Investigation of Bismuth Doping Effects on CZTS Properties: A Density Functional Theory Study

Post-transition metal-doped quaternary chalcopyrite CZTS film has been proposed as a novel approach to improve solar energy capturing and conversion strategy. In this paper, lattice structural and optical properties of CZTS (Cu
2
ZnSnS
4
) in pure and Bi-doped conditions are theoretically investigated by employing first-principle density functional theory (DFT). Results indicate that lattice constant and bonding lengths do not change after adding bismuth substitution for a tin site. Projected density of states has been calculated for sulfur atoms on the first layer of adjacent Bismuth. Hybridization occurs between 6p state of bismuth and 3s state of sulfur atom. Furthermore, the PDOS diagram represents energy bandgap of 0.7eV and Fermi energy shift to the conduction band edge. An acceptor state is induced due to bismuth-tin substitution about 0.5 eV above the valance band edge. According to optical results extracting from dielectric functions, the absorption coefficient for bismuth-doped structure increases to 5.5×10
5
cm
−1
for low energy photons whereas, pure CZTS has no absorbed photon in this range. A slight decrement is exhibited in visible light reflection. Due to the acceptor state, the conductivity of carrier is facilitated for lower energy photons. Consequently, the substitution between bismuth and tin improved optical properties of CZTS as an absorber layer.