Natural Intermediate Band in I ₂ -II-IV-VI₄ Quaternary Chalcogenide Semiconductors

An intermediate band in the band gap of semiconductors is fundamental to the development of the intermediate band solar cells, but it is usually produced artificially, which imposes technical challenges on the experimental realization. Here we found that there are natural intermediate bands in the band gaps of the I₂-II-IV-VI₄ quaternary chalcogenide semiconductors such as Cu₂ZnSnS₄ and Ag₂ZnSnSe₄, which had been proposed as promising light-absorber semiconductors in thin film solar cells. By first-principles calculations, we found the lowest conduction band of these I₂-II-IV-VI₄ semiconductors in the kesterite structure is isolated (a lone band, resulting from the energy separation between Sn 5s and 5p states), which can be viewed as a natural intermediate band. The gap between the intermediate band and higher-energy conduction band can be increased through changing the crystal structure from the zincblende-derived kesterite structure to the wurtzite-derived wurtzite-kesterite structure. In contrast, the intermediate-conduction band gap shrinks when the component element Sn is replaced by Ge (Cu₂ZnGeS₄), and the gap even disappears (intermediate band disappear) when Sn is replaced by Si (Cu₂ZnSiS₄). Through tuning the intermediate-conduction and intermediate-valence band gaps, we show that the wurtzite-kesterite structured Ag₂ZnSnSe₄ may be a potential light-absorber semiconductor in intermediate band solar cells.