Sodium-doped molybdenum back contact designs for Cu(In,Ga)Se2 solar cells

The reliable and homogenous addition of Na to the Cu(In,Ga)Se 2 (CIGS) solar cell absorber represents one major challenge for the industrial implementation of the CIGS technology faces. In this study, an alternative Na source from a Na-doped Mo (MoNa) layer is compared to the conventionally used soda-lime glass (SLG) substrate. In particular, the Na out-diffusion mechanism from different MoNa back contacts into the CIGS layer is investigated for different multilayer designs and magnetron sputtering conditions. From the obtained experimental results a Na diffusion model for the MoNa back contact system is developed, which proposes that Na is mainly collected in the MoNa grain boundary region and then diffuses from these grain boundaries towards the CIGS layer. It is demonstrated that with increasing porosity of the MoNa layer the Na diffusion into the CIGS layer is enhanced. In addition, the low activation energy for Na diffusion found for the MoNa back contact proposes that this Na doping technology is suitable for both low and high substrate temperature CIGS processes. For CIGS solar cells with MoNa back contacts, best cell efficiency of 15% was achieved without anti-reflection coating, which exceeded the performance of the reference sample on SLG with standard Mo back contact.