Band Gap Engineering of Alloyed Cu2ZnGexSn1–xQ4 (Q = S,Se) Films for Solar Cell

We fabricated polycrystalline Cu 2 ZnGe x Sn 1– x Q 4 (Q = S or Se) thin films by using spray-based deposition. The effects of Ge alloying were studied by X-ray diffraction (XRD), Raman spectroscopy, and UV–visible spectroscopy. XRD and Raman spectroscopy revealed that lattice parameters decreased linearly and characteristic Raman peaks shifted to higher frequency with increasing Ge alloying. The band gap energies of postsulfurized CZGTS films (1.51 ± 0.05 to 1.91 ± 0.05 eV) and postselenized CZGTSe films (1.07 ± 0.05 to 1.44 ± 0.05 eV) increased almost linearly with an increase of Ge alloying in the respective films. Analysis of band gap bowing model showed a small bowing constant b ∼ 0.1 ± 0.02 eV, indicating high miscibility of alloyed elements. The band gap tuning of CZGTS(Se) thin films can be utilized for tuning band gap of subcell in multijunction cell and for band gap graded photoabsorber of high efficient solar cell.