Influence of argon pressure on sputter-deposited molybdenum back contacts for flexible Cu(In,Ga)Se2 solar cells on polyimide films

Abstract

In this study, polyimide films with various thicknesses and coefficients of thermal expansion were used as candidates of lightweight and flexible substrates for Cu(In, Ga)Se2 (CIGS) solar cells. The influence of the Ar background pressure (PAr) during deposition on the properties of the Mo back contact on the polyimide films was investigated. It was found that the stress condition of the Mo contact was the sum of thermal and intrinsic stresses. The thermal stress depended on the coefficient of thermal expansion (CTE) of polyimide films and induced compressive stresses. The intrinsic stress was largely influenced by the PAr. High PAr caused large tensile intrinsic stress and the subsequent formation of cracks in the Mo layer deposited on the polyimide films, whereas no apparent formation of cracks was observed on the reference rigid-glass substrates. The crack formation in Mo deposited on the polyimide films subsequently caused remarkable increase in sheet resistance, which deteriorated the fill factor of the CIGS solar cells. The decreasing PAr reduced the tensile intrinsic stress in the Mo layer, preventing the detrimental crack formation and improving the fill factor of the CIGS solar cells on the flexible polyimide substrates. The stress condition of the Mo was also found to influence thermal diffusion of alkali metal through the contact. The impact of the Mo stress on alkali metal diffusion was adjustable by varying the thickness of alkali silicate thin layers and/or alkali-metal postdeposition treatment conditions, improving conversion efficiency.