Addition of Na into CuInS2 thin film via co-evaporation

While most of studies focus on the addition of Na into CuInGaSe2 as well as CuInGaS2 thin films, this study examines the addition of Na into CuInS2 (CIS) thin films. Moreover, an alternative approach has been used to incorporate Na into CIS thin films. Two source evaporation (Cu and In) was first performed to obtain Cu–In layers with desired thicknesses. Three source evaporation (Cu, In, and NaF) then followed subsequently to produce Na-doped Cu–In precursor films having different Na concentrations. The precursor films were immediately sulfurized in the same evaporation chamber to form CIS thin films. The addition of Na was found to enhance (112) preferred orientation and reduce the grain size. Raman spectra show that the addition of Na does not alter the needed phase transformation from CuInS2–CuAu structure to CuInS2–chalcopyrite structure during the sulfurization. Blue shift of the CIS Raman CH mode occurs as a result. The doping of Na was also found to decrease the film resistivity or increase the hole concentration in the films.