Theoretical Study on the Ambivalent Roles of the Point Defect on Solid Ionic Compounds via Multi-scale Simulation

Abstract

Point defects such as vacancies, as well as interstitial and substitutional defects, naturally form in ionic compounds and are utilized in various engineering fields. Particularly, point defects act as active sites for catalytic reactions and trap states for electrons and holes. Alternatively, they can induce structural collapse and phase transitions. The effects and ambivalent roles of point defects have been precisely studied with developed experimental equipment. However, there are still many phenomena that can be explained only by theoretical analysis. Therefore, using density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we studied the effects of point defects in representative ionic compounds such as transition metal oxide, transition metal hydroxide, and organic–inorganic hybrid perovskite structures. Herein, the ambivalent roles of point defects in the stability and catalytic activity of such systems were investigated, and the control methods of the point defects were studied. In Chapter 1, a background regarding point defects and solid ionic compounds, which are used in various research applications such as batteries, catalysts, and optoelectronic device systems, is provided. Additionally, multi-scale simulations, namely, DFT calculations and MD simulations, are introduced. They are applicable at the atomic and molecular levels, respectively. In Chapter 2, the role of point defects in transition metal oxide systems is introduced; such defects are employed in solid electrolyte and cathode materials. We have investigated the effect of point defects on a solid electrolyte, namely, a sodium (Na) super ionic conductor (NASICON), and spinel-type LiMn2O4 cathode for a Li-ion battery. In the NASICON, we found that P-vacancies induced a dumbbell-shaped configuration of oxygens, which could block the Na+ ion migration path. By contrast, in the LiMn2O4 cathode, we found that microcrack evolution in a single crystal occurs due to oxygen vacancy condensation in specific crystallographic orientations generated by the continuous migration of oxygen vacancies and TM-ions (especially Mn-ions). Additionally, we proposed that Ca or Mg doping could prevent the migration of the vacancies. In Chapter 3, we present the investigation of the role of point defects in transition metal hydroxides. Transition metal hydroxides, important materials in physics and chemistry, have been applied in many engineering fields, including fuel cells. However, the pristine Ni(OH)2 monolayer, which is a strong candidate fuel cell catalyst, has relatively low catalytic activity and low stability for the oxygen evolution reaction (OER), which is an important reaction for fuel cells. To improve the efficiency of Ni(OH)2, the effect of V and Fe dopants was investigated for a Ni(OH)2 monolayer system. Based on the DFT calculations, we expect that the V and Fe co-doped system will show the highest efficiency, which can be attributed to the independent characteristics of the doping element, the relatively high conductivity and stabilizing ability of the Fe dopant, and the high efficiency of the V dopant for the OER. In Chapter 4, we elucidate the role of point defects in organic–inorganic perovskite materials, which are widely used in optical devices (e.g., photodetector and light-emitting diodes (LEDs)) owing to their relatively high light absorption capability, long carrier lifetime, and adjustable band gap. Using DFT calculations, we investigated the effect of point defects on MAPbI3 and FAPbBr3, which are used in photodetectors and LEDs, respectively. We found that the formation of point defects at the surface system is easier than that in the bulk system for MAPbI3. Therefore, more exposed surfaces can generate more deep trap states, thereby increasing the detectivity of the system. Consequently, we propose that the generation of a large surface and abundant grain boundaries can improve the efficiency of the photodetector. On the other hand, the deep trap state, induced by the formation of point defects, decreases the efficiency of the LED. Herein, we suggest the passivation method of the deep trap state using molecular cations, such as tert-methyl ammonium (TMA). The adsorption of TMA on the surface of the FAPbBr3 shifts the trap state to a lower level, indicating the charge trapping at trap states can be effectively suppressed.