Unveiling the fundamental physical properties of Cu2-xNaxZnSnX4 (X = S, Se) alloys for solar cell applications: a theoretical investigation

Abstract

Quaternary chalcogenide compounds Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are the promising absorber materials for solar cell applications. The impact of Na as an isovalent dopant for Cu on the structure, thermodynamic stability, mechanical, electronic, and optical properties of CZTS and CZTSe is examined for the first time. The computed lattice constants of Cu2-xNaxZnSnS4 (CNZTS) and Cu2-xNaxZnSnSe4 (CNZTSe) (x = 0, 0.5, 1, 1.5, and 2) are increased and decreased as a function of x for a and c, respectively. The stability of CNZTS and CNZTSe is ensured by their negative formation energies. The results show that all alloys are mechanically stable and ductile materials. The band gaps of CZTS and CZTSe are greatly broadened when Cu is replaced by Na. The tunable band gaps of CNZTS and CNZTSe are in the range of 1.19–3.08 eV and 0.65–2.30 eV, respectively. It is found that two novel compounds have suitable band gap values for solar cell applications, such as Cu1.5Na0.5ZnSnS4 (1.47 eV) and CuNaZnSnSe4 (1.43 eV). In addition, Cu1.5Na0.5ZnSnS4 and CuNaZnSnSe4 show good carrier mobility. The analysis of optical properties displays that CuNaZnSnSe4 exhibits a stronger absorption coefficient than those of CZTS and Cu1.5Na0.5ZnSnS4 at 300–500 nm. Combined with the band gap and light absorption, CuNaZnSnSe4 is the most promising material for solar cell applications.