Investigation of intergrain compounds in sputtered Mo films applied in CuIn1 − xGaxSe2-based solar cell

Sputtered molybdenum (Mo) thin film is commonly used as back contact in CuIn1 − xGaxSe2 (CIGSe)-based solar cell. However, when grown directly on soda-lime glass (SLG) substrate, such Mo layer does not only act as electrical contact but also as tuner for the Na availability during CIGSe growth. Control of the diffusion of sodium from SLG to CIGSe is possible by adjusting the Mo intergrain properties. Samples were prepared by direct current magnetron sputtering with argon pressure of 0.3, 1.2 and 2.0 Pa. Transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) have been used to characterize intergrain areas; it is observed that they are amorphous and their width increases from few tenths to several nanometers with the Ar sputtering pressure. Low loss region EELS spectra acquired within these intergrain zones have been compared to reference spectra acquired in the same experimental conditions on well known compounds. The presence of sodium within the intergrain areas, most probably in sodium molybdate form and intercalated in molybdenum trioxide, has been confirmed.