Ex- and in-situ investigations of sulfur diffusion into Cu(In,Ga)Se2 thin films

In this article we investigate the incorporation of sulfur into Cu(In,Ga)Se2 thin films by controlled diffusion. For this purpose, the conventional stacked elemental layer process has been extended by a subsequent sulfur diffusion step. The sulfur gradient is desirable for achieving a graded band gap of the absorber layer which allows to optimize the absorption properties. The sulfur incorporation can be influenced by the sulfurization temperature, the sulfur supply (partial pressure) and the temperature of the Cu(In,Ga)Se2 pre-treatment. The presence of binary selenides and small grains enhances the speed of sulfur incorporation. A combination of Raman spectroscopy and in-situ as well as ex-situ X-ray diffraction has been used in order to obtain detailed information about the diffusion process. The sulfur incorporation occurs at sulfurization temperatures in the range of 575 °C in step 1. By decreasing the selenization temperature in step 2 the amount of sulfur incorporation is enhanced. Furthermore an additional defect spinel phase CuIn5S8 is obtained when sulfur is supplied in excess.