Band Gap Tuning of Cu2ZnGeSxSe4-x Absorbers for Thin-Film Solar Cells

In this work, kesterite-type Cu2ZnGeSxSe4-x absorbers were prepared by a two-step process for use in thin-film solar cells. Their high band gap makes them an interesting candidate as top cells in multijunction solar cells. However, an exact tuning of the band gap is essential. Therefore, for the first time, the[S]/([S] +[Se]) ratio was controlled via addition of a variable amount of GeS during the annealing step, which allowed precise control of the band gap between 1.5 and 1.7 eV. The changes in morphology and crystallinity of the absorber are discussed in detail. An additional focus was directed toward the parameters of the resulting solar cells. Although the efficiency declined as the [S]/([S] +[Se]) ratio increases, the open-circuit voltage was considerably increased.