Theoretical Study Of The Electronic And Optical Properties Of FeS₂ Under Strain And Doping

By applying the first-principles method based on density functional theory, we have systematically studied the atomic structure, as well as electronic and optical properties of FeS2 under strain and Zn-doping. The band gap and optical properties of FeS2 have been improved to the benefit of enhancing theoretical photoelectric conversion efficiency of FeS2 solar cell.First, we have systematically studied the atomic structure, as well as electronic and optical properties of FeS2 under strain. The results show that, under uniaxial and biaxial strain, the band gap of FeS2 decreases with the increasing compressive strain, and the band gap increases and then decreases with the increasing tensile strain. Besides, the band gap increases to its maximum value 1.13 e V at 10% uniaxial tensile strain, while increases to its maximum value 1.15 eV at 6% biaxial tensile strain with enlargement of 0.30 e V compared with the free strain FeS2. In addition, the low intensity states at conduction band extend under the increasing compressive strain and the onset of sharp rise moves to right, which results in the blue shift of the absorption edge. While, when FeS2 is subjected to tensile strain, the low intensity conduction states disappear and the sharp rise moves to left, which makes the absorption edge red shift. The red shift makes the optmum energy scope match with the optimum solar spectroscope(1.5~3.1 eV) and enhance the overall optical absorption so that increases theoretical photoelectric conversion efficiency of solar cell. Compared the results of uniaxial and biaxial strain, we find that biaxial tensile strain can effectively improve the electronic and optical properties of FeS2 in photovoltaic applications than uniaxial strain.Then, we have studied the effect of Zn-doping on atomic structure, as well as electronic and optical properties of FeS2. The results illustrate that the lattice constant of Fe1-xZnxS2 increases monotonically with increasing Zn concentration, that's because Zn ions are larger than Fe ions. Besides, the formation energy is positive and increases with increasing Zn concentration, implying that it is difficult to incorporate Zn in pyrite and it becomes more difficult at higher Zn concentration. Additional, the band gap of Fe1-x Znx S2 increases and then decreases with the increasing Zn concentration, and reachs its maximum value 0.95 eV at 6.25% concentration with enlargement of 0.10 e V compared with the undoped FeS2. Moreover, the optical absorption threshold shifts toward left continually with the increasing Zn concentration, which means the Zn-dopped FeS2 alloys are indeed stronger light absorbers relative to pure FeS2.Finally, we have studied the combined effect of Zn-doping and biaxial strain on atomic structure, as well as electronic and optical properties of FeS2. The results indicate that the band gap of Zn-doped FeS2 decreases with the increasing compressive strain, and the band gap increases and then decreases with the increasing tensile strain. Besides, the band gap increases to its maximum value 1.14 eV at 5% biaxial tensile strain, which increases by 0.19 e V compared with the free strain Zn-doped FeS2. In addition, the low intensity states at conduction band extend under the increasing compressive strain and the onset of sharp rise moves to right, which results in the blue shift of the absorption edge. While, when Zn-doped FeS2 is subjected to tensile strain, the low intensity conduction states disappear and the sharp rise moves to left, which makes the absorption edge red shift. Compared the effect of strain or doping alone, the combined effect of Zn-doping and biaxial strain can futher enhance the overall optical absorption.