NaSbSe2 as a promising light-absorber semiconductor in solar cells: First-principles insights

NaSbSe2 has recently shown great potential to be a light-absorber semiconductor in thin-film solar cells. Our first-principles calculations show that NaSbSe2 has a quasi-direct bandgap (1.11 eV indirect vs 1.18 eV direct gap), which is beneficial for increasing the lifetime of minority carriers. The optical absorption coefficient is high (exceeding 10−4 cm−1 for visible light) because of the direct band-edge transition from the (Sb-5s/5p + Se-4p) valence band to (Sb-5p + Se-4p) conduction band. The formation of the dominant acceptor defects such as NaSb, VNa, and VSb makes it difficult to dope NaSbSe2 to n-type, and thus, only the intrinsic p-type conductivity has been observed. Se-rich conditions are found to produce high concentration of hole carriers and low concentration of recombination-center defects, so we propose that the Se-rich conditions should be adopted for fabricating high efficiency NaSbSe2 solar cells. Furthermore, the mixed-anion NaSb(S,Se)2 alloys are predicted to be highly miscible with a low formation enthalpy and a low miscibility temperature (below room temperature), and their bandgaps can be tuned almost linearly from 1.1 to 1.6 eV, covering the optimal bandgap range for single-junction solar cells. Therefore, we propose that alloying provides a promising method for optimizing the performance of NaSbSe2-based solar cells.