Comparison of Sulfur Incorporation into CuInSe2 and CuGaSe2 Thin-Film Solar Absorbers

Abstract

Herein, sulfurization of CuInSe2 and CuGaSe2 (CGSe) absorber layers is compared to improve the understanding of sulfur incorporation into Cu(In,Ga)Se2 films by annealing in a sulfur atmosphere. It is found for Cu-poor CuInSe2 that for an annealing temperature of 430 °C, sulfur is incorporated into the surface of the absorber and forms an inhomogeneous CuIn(S,Se)2 layer. In addition, at 530 °C, a surface layer of CuInS2 is formed. In contrast, for Cu-poor CuGaSe2 samples, S can only be introduced at 530 °C, mainly forming an alloy of CuGa(S,Se)2, where no closed CuGaS2 layer is found. In Cu-rich CuGaSe2 samples, however, selenium is substituted by S already at 330 °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 CuInSe2 solar cell performance is improved by the sulfurization step at 430 °C, whereas for the 530 °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 CuGaSe2 cells.