Earth-abundant photovoltaic semiconductor NaSbS2 in the rocksalt-derived structure: A first-principles study

NaSbS2 was recently proposed as a novel photovoltaic semiconductor with earth-abundant component elements, but its fundamental material properties have not been well studied. The systematical first-principles calculations for its electronic, optical and defect properties were carried out in the present study, and the results show that: i) NaSbS2 in the rocksalt-derived structure has a quasi-direct band gap and thus may have long minority carrier lifetime; ii) its absorption coefficients are as high as 104∼105cm−1 for the visible light and almost isotropic despite that the structure is distorted relative to the high-symmetry rocksalt structure; iii) the effective masses of the electron and hole carriers are anisotropic with much larger values along the z direction than in the x-y plane, and hence the orientational control of thin films should be important for enhancing the photovoltaic performance; iv) the valence and conduction band edges of NaSbS2 are close to those of CuGaSe2, so the n-CdS/p-CuGaSe2 device structure can be inherited to form the n-CdS/p-NaSbS2 solar cells; v) the acceptor defects (NaSb antisites and Na vacancies) have very high concentration, making the synthesized NaSbS2 always be p-type; vi) the S-rich condition can suppress the formation of deep-level donor defects (S vacancies and SbNa antisites) and therefore should be adopted for fabricating high-efficiency NaSbS2 solar cells.