| With the rapid growth of society's energy demand and the environmental problems caused by massive energy consumption,the development of sustainable,low-cost green energy sources and their energy utilisation technologies has become particularly important.Hybrid Organic-inorganic halide perovskite materials due to their excellent photovoltaic properties,such as high light absorption coefficient,suitable band gap,high carrier mobility,long carrier diffusion length,low trap density and exciton binding energy,have attracted a lot of attention in the field of solar cells.After just a few years of research,the power conversion efficiency of solar cells based on hybrid organic–inorganic halide perovskites has exceeded 25%.Currently,the main obstacles limiting the commercial application of halide perovskites are:poor intrinsic thermal stability and the toxicity of lead.In perovskite material system,as the structural dimension changes from three-dimension to two-dimension,the thermal stability of the material gradually improves.The unique physical properties caused by quantum confinement effect extend the potential applications of halide perovskite materials.However,the stability of two-dimensional hybrid organic–inorganic halide perovskites still needs to be further improved.Meanwhile,the two-dimensional halide perovskite materials also suffer from poor interlayer charge transport performance and lead toxicity.Therefore,exploring the intrinsic correlation between structures and properties and gaining the knowledge of the hidden physical laws in the perovskite material can help optimize the thermal stability and optoelectronic properties of perovskite materials,as well as discover and design novel perovskite materials,which is an essential promotion for the development of perovskite materials.In summary,in this thesis we have selected two-dimensional hybrid organic–inorganic halide perovskite as the object of study and used first-principles calculation based on Density Functional Theory to carry out systematic research work and obtain the following innovative results:1.Revealing the influence of morphology on the electronic properties and the structures of two-dimensional hybrid organic–inorganic halide perovskite materials.We have systematically investigated the effect of morphology on the electronic properties and structures of two-dimensional hybrid organic–inorganic halide perovskites in terms of surface engineering,composition engineering and structural engineering.It was found the two-dimensional perovskites with different organic ligands exhibit different quantum confinement effect for the same thickness of the two-dimensional perovskites.The strength of the quantum confinement effect weakens with increasing thickness of the two-dimensional perovskites with the same organic ligand.In addition,monolayers of two-dimensional perovskite have a larger band gap than the corresponding Ruddlesden-Popper(RP)phases.Planar-averaged squared magnitude of wave functions and the distribution of bond lengths and bond angles of the perovskites suggest there is stronger quantum confinement effect in monolayer two-dimensional perovskites.Our results show morphology engineering can be a potential method to modulate the electronic properties of two-dimensional perovskites,providing theoretical support for their applications in optoelectronic fields.2.Designe for novel two-dimensional Janus hybrid organic–inorganic halide perovskite material and the investigation on its stability and optoelectronic properties.We have systematically investigated the stability and electronic properties of two-dimensional Janus hybrid organic–inorganic halide perovskites(2D-JPs)via first-principles calculations.Compared with the corresponding non-Janus perovskite,we found 2D-JPs exhibit different electronic properties.Firstly,the enthalpies of formation indicate 2D-JPs are thermodynamically stable.Secondly,we found the band gaps and effective masses of 2D-JPs can be tuned in the range of 0.29 e V and 0.27 m_e,respectively.The out-of-plane asymmetry of the structure of 2D-JPs led to an asymmetric distribution of electrostatic potential and band edge charge density,potentially leading to the separation of electron-hole pairs.Bader charge analysis showed two different organic ligands on opposite sides of the inorganic octahedra plane exhibit large variability in the amount of charge transfer.By investigating RP-JPs with two different stacking modes,we revealed the stability of RP-JPs with different ligands.The unique Jansu structure in RP-JPs confered the ability to modulate the band gap.Our results suggested this novel 2D-JPs has unique properties that could have potential applications in the field of electronics and optoelectronics.3.Revealing the role of organic ligands in the transformation of three-dimensional hybrid organic–inorganic halide perovskite into two-dimensional phase.Experimentally the introuduce of unprotonated organic ligands to three-dimensional perovskite can transform them into two-dimensional phase.The role of organic ligands in this process has not yet fully understood.Based on this experimental observation,we have constructed a theoretical model for the transformation of three-dimensional perovskite into two-dimensional phase under the action of organic ligands.Using first-principles calculations,we have calculated the formation energies of two-dimensional perovskite formed by the introuducing of alkylamines with different lengths.It was found that the formation energy of the material decreases with increasing alkylamine chain length.This indicated that long chain alkylamines are more favourable for the formation of two-dimensional perovskites.In addition,we have simulated the proton transfer process using the Nudged Elastic Band(NEB)method.The calculations showed the energy barrier of the proton transfer process is very low,indicating that the transition from three-dimensional perovskite to two-dimensional phase occurs very easily.This work provides strong theoretical support for the experimentally observed transformation of three-dimensional perovskite into two-dimensional phase,revealing the role of organic ligands on the formation of two-dimensional perovskite.4.Investigation on the effect of organic ligand orientation on the structural and electronic properties of two-dimensional hybrid organic–inorganic halide perovskites.By controlling the number of inorganic octahedra layer of the two-dimensional perovskites,the angle of the organic ligands on the two-dimensional perovskite surface and the dipole configuration of the organic ligands,we have constructed 160 two-dimensional perovskite structures.The structural features and electronic properties of these two-dimensional perovskites were systematically analysed and studied by first-principles calculations.We found organic ligand dipoles enhances the octahedra distortion in two-dimensional perovskites.The difference in energy between these two-dimensional perovskite structures is small,but the corresponding band gap varies considerably.Furthermore,we also revealed the linear relationship between the average bond angle of Pb-I-Pb and the band gap in two-dimensional perovskites.This work provides a theoretical reference for exploring the relationship between organic ligand orientation and the structural and electronic properties of two-dimensional perovskites,which facilitates the development of ligand engineering in the regulation of the structure geographies and properties of two-dimensional perovskites. |
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