Structural and chemical analyses of sputtered InxSy buffer layers in Cu(In,Ga)Se2 thin-film solar cells

Sputtered InxSy layers deposited on borosilicate glass and Si at substrate temperatures ranging from about 60 °C to 340 °C were analyzed by means of X-ray diffraction, energy-dispersive X-ray spectrometry, and optical transmission and reflection measurements. With increasing substrate temperature, the InxSy layers exhibit increasing sulfur concentration and also increasing absorption-edge energies. InxSy layers on Cu(In,Ga)Se2(CIGS)/Mo/glass stacks were additionally studied by scanning and transmission electron microscopy. With increasing substrate temperature, Cu, Ga, and In interdiffusion between CIGS and InxSy becomes more enhanced. At 340 °C, CuIn5S8 forms instead of InxSy. The CuIn5S8 formation at elevated temperatures may be the reason for the very low efficiency of solar cells with indium sulfide buffers deposited at temperatures above about 250 °C by various techniques.