Material and device properties of Cu(In,Ga)Se2 deposited in-line at different deposition temperatures

We investigated the influence of decreased deposition temperature and sodium content on in-line deposited Cu(In,Ga)Se₂. CIGS thin-films were prepared at T_Dep.=420°C and Na was co-evaporated during CIGS deposition. Structural and electrical characterisation was performed and results were compared with standard films and devices prepared at T_Dep.=550°C. For T_Dep.=420°C, Na diffusion from the substrate is suppressed, leading to high resistivity and low energy conversion efficiency as well as to a less pronounced (112) preferential orientation and smaller grains. Na co-evaporation compensates for reduced Na diffusion from the substrate at low deposition temperatures and benefits resistivity and solar cell output parameters. However, a loss of current density at longer wavelengths is detected for low temperature devices. Using an optimised Na co-evaporation process, these losses can be reduced but not totally avoided.