Na-incorporation on the structural, optical and electrical properties of CuInS2:Na-doped thin films grown by the single source thermal evaporation method. In the resulting Bridgman-grown ingots brittleness were increased with increased Na and measured hot point probe changed sign from p- to n-type in stoechiometric ingots above 0.5 at.%. The samples having a low Na atomic percentage exhibit better crystallinity that shows the effect of the sodium for the larger percentage to degrade the crystallinity of layers (atomic percentage greater than 2 at.%). The film conductivity was strongly affected by Na-doping, which increased from 10− 6 to 10− 3 S/cm by increasing [Na]/[CuInS2] ratio from 0.1 to 3 at.%. Indeed for an atomic percentage Na of 1% we obtained the biggest thickness (1 μm) corresponds to the lowest refractive index value (2.08).