Diffusion control of Na in Cu(In,Ga)Se₂ by the growth Mo:Na layer and variation of its growth pressure

Flexible Cu(In,Ga)Se₂ (CIGS) solar cells face the problem of efficiency deterioration when stainlesssteel (STS) is used as substrate without additional Na incorporation. Na plays an important role in the performance of CIGS thin film solar cells by the increase of current density and improvement of electrical properties. In our present work, sodium-doped molybdenum (Mo:Na) layers are deposited by DC magnetron sputtering system as diffusion layers of Na on the STS substrate for various growth pressures. The deposited Mo:Na diffusion layers and fabricated CIGS solar cells using them are analyzed by Scanning Electron Microscopy (SEM), X-ray diffractometer (XRD), UV-Vis spectrophotometer and solar simulator. X-ray diffraction shows that the CIGS absorption layers grown with Mo/Mo:Na/STS substrates have a pure chalcopyrite phase with a preferred (112) orientation. Na concentration diffused in the CIGS absorption layer is also measured by secondary ion mass spectroscopy (SIMS). The SIMS depth profile and optical measurement results clearly demonstrate that the diffusion of Na into the CIGS absorber layer is controlled by varying the growth pressure of Mo:Na diffusion layer. Also, I-V curves showed that the performance of CIGS solar cell is improved with the increase of Na incorporation. In conclusion, Na concentration in the CIGS absorption layer and efficiency of the fabricated flexible cell can be efficiently controlled by the growth of Mo:Na diffusion layer and the variation of its growth pressure.