Optimization of Ti/TiN/Mo back contact properties for Cu(In,Ga)Se 2 solar cells on polyimide foils

Abstract

Molybdenum is conventionally used as electrical back contact for Cu(In,Ga)Se 2 (CIGS) solar cells. In this work, a multifunctional stack of Ti/TiN/Mo is introduced as back contact for flexible CIGS solar cells. The multilayer back contact was deposited on 25 μm thick polyimide foil by means of DC reactive sputtering. To optimize electrical conductivity and film stress of the alternative back contact sputter parameters such as total gas pressure, sputtering power, substrate temperature and RF substrate bias have been varied. XRD measurements and quantitative analysis of foil curvature revealed that the film stress is significantly influenced by the argon gas pressure and sputtering power. The electrical conductivity was improved by applying higher sputtering power or RF substrate bias. Analysis of the film microstructure with SEM shows that applied substrate bias influences the density of the sputtered film. The solar cells processed on Ti/TiN/Mo as well as on a conventional Mo bilayer back contact have been compared using standard current density to voltage (J–V) measurements and external quantum efficiency measurements. Conversion efficiencies of 13.4% for the alternative and 14.9% for the conventional design have been obtained.