Comparison of Sulfur Incorporation into CuInSe2and CuGaSe2Thin-Film Solar Absorbers

Herein, sulfurization of CuInSe(2)and CuGaSe2(CGSe) absorber layers is compared to improve the understanding of sulfur incorporation into Cu(In,Ga)Se(2)films by annealing in a sulfur atmosphere. It is found for Cu-poor CuInSe(2)that for an annealing temperature of 430 degrees C, sulfur is incorporated into the surface of the absorber and forms an inhomogeneous CuIn(S,Se)(2)layer. In addition, at 530 degrees C, a surface layer of CuInS(2)is formed. In contrast, for Cu-poor CuGaSe(2)samples, S can only be introduced at 530 degrees C, mainly forming an alloy of CuGa(S,Se)(2), where no closed CuGaS(2)layer is found. In Cu-rich CuGaSe(2)samples, however, selenium is substituted by S already at 330 degrees C, which can be explained by a rapid phase transformation of Cu2 - xSe into Cu2 - x(S,Se). This transformation facilitates S in-diffusion and catalyzes CuGa(S,Se)(2)formation, likewise that previously reported to occur in CuInSe2. Finally, the Cu-poor CuInSe(2)solar cell performance is improved by the sulfurization step at 430 degrees C, whereas for the 530 degrees C sample, a decreasing fill factor and short-circuit current density are observed, indicating lower diffusion length accompanied by possible formation of an electron transport barrier. In contrast, the electrical characteristics deteriorate for all sulfurized Cu-poor CuGaSe(2)cells.