Photovoltaic properties of alkali metal X (X=Li, Na, K, Rb, Cs) doped defective molybdenum diselenide structures: First principles study

The electronic structure, work function, complex dielectric function, absorption coefficient and reflectivity of defective MoSe2 models containing point defects as well as alkali metal doping have been calculated based on the first principles of density flood theory. The effect of alkali metal doping on the optoelectronic properties of MoSe2 under defect conditions is investigated. Alkali metal elements were doped separately on the basis of a single Se atom defect, and the results show that the doped systems can all be stable. The introduction of the defect decreases the band gap of MoSe2 from 1.498 eV to 0.816 eV. The alkali-metal-doped defective MoSe2 systems have semiconductor-metal transitions with collisions between the bottom of the conduction band and the top of the valence band for each of the systems, except for the K doping. The K-doped defective MoSe2 is a narrow band gap semiconductor with a band gap of 0.202 eV. Overall, alkali metal doping under defective conditions improves the electrical conductivity of MoSe2, resulting in enhanced conductivity of the crystal. The work function analysis reveals that the work function of the alkali metal doped defective MoSe2, except for K doping, decreases gradually with the increase of its atomic radius, while the electrical conductivity of each system is enhanced. Optical property analyses revealed that the absorption coefficient of alkali-metal doped defective MoSe2 was significantly higher than that of intrinsic MoSe2 in the infrared light region, which compensated for the lack of zero absorption of the intrinsic material in the infrared light region, and enhanced its light-absorbing ability in the infrared light region and part of the visible light region. The reflectivity of each defective MoSe2 system doped by alkali metals is much smaller than that of the intrinsic, and the reflection peaks are red-shifted in the direction of the low-energy region. Overall, the doping of alkali metal atoms reduces the average reflectivity of the defective MoSe2 materials, which is conducive to the enhancement of light absorption and photoelectron emission from the materials. It is hoped that the research results in this paper will contribute to expanding the potential applications of MoSe2 materials in solar cells, photovoltaic devices, and photodetectors.