(Li,Na)SbS₂ as a promising solar absorber material: A theoretical investigation

NaSbS₂ has been proposed as a novel photovoltaic material, but its band gap is not suitable for single-junction solar cells. In the present study, the systematic first-principles calculations were carried out to investigate the structural, mechanical, electronic and optical properties of ASbS₂ (A = Li, Na, K) and Na_1-xLi_xSbS₂ solid solutions. These structures show good structural stability compared to CH₃NH₃PbI₃. The results indicate that all the structures are indirect band gap semiconductors. The band gap of ASbS₂ increases gradually when the alkali metal changes from Li to K. The band gap of NaSbS₂ can be tuned by manipulating the amount of Li doping. The Na_1-xLi_xSbS₂ solid solutions have suitable band gaps for light-absorber semiconductors in solar cells. Moreover, the suitable band gap of NaSbS₂ can be also obtained under moderate pressure. The mechanical properties of these materials are also analyzed, and the results indicate that they are brittle materials except for KSbS₂. The optical absorption coefficients of these compounds are large over 10^-5 cm^-1 in the visible light region. We find that alloying can provide a feasible and effective approach for improving the photovoltaic performance of NaSbS₂-based solar cells.